Delta RMC151 User Manual download pdf (Page 102)

RMC70/150 and RMCTools User Manual

Partial Open Loop Commands

The following commands use open-loop control for part of the motion. As the axis reaches

the requested position, the axis switches to closed loop control, decelerates, and holds

position. These commands are useful for fast motion.

• Quick Move Absolute (15)

• Quick Move Relative (16)

3.5.4. Position PID

Position PID is the algorithm typically used to perform closed-loop motion control on a

position feedback axis. PID stands for the central gains used in this mode: Proportional,

Integral, and Differential. The Position PID provides very good control and is suitable for

nearly all motion control systems with position feedback.

The Position PID works on a position feedback only and controls both position and velocity. In

certain advanced applications, other control modes may be preferred, such as Position I-PD

Velocity PID, or Velocity I-PD.

Position PID Advantages

• Tracks position very well.

• Is very well understood by most people in the motion control industry.

Position PID Disadvantages

• May chatter or oscillate when following an irregular target, such as a step jump or a

noisy reference signal.

• Tendency to overshoot final position on some systems.

Motion Commands in Position PID Mode

The default control mode of a position axis is Position PID. If the axis is not in Position

PID, use the Set Pos/Vel Ctrl Mode (68) command to set the Next Pos/Vel Control Mode

to Pos PID. The next closed-loop motion command will use the control mode specified in

the

Next Pos/Vel 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 PID control.

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 RMC uses the specified target profile to calculate the desired

position of the axis at that moment (called the Target Position

) and subtracts the Actual

Position to determine the Position Error. The Position PID algorithm then uses this

information, together with the gains and feed forwards, to calculate how much Control

Output should be generated to move the axis to the Target Position. The values of the

gains and feed forwards must be set to achieve proper control. The process of setting the

gains is called

tuning and is done as part of the setup procedure.

The Position PID uses the gains and feed forwards listed below. Each gain or feed forward

is multiplied by some quantity related to the Target Position and Actual Position to come

up with a percentage. The resulting percentages are all summed and then multiplied by

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

loop time.

• Proportional Gain

The Proportional Gain is multiplied by the Position Error. This is the most important

gain.

82 Delta Computer Systems, Inc.

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Delta RMC151 User Manual Page 102

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