Yokogawa YVP110 - Manual (Page 83)

<16. PID Function Block>
16-2
IM 21B04C01-01E
16.5 PID Computation
For PID control, the PID block in a YVP110 employs
the PV-proportional and PV-derivative type PID
control algorithm (referred to as the I-PD control
algorithm) for Auto and RCas mode. This algorithm
mensures control stability against sudden changes
in the setpoint, such as when the user enters a new
setpoint value. At the same time, the I-PD algorithm
ensures excellent controllability by performing
proportional, integral, and derivative control
actions in response to changes of characteristics
in the controlled process, changes in load, and
occurrences of disturbances.
For Cas mode, PV-derivative type PID control
algorithm (referred to as the PI-D control algorithm)
is employed in order to obtain better performance
against the changes in the setpoint.
The algorithm is automatically changed by the
block according to the mode. A basic form of each
algorithm is expressed in the equation below.
In Auto / RCas mode
∆MVn=K ∆PVn+ ∆(∆PVn)
∆T
Ti
Td
∆T
(PVn−SPn)+
In Cas mode
∆MVn=K ∆(PVn−SPn)+ ∆(∆PVn)
∆T
Ti
Td
∆T
(PVn−SPn)+
Where ;
ΔMVn = change in control output
ΔPVn = change in measured (controlled) value
= PVn – PVn–1
ΔT = control period
= period_of_execution in block header
K = proportional gain
= GAIN (= 100/proportional band)
TI = integral time = RESET
TD = derivative time = RATE
The subscripts, n and n–1, represent the sampling
time and thus PVn and PVn
–1
denote the PV value
sampled most recently and the PV value sampled
at the preceding control period respectively.
The table below shows the PID control parameters.
Table 16.1 PID Control Parameters
Parameter Description Valid Range
GAIN Proportional gain 0.05 to 20
RESET Integral time 0.1 to 10,000 (seconds)
RATE Derivative time 0 to infinity
16.6 Control Output
The final control output value, OUT, is computed
based on the change in control output ΔMVn, which
is calculated at each control period in accordance
with the aforementioned algorithm. The PID block
in a YVP110 performs the velocity type output
action for the control output. This means that the
PID block determines the value of the new control
output(OUT) by adding the change in control output
calculated in the current control period, ΔMVn, to
the current read-back value of the MV(OUT), MV RB
(BKCAL_IN). This action can be expressed as:
OUT = BKCAL_IN – ΔMVn'
ΔMVn' = ΔMVn which is scaled by PV_SCALE
and OUT_SCALE
16.7 Direction of Control Action
The direction of the control action is determined by
the Direct Acting setting in CONTROL_OPTS.
Table 16.2 Direction of Control Action
Value of
Direct Acting
Resulting Action
True The output increases when the input
PV is greater than the setpoint SP.
False The output decreases when the input
PV is greater than the setpoint SP.
16.8 Control Action Bypass
The PID control computation can be bypassed so
as to set the SP value in the control output OUT as
shown below. Setting BYPASS to on bypasses the
PID control computation.
F1603.ai
Setpoint
RCAS_IN
OUT
SP
CAS_IN
Filter IN PV
Control
BYPASS
Feed
Forward
Output
Figure 16.3 Control Action Bypass