I-PD Position Move Command

Character: !
Decimal: 33
Hexadecimal: 0x21
Command Value: Requested Position, in position units

The I-PD Position Move command initiates a position move using the I-PD control algorithm. All other motion commands in the RMC100 use the PID control algorithm.

The Acceleration and Deceleration fields are not used. The Speed field is the maximum velocity allowed during the move. The Command Value is the requested position.

I-PD Explained

The I-PD control algorithm does not generate a target motion profile like the PID. Instead, the Target Position is always the same as the Command Position. This causes a step jump in the Target Position, which would normally cause a large step jump in the drive with the PID. The I-PD does not cause a large jump in the drive.

I-PD moves are simpler than PID moves, but are not as precise in terms of specifying the profile or the time it will take to reach final position.

Using I-PD to Follow Another Axis

The I-PD can be used to follow another axis. The I-PD will lag slightly behind the master during motion, but will get to the same position as the master when stopped. The I-PD can only follow the Actual or Target Position of the master with an offset. One suitable use for the I-PD is following a moving axis when the initial difference between the master and slave positions is large.

To follow an axis:

Select the Gearing bit in the Mode word and select whether to gear to the master's Actual or Target Position.
The Accel, Decel, and Speed fields are not used.
Specify the offset in the Command Value field.
Issue the I-PD command.

Tuning I-PD Motion

The I-PD uses the Proportional, Integral and Differential gains, but not the Feed Forwards or Accel Feed Forwards. The gains determine how quickly the I-PD gets into final position, as described below.

Use the following tips when tuning I-PD motion:

The Proportional Gain works the same as in the PID.
The ratio of the Integral Gain to Proportional Gain determines the response. For a faster response (to get into position quicker), increase the Integral Gain/Proportional Gain ratio.
The Differential Gain works the same as in the PID.
Feed Forwards and Accel Feed Forwards are ignored.

Why Bother?

The I-PD is simpler than the PID for basic moves where the goal is simply to get to position.
Some gearing applications may benefit from the I-PD, as explained above.