Image credit: PMC Corporation Overshoot limitsĭerivative gain is used to set the overshoot limit, or the acceptable amount by which the servo can exceed the target position. The use of derivative gain in conjunction with proportional gain reduces settling time and overshoot. To avoid this, the derivative sampling period can be increased. Servo instability can occur if the derivative gain value is calculated too frequently, as it will begin to work against, rather than with, the proportional gain. It works in conjunction with proportional gain to dampen the system response and reduce overshoot and oscillations. However, it is useful when steady-state (static) positioning is difficult to hold due to system disturbances, or when constant velocity motion is required.ĭerivative gain (K d) determines the restoring force that is proportional to the rate of change (derivative) of the positioning error. If sufficient positioning accuracy is achieved with the proportional gain, then integral gain may not be necessary. The term “integral gain” is used because its command increases over time at the end of the move. Integral gain (K i) overcomes this by producing a command that “pushes” the system to zero positioning error at the end of the move. Often, as the servo controller works to decelerate the motor by reducing the command output, system friction overcomes the command voltage and causes the motor to fall short of the target. Proportional gain is the most important component of the PID algorithm, but a K p value that is too high can cause the system to oscillate, to become under-damped, or to become unstable. For example, if the proportional gain is 1.2 volts per encoder count, and the motor is 10 encoder counts from the commanded position, the command voltage will be 12.0 volts. The term “proportional gain” is used because its value is directly proportional to the positioning error. Proportional gain (K p) determines the amount of restoring force (generated by the command voltage) that is applied to overcome the position error. Proportional gain is dependent on present error, integral gain is dependent on past error, and derivative gain is dependent on predicted future error. Image credit: Thorlabs, Inc.įeedback gains determine how hard the servo tries to correct or reduce the error between the commanded position and the actual position. The PID algorithm uses three feedback gains-proportional gain, integral gain, and derivative gain-to compare the commanded position (or velocity) with the actual value and issue commands to correct errors between the two. Servo tuning can be accomplished by several methods, but the most common way is to use a PID algorithm.
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