Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 225)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 225
Motor Control Chapter 4
Load Observer The PowerFlex 755 load observer feature compensates for and greatly reduces
load disturbances and gives quicker system response. It minimizes the load torque
requirements of the speed regulator. The load observer attempts to determine a
load estimate value that matches the load torque present in the simplified load
model. This is a simplified motor/load model.
From a control point of view, load torque is an input that is just as real as velocity
reference, but lacks a parameter. Load torque is unavoidable because it is
effectively torque times speed that creates the power to run a load. Considering
this load model, the applied torque is the electromagnetic torque generated by
the motor control and load torque is clearly shown. M is the combined motor/
load mass (inertia). The applied torque must be greater than the load torque to
accelerate the system.
Load torque is not a parameter and thus is not directly accessible, but it can be
indirectly measured. Referring to the plant model we can directly measure the
applied torque, output velocity and the inertia is generally known or calculated.
This leaves load torque as the only unknown value. If we place all three known
quantities in a box that we call an observer, load torque can be estimated.
As the output of the load observer is added to the output of the speed regulator
the function minimizes the load torque requirement for the output of the speed
regulator. Because load observer affects the torque reference and the acceleration
feedback is required, this method can only be applied on P35 [Motor Ctrl Mode]
Flux Vector control modes with motor feedback device. This feature is available
only on PowerFlex 755 drives.
+
–
1
M*s
Load Torque (Disturbance)
Applied Torque Velocity Output
II
R
+
+
+
+
+
–
689 685
704
76
704
711
707
2
0
0
From
Speed Reg
Output
Selected
Trq Ref
Notch
Filtered Trq
Ref
Load
Estimate
Total Inertia
Motor Acceleration Feedback
Load Observer BW
Load
Observer
Disabled
InAdp LdObs Mode
Load
Observer/
Estimator