What is a Servo Motor?
A Servo Motor is a motor which is part of a servomechanism. It is typically paired with some type of encoder to provide positioning and speed feedback.
Servo Motor Basics
A Servo Motor is defined as an automatic device that uses an error-correction routine to correct its motion. The term servo can be applied to systems other than a Servo Motor; systems that use a feedback mechanism such as an encoder or other feedback device to control the motion parameters. Typically when the term servo is used it applies to a 'Servo Motor' but is also used as a general control term, meaning that a feedback loop is used to position an item.
A servomechanism may or may not use a servo motor. For example, a household furnace is a servomechanism that is controlled by a thermostat. Once a set temperature is reached, there is feedback signaling it to shut off; making it a "servo" in nature. The term "servo" describes more of a function or task, than it does a specific product line. For this guide, we will discuss servo motors specifically.
A servo motor can be a DC, AC, or brushless DC motor, combined with a position sensor; in most cases, a digital encoder. A servo motor is typically the motor selected when it is essential that there is a high degree of confidence that the servo motor and drive system will closely track what is asked of it. There is typically a higher cost to a servo motor system than a stepper motor system, due to the servo motor's feedback sensor and processing electronics.
How to Operate Servo in JogHow to Operate Servo in Speed Control
Physical Properties of a Servo Motor
A Servo Motor consists of three major parts: a motor, control board, and potentiometer (variable resistor) connected to the output shaft. The motor utilizes a set of gears to rotate the potentiometer and the output shaft at the same time. The potentiometer, which controls the angle of the servo motor, allows the control circuitry to monitor the current angle of the servo motor. The motor, through a series of gears, turns the output shaft and the potentiometer simultaneously. The potentiometer is fed into the servo control circuit and when the control circuit detects that the position is correct, it stops the servo motor. If the control circuit detects that the angle is not correct, it will turn the servo motor the correct direction until the angle is correct. Normally a servo motor is used to control an angular motion of between 0 and 180 degrees. It is not mechanically capable (unless modified) of turning any farther due to the mechanical stop build on to the main output gear.
Where are Servo Motors used?
Servos are extremely useful in robotics and automation. Servo motors are used across various automation fields specifically where the motor must be able to operate at a range of speeds without overheating, operate at zero speed while being able to retain its load in a set position, as well as operate at low speeds. Servo motors are utilized in industrial machine tools, CNC manufacturing machines and processes, and packaging applications. Robots utilize servo motors because of their smooth commutation and accurate positioning. The aerospace industry makes use of servo motors in their hydraulic systems to contain system hydraulic fluid. The servo motor is relatively small in size, yet very powerful. A servo motor also draws power proportional to the mechanical load.
Anaheim Automation's customer base for the Servo Motor product line is diverse: companies operating or designing automated machinery or processes that involve food, cosmetics or medical packaging, labeling or tamper-evident requirements, cut-to-length applications, assembly, conveyor, material handling, robotics, special filming and projection effects, medical diagnostics, inspection and security devices, pump flow control, metal fabrication (CNC machinery), and equipment upgrades. Many OEM customers request that we "private-label" the Servo Motor, so that their customers stay loyal to them for servicing, replacements and repairs.
What Industries are Servo Motors used in?
Servo motors are seen in applications such as factory automation, robotics, CNC machinery, and packaging. The feedback lets the drive know its position, speed, and torque to detect unwanted motion. Pharmaceutical industries are driven be the need to create smaller devices; ones that are easier to operate and function more efficiently.
How Does a Servo Motor Work?
Typical servo motor mechanism is not complex. The servo motor has control circuits and a potentiometer that is connected to the output shaft. The shaft, which is the output device, links to a potentiometer and control circuits that are located inside the servo. The potentiometer, coupled with signals from the control circuits, control the angle of the shaft – anywhere from 0 to 180 degrees, sometimes further. The potentiometer allows the control circuitry to monitor the current angle of the servo motor. If the shaft is at the correct angle, the servo motor idles until next positioning signal is received. The servo motor will rotate the correct direction until the angle is correct.
Each servo motor works off of modulation known as Pulse Coded Modulation, or PCM. The motor has a control wire that is given a pulse application for a certain length of time. The angular degree of the shaft is determined by the length of the pulses, which the servo motor anticipates every couple seconds. A normal servo is mechanically not capable of rotating further due to a mechanical stop built into the main output gear. The amount of power applied to the motor is proportional to the distance it needs to travel. So if the shaft of the servo motor needs to turn a large distance, the servo motor will run at full speed. If the servo motor needs to rotate only a small amount, the motor will run at a slower speed. This is referred to as Proportional Control. The servo motor expects to see a pulse every 20 milliseconds, (.02 seconds) and the length of each pulse will determine how far the servo motor will rotate.
How to Select a Servo Motor
The simplified definition of a servo system is that it consists of several components which together control or regulate speed/position of a load. The servo motor is one of these components in the system. When it comes time to select an appropriate servo motor for an application some people may be naïve in thinking that they can just check size the motor based on the horsepower rating of the presently installed motor, or exclusively based on the application's torque requirements. The following factors must all be taken into account when selecting the appropriate motor: inertia ratio, speed, and max torque at desired speed.
Any rotating object has a moment of inertia which is a measurement of how difficult it is to change the rotating velocity of that object. Moment of inertia in a servo system can be divided into two parts; load inertia and motor inertia. The motor inertia is part of the servo design and is typically listed in the manufacturers' specification sheet. Load inertia is more complicated because it involves every component that is moved by the motor, and is calculated using proper equations for each component. A typical inertia ratio for most applications is 5:1, but the lower the ratio is, the higher performance will be, and vice versa.
Since there may be a variety of servo motors that meet the required inertia ratio specifications, the next step is to find the smallest, most cost-effective servo motor that will meet the speed and torque demands. Servo motor manufacturers normally provide speed-torque curves for each series of motors, which illustrate several interesting points of the servo motor's characteristics. The speed-torque curve contains two regions; continuous and intermittent, which can translate to correct match or incorrect match (respectively) for the application. If the speed-torque required for a specific application falls into the continuous region of the speed-torque curve, then that motor can produce that torque and speed without overheating. If the speed-torque required for the application falls into the intermittent region of the speed-torque cure, then that motor can only produce that speed and torque for a limited amount of time before overheating.
How are Servo Motors controlled?
Servo motors operate on negative feedback, meaning that the control input is closely compared to the actual position via a transducer. If there is any variance between physical and wanted values, an error signal is amplified, converted, and used to drive the system in the direction necessary to reduce or eliminate error. Servo motors are controlled by a pulse of variable width that is sent from a micro-controller output pin to the servo motor's control wire. The shaft angle is determined by the duration of the pulse, also known as pulse width modulation (pwm). This pulse has to have specific parameters such as; minimum pulse, a maximum pulse, and a repetition rate. Given these constraints, neutral is defined to be the position where the servo has exactly the same amount of potential rotation in the clockwise direction as it does in the counter clockwise direction. It is important to note that different servo motors will have different constraints on their rotation, but they all have a neutral position, and that position is always around 1.5 milliseconds (ms).
Anaheim Automation offers AC Servo Drives providing high speed DSP. These servo motors are equipped with auto disturbance rejection control and speed observation control algorithm, in addition to compensation servo delays, forward feed control, and reference smoothing techniques. Anaheim Automation Servo Drives are equipped with a range of dynamic features:
High Overload Capacity
The industrial grade Intelligent Power Modules (IPM) utilized in the EDB/EDC AC Servo Drivers are one step higher in capacity than other servo products that are specified at the same power.
Standard CAN bus interfaces are available in the EDC AC Servo Driver, simplifying the integration process. Based on Modbus protocols from either RS485 or RS232 interfaces, up to 32 servo motors can be connected together. When RS485 interface is used, the transmission distance can reach up to 4000 feet. Anaheim Automation AC Servo Drivers can also communicate with a PLC, DCS, intelligent instruments, touch screens, and more.
ESView Communication Software
Anaheim Automation software is capable of the following:
|-||Parameter Management - Fast and convenient operations to all parameters available, including editing, transmission, comparison, and initialization.|
|-||Monitoring - Real time monitoring of all I/O signals, alarms of the present and history records, and system status|
|-||Real Time Management - Real time sampling of the torque vs. speed curves for simple, rapid analysis.|
|-||Adjusting - Quick adjustment of gains.
Servo Motor Types
There are two main types of Servo Motors: Rotary and Linear.
Rotary Servo Motor
A rotary Servo Motor is what most people think of when they think of a Servo Motor. The three types of Rotary Servo Motors are: AC Servo Motor, Brush DC Servo Motor, and Brushless DC Servo Motor. The motion of a rotary Servo Motor is often converted into linear motion by the use of a screw thread (ball screw or lead screw), or with the use of belts and pulleys.
A Rotary AC Servo Motor is an AC type motor that is used with a feedback device. These are typically used in smaller applications, because a large AC Servo Motor is typically inefficient when compared to its DC or Brushless counterparts.
Linear Servo Motor
A linear Servo Motor is a flattened out Servo Motor where the rotor is on the inside, and the coils are on the outside of a moveable u-channel. Both Servo Motor types are becoming more popular as Servo Motor prices continue to come down.
Servo Motor Cost
A Servo Motor is considered one of the more expensive motors when compared to AC, Brushless, DC, Stepper, and other motor types. The reason for the expense of a Servo Motor is the precision required to make a Servo Motor, and the added components that go along with a Servo Motor. Generally speaking, a Servo Motor is intended to be used as a precise positioning or speed control device. The motion should be smooth and very precise. To accomplish these features, the Servo Motor is manufactured under very tight control parameters. In addition to the cost of the Servo Motor are the case, bearings, connectors, and feedback devices. The case is usually industrial grade, often sealed to achieve an IP65 rating or better. The bearings are high quality to ensure that the Servo Motor can run the speeds desired and can handle the appropriate axial and radial loads. The connectors are typically mil-style connectors that can be detached at the Servo Motor, but are very reliable and industrial grade. The feedback devices are typically differential encoders and or resolvers. These devices are very expensive and add cost to the Servo Motor.
Servo Motor Feedback
There are two options for Servo Motor feedback controls, either a servo encoder or a servo resolver. A servo encoder and a servo resolver provide the same solution in many applications, but are vastly different. They are both used to sense speed, direction, and position of the Servo Motor output shaft.
The resolver on the Servo Motor uses a second set of rotor and stator coils called the transformer to induce rotor voltages across an air gap. The resolver does not use any electronic components, therefore it is very robust with a high temperature range, and is inherently shock-resistant due to its design. A resolver is mostly used in harsh environments.
The optical encoder on the Servo Motor uses a rotating shutter to interrupt a beam of light across an air gap between a light source and a photodetector, over time the wear associated with the rotating shutter reduces the longevity and reliability of the encoder. The application will determine whether a resolver or an encoder is needed. Encoders are more accurate and are easier to implement so they should be the first choice for any application. The only reason to choose a resolver is environmental concerns and longevity requirements.
Servo Motor Accessories
Anaheim Automation provides many different accessories for its Servo Motor product line. These accessories include brakes, encoders, connectors, cables and a handheld interface unit, as well as a full line of servo motor drives.
The Servo Motor brake is a 24VDC system. These Servo Motor brakes are perfect for holding applications and are available for Anaheim Automation Servo Motors. They can be purchased separately or are attached to the rear of the Servo Motor. The Servo Motor brakes have a low voltage design for applications that are susceptible to weak batter, brown out, or long wiring runs. When electric power is applied to the Servo Motor brake, the armature is pulled by the electromagnet force in the magnet body assembly, which overcomes the spring action. This allows the friction disc to rotate freely. When electrical power is interrupted, the electromagnetic force is removed and the pressure spring mechanically forces the armature plate to clamp the friction disc between itself and the pressure plate.
Anaheim Automation's Servo Motor is designed with a 2500 counts per revolution quadrature encoder, with a resolution of 10,000 pulses per revolution.
Anaheim Automation's Servo Motor comes with the necessary connectors to connect to another company's servo drive or an Anaheim Automation servo drive. These Servo Motor connectors can also be purchased separately should they become lost. Please refer to the servo motor user's guide for a specific part numbers.
Servo Motor cables can be made with the supplied Servo Motor connector, or can be purchased from Anaheim Automation. The Servo Motor cable comes with a standard length of 5M but can be adjusted to any length required.
NOTE: Anaheim Automation strongly recommends the purchasing of the cables with servo motors and drives to ensure cable integrity. These cables are perfectly matched and a stable form of connection.
Servo Motor History
The steam engine governor is considered the first powered feedback system that used a gain value so it is considered the first servo mechanism. The word Servo Motor comes from the French phrase "Le Servomoteur" or the "slave motor". The first known record of its use was by JJL Farcot in 1868 to describe steam engines and hydraulics for use in steering a ship.
Servo Motor Customization
Anaheim Automation was established in 1966 as a manufacturer of "turnkey" motion control systems. Its' emphasis on R&D has ensured the continued introduction of advanced motor drivers and controllers, such as the Servo Motor product line. Today, Anaheim Automation ranks among the leading manufacturers and distributor of motion control products; a position enhanced by its excellent reputation for quality products at competitive prices. The Servo Motor product line is no exception to the Company's goal.
Anaheim Automation offers a wide variety of standard Servo Motor products. Occasionally, OEM customers with mid to large quantity requirements prefer to have a Servo Motor that is custom or modified to meet their exact design requirements. Sometimes the customization is as simple as a shaft modification, brake, oil seal for an IP65 rating, mounting dimensions, wire colors, or simple label requirement. Other times, a customer might require that a Servo Motor meet an ideal specification such as, speed, torque, and/or voltage. Engineers appreciate that Anaheim Automation's Servo Motor product line can answer their desire for creativity, flexibility and system efficiency. Buyers appreciate the simplicity of the "one-stop shop," and the cost savings of a custom Servo Motor design, while engineers are pleased with Anaheim Automation's dedicated involvement in their specific servo motor system. Note: There are minimum purchase requirements for all special/custom orders.
Anaheim Automation's standard Servo Motor product line is a cost-effective solution; they are known for their rugged construction and excellent performance. This is a result of dedicated engineering, friendly customer service and professional application assistance, often surpassing the customer's expectations for fulfilling their custom requirements. Anaheim Automation takes pride in its standard stock base located in Anaheim, California, USA. To make customization of a Servo Motor affordable, a minimum quantity and/or a Non-Recurring Engineering (NRE) fee is required. Contact the factory for details, should you require a custom Servo Motor in your design.
All Sales for a customized or modified Servo Motor are Non-Cancelable-Non-Returnable, and a NCNR Agreement must be signed by the customer, per each request. All Sales, including a customized Servo Motor, are made pursuant to Anaheim Automation's standard Terms and Conditions, and are in lieu of any other expressed or implied terms, including but not limited to any implied warranties.
PLEASE NOTE: Technical assistance regarding its Servo Motor product line, as well as all the products manufactured or distributed by Anaheim Automation, is available at no charge. This assistance is offered to help the customer in choosing Anaheim Automation products for a specific application. However, any selection, quotation, or application suggestion for a Servo Motor, or any other product, offered from Anaheim Automation's staff, its' representatives or distributors, are only to assist the customer. In all cases, determination of fitness of the Servo Motor in a specific system design is solely the customers' responsibility. While every effort is made to offer solid advice regarding the Servo Motor product line, as well as other motion control products, and to produce technical data and illustrations accurately, such advice and documents are for reference only, and subject to change without notice.
Servo Motor Mounting
Mounting - Introduction
Proper installation will achieve the best results from the production capability of the servo motor and drive system. This can only be accomplished if several important steps are implemented and some precautions are taken. Note: Local codes may suggest different requirements, but those given in this section must be satisfied as much as possible.
CAUTION - Only qualified personnel should be allowed to open and work on the servo motor and drive and other components inside electrical enclosures.
Equipment and machinery should never be run unless the electrical enclosure door is closed and locked.
The electronics inside the main electrical enclosure are sensitive to metal chips and filings. During the installation and use of the servo motor and drive system, great care must be given to make sure metal chips or filings cannot fall onto or into any of the electrical devices.
Safety is the number one concern when performing the electrical connection of the servo motor and drive, as well as all motion control products and electrical equipment. Therefore, check every step at least once after it has been taken. During the installation of the servo motor and drive, it is important to minimize the possibility of electrical noise entering critical sensitive circuits. This is best accomplished by following the electrical installation procedures precisely. Considerable attention has been given to noise immunity in the basic design and manufacture of the servo motor and drive system. However, it is essential that great care and attention be given during the installation of the servo motor and drive in your machine or in your facility.
Before attempting any electrical installations, gather any drawings, instructions, or procedural documents you have on the servo motor and drive, as well as other components in your system. Reading and studying the servo motor and drive documentation before starting the project will alert you to any special situations, such as the need for specific tools. Also, you will know where to begin and where to go from there. Always keep the specific servo motor and drive documentation with you while completing the installation, as you should regularly refer to them. Documentation among servo motor and drive manufacturers will vary greatly, as their design, layout, and connections are not the same. Match the part numbers to the documentation before attempting the installation. Even seasoned professionals need guidance and advice while performing complicated electrical installations. This ensures the safest results for everyone.
Human safety and equipment safety must be the first considerations when performing the installation procedures for the servo motor and drive system. When it comes to electronics in your factory or workplace, you want to make sure both your facility and the employees in it are safe. The following is an electrical safety checklist, courtesy of the National Electric Safety Foundation:
General Electrical Safety Checklist:
Cords and Cables: Make sure cords and cables are in good condition. Check cords, cables and other wiring for frays and cracks. Make sure that all wiring and cabling is placed out of reach and out of traffic areas. Cords/cables should never be nailed or stapled to the wall, baseboard or to another object. Do not place cords under carpets or rugs. Anaheim Automation recommends using its product-specific cables for its servo motors and drives product line, and extreme care should be taken if the installer decides to use their own cabling system.
Plugs and Terminals: Make sure that all plugs fit the outlets, and that the terminals of the servo motor and drive are correctly matched and fit snug. Never remove the ground pin (the third prong) to make a three-prong fit a two-conductor outlet, because it could lead to an electrical shock. Avoid overloading outlets with too many electronic components. Never force a plug into an outlet if it doesn't fit, nor should you ever modify terminal blocks or cables for the servo motor and drive.
Electrical Outlet Safety: Routinely check for loose-fitting plugs, which can overheat and lead to fire. Replace broken or missing wall plates
Ground Fault Circuit Interrupters (GFCIs): These can help prevent electrocution and are used in any areas where water and electricity may come into contact. When a CFCI senses leakage in an electrical circuit, it assumes a ground fault has occurred. It then interrupts power fast enough to help prevent serious injury from electrical shock. Test GFCIs regularly, according to the manufacturer's instructions to make sure they are working properly.
Circuit Breakers/Fuses: Should be the correct size current rating for their circuit. If you do not know the correct size, have an electrician identify and label the size to be used. Always replace a fuse with the same size fuse.
Computer, Controller, HMI, PLC and Drive Products: Check to see that the equipment is in good condition and working properly. Look for cracks or damage in wiring, terminals, plugs and connectors. Use a surge protector bearing the seal of a nationally recognized certification agency.
Lightning: During an electrical storm, make sure you use surge protectors on electronic devices.
Mounting, Bonding and Grounding
After establishing all layouts, you can begin mounting, bonding, and grounding each chassis/enclosure. Bonding is the connecting together of metal parts of chassis, assemblies, frames, shields, and enclosures to reduce the effects of EMI and ground noise. Grounding is the connection to the grounding-electrode system to place equipment at earth ground potential.
IMPORTANT NOTE: These guidelines assume that you follow surge-suppression guidelines. While these guidelines apply to the majority of servo motor and drive installations, as well as other motion control applications, certain electrically harsh environments may require additional precautions.
Grounding of equipment and machinery is required for two reasons.
1. To prevent hazards to personnel in case of a breakdown between current electrical components and the exposed metal surfaces.
2. To minimize the effects of electrical noise on the control system.
Mounting and Bonding the Enclosure – General Practices
Generally speaking, you can mount the chassis with either bolts or welded stud:
• Stud-mounting a ground bus or chassis to the back panel of the enclosure
• Stud-mounting a back panel to the enclosure
• Bolt-mounting a ground bus or enclosure to the back panel of the enclosure
If the mounting brackets of a chassis do not lay flat before the nuts are tightened, use additional washers as shims so that the chassis does not bend when you tighten the nuts.
Important Note: Do not bend the chassis. Bending the chassis might damage the backplane and result in poor connections. Make good electrical connection between each chassis, back-panel, and enclosure through each mounting bolt or stud. Wherever contact is made, remove paint or other non-conductive finish from around studs or tapped holes.
With motors, drives and controls, proper bonding and grounding helps reduce the effects of EMI and ground noise. Also, since bonding and grounding are important for safety in electrical installations, local codes and ordinances dictate which bonding and grounding methods are permissible. Anaheim Automation supplies motion control components only, therefore, it is imperative that the user know all pertinent safety practices and local codes and ordinances for where the machinery or system is built, and/or their machinery is shipped.
For example, for U.S. installations, the National Electrical Code (NEC) will provide the requirements for safe bonding and grounding, such as information about the size and types of conductors and methods of safely grounding electrical components.
Use a steel enclosure to guard against EMI. If the enclosure door has a viewing window, it should be a laminated screen or a conductive optical substrate to block EMI.
Important Note: Do not rely on the hinge for electrical contact between the door and the enclosure; install a bonding wire.
How Good Is Your Earth Grounding System?
The existing factory earth and power systems of the plant, into which a new machine or motion control system is to be installed, should be checked for at least 24 hours before the machine/system arrives. This should be done as soon as the location is known to allow as much time as possible to make any changes that may be required. A good and reliable system that has been used for this purpose for many years is a Dranetz line analyzer. The power line disturbances should not exceed + or - 15% of the machine, or motion control components specification power requirements. This includes all forms of noise, voltage drop out or voltage spikes. While most machinery and motion control systems can usually tolerate more deviation than this, it is best to maintain these limits to protect people and the machine/system performance.
Water Pipes May Not work
Although a utility ground, such as a cold water pipe or the metal frame of a building, is generally an adequate ground for safety purposes, IT IS NOT usually recommended for minimizing the effects of electrical noise.
Sources of Noise
Normally, other electrical equipment is connected to water pipe grounds or building steel and, therefore, carries the transient electrical noise currents associated with all of the attached equipment. These combined electrical noise currents cause a voltage gradient to be developed within the pipe or structural member because of its inherent resistance and reactance. Therefore, a function of the total noise current flowing at any one instant may cause a disturbance. This transient ground shift voltage disturbances are set up which may be coupled into the electronics and cause the drives and controllers to malfunction.
What Is An Earth Rod?
A separate earth ground should always be used to ground a computer-controlled machine tool, or motion control system that uses drives, controllers, PLCs and/or HMIs. It may consist of a driven rod, driven pipe, buried plate, or any other device approved for this purpose. However, they should be kept out of any oily areas.
It is preferred that Earth Rods are located where saltwater can periodically be poured down the side of the rod. If a parallel water pipe is provided, the rod should be located where it can easily be filled. This type of ground usually provides the low-impedance, stable, noise-free ground required for minimizing the effects of electrical noise on the control system and will also provide personnel safeguards. At no time should more than one machine be connected to one ground rod. The cable connecting the control panels' ground point to the earth rod should be continuous, as short as practical, and of at least the size of the conductors used to connect the electrical power to the machine tool or process line.
Installation of Earth Ground Rod
The length and diameter of the ground rod is dependent upon the soil in the area of machine site. A good starting point would be to use a ten foot long by 5/8" diameter rod. The actual length and diameter of the earth ground rod should be determined by the length, and hence the diameter, required to reach the water, or moisture table in the subsoil. However, the local grounding conditions should be well-known by the plant electrical engineers and local electric company or electrical authority engineers; Anaheim Automation recommends consulting with them. It is best to weld a steel spike or cone to the end of the rod to help it penetrate the soil.
Sizing the Transformer - General Practices
To determine the required rating of the transformer, add the external-transformer load of the power supply and all other power requirements (input circuits, output circuits). The power requirements must take into consideration the surge currents of devices controlled by the processor. Choose a transformer with the closest standard transformer rating above the calculated requirements. For example, a 500VA transformer should be used if there were 360VA of load.
Isolation Transformer — For applications near excessive electrical noise generators, an isolation transformer (for the second transformer) provides further suppression of electromagnetic interference (EMI) from other equipment.
Constant-Voltage Transformer — In applications where the AC power source is especially "soft" and subject to unusual variations, a constant-voltage transformer can stabilize the AC power source to the processor and minimize shutdowns. The constant-voltage transformer must be of the harmonic neutralizing type. If the power supply receives its AC power through a constant-voltage transformer, the input sensors connected to the I/O chassis should also receive their AC power from the same constant-voltage transformer. If the inputs receive their AC power through another transformer, the AC source voltage could go low enough that erroneous input data enters memory while the constant-voltage transformer prevents the power supply from shutting down the processor. The output actuators being controlled should draw power form the same AC sources as the constant-voltage transformer, but not from the secondary of the constant-voltage transformer.
The following information is intended as a general guideline for the installation and mounting of a Servo Motor. WARNING - Dangerous voltages capable of causing injury or death may be present in the Servo Motor system. Use extreme caution when handling, testing, and adjusting during installation, set-up, tuning, and operation. It is very important that the wiring of the Servo Motor be taken into consideration upon installation and mounting.
Subpanels installed inside the enclosure for mounting Servo Motor system components, must be a flat, rigid surface that will be free from shock, vibration, moisture, oil, vapors, or dust. Remember that the Servo Motor and drive will produce heat during work. Therefore, heat dissipation should be considered in designing the system layout. Size the enclosure so as not to exceed the maximum ambient temperature rating. It is recommended that the servo drive be mounted in an upright position, providing adequate airflow. The Servo Motor should be mounted in a stable fashion, secured tightly.
NOTE: There should be a minimum of 10mm between the servo drive and any other devices mounted in the system/electric panel or cabinet. There should be at least 10mm space in the lateral direction and 50mm space in the longitudinal direction, between the servo motor drive and other electronic/electrical devices. For multi-axis systems, mount in the panel left to right according to power utilization (highest to lowest). If power utilization is unknown, mount from left to right based on Amp rating.
NOTE: In order to comply with UL and CE requirements, the servo motor drive must be grounded in a grounded-conducive enclosure offering protection as defined in standard EN 60529 (IEC 529) to IP55 such that they are not accessible to the operator or unskilled person. As with any moving part in a system, the Servo Motor should be kept out of the reach of the operator. A NEMA 4X enclosure exceeds those requirements providing protection to IP66. To improve the bond between the power rail and the subpanel, construct your subpanel out of zinc-plated (paint-free) steel. Additionally, it is strongly recommended that the servo motor drive be protected against electrical noise interferences. Noise from signal wires can cause mechanical vibration and malfunctions.
Servo Motor Environmental Considerations
The following environmental and safety considerations must be observed during all phases of operation, service, and repair of a Servo Motor system. Failure to comply with these precautions violates safety standards of design, manufacture, and intended use of the Servo Motor and drive. Please note that even well-built servo motor products operated and installed improperly can be hazardous. Precaution must be observed by the user with respect to the load and operating environment. The customer is ultimately responsible for the proper selection, installation, and operation of the Servo Motor system.
The atmosphere in which a Servo Motor is used must be conducive to good general practices of electrical/electronic equipment. Do not operate the Servo Motor in the presence of flammable gases, dust, oil, vapor, or moisture. For outdoor use, the Servo Motor and drive must be protected from the elements by an adequate cover, while still providing adequate air flow and cooling. Moisture may cause an electrical shock hazard and/or induce system breakdown. Due consideration should be given to the avoidance of liquids and vapors of any kind. Contact the factory should your application require specific IP ratings. It is wise to install the Servo Motor and drive in an environment which is free from dust, metal chips, condensation, electrical noise, vibration and shock.
Additionally, it is preferable to work with the Servo Motor and Drive system in a non-static protective environment. Exposed circuitry should always be properly guarded and/or enclosed to prevent unauthorized human contact with live circuitry. No work should be performed while power is applied. Do not plug in or unplug the connectors when power is ON. Wait for at least 5 minutes before doing inspection work on the Servo Motor system after turning power OFF, because even after the power is turned off, there will still be some electrical energy remaining in the capacitors of the internal circuit of the servo motor drive.
Plan the installation of the Servo Motor and drive in a system design that is free from debris, such as metal debris from cutting, drilling, tapping, and welding, or any other foreign material that could come in contact with circuitry. Failure to prevent debris from entering the Servo Motor system can result in damage and/or shock.
NOTE: Meeting CE Requirements mandates a ground system; and the method of grounding the AC line filter and the servo motor drive must match. Failure to do this renders the filter ineffective and may result in damage.
Servo Motor Wiring - Safety First!
Extension Cord Safety
When installing new technology, you may be dealing with extension cords to help plug your Anaheim Automation products on a test bench for prototyping. Use them on a temporary basis. Extension cords are not meant for permanent wiring.
Here are some general electrical cord safety tips when dealing with extension cords, in part, courtesy of the National Electrical Safety Foundation, which develops safety policies and procedures for electronics.
|•||Should not be used as a substitute for permanent wiring.|
|•||Should not be used on equipment drawing more than 15 amps.|
|•||Should not run through, behind or in walls, ceilings or floors or other concealed space.|
|•||Should not be run through ventilation ducts, under carpets, under doors or other locations that will subject them to abrasion or damage.|
|•||Should not be placed across walkways or doorways because they will become a tripping hazard.|
|•||Should not be spliced or taped, nor should broken cords or cords with damaged insulation be used to power Anaheim Automation products.|
|•||Should not be used near flammable gases or vapors or explosive dusts.|
|•||Should not be overloaded.|
NOTE: Water and electricity don't mix: don't leave plugged-in electronics where they might come into contact with water. If they do fall in water, never reach in and pull them out, even if they are turned off. First, turn off the power source and then unplug the unit. If you have a servo motor, drive or controller that has gotten wet, don't use it until it has been checked by our qualified repair staff.
High-Temperature Braided Sleeving
The life of cables, wires and hoses can be greatly extended with high-temperature braided sleeving. Braided sleeving is a protective cover for the vulnerable material of common wires. High temperatures can cause cracks, frays, or fires, especially for wires and cables that are used in industrial settings or exposed to outdoor elements. In addition to protecting the wires from high temperatures, braided sleeving can shield wires and cables from abrasions, chemicals, dirt, and even freezing temperatures.
Electrical Safety in the Workplace
When establishing electrical safety policy in the workplace, here are some points to consider, in part, courtesy of the National Electrical Safety Foundation:
|1.||Have a good idea of what could go wrong.|
|2.||Use the right tools for the job.|
|3.||Always follow procedures, drawings and other product documentation.|
|4.||Isolate equipment from energy sources.|
|5.||Identify hazards that may be present.|
|6.||Establish approach limitations to machinery and moving parts to minimize hazards.|
|7.||Be sure you are properly trained for the job.|
|8.||Work on the servo motor and drive, as well as all other Anaheim Automation products and all other electrical equipment only when de-energized.|
|9.||Check and double-check safety regulations and product documentation|
|10.||Treat de-energized equipment as energized until performing a lockout/tagout test (a test used to disable machinery or equipment to prevent the release of potentially hazardous energy while the machine is being serviced)|
Why So Many Electrical Safety Requirements
Organizations, such as the Standard for Electrical Safety Requirements for Employee Workplaces, outlines the steps companies must take to be in federal compliance with safety. They include:
|1.||A safety program with defined responsibilities|
|2.||Calculations for the degree of an arc flash hazard|
|3.||Electrical safety equipment for workers|
|4.||Training for workers|
|5.||Electrical safety tools|
|6.||Electrical safety labels on equipment|
An emphasis on safety is largely due to the fear of what an arc flash can do. An arc flash is a short circuit through the air that can happen when conductors can't support the voltage. An arc flash can be as hot as 5,000 F and creates a brilliant flash of light and loud noise. As radiant energy explodes out of the electrical equipment, hot gases and melted metal can endanger human life. This is why there are four separate industry standards or electrical safety requirements in place to protect workers against arc flashes and electrical safety equipment on the market in the form of boots, suits, gloves and more. It is the responsibility of the installer/user of servo motors and drives, and all other Anaheim Automation products, to become familiar with all safety requirements.
Avoid Working with Live Wires
A "live" wire is one that has electricity running through it. If you are installing or repairing anything electrical, always isolate the equipment from the power source. In addition to turning any circuit breakers off, it is always good to test any circuit or conductor before you touch it. This can be done very simply with a hand-held voltage tester. Use this multi-meter every time you must handle something that is potentially live.
The following are the four main hazards involved with the installation of electrical equipment:
|1.||Electric Shock- An electric shock or burn occurs when an electric current comes into contact with the skin and conducts through the body. If high-voltage electricity runs through the head or chest, death can occur instantly.|
|2.||Arc Flash Burn- An arc flash occurs when a conductive object gets too close to a high voltage, electrified object. This flash can cause intense heat in the surrounding air, possibly causing clothes to catch fire.|
|3.||Arc Blast Impact- When a metal object triggers an arc flash, a subsequent blast can cause hearing loss and concussion. Also, this blast can cause lacerations from flying metal pieces. Falling- Shocks and arc blasts can easily knock a worker off a high platform, such as a ladder or pole.|
Surge-Suppression – General Practices
Transient Electromagnetic Interference (EMI) can be generated whenever inductive loads such as relays, solenoids, motor starters, or motors are operated by "hard contacts" such as pushbutton or selector switches. The wiring guidelines are based on the assumption that you guard your system against the effects of transient EMI by using surge-suppressors to suppress transient EMI at its source. Inductive loads switched by solid-state output devices alone do not require surge-suppression. However, inductive loads of AC output modules that are in series or parallel with hard contacts require surge-suppression to protect the module output circuits as well as to suppress transient EMI.
Electrical noise from any source, whether it is the power line, an electrical arc generated in an adjacent machine or process, or crosstalk within the control, is transmitted by conduction, inductive or capacitive coupling, or radiation. It is extremely important to maintain the electrical enclosures and panels, conduits, wiring shields, and machine members at zero potential and to provide a return path to the earth for noise currents so as to effectively shield the sensitive logic from electrical noise.
EMI – Electromagnetic Interference
Electromagnetic Interference is the radiation or induction of electromagnetic noise on a system or machine. Servo motors, and motors in general, are a common source of EMI, due to their electromagnetic circuit components. An electromagnetic disturbance which may degrade the performance of equipment (device, system or sub-system), or causes malfunction of the equipment, is called electromagnetic interference (EMI). Motors are potential sources of noise and can generate common-mode currents. EMI can result in degraded system performance and/or data corruption. When it is very strong, it can cause the system to fail completely. EMI can be radiated or conducted comes from magnetic and electrical sources, respectively. In the case of most motors, both radiated and conducted emissions are present.
Causes of EMI – Typical sources of EMI
Industrial – ARC welding, motors, computers, fast-switching digital devices, Integrated Circuits, power cords
Military - Aircraft navigation and equipment
Household – Refrigerators, washers, dryers, dishwashers, electric shavers, personal computers, air-conditioning and heating systems
Susceptible to EMI – Can cause poor functioning and/or damage
Communication receivers, microprocessors, industrial drives and controls, medical devices, household appliances
EMC – Electromagnetic Compatibility
Electromagnetic Compatibility is the practice of monitoring and reducing unwanted EMI. Electromagnetic compatibility is a near-perfect state in which a receptor (device, system, or sub-system) functions well in a common electromagnetic environment, without introducing intolerable electromagnetic disturbance to any other devices, equipment or systems that share that environment.
IMPORTANT NOTE: EMC performance concerns the complete servo motor and drive system, as well as all other motion control systems and machinery. Therefore it is the responsibility of the OEM (original equipment manufacturer) to monitor and reduce undesirable EMI, and not of those who supply motion control products. There are often varying regulations on EMC, depending on the purpose of the system and the country it is used or sold.
EMI Reduction Techniques for motors to improve systems EMC performance.
Arcing (sometimes referred to as arc discharge or voltaic arc) is an electrical characteristic where current can flow through the air, or other normally non-conductive materials. You may have seen instances of arcing between two wires, or on the power rails of trains or trams. This is not to be confused with an electrical spark, as an electrical arc is continuous, however they do look similar.
Whilst arcing can be useful, used in both welding and strip lighting, in some cases it can be a source of EMI. For Example: With DC motors, arcing can be common because of the periodic interruption of the current in the rotor windings. This very high-frequency spectra content, which can appear as wideband noise superimposed onto other signals. DC motors also provide paths for common-mode currents through their frames.
Another example of radiated and conducted emissions can come from the driver circuit. A typical H-bridge circuit should ideally provide a constant current to the motor, but this current has fast rise time spikes due to the fast and frequent switching of the current in the driver circuit. Another significant problem is when the motor is located far from the driver, as this creates a fairly large loop area between motor leads and device frame. The radiation potential is a direct function of the loop area; the larger the loop, the larger the emissions.
Prevent EMI problems:
|•||Suppress possible emissions at the source point.|
|•||Make the servo motor and drive and other components in the system less susceptible to EMI emissions|
|•||Make the coupling path as inefficient as possible|
EMI control techniques at source point:
|•||Proper Grounding – Single point, multi-point or hybrid grounding depending upon the frequency of the operation|
|•||Shielding – Metal barrier used to suppress coupling of radiated EM energy into the equipment|
|•||Filtering – Filtering techniques are used to suppress conducted interference on Power, Signal, and Control lines.|
|•||Printed Circuit Board (PCB) – Proper design and layout of printed circuit boards from the early design stage is essential to eliminate EMI issues in the future|
|•||Noise Coupling – Conductive coupling through cables, coupling through common impedance and ground loop coupling techniques are good preventive measures against EMI issues.|
The easiest solution is to place a ceramic capacitor between the motor terminals, as close as possible to the motor. This is known as a decoupling capacitor, and reduces EMI by removing some of the high frequency noise signals. The common value used for these decoupling capacitors is between 100pF and 100nF, depending on the size of the motor.
IMPORTANT NOTE: EMC is an important field in electronics with strict regulations, and motors and their circuits are significant sources of EMI. It is therefore essential that engineers take the appropriate actions to reduce EMI improve EMC as much as possible.
Power Line Interference
This section provides basic information that should help achieve a safe, successful and reliable servo motor and drive installation. It does not cover all possibilities, but does give good basic information and guidance to servo motor and drive applications, as well as other motion control systems.
Power lines are one of the most troublesome sources of electrical noise. The power lines to which the motion control components are connected, may also supply power to equipment such as arc welders, high current (induction) furnaces, or large horsepower electric motors. Starting or stopping these large consumers of power, or changing the load conditions on them, may cause transient voltages, which may take the form of voltage surges or dips accompanied by high frequency noise superimposed on the incoming voltage waveform. This electrical noise may cause a digital electronic control system to count incorrectly, lose stored data, store incorrect data, or lose axis synchronization.
Power lines in an ungrounded Delta power system are inherently noisy. This system floats with respect to ground and may also cause excessively high voltages to be applied to equipment connected to it. For these reasons, a grounded Wye power system is preferred for supplying power to a computer controlled machine tool or other motion system.
To minimize the effects of power line noise on computer-controlled machine tools or other motion control system components, the power wiring is physically and electrically separated from the logic signal wiring. Also, shielded cables are used for logic signal wiring where appropriate, and an effective common point ground system is provided. Even though these precautions have been taken, power line noise may still be coupled into the logic in extreme cases and cause the control to malfunction as described above.
To eliminate controller, HMI, PLC, and computer malfunctions that are caused by excessive power line noise, one or more of the following may be necessary:
|1.||Reduce existing power line noise or install a separate incoming power line to the machine or process.|
|2.||If the only available power source is an ungrounded Delta type, install a Delta-to-Wye isolation transformer ahead of the control and ground the neutral of the Wye to improve noise rejection and to better regulate the input voltage to the controls.|
|3.||Install a motor alternator set ahead of the control to isolate it from the incoming power line.|
Possible Effects of EMI in a servo motor and drive system:
|•||Intermittent disturbances in the servo drive and controllers|
|•||Computers and HMIs may falter and lose data, causing to reset/start-up|
|•||Burn-out of sensitive components|
|•||Disturbs the settings and status registers of control equipment|
A servo motor and drive system is considered to be electro-magnetically compatible if:
|•||If the servo motor and drive do not cause interference with other components in the system or machinery.|
|•||The servo motor and drive are not susceptible to EMI emissions from other components in the system or machinery.|
|•||The servo motor and drive do not cause interference within itself.|
Qualified Professionals Only
Electrical wiring is complicated and potentially life-threatening if installed incorrectly. Do not attempt to install a servo motor and drive, or any other Anaheim Automation product, or other electrical equipment, unless you are certain you have received the proper training and education for the task. This is one area of business you can't afford to be frugal about. If you are not sure how to do the servo motor and drive installation, always hire a qualified electrician/panel builder/integrator. A servo motor and drive should never be used around water, dust, or flammable materials. Adherence to proper safety guidelines can save lives and valuable property by preventing electric shock and fires.
When installing a servo motor and drive, and any other wiring dealing with any electrical equipment, it is important to use the proper tools. A non-conductive tool will have a rubber grip for you to hold it by. Never use a tool for installation that is solid metal, even if you are wearing gloves and have the power source turned off. There is no such thing as being too cautious when it comes to electrical shock or burns.
High-Temperature Applications – Use Braided Sleeving
The life of a cable can be greatly extended with high-temperature braided sleeving. Braided sleeving is a protective cover for the vulnerable material on common wires. High temperatures can cause cracks or frays, even fires, without braided sleeving, especially in wires and cables that are used in industrial settings or exposed to outdoor elements. In addition to protecting the wires from high temperatures, braided sleeving can shield wires and cables from abrasions, chemicals, dirt, and even freezing temperatures. Consult with your representative should your servo motor and drive application involve high temperatures.
Shielded I/O Cables — General Practices
Certain I/O connections require shielded cables to help reduce the effects of electrical noise coupling. Ground each shield at one end only. A shield grounded at both ends forms a ground loop which can cause a processor to fault. Never connect a shield to the common side of a logic circuit (this would introduce noise into the logic circuit). Connect each shield directly to a chassis ground. For some communication network cables, the shield connections are unique to the particular cabling system. In some such cases, a DC short to ground is not needed because a low-impedance AC path to ground and a high-impedance DC path to ground are provided internally at each node. Follow the specific instructions in the manufacturer's publication provided for each specific product.
Enclosure Lights – General Practices
Fluorescent lamps are also sources of EMI. If you must use fluorescent lamps inside an enclosure, the following precautions may help guard against EMI problems from this source:
|•||Install a shielding grid over the lamp|
|•||Use shielded cable between the lamp and its switch|
|•||Use a metal-encased switch|
|•||Install a filter between the switch and the power line, or shield the power-line cable|
Basic Electrical Safety Rule(s)
The OSHA regulation regarding electrical safe practices states two very important basic points. The first is that live parts shall be de-energized before working on or near them. The second point is that even after the exposed parts have been de-energized, they shall still be treated as energized until they are locked out and/or tagged out. That is why the BASIC RULE for electrical safe practices procedure is stated as follows:
ALL ELECTRICAL CIRCUIT CONDUCTORS, BARE OR INSULATED, ARE ASSUMED TO BE ENERGIZED UNTIL PROVEN OTHERWISE. THEY SHALL BE DE-ENERGIZED, LOCKED OUT AND TESTED FOR THE ABSENCE OF VOLTAGE BEFORE WORKING ON THEM OR WORKING NEAR THEM. WORK ON ELECTRICAL CIRCUIT CONDUCTORS MAY ONLY BE PERFORMED BY QUALIFIED PERSONNEL WHO HAVE BEEN AUTHORIZED TO DO THE WORK.
For persons unfamiliar with servo motor and drive products, as well as other motion control components, Anaheim Automation recommends consulting with a systems integrator for all installations.
The following information is intended as a general guideline for wiring of the Anaheim Automation Servo Motor product line. Be aware that when you route power and signal wiring on a machine or system, radiated noise from the nearby relays, transformers, and other electronic devices can be induced into servo motor and encoder signals, input/output communications, and other sensitive low voltage signals. This can cause systems faults and communication errors.
WARNING - Dangerous voltages capable of causing injury or death may be present in the servo motor driver. Use extreme caution when handling, wiring, testing, and adjusting during installation, set-up, tuning, and operation. Do not make extreme adjustments or changes to the servo motor drive parameters, which can cause mechanical vibration and result in failure and/or loss. Once the Servo Motor is wired, do not run the servo drive by switching On/Off the power supply directly. Frequent power On/Off switching will cause fast aging of the internal components in the servo drive, which will reduce the lifetime of servo motor system. It's required to use reference signals to control the running of the servo motor drive.
Strictly comply with the following rules specific to Anaheim Automation:
|-||Route high-voltage power cables separately from low-voltage power cables.|
|-||Segregate input power wiring and Servo Motor power cables from control wiring and motor feedback cables as they leave the servo drive. Maintain this separation throughout the wire run.|
|-||Use shielded cable for power wiring and provide a grounded 360 degree clamp termination to the enclosure wall. Allow room on the sub-panel for wire bends.|
|-||Make all cable routes as short as possible.|
|-||Single point grounding is required when mounting the Servo Motor and servo drive, and grounding resistance should be lower than 100Ω.|
|-||It is prohibited to apply a power input noise filter between servo drive and Servo Motor.|
NOTE: Meeting CE Requirements for a Servo Motor and drive system requires a ground system, and the method of grounding the AC line filter and the servo drive must match. Failure to do this renders the filter ineffective and may cause damage to the filter. For grounding and filter suggestions, please contact the factory.
WARNING - To avoid the possibility of electrical shock, perform all mounting and wiring of the servo motor and drive system prior to applying power. Once power is applied, connection terminals may have voltage present, even when the servo motor and drive are not in use.
Required Maintenance for a Servo Motor
Servo Motors are not prone to wear over time, and therefore require little maintenance. However, periodic maintenance checks should be performed so that the servo motor keeps running like new. Upon first arrival of the servo motor one should double-check the following: the motor is the correct model, motor does not have any visible damage, shaft can be rotated by hand, the brake works correctly, and there are no loose bolts. Operators should periodically check the motor for vibration and noise while the motor is not rotating, rotating at low speeds, and accelerating and decelerating. Inspect the motor for scratches or cracks on the motor casing. If crevices or cracks are found on the motor, action should be taken immediately by repairing or replacing the damaged unit. Check the motor casing for oil or cutting fluid because this can corrode the coating – possibly leading to future failure. Use an insulation level tester to check insulation resistance between motor coil and motor frame and refer to the owner's manual to see if insulation value falls within an operable range. Observe the normal voltage waveforms on an oscilloscope periodically and take notes for future comparison purposes and report any inconsistencies to manufacturer. Check cables and wiring for cracks and frays. Replace if found worn, as this could be dangerous (See the Wiring section in this guide for more details).
Servo Motor Applications
Anaheim Automation's cost-effective AC Servo Motor product line is the wise choice for both OEM and user accounts. Anaheim Automation's customers for the AC Servo Motor product line is diverse: industrial companies operating or designing automated machinery or processes that involve food, cosmetics or medical packaging, labeling or tamper-evident requirements, cut-to-length applications, assembly, conveyor, material handling, robotics, special filming and projection effects, medical diagnostics, inspection and security devices, pump flow control, metal fabrication (CNC machinery), and equipment upgrades. An AC Servo Motor is found in motion systems that require High torque, position, velocity and/or torque control.
NOTE: Technical assistance regarding the AC Servo Motor product line is available at no charge. This assistance is offered to help the customer in choosing Anaheim Automation products for a specific application. However, any selection, quotation, or application suggestion for an AC Servo Motor, or any other product, offered from Anaheim Automation's staff, its' representatives or distributors, are only to assist the customer. In all cases, determination of fitness of the AC Servo Motor in a specific system application is solely the customers' responsibility. While every effort is made to offer solid advice regarding the AC Servo Motor in a specific application, and to produce technical data and illustrations accurately, such advice and documents are for reference only, and subject to change without notice. Anaheim Automation is in no event responsible or liable for indirect or consequential damages resulting from the use or application of the AC Servo Motor. Improper use of an AC Servo Motor in an application can result in personal injury or death, property damage, and/or economic loss.
|Stepper Motor Versus Servo Motor|
|Stepper Motor Advantages||Servo Motor Advantages|
|Stability: A stepper motor can drive a wide range of frictional and inertial loads||High output: Power in relation to the servo motor size and weight|
|Does Not Require Feedback: The stepper motor also acts as the position transducer||Encoder: Determines the accuracy and resolution of the servo motor|
|Price: Relatively inexpensive||High-efficiency: The servo motor can approach 90% efficiency at light loads|
|Standardized: NEMA standard frame size and performance||High Torque to Inertia Ratio: The servo motor can rapidly accelerate loads|
|Plug and Play features: Easy to set up and use||A servo motor has 2-3 times continuous power for short periods|
|Safety: The stepper motor stops if there is a malfunction or interference||A servo motor has 5-10 times rated torque for short periods|
|Excellent Low Speed Torque: The stepper motor has the ability to drive several loads without gearing||A servo motor stays cool because the current draw is proportional to load|
|Repeatability: Accurately returns to the same location - open loop system||A servo motor maintains usable high speed torque of 90% of NL RPM|
|Overload safe: A mechanical overload cannot damage the stepper motor||A servo motor performs quietly at high speeds silently|
|Longevity: If the specifications of the motor are not exceeded, the stepper is good for 10,000 hours of operation||A servo motor has a resonance-free and vibration-free operation|
|Stepper Motor Disadvantages||Servo Motor Disadvantages|
|Low efficiency: A stepper motor draws enough power regardless of the load||Higher cost|
|Torque drops quickly with speed||The servo motor requires "tuning" to stabilize feedback loop|
|Requires microstepping to move smoothly; prone to resonance problems||Safety circuits are required; motor "runs away" when something breaks|
|There is no feedback to indicate missed steps Low torque to inertia ratio; cannot accelerate loads very quickly||A servo motor is more complex and requires an encoder|
|In high performance configurations, the stepper motor can get extremely hot (requires heat sink or fan-cooled installation||The life of a servo motor is only 2,000 hours, after that, service is required|
|Motor does not "pick up" after momentary overload||Peak torque is limited o a 1% duty cycle|
|In moderate to high speeds, the stepper motor can be noisy, which limits its applications||Sustained overload can damage the servo motor|
|Low output power for size and weight||Overwhelming choice of motors, encoders, and servo drivers|
|Power supply current 10 times average to use peak torque|
|The servo motor develops peak power at higher speeds; gearing is often needed|
|Poor Motor Cooling: Ventilated servo motors are easily contaminated|
Formulas for a Servo Motor
Formulas for Operating Patterns
Determine the relationship between time and speed for each item that is being controlled, then convert the operating pattern for these items into a servo motor shaft operating pattern.
Triangular Operating Pattern
Maximum Speed: v0 = X0
Acceleration/Deceleration Time: tA = X0
Travel Distance: X0 = v0 * tA
Trapezoid Operating Pattern
Maximum Speed: v0 = X0
Acceleration/Deceleration Time: tA = t0 - X0
Total Travel Time: t0 = tA + X0
Constant-Velocity travel time: tB = t0 - 2 * tA
Total Travel Distance: X0 = v0 (t0 - tA)
Acceleration/Deceleration Travel Distance XA = V0 * tA
Constant-Velocity travel time: XB = v0 - tB
Speed and Slope When Ascending Operating Pattern
Ascending Time: tA = v0-v1
Ascending Time (tA) including distance moved:XA = ½ α*tA² + v1 * tA
Speed after ascending: v0 = v1+α*tA
Speed Gradient: α = vg