Servomechanisms
Core concepts
A servomechanism is a closed-loop control system in which feedback makes an output follow a reference. The comparator subtracts the feedback signal from the reference to produce an error signal. A controller amplifies or shapes this error, an actuator drives the load, and a sensor measures the output. Negative feedback reduces error and makes the output robust to disturbances and component variation. Position servos often use potentiometers, synchros or resolvers for position feedback; velocity servos use a tachogenerator. A proportional controller gives a command proportional to error, integral action removes steady-state offset, and derivative action adds damping. Important properties include stability, damping, response speed, steady-state error, deadband, backlash and hysteresis. Excessive gain can cause oscillation or hunting; too little gain gives sluggish response and large error.
Traps
Do not call a tachogenerator a position sensor: it gives velocity feedback. Do not confuse the error signal with the output signal. Positive feedback is not normal servo feedback; it drives the output away from the reference. An open-loop system has no automatic correction. Remember that gain affects both speed of response and stability, and that mechanical backlash or deadband can prevent precise positioning even when the electronics are correct.
Worked example
A position reference is 7.0 V and the feedback potentiometer gives 6.0 V. The error is 7.0 - 6.0 = 1.0 V. With a controller gain of 5, the command to the actuator is 5 x 1.0 = 5.0 V. If the gain is raised too far, the drive may overshoot and oscillate; if it is too low, a steady offset may remain. Rule of thumb: increase proportional gain for faster correction, but add derivative damping or reduce gain if hunting appears.