Delta RMC151 User Manual download pdf (Page 112)

RMC70/150 and RMCTools User Manual

Control Mode status register. The Current Control Mode register indicates the mode

currently in use.

See the Closed Loop Control topic for details on which commands are supported in

Position I-PD control.

Special Notes

Decreasing Jerk at Start of Motion

When using the Move Absolute (I-PD)(28) and Move Relative (I-PD)(29) commands, the

system will start moving with a sudden jerk. This is because the Target Position is set to

the Command Position immediately. In many systems this is acceptable. If it is not

acceptable for your system, you can instead use the Move Absolute (20) or Move Relative

(21) commands in I-PD mode. With these commands, the Target Position is ramped

toward the Command Position at the speed you specify (set the Accel and Decel to a high

value, such as 1000). This will essentially eliminate the sudden jerk. This may increase

the time it takes to get into position at the end of the move.

Fast Moves (Saturating the Output)

If the Control Output saturates, the I-PD is not disrupted like the PID is (with the PID, the

Integrator doesn't handle it very well). Therefore, the I-PD can be used to move the

system at it's maximum speed (typically the speed at 10V Control Output). To achieve

this, with the Move Absolute (I-PD)(28) and Move Relative (I-PD)(29) commands, set the

Maximum Speed command parameter to a value greater than you system's maximum

speed. The moves will then saturate the Control Output during the move, indicating that

it is moving at it's maximum speed. You will, of course, need to set the Output Saturated

Auto Stop

to Status Only.

Saturating the output can be very useful with non-linear valves where the gain "rolls off"

at the upper end. With a PID, it is especially difficult to get close to maximum speed,

because a small increase in speed can suddenly saturate the output. The I-PD makes it

easy to get to maximum speed.

Position I-PD Algorithm

Each closed loop motion command issued to the RMC specifies a target profile, which

defines where the axis should be at any given moment. For each loop time when the axis

is in closed loop control, the Position I-PD algorithm calculates the values from each gain,

as described below. Then, the terms from the Proportional and Differential gains are

subtracted from the Integral Gain term. The resulting value (in percent) is multiplied by

the maximum output (typically 10V), to come up with the Control Output voltage for that

loop time.

Gains and Feed Forwards

The Position I-PD uses the gains listed below. It does not use any Feed Forwards.

• Integral Gain

The Integral Gain is multiplied by the accumulated Position Error.

• Proportional Gain

In this control mode, the Proportional Gain multiplied by the change in the Actual

Position is subtracted from the Control Output each control loop.

• Differential Gain

In this control mode, the Differential Gain multiplied by the change in the Actual

Velocity is subtracted from the Control Output each control loop.

In addition, higher-order gains may be used if Acceleration Control or Active Damping

are

selected.

Tuning Position I-PD

The tuned position I-PD gains are typically the same values as the tuned position PID

gains. Therefore, you can use the same tuning procedures for as for position PID. See the

92 Delta Computer Systems, Inc.

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