Step Bias Proportional To Step Change In Sp; Eliminating Proportional, Integral Or Derivative Action; Velocity Form Of The Pid Equation - Automation Direct DL05 User Manual

Chapter 8: PID Loop Operation

Step Bias Proportional to Step Change in SP

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This feature reduces oscillation caused by a step change in setpoint when the adjusting bias
feature is used.
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Eliminating Proportional, Integral or Derivative Action

It is not always necessary to run a full three mode PID control loop. Most loops require only
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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.
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Eliminating Integral Action
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Eliminating Derivative Action
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Eliminating Proportional Action Although rarely done, the effect of proportional term on
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A

Velocity Form of the PID Equation

The standard position form of the PID equation computes the actual actuator position. An
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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
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equation at time "n" from the equation at time "n-1".
The velocity equation is given by:
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DL05 Micro PLC User Manual, 6th Edition, Rev. A
Mx = Mx * SP / SP
n n-1
Mx = Mx * SP / SP
n-1 n
Mx = 0
n
Mx = Mx
n
Mx = 1
n
M = M - M
n n-1
M = Kc * (e - e ) + Ki * (PV
n n n-1
"if the loop is direct acting"
"if the loop is reverse acting"
"if Mx < 0"
"if 0 Mx 1"
"if M > 1"
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.
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).
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) "if Kc
Ki = Ts / Ti
Kr = Kc * (Td / Ts) "if Kc
Kr = Td / Ts
- 2 * PV + PV
n n-1 n-2
0"
"if Kc = 0 (I or ID only)"
0"
"if Kc = 0 (ID or D only)"
)