User guide

Xenus Plus User Guide Operational Theory
Copley Controls 19
2.5.2: Current Mode and Current Loop
Current Loop Diagram
As shown below, the “front end” of the current loop is a limiting stage. The limiting stage accepts a
current command, applies limits, and passes a limited current command to the summing junction.
The summing junction takes the limited current command, subtracts the actual current
(represented by the feedback signal), and produces an error signal. This error signal is then
processed using the integral and proportional gains to produce a command. This command is then
applied to the drive’s power stage.
Current Loop Inputs
The drive’s analog or PWM inputs.
A network command, CAN, or RS-232 Serial.
A CVM control program.
The drive’s internal function generator.
In velocity or position modes, the current command is generated by the velocity loop.
Offset
The current loop offset is intended for use in applications where there is a constant force applied
to, or required of, the servomotor and the system must control this force. Typical applications
would be a vertical axis holding against gravity, or web tensioning. This offset value is summed
with the current command before the limiting stage.
Limits
The current command is limited based on the following parameters:
Limiter
Description
Peak Current Limit
Maximum current that can be generated by the drive for a short duration of time. This value
cannot exceed the peak current rating of the drive.
Continuous Current
Limit
Maximum current that can be constantly generated by the drive.
I
2
T Time Limit
Maximum amount of time that the peak current can be applied to the motor before it must be
reduced to the continuous limit or generate a fault.
For more details, see I
2
T Time Limit Algorithm (p. 129).
Note: Although the current limits set by the user may exceed the drive's internal limits, the
drive operates using both sets of limits in parallel, and therefore will not exceed its own
internal limits regardless of the values programmed.
Ramp
Rate of change in current command.
Current Command
PWM
Command
Current Limiter
Current Loop
Feedback (Actual Current)
Limits:
Peak Current
Continuous Current
Peak Current Limit Time
Current Integral Gain (Ci)
Current Proportional Gain (Cp)
Limited Current
+
-
+
+
Motor
Current Offset