Yokogawa RotaMASS 3-Series - Manual (Page 150)

APPENdIX 5. PId BLOCK
a-50 IM 01R04B05-00E-E 3rd edition July 30, 2010 -00 all Rights Reserved. Copyright © 2005, Rota Yokogawa
A5.4 PId Computation
details
For PID control, the PID block in a RotaMaSS
employs the PV-proportional and -derivative type
PID control algorithm (referred to as the I-PD
control algorithm), or the PV-derivative type PID
control algorithm (referred to as the PI-D control
algorithm) depending on the mode, as described
below.
A5.4.1 PV-proportional and -derivative
Type PId (I-Pd) Control Algo-
rithm versus PV-derivative Type
PId (PI-d) Control Algorithm
the I-PD control algorithm, which is expressed
by the equation below, ensures control stability
against sudden changes in the setpoint, such as
when the user enters a new setpoint value. the
I-PD algorithm also ensures excellent controllabil-
ity by performing proportional, integral, and deriva-
tive control actions in response to changes of
characteristics in the controlled process, changes
in load, and occurrences of disturbances.
When the PID block is in auto or RCas mode, this
I-PD algorithm is used for control. In Cas mode,
however, the PV-derivative type PID (PI-D) algo-
rithm takes over since the response to setpoint
changes is more important. the control algorithm
in use thus switches over automatically in line with
the mode transitions. the following shows the ba-
sic computation formulas of these algorithms.
PV-proportional and -derivative (I-Pd) control
algorithm:
∆MVn = K ∆PVn + (PVn – SPn) + ∆(∆PVn)
∆T
Ti
T d
∆T
PV-derivative (PI-d) control algorithm:
∆MVn = K ∆(PVn – SPn) + (PVn – SPn) + ∆(∆PVn) T d
∆T
∆T
Ti
Where,
∆MVn = change in control output
∆PVn = change in measured (controlled) value
= PVn - PVn-1
∆t = control period = period_of_execution
in Block Header
K = proportional gain = GaIn (= 100/pro-
portional band)
tI = integral time = RESEt
t
D
= derivative time = RatE
the subscripts, n and n-1, represent the time of
sampling such that PVn and PVn-1 denote the PV
value sampled most recently and the PV value
sampled at the preceding control period, respec-
tively.
A5.4.2 PId Control Parameters
the table below shows the PID control param-
eters.
Parameter Description Valid Range
GaIn
RESEt
RatE
Proportional gain
Integral time
Derivative time
0.05 to 20
0.1 to 10,000 (seconds)
0 to infinity (seconds)
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A5.5 Control Output
the final control output value, MV, is computed
based on the change in control output ÐMVn,
which is calculated at each control period in ac-
cordance with the aforementioned algorithm. the
PID block in a RotaMaSS performs the velocity
type output action for the control output.
A5.5.1 Velocity Type Output Action
the PID block determines the control output
(out) value by adding the change in control out-
put calculated in the current control period, ∆MVn,
to the value read back from the output destination,
BKCaL_In. this velocity type output action can
be expressed as:
out = BKCaL_In – ∆MVn’
where ∆MVn’ is ∆MVn scaled based on PV_
SCaLE and out_SCaLE.
note: MV indicates the PID computation result.
A5.6 direction of Control
Action
the direction of the control action is determined
by the Direct acting setting in ContRoL_oPtS.
Value of Direct acting Resulting action
the output increases when the input
PV is greater than the setpoint SP.
the output decreases when the input
PV is greater than the setpoint SP.
true
False
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