PID Chap. 4 - PID Concepts
| John A. Shaw |
| Process Control Solutions |
| PID Control |
| Description |
| Tutorial |
| Ch. 1 |
| Ch. 2 |
| Ch. 3 |
| Ch. 4 |
| Ch. 5 |
| Ch. 6 |
| Code |
| Multiple Loop |
| Cascade |
| Ratio |
| Override |
| Trim |
Additional PID Concepts
Interactive or Noninteractive PID Algorithm
"Interactive" and "Noninteractive" refer to interaction between the reset and derivative terms. This is also known as "series" or "parallel" derivative.
Almost all analog controllers are interactive.
Many digital controllers are non-interactive, some are interactive
The only difference is in the tuning of controllers with derivative.
Non-Interactive (Parallel):
Out = G
(e + R+ D ) Interactive (series):
Out = (RD+1)G
(e + R+D )Converting between interactive and non-interactive
Applies only to 3-mode controllers
To convert from non-interactive to interactive:
Gn = Gi (1 + Ri Di)
Rn = Ri/(1 + Ri Di)
Dn = Di/(1 + Ri Di)
• In other words, with a non-interactive controller the gain should be higher, the reset rate lower, and the derivative lower than on a commercial interactive controller.
External feedback
The integral function implemented using a positive feedback.
If the input to the positive feedback loop is taken from the signal to the process, it is called "external feedback" or "reset feedback". At steady state the controller output is the Gain multiplied by Error added to external feedback. If the error is zero, the output is equal to the external feedback.
Saturation Properties
Another difference is in the "Saturation Properties"
eg. what happens when output has been at the upper or lower limit.
Standard algorithm
Described on previous page.
Output stays at limit until measurement crosses setpoint.
"Integrated velocity form"
Similar to equation:
Output = Last output + gain
x (error - last error + reset x error)Output pulls away from limit one reset time before measurement crosses setpoint.
• For most applications, there is no difference. For some batch startup problems, the "integrated velocity form" algorithm works best.
• Standard works best for high gain/low reset rate applications.
