A servo drive is an

electronic amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost th ...

used to power electric

servomechanism In control engineering a servomechanism, usually shortened to servo, is an automatic device that uses error-sensing negative feedback to correct the action of a mechanism. On displacement-controlled applications, it usually includes a built-in ...

s. A servo drive monitors the

feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...

signal from the servomechanism and continually adjusts for deviation from expected behavior.

Function

A servo drive receives a command signal from a control system, amplifies the signal, and transmits electric current to a

servo motor A servomotor (or servo motor) is a rotary actuator or linear actuator that allows for precise control of angular or linear position, velocity and acceleration. It consists of a suitable motor coupled to a sensor for position feedback. It also ...

in order to produce motion proportional to the command signal. Typically, the command signal represents a desired velocity, but can also represent a desired torque or position. A

sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...

attached to the servo motor reports the motor's actual status back to the servo drive. The servo drive then compares the actual motor status with the commanded motor status. It then alters the voltage,

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

or pulse width to the motor so as to correct for any deviation from the commanded status. In a properly configured control system, the servo motor rotates at a velocity that very closely approximates the velocity signal being received by the servo drive from the control system. Several parameters, such as stiffness (also known as proportional gain), damping (also known as derivative gain), and feedback gain, can be adjusted to achieve this desired performance. The process of adjusting these parameters is called performance tuning. Although many servo motors require a drive specific to that particular motor brand or model, many drives are now available that are compatible with a wide variety of motors..

Digital and analog

All servo drives used in industry are digital, analog, or both. Digital drives differ from analog drives by having a microprocessor, or computer, which analyses incoming signals while controlling the mechanism. The microprocessor receives a pulse stream from an encoder which can determine parameters such as velocity. Varying the pulse, or blip, allows the mechanism to adjust speed essentially creating a speed controller effect. The repetitive tasks performed by a processor allows a digital drive to be quickly self-adjusting. In cases where mechanisms must adapt to many conditions, this can be convenient because a digital drive can adjust quickly with little effort. A drawback to digital drives is the large amount of energy that is consumed. However, many digital drives install capacity batteries to monitor battery life. The overall feedback system for a digital servo drive is like an analog, except that a microprocessor uses algorithms to predict system conditions. Analog drives control velocity through various electrical inputs usually ±10 volts. Often adjusted with potentiometers, analog drives have plug in “personality cards” which are preadjusted to specific conditions. Most analog drives work by using a tach generator to measure incoming signals and produce a resulting torque demand. These torque demands request current in the mechanism depending on the feedback loop. This amplifier is referred as a four-quadrant drive because can accelerate, decelerate and brake in either rotating direction. Traditional analog drives consume less energy than digital drives and can offer very high performance in certain cases. When conditions are met, analog drives offer consistency with minimal “jitter” at standstills. Some analog servo drives do not need a torque amplifier and rely on velocity amplifiers for situation where speed is more important.

Use in industry

Servo systems can be used in CNC machining, factory automation, and robotics, among other uses. Their main advantage over traditional DC or

AC motor An AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of two basic parts, an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor ...

s is the addition of motor feedback. This feedback can be used to detect unwanted motion, or to ensure the accuracy of the commanded motion. The feedback is generally provided by an encoder of some sort. Servos, in constant speed changing use, have a better life cycle than typical AC wound motors. Servo motors can also act as a brake by shunting off generated electricity from the motor itself.

References

Control devices {{tech-stub

Function

Digital and analog

Use in industry

See also

References