40
Setting the PID Gains
PID Parameters, Install
Conditions
At startup, the PID gain settings are
zero.
PID gains are AXIS GLOBAL VALUES
which are set from the CFG mode – Axis
Gains (see Axis Parameters section,
page ##). Two PID gains can be altered
in any segment by using the GAIN
feature when programming the segment.
Open Loop Mode
If the loop has been deliberately opened
using the screen available in Run mode,
the PID algorithm is disabled. An offset
in counts (range 0 to +/–8191 counts)
can be entered which directly generates
an output. The output produced will be:
(Maximum for the Selected Output) *
(input counts)/8191
WARNING
Open loop operation of a normally
closed loop can be hazardous.
Since the actuator will generally move at
maximum velocity with full controller
output and since an open loop can
easily run to the maximum available
position, it is suggested that a small
offset be used and advance precautions
should be taken to prevent injury or
damage.
Closed Loop Mode, General
Discussion
At each update interval, the difference
between reference position and actual
position is computed in digital form. This
is the “Position Error”.
If only proportional gain (Kp) is used,
constant SMC20 output corresponds to
a constant position error. This required
Position Error is called “Following Error”
and is the amount by which measured
position is behind the reference position
for a given velocity.
The Position Error is routed through a
PID digital compensation algorithm. The
version implemented in this controller
has a number of features specifically
aimed at common electric servo loop
applications.
FS
Means full scale controller
output
EU
Means engineering units
(counts)
Kp
Proportional gain
Ki
Integrator gain
Kd
Derivative gain (Velocity)
Integrator The integrator in the PID
compensation in each axis
of this controller is a “part
time” integrator. After the
AXIS IN POSITION
function has been TRUE
for a time equal to DELAY,
the remaining position
error (within the
INTEGRATOR BAND) is
integrated. The integrator
output is limited to a
maximum of plus or minus
the INTEGRATOR LIMIT.
Adjustable parameters
are:
DELAY (SEC)
BAND (EU)
LIMIT (%FS)
Table A-1. PID Parameters
Figure A-1. Control Algorithm
The output of the PID module is loaded
into a 14 bit Digital–to–Analog Converter
(D/A) at each update (+/– 8191 counts).
Any D/A offset programmed as an axis
parameter (CFG mode) is added to the
PID output in the D/A converter.
The Maximum Position Error which can
be processed is limited. Note that
Maximum Position Error programmed in
CFG or PRG mode, sets up a value
beyond which an error routine is
triggered. It DOES NOT set the internal
limits in the PID algorithm.
