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.

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.

Kinco 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).

Kinco 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. Kinco 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.

Communication Interface

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. Kinco Automation AC Servo Drivers can also communicate with a PLC, DCS, intelligent instruments, touch screens, and more.

ESView Communication Software

Kinco 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

Kinco 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 Kinco 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.

Kinco Automation's Servo Motor is designed with a 2500 counts per revolution quadrature encoder, with a resolution of 10,000 pulses per revolution.

Kinco Automation's Servo Motor comes with the necessary connectors to connect to another company's servo drive or an Kinco 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 Kinco Automation. The Servo Motor cable comes with a standard length of 5M but can be adjusted to any length required.

NOTE: Kinco 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 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.

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

Kinco Automation's cost-effective AC Servo Motor product line is the wise choice for both OEM and user accounts. Kinco 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 Kinco Automation products for a specific application. However, any selection, quotation, or application suggestion for an AC Servo Motor, or any other product, offered from Kinco 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. Kinco 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.