Incremental encoders are simpler, cheaper and work at faster speeds. Absolute encoders can determine their position at power-on, but are more complicated and expensive. Modern servomotors use rotary encoders, either absolute or incremental. Although it would be possible to electrically differentiate their position signal to obtain a speed signal, PID controllers that can make use of such a speed signal generally warrant a more precise encoder. They suffer from wear and electrical noise in the potentiometer track. These are only used at the very simplest and cheapest level, and are in close competition with stepper motors. Simple servomotors may use resistive potentiometers as their position encoder. Much work was done with these systems in the development of radar and anti-aircraft artillery during World War II. The first servomotors were developed with synchros as their encoders. Many applications, such as laser cutting machines, may be offered in two ranges, the low-priced range using stepper motors and the high-performance range using servomotors. There is also no need to tune the PID controller on a closed loop stepper system. The main benefit of a closed loop stepper motor is its relatively low cost. They act like servomotors but have some differences in their software control to get smooth motion. There has been increasing popularity in closed loop stepper motors in recent years. With larger systems, where a powerful motor represents an increasing proportion of the system cost, servomotors have the advantage. The encoder and controller of a servomotor are an additional cost, but they optimise the performance of the overall system (for all of speed, power and accuracy) relative to the capacity of the basic motor. The lack of feedback of a stepper motor limits its performance, as the stepper motor can only drive a load that is well within its capacity, otherwise missed steps under load may lead to positioning errors and the system may have to be restarted or recalibrated. A servomotor will immediately turn to whatever angle the controller instructs it to, regardless of the initial position at power up. This can be observed when switching on an inkjet printer the controller will move the ink jet carrier to the extreme left and right to establish the end positions. Therefore, on first power up, the controller will have to activate the stepper motor and turn it to a known position, e.g. This often allows them to be used as an open-loop position control, without any feedback encoder, as their drive signal specifies the number of steps of movement to rotate, but for this the controller needs to 'know' the position of the stepper motor on power up. Stepper motors have some inherent ability to control position, as they have built-in output steps. Servomotors are generally used as a high-performance alternative to the stepper motor. JSTOR ( March 2016) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. This Section needs additional citations for verification. Both of these enhancements, usually in combination with a PID control algorithm, allow the servomotor to be brought to its commanded position more quickly and more precisely, with less overshooting. More sophisticated servomotors use optical rotary encoders to measure the speed of the output shaft and a variable-speed drive to control the motor speed. This type of servomotor is not widely used in industrial motion control, but it forms the basis of the simple and cheap servos used for radio-controlled models. The very simplest servomotors use position-only sensing via a potentiometer and bang-bang control of their motor the motor always rotates at full speed (or is stopped). As the positions approach, the error signal reduces to zero and the motor stops. If the output position differs from that required, an error signal is generated which then causes the motor to rotate in either direction, as needed to bring the output shaft to the appropriate position. The measured position of the output is compared to the command position, the external input to the controller. In the simplest case, only the position is measured. The motor is paired with some type of position encoder to provide position and speed feedback. The input to its control is a signal (either analogue or digital) representing the position commanded for the output shaft. A servomotor is a closed-loop servomechanism that uses position feedback to control its motion and final position.
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