User manual

PID Loop Operation
(DL450 Only)
Maintenance
and Troubleshooting
8--12
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
This feature reduces oscillation caused by a step change in setpoint when the
adjusting bias feature is used.
Mx = Mx
*
SP
n
/SP
n--1
if the loop is direct acting
Mx = Mx
*
SP
n--1
/SP
n
if the loop is reverse acting
Mx
n
=0 “ifMx<0”
Mx
n
=Mx “if0<Mx>1”
Mx
n
=1 “ifM>1”
It is not always necessary to run a full three mode PID control loop. Most loops
require only the PI terms or just the P term. Parts of the PID equation may be
eliminated by choosing appropriate values for the gain (Kc), reset (Ti) and rate (Td)
yielding a P, PI, PD, I and even an ID and a D loop.
Eliminating Integral Action The effect of integral action on the output may be
eliminated by setting Ti = 9999 or 0000. When
this is done, the user may then manually control
the bias term (Mx) to eliminate any steady--state
offset.
Eliminating Derivative Action The effect of derivative action on the output may
be eliminated by setting Td = 0 (most loops do
not require a D parameter; it may make the loop
unstable).
Eliminating Proportional Action Although rarely done, the effect of proportional
term on the output may be eliminated by setting
Kc = 0. Since Kc is also normally a multiplier of
the integral coefficient (Ki) and the derivative
coefficient (Kr), the CPU makes the computation
of these values conditional on the value of \Kc as
follows:
Ki=Kc*(Ts/Ti) “ifKc¸ 0”
Ki=Ts/Ti “ifKc=0(IorIDonly)”
Kr = Kc * (Td / Ts) “if Kc ¸ 0”
Kr = Td / Ts “if Kc = 0 (ID or D only)”
The standard position form of the PID equation computes the actual actuator
position. An alternative form of the PID equation computes the change in actuator
position. This form of the equation is referred to as the velocity PID equation and is
obtained by subtracting the equation at time “n“ from the equation at time “n--1“.
The velocity equation is given by:
nM
n
=M--M
n--1
Step Bias
Proportional to
Step Change SP
Eliminating
Proportional,
Integral or
Derivative Action
Velocity Form of
the PID Equation