Operational Theory
Xenus XSL User Guide
20
Copley Controls Corp.
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 commanded current, 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
amplifier’s power stage.
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
Current Loop Inputs
•
The amplifier’s analog or PWM inputs.
•
A CANopen network via the amplifier’s CAN interface.
•
A Copley Virtual Motion (CVM) control program.
•
The amplifier’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 amplifier for a short duration of time. This
value cannot exceed the peak current rating of the amplifier.
Continuous Current
Limit
Maximum current that can be constantly generated by the amplifier.
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. 161)
.
Note: Although the current limits set by the user may exceed the amplifier's internal limits,
the amplifier 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. Used to limit jog moves initiated from the Control
Panel Jog function in current mode, and in advanced Indexer Program functions.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
