Setting the Control Parameters
zub machine control AG
Manual MK1/100
·
Hardware Installation
Seite 8
MK1/100
The programs are started in exactly the same way as in the previous example.
The demo programs contain a commentary which explains which commands are being
performed and what should happen. In general, all position specifications in the programs are
given in units in multiples of four of the encoder resolution (hereinafter referred to as qc =
quadcounts). For instance, if a encoder with 512 lines is being used, the command
POSR 2048 (= 4 * 512) causes a relative movement of a single revolution. Similarly, a
command POSA 512 means that the motor will be located at a quarter revolution from the
machine's zero calibration. If a encoder with a division of 1,000 lines is being used, the same
results are achieved with the commands POSR 4000 or POSA 1000 respectively. See also
the explanatory notes to the parameters.
This section describes the simple procedure for empirically determining the control
parameters. The values determined in this matter have proven to be sufficient in most
applications; in order to make setting the parameters easier we also offer the program OPAL.
In order to be able to perform the adjustment tests described here the drive must be able to
run, with a normal load, a test distance that is long enough so that the majority of the distance
can be run at the necessary top speed. If the motor is driven at load torque's which differ
greatly (e.g. movements with and without tools), different control settings must be determined
for the various loads and the parameters of the control unit are to be set accordingly in
subsequent application programs.
The control algorithm of the PID filter used is described by the following formula:
u (n)
= K
P
* e(n) + K
I
∑
e(n) + K
D
[ e(n') - e (n'-1)]
u (n) = Correction factor in n
e (n) = Control error in n
n
= Sample cadence
n'
= Sample cadence for differential
factor
K
P
= Proportional factor
K
I
= Integration factor
K
D
= Differential factor
K
ILIM
= Integration limit factor
For the initial connection of the system it is advisable to set a very low value for the
proportional factor, e.g. K
P
= 1 (K
I
, K
D
and K
ILIM
= 0); thus it is certain that the cycle of the
closed loop control circuit is correct. Now K
P
can be set to values ranging from 20 to 200
(typical values) in order to check the basic functions of the system. The values are set
individually, with the aim that vibration-free operation is possible. Then it is possible to begin
with the setting of the other filter parameters.
The setting of the filter parameters directly in the application is necessary since the mechanical
systems can change sufficiently due to fluctuations in temperature, etc., to make an accurate
model recording impossible. However, since it is easy to change the filter parameters, only a
small amount of effort is required. Various methods can be used to empirically determine the
optimal filter values. The method described in the following section is most suitable to make
the settings without having any measured values on hand.
In order to logically comprehend how the settings are made, it is necessary to first understand
the functions of the various filter parameters. The proportional factor K
P
causes a gain, the
differential factor K
D
dampens, the integral factor K
I
eliminates constant positioning errors.
K
D
and K
I
are set to zero, and K
P
is increased until an overcorrection becomes visible. Reading
of the speed over time can be made over the measurement of the motor voltage. By
increasing K
D
a reduction in the overcorrection can be observed. With ITERATION K
P
and K
D
Setting the Control
Parameters
