Process Control
January 2023
•
Rev. 0
• Standard Devices User Manual
21
PD/PDF or PD²I Control Algorithms
MENU
→
CONTROL
→
Control Loop
→
Loop Type
Your controller uses an electronic closed loop controller to
determine how to actuate its valve(s) in order to achieve the
commanded setpoint. These settings are tuned at production
for your specific operating conditions, but changes sometimes
require on-site adjustments to maintain optimal control
performance. Fine-tuning your closed loop control may help
correct issues with control stability, oscillation, or speed of
response.
For most applications, the PD/PDF algorithm is recommended.
When controlling pressure with a dual valve controller (MCD
or PCD devices), the PD²I algorithm is recommended.
Tuning the PD/PDF Control Algorithm
The controller’s default control algorithm (PD) employs
pseudo-derivative feedback (PDF) control, which uses two
editable variables:
•
The larger the
D
gain, the faster the controller
corrects errors between the commanded setpoint
and the measured process value. This is equivalent
to the
P
variable in common PDF controllers.
•
The larger the
P
gain, the faster the controller will
correct for offsets based on the size of the errors
and the amount of time they have occurred. This is
equivalent to the
I
variable in common PDF controllers.
✓
Note:
The D and P variables in the PD/PDF
control algorithm are more typically referred to
as P and I, respectively, in PDF controllers.
Tuning the PD²I Control Algorithm
PD
2
I usually provides a faster response in dual-valve flow and
pressure controllers. This algorithm uses typical PI terms and
adds a squared derivative term (D):
•
The larger the
P
gain, the more aggressively the
controller will correct errors between the commanded
setpoint and the measured process value.
•
The larger the
I
gain, the faster the controller will
correct for offsets based on the size of the errors
and the amount of time they have occurred.
•
The larger the
D
gain, the faster the controller
will predict needed future corrections based
on the current rate of change in the system.
This often results in slowing the system down
to minimize overshoot and oscillations.
Troubleshooting Valve Performance with PID Tuning
The following issues may be resolved by adjusting the PID
gain values for your controller.
Fast oscillation around the setpoint:
• PD:
Reduce the
P
gain in decrements of 10%.
• PD²I:
Increase the
P
gain in increments of 10%,
and then adjust the
I
gain to fine-tune.
Overshot setpoint:
• PD:
Reduce the
P
gain in decrements of 10%.
• PD²I:
Increase the
P
gain in increments of 10%.
Delayed or Unattained Setpoint:
• PD:
Increase the
P
gain in increments of 10%, and then
decrease the
D
gain by small amounts to fine-tune.
• PD²I:
Increase the
P
gain in increments of 10%,
and then increase the
I
gain to fine-tune.
✓
Valve tuning can be complex. More detailed
information is available at
Flow Limit while Controlling
Pressure
Flow Controllers
MENU
→
CONTROL
→
Control Loop
→
Flow Limiter
Limiting the flow rate while controlling pressure can help to
avoid exceeding the measurable range of the device as well
as prevent damage of sensitive devices later in the process.
To limit flow:
1.
Choose either mass flow or volumetric flow to limit by
selecting
TYPE
.
2.
Set the maximum value of flow rate desired by selecting
MAX
FLOW
and entering the maximum value in the engineering
units displayed.
3.
Set the
Limiter Gain
to 500 and adjust as needed.
Limiter Gain
determines how aggressively the
proportional control function corrects the error when the
flow rate exceeds the maximum flow setting. A higher
value corrects more aggressively, but it is also more likely
to oscillate near the flow limit.
✓
Note:
If both flow limiting and pressure setpoint
ramping are active when controlling pressure, the
more restrictive function regulates the controller’s
operation as it attempts to attain the setpoint.
✓
Note:
For bidirectional controllers, the flow limit
will be symmetric. For example an MCD-20SLPM
with a maximum set to 10 SLPM will be limited
to between −10 SLPM and 10 SLPM.
The
control loop menu
in PD
²
I control mode.
Setting a P Gain.
Flow limiter menu
.
Setting a maximum flow limit.
