Programming Your Application
6-57
590+ Series DC Digital Converter
PID
Functional Description
The following block diagram shows the internal structure of the PID block.
PID is used to control the response of any closed loop system. It is used specifically in system
applications involving the control of drives to allow zero steady state error between Reference
and Feedback, together with good transient performance.
The block executes a Proportional Gain + In Derivative control algorithm, with an
added filter to attenuate high-frequency noise. You can select P, PD, PI or PID as required.
P
roportional Gain (PROP. GAIN)
This is used to adjust the basic response of the closed loop control system. It is
defined as the portion of the loop gain fed back to make the complete control loop
stable. The PID error is multiplied by the Proportional Gain to produce an output.
I
ntegral (INT. TIME CONST.)
The Integral term is used to give zero steady state error between the setpoint and
feedback values of the PID. If the integral is set to a small value, this will cause an
underdamped or unstable control system.
D
erivative (DERIVATIVE TC)
This is used to correct for certain types of control loop instability, and therefore
improve response. It is sometimes used when heavy or large inertia rolls are being
controlled. The derivative term has an associated filter to suppress high frequency
signals.
The algorithm modifies the error between the setpoint and the feedback with the proportional,
integral, and derivative terms. The error is clamped internally to ±105% maximum.
The proportional, integral, and derivative terms are scaled by PROP. GAIN, INT. TIME
CONST., and DERIVATIVE TC respectively. An additional gain profiler can modify the
proportional gain as the roll diameter changes. The block diagram shows how the proportional
gain changes when using the profiler.
Proportional Gain
Proportional gain scales the
output based upon the input
error. Increasing PROP.
GAIN will improve the
response time while
increasing overshoot. MODE
selects the proportional gain
profile. When set to 0, the
proportional gain remains
constant over the entire roll.
Changing the value of
MODE increases the profile
as shown opposite.
You should try to achieve a
critically damped response
which allows the mechanics
to track as precisely as
possible a step change on the
setpoint.
Integral Gain
Integral eliminates steady-state error. Reducing INT. TIME CONST. improves the response,
however, if it is set too short it will cause instability. The integral value is clamped internally
by the settings of POSITIVE LIMIT and NEGATIVE LIMIT. It is also held at the last value
when the PID CLAMPED output is TRUE. Setting INT. DEFEAT to ON will disable the
integral gain term.
Critically Damped Response
Underdamped
Critically damped
Overdamped
Setpoint
Value
Setpoint
Time
Time