Velocity Feedback Gain (Kv)

Determines the contribution of restoring force (damping) proportional to the velocity of the motor/actuator. You can use a primary or secondary feedback encoder for velocity feedback. Setting velocity feedback gain (Kv) to a value other than zero (0) enables velocity feedback using the secondary encoder, if configured, or the primary encoder if a secondary encoder is not configured.

Kv is used to scale this velocity feedback before it is added to the other components in the 16-bit DAC command output. Kv is similar to derivative gain (Kd) except that it scales the velocity estimated from encoder resources only. The derivative gain scales the derivative of the position error, which is the difference between the instantaneous trajectory position and the primary feedback position. Like the Kd term, the velocity feedback derivative is calculated every derivative sampling period and the contribution is updated every PID sample period.

The formula for calculating the velocity feedback contribution is as follows:

where the time between t1 and t0 is the derivative sample period.

Velocity feedback is estimated through a combination of speed dependent algorithms. At high speeds, velocity is simply the change in position per sample. At low speeds, the estimator seamlessly transitions to a 1/T method that measures the time between encoder counts and then calculates the inverse. This method is used for smoother performance when estimating velocities less than one encoder count per sample derivative sample period.

Using Kv and a secondary feedback encoder creates a minor velocity feedback loop. This is very similar to the traditional analog servo control method using a tachometer and a velocity block amplifier and is commonly referred to as dual-loop feedback. Dual-loop feedback is most useful when the primary position sensor (encoder or analog transducer) is located on the end-effector for improved accuracy, and is separated from the motor by gears, ballscrews, belt drives, and/or other mechanical apparatus with potentially poor dynamics. In this case, it can be difficult to achieve a high performance, stable control system without using the minor loop velocity feedback from an encoder mounted directly on the back of the motor.

Typically, Kd is set to zero when Kv is used. However, NI-Motion allows you to use both Kv and Kd terms simultaneously for improved performance.

Note  Operating with zero derivative gain (Kd) and either velocity feedback or a velocity block amplifier is often referred to as PIVff mode.

You can operate NI-Motion in PID mode, PIVff mode, or in a combination of both modes, by using Kd, Kv, or both.