NI Motion Controllers with Velocity Amplifiers

Velocity amplifiers close the velocity loop using a tachometer on the amplifier itself, as shown in the following figure. In this case, the controller must ensure that the voltage output is proportional to the velocity. Use the velocity feedforward term (Vff) to ensure that there is minimum following error during the constant velocity profiles.

The following figure describes how to use NI motion controllers with velocity amplifiers.

You typically use velocity feedforward when using controllers with velocity amplifiers. The uncompensated following error is directly proportional to the specified velocity. You can reduce the following error by applying velocity feedforward. Increasing the integral gain (Ki) also reduces the following error during constant velocity, but at the expense of increased following error during acceleration and deceleration and reduced system stability.

Note  National Instruments does not recommend increasing Ki.

Velocity feedforward is rarely used when operating in PID mode with torque block amplifiers. In this case, following error is typically much smaller because it is proportional to the torque required rather than to the velocity. When operating in PID mode with torque block amplifiers, velocity feedforward is not required.