Example Programs
Appendix B
B-4
Basic Positioning template program
The PLC option board for 3G3MV inverters provides the needed hardware to perform a basic position control
software. We can read an encoder with A and B phase signals, digital inputs and have complete control on
the inverter speed and Run commands. We have additional I/O and fully programmable PLC. That’s all
needed for a position controller application.
The control loop is performed by the ladder program. The selected control loop performs a very simple P
controller on the position error between demanded and real positions.. Then it limits and applies a frequency
reference proportional to it. With this setup, without profile generator (acceleration, deceleration generated by
position reference calculations), we have a compact position controller software, that will solve a lot of simple
point to point applications.
B-4-1
Method
•
We apply directly the position difference as speed reference , we have programmed some accelera-
tion on the inverter (so it will ramp up at that defined rate). The inverter has zero deceleration, so
when the position is reaching the point automatically the speed is reduced gradually, generating some
non-linear ramp, but stopping in the correct position.
•
As the PLC can not handle big negative numbers we have to apply an offset position and work
around an intermediate point, scaling for the user.
B-4-2
Features
•
Easy to use
•
Continuous loop
•
Scaled setpoint by N1/N2 factor.
•
2 InPosition windows. The second one can be defined bigger for faster sequence control.
•
Variable P Gain
•
Position_Reset available
•
Home(origin) search sequence, with fast forward and slow backwards seek. Definable speeds
•
Home(origin) timeout control
B-4-3
Limitations
•
As the 3G3MV is an open loop inverter, with only P type of controller, inertial loads might not be well
handled by the software, leading to oscillation. Lowering P gain can help, but lowers dynamics. It is
preferred some kind of frictional load. Most applications that use a high gear-ratio gear-motor will be
mostly controlled. To control inertial loads a more sophisticated control loop should be programmed.
Using a free motor can lead to instability.
•
Deceleration profile will be exponential due to the method of using the position difference to generate
speed reference.
•
We are limited to two word position references. So 80000000 quadrature pulses approximately.
•
The values allowed for the fractional factor limit the reference position range. Scaling intermediate
results can only be two word values. The bigger the factor, the shortest the position reference al-
lowed. It is recommended to use values from 1 to 10 in N1 and N2.
•
We don’t have the real concept of following error as the program does not perform a real positioning
profile. We only have the “demanded-real position” error.
B-4-4
Inverter/PLC Setup
We need some specific settings in the inverter for a correct positioner work :
•
We will use two sets of ramps… one is for the positioning with acceleration defined and deceleration
set to zero…
•
n021=1 sec n022=0 sec for position control (n022 must be always zero).
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