Velocity proportional integral regulator with negative feedforward to control response of torque disturbances

A method and apparatus for control of one or more torque-regulated motors in a controlled system in which a feedforward velocity component is subtracted from an output of proportional-integral velocity regulator to soften the command response of the motor control, independent of torque disturbances reflected in the velocity feedback path. The velocity feedforward path is decoupled from the velocity feedback path. In a preferred embodiment, a digital-type motor controller is used to practice the method in controlling a paper processing line.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The field of the invention is electronic motor drives for controlling speed 
and torque of an AC motor. 
2. Description of the Background Art 
Motor drives typically employ a velocity regulator in which proportional 
and integral functions are applied to an error signal and the results are 
summed to form an output. The error signal is an algebraic sum of inputs 
of position error or velocity reference and velocity feedback, and the 
resultant output is a torque command. 
In one particular application of motor drives to motors on a paper 
processing line, a long web of material is wound over rollers at a certain 
tension. During tuning of the system for response to line transients, the 
change in position of a swing bar on a rolling member, known as a 
"dancer", may be considered to simulate a typical disturbance. 
The general technical problem is the maintenance of overall line speed of 
the web feeding operation while smoothly damping out tension disturbances 
applied to the web. 
The difficulty in applying prior approaches has been the coupling or 
interdependence of position and velocity feedback loops, which makes 
tuning of the control circuit very iterative and difficult. 
Feedforward has been used for a variety of control purposes, but has 
traditionally been a factor that is added to regulator output signals. 
SUMMARY OF THE INVENTION 
In its broadest aspects, the invention relates to a method for providing a 
proportional feedforward factor from one input of a velocity regulator and 
subtracting the feedforward factor (negative feedforward) from the value 
of the torque command output from the velocity regulator such that the two 
proportional paths from velocity reference tend to cancel each other at 
the input to the torque controller. The velocity feedforward factor is 
decoupled from any velocity feedback and from any torque disturbance which 
is reflected in the velocity feedback. 
The invention is practiced in a method for controlling an AC motor in a 
controlled system, including the steps of: 1) generating a velocity 
reference command; 2) comparing the velocity reference command and 
velocity feedback to produce a velocity error; 3) inputting the velocity 
error as an input to a proportional-integral velocity regulator; 4) 
applying a feedforward gain factor to the velocity reference command 
independent of the velocity feedback path to produce a velocity 
feedforward component that is independent of the velocity feedback path; 
and 5) subtracting the velocity feedforward component from an output from 
the proportional-integral velocity regulator to provide negative 
feedforward of the velocity reference command to control torque being 
applied to the AC motor in the controlled system. 
In a more detailed aspect of the invention, the velocity reference command 
may represent position error developed from a position control loop. 
In another more detailed aspect, the invention allows one or more inputs to 
a proportional-integral velocity regulator to be fed forward in the above 
described manner without affecting response to other inputs using the same 
regulator. 
The invention can be practiced in a digital controller which is interfaced 
to a velocity-regulated motor with inner torque-regulated control. 
Other objects and advantages, besides those discussed above, shall be 
apparent to those experienced in the art from the description of the 
preferred embodiment which follows. In the description, reference is made 
to the accompanying drawings, which form a part hereof, and which 
illustrate examples of the invention. Such examples, however, are not 
exhaustive of the various embodiments of the invention, and therefore 
reference is made to the claims which follow the description for 
determining the scope of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the method of the invention is illustrated in the form 
of a control diagram showing the operation of a digital controller in 
controlling an AC motor 28. A speed feedback device (not shown in FIG. 1) 
is coupled to an output of the motor 28, to provide a velocity feedback 
(V.sub.FDBK) indicative of the output velocity (V.sub.OUT) of the motor 
28. 
At the input to the motor control, a velocity reference command (V.sub.REF) 
10 is algebraically summed with velocity feedback (V.sub.FDBK) 12 to 
generate a velocity error input 13 to a velocity regulator 17. In one 
branch of the regulator 17, a proportional gain factor (K.sub.P), 
represented by block 14, is applied to the velocity error input 13. In a 
second, parallel branch, an integral gain factor (K.sub.I) and an integral 
function represented by its Laplace transform (1/s), represented by block 
15, are applied to the velocity error input. The two results are summed, 
as represented at summing junction 16 to a produce torque component 18 of 
torque command 22. 
The velocity reference input (V.sub.REF) 10 is also fed forward through 
feedforward gain block 19, where the V.sub.REF input is multiplied by 
feedforward gain (G.sub.FF) to produce a torque adjustment factor 20. The 
torque adjustment factor 20 is algebraically summed with the torque 
component 18 to produce the torque command 22. In performing this 
summation, the torque adjustment factor 20 is of opposite polarity from 
the proportional branch component of the PI regulator 17 that is 
transmitted through signal path 10, 13, 14 and 18 to junction 21. This 
summation of the torque adjustment factor 20 tends to reduce the effect of 
the proportional branch component on the torque command 22. 
The resultant adjusted torque command 22 then becomes an input to a torque 
regulator 23, which may again be a proportional-integral type regulator. 
The output 24 of the torque regulator 23 becomes a signal to drive AC 
motor 28. The torque output 24 from regulator 23 is effectively summed 
with any torque disturbance (F.sub.D) 27 applied to the motor, as 
represented at summing junction 25. The resultant electromagnetic torque 
after the disturbance is input 26 to the motor 28. 
A Laplace transform transfer equation for this system is therefore as 
follows: 
##EQU1## 
where 
K.sub.P is the proportional gain factor, 
K.sub.I is the integral gain factor, 
J is the inertial constant for the motor, and 
G.sub.FF is the feedforward gain factor. 
The feedforward block provides a frequency-response "zero" in the numerator 
of the first term on the right side of the equation. The feedforward gain 
(G.sub.FF) can be adjusted independently of tuning to reduce the effect of 
the disturbance torque (F.sub.D) 27 on the speed of the system. In FIG. 1, 
it will be observed that the feedforward gain (G.sub.FF) and feedforward 
path are not affected by, or connected to receive feedback (V.sub.FDBK) 
from the feedback loop. Any torque disturbance is therefore isolated from 
the feedforward loop. 
The above method is applied to a paper processing operation, a portion of 
which is represented schematically in FIG. 2. A web of material 31 is fed 
through a first pair of calendar rolls 32, 33 which are driven by one AC 
motor (not shown). The web passes over auxiliary rollers 34 and 36 and 
around "dancer" roll 35, which is used to maintain proper tension on the 
web 31. The web 31 is then fed to a second pair of calendar rolls 38 and 
39 which are driven by a second AC motor 28 shown in FIG. 3. In an actual 
paper processing operation there may be a plurality of cascaded stages 
with AC motors driving various rotating members, including a motor driving 
a reel for winding up the paper at the end of the line. 
FIG. 3 shows a controller which uses the general method of FIG. 2, and 
includes the feedforward block 19, the proportional-integral velocity 
regulator 17 and the torque regulator as described above for FIG. 1. The 
same numbers have been used to identify elements in FIG. 3 that are also 
found in FIG. 1. FIG. 3 also shows a speed feedback device 30 which is 
coupled to an output shaft of the AC motor 28 to provide velocity feedback 
information (V.sub.FDBK) for determining the actual rotational velocity of 
the motor 28. 
As shown in FIG. 3, the velocity reference command (V.sub.REF) of FIG. 1, 
may itself be an output from a position control loop in which a position 
reference command (POS.sub.REF) and position feedback (POS.sub.FDBK) are 
algebraically summed at junction 42 to provide a position error that 
becomes V.sub.REF. 
In this control application, position sensor 37 is coupled to a swing bar 
on the dancer roller 35 as represented by a vertical dashed line. The 
swing bar can move laterally as represented by the dashed double-arrow in 
FIG. 2. Movement of the swing bar occurs in response to transient 
operating forces on the web 31 and causes the position sensor 37 to 
transmit feedback signals 40. Position feedback (POS.sub.FDBK) is 
transmitted from the position sensor 37 to a digital reference controller 
(elements 42, 43, and 19 in FIG. 3). 
Within the digital reference controller, the position feedback 40 is summed 
with the position reference command 41 at junction 42. The resulting 
V.sub.REF command is then fed to two inputs, one on the feedforward gain 
block 19 and the other to an input to a compensation block 43. This 
compensation block 43 may include several derivative functions, but 
preferably includes at least one branch that applies only a proportional 
gain factor. After these functions are applied to input 10, it is referred 
to as a velocity trim (V.sub.TRIM) input 44 to the velocity controller 17. 
The velocity trim (V.sub.TRIM) input 44 and the feedforward torque 
adjustment 20 input are preferably transmitted as serial data from the 
digital reference controller to a digital speed-torque regulator (elements 
17 and 23). 
Within this regulator, at junction 11, the velocity trim (V.sub.TRIM) input 
44 is summed with a line speed reference velocity (V.sub.LINE SPEED) 45, 
and the negative velocity feedback (V.sub.FDBK). The relative magnitude of 
the line speed reference velocity (V.sub.LINE SPEED) is great compared to 
the other positive inputs, and its purpose is to keep all the motors in 
the paper processing line running at essentially the same speed, in the 
absence of transient disturbances. Finally, the feedforward torque 
adjustment 20 is subtracted from the torque command 18 at junction 21 to 
produce input 22 to torque regulator 23. 
Before tuning the system, it is operated to obtain an operating mode that 
is generally satisfactory before final tuning adjustments are made. For 
tuning purposes, and to simulate transients on the paper line, a position 
change for the dancer 35 can be input as position feedback, or a 
disturbance torque (F.sub.D) can applied to the web at a location near 
rollers 38 by the engagement of nip rolls (not shown) with the web 31. 
The result of applying negative feedforward in the above described control 
method and apparatus is to decouple position error tension transients from 
line speed, while maintaining quick response to line speed commands and to 
disturbances. This is attributable to the negative feedforward control 
loop structure and its decoupling from the line disturbances. 
FIG. 3 also shows the advantage of the invention in being applicable to one 
of a plurality of inputs (V.sub.TRIM) without affecting the response to 
line speed reference velocity (V.sub.LINE SPEED). 
This description has been by way of example of how the invention can be 
carried out. Those with knowledge in the art will recognize that various 
details may be modified in arriving at other detailed embodiments, and 
that many of these embodiments will come within the scope of the 
invention. For example, although the invention has been described in 
connection with an AC motor, a motor control as described is also 
applicable to a DC motor. Therefore to apprise the public of the scope of 
the invention and the embodiments covered by the invention the following 
claims are made.