If Kp has a value and Ki is set to zero the controller will only have a proportional term, and there must be a speed error to produce a torque reference.

Therefore as the motor load increases there will be a difference between the reference and actual speeds. This effect, called regulation, depends on

the level of the proportional gain, the higher the gain the smaller the speed error for a given load. If the proportional gain is too high either the acoustic

noise produced by speed feedback quantization (using digital encoders, resolvers, etc.) becomes unacceptable, or the closed-loop stability limit is

reached (using SINCOS encoders).

Integral gain (Ki)

The integral gain is provided to prevent speed regulation. The error is accumulated over a period of time and used to produce the necessary torque

demand without any speed error. Increasing the integral gain reduces the time taken for the speed to reach the correct level and increases the

stiffness of the system, i.e. it reduces the positional displacement produced by applying a load torque to the motor. Unfortunately increasing the

integral gain also reduces the system damping giving overshoot after a transient. For a given integral gain the damping can be improved by

increasing the proportional gain. A compromise must be reached where the system response, stiffness and damping are all adequate for the

application. The integral term is implemented in the form of (Ki x error), and so the integral gain can be changed when the controller is active without

causing large torque demand transients.

Differential gain (Kd)

The differential gain is provided in the feedback of the speed controller to give additional damping. The differential term is implemented in a way that

does not introduce excessive noise normally associated with this type of function. Increasing the differential term reduces the overshoot produced by

under-damping, however, for most applications the proportional and integral gains alone are sufficient. It should be noted that the differential term is

limited internally so that it is ineffective if speed in rpm x Kd x Ki is greater than 170.

Keypad and

Txt VM DP ND

Torque

reference

(Te*)

Speed

feedback

(wr)

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