A servomechanism, or servo is an automatic device that uses error-sensing feedback to correct the performance of a mechanism. The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position or other parameters. For example, an automotive power window control is not a servomechanism, as there is no automatic feedback which controls position—the operator does this by observation but the car’s cruise control uses closed loop feedback, which classifies it as a servomechanism.
A servomechanism is unique from other control systems because it controls a parameter by commanding the time-based derivative of that parameter. For example a servomechanism controlling position must be capable of changing the velocity of the system because the time-based derivative (rate change) of position is velocity. A hydraulic actuator controlled by a spool valve and a position sensor is a good example because the velocity of the actuator is proportional to the error signal of the position sensor.
common type of servo provides position control. Servos are commonly electrical or partially electronic in nature, using an electric motor as the primary means of creating mechanical force. Other types of servos use hydraulics, pneumatics, or magnetic principles. Usually, servos operate on the principle of negative feedback, where the control input is compared to the actual position of the mechanical system as measured by some sort of transducer at the output. Any difference between the actual and wanted values (an “error signal”) is amplified and used to drive the system in the direction necessary to reduce or eliminate the error. An entire science known as control theory has been developed on this type of system.
Servomechanisms were first used in military fire-control and marine navigation equipment. Today servomechanisms are used in automatic machine tools, satellite-tracking antennas, remote control airplanes, automatic navigation systems on boats and planes, and antiaircraft-gun control systems. Other examples are fly-by-wiresystems in aircraft which use servos to actuate the aircraft’s control surfaces, and radio-controlled models which use RC servos for the same purpose. Many autofocuscameras also use a servomechanism to accurately move the lens, and thus adjust the focus. A modern hard disk drive has a magnetic servo system with sub-micrometre positioning accuracy.
Typical servos give a rotary (angular) output. Linear types are common as well, using a screw thread or a linear motor to give linear motion.
Another device commonly referred to as a servo is used in automobiles to amplify the steering or braking force applied by the driver. However, these devices are not true servos, but rather mechanical amplifiers. (See also Power steering or Vacuum servo.)
In industrial machines, servos are used to perform complex motion.
In many applications, servomechanisms allow high-powered devices to be controlled by signals from devices of much lower power. The operation of the high-powered device results from a signal (called the error, or difference, signal) generated from a comparison of the desired position of the high-powered device with its actual position. The ratio between the power of the control signal and that of the device controlled can be on the order of billions to one.
All servomechanisms have at least these basic components:
- a controlled device,
- a command device,
- an error detector,
- a comparator,
- a device to perform any necessary error corrections (the servomotor).
In the controlled device, that which is being regulated is usually the position. This device must, therefore, have some means of generating a signal (such as a voltage), that represents its current position, which is send to the feedback elements. These elements generate a signal called the feedback signal, which is in a form which is comparable with the input. Now, the comparator compares the two signals and sends the change in the two values as the error to the error detector. Now, this error detector sends a signal to the command device which on the nature of the signal drives the servo motor, which repositions the controlled device.
Applications
Servomechanisms were first used in gun laying (aiming), military fire-control and marine-navigation equipment. Today, applications of servomechanisms include their use in
- Automatic machine tools
- Satellite-tracking antennas
- Celestial-tracking systems on telescopes
- Automatic navigation systems
- Antiaircraft-gun control systems
- Roll stabilization of ships
- Radar servo tracking systems
Radar Servo Tracking System
The purpose of a tracking system is to determine the location or direction of a target on a near-continuous basis. An ideal tracking system would maintain contact and constantly update the target’s bearing (azimuth), range and elevation. The output of the tracking system can be sent to a fire control system, which stores the information and derives the target’s motion and therefore its future position.
In a servo tracking system, the radar antenna is initially trained on a target after which it automatically remains pointed at the target as it follows its motion. Furthermore, the system provides continuous position information to the operator and possibly to a fire control system. The antenna is rotated by a motor which provides a negative position feedback signal to a controller.
The commanded input signal is the desired azimuth of the antenna. The error signal drives the motor to reposition the antenna until the position feedback indicates the antenna is at the desired azimuth, at which point the error signal is zero and the motor stops. This servo-mechanism can be combined with a tracker, which determines the azimuth as the target, which the system now uses as the input.
Here, the input comes from the tracker. The combination is called a radar servo-tracking system. The tracker takes the return signal and position information and determines the location of the target.
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