Delta RMC151 User Manual
Page 104
RMC70/150 and RMCTools User Manual
nearly all motion control systems with velocity feedback. In certain cases, Velocity I-PD
control may be preferred.
Velocity PID Advantages
•
Excellent for controlling an axis that follows a smooth target, such as one generated
by the RMC motion commands.
Velocity PID Disadvantages
•
May not control very well with an irregular target, such as step jumps or a joystick.
Motion Commands in Velocity PID Mode
To use Velocity PID, use the Set Pos/Vel Ctrl Mode (68) command to set the Next Pos/Vel
Control Mode to Vel 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
Velocity 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
velocity of the axis at that moment (called the Target Velocity) and subtracts the Actual
Velocity to determine the Velocity Error. The Velocity 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 Velocity. 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 Velocity PID uses the gains and feed forwards listed below. Each gain or feed forward
is multiplied by some quantity related to the Target Velocity and Actual Velocity 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.
•
The Proportional Gain is multiplied by the Velocity Error. This is the most important
gain.
•
The Integral Gain is multiplied by the integrated (sum of value x time) Velocity Error.
This helps the axis get to velocity over time.
•
The Differential Gain is multiplied by the difference between the Target and Actual
Accelerations. This helps the axis keep up with quick changes in velocity.
•
The Velocity Feed Forward is multiplied by the Target Velocity.
•
The Acceleration Feed Forward is multiplied by the Target Acceleration.
•
The Jerk Feed Forward is multiplied by the Target Jerk. The Jerk Feed Forward is not
necessary for most applications.
In addition, higher-order gains may be used if Acceleration Control or Active Damping are
selected.
Tuning Velocity PID
The velocity PID gains must be tuned manually. The Tuning Wizard cannot be used to
tune velocity PID control.
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