9-1
Chapter 9 Using PID Control for Constant Flow / Pressure Applications
9.1 What is PID Control?
The PID function in the EQ7 drive can be used to maintain a constant process variable such as pressure, flow,
temperature by regulating the output frequency (motor speed). A feedback device (transducer) signal is used to
compare the actual process variable to a specified setpoint. The difference between the set-point and feedback
signal is called the error signal.
The PID control tries to minimize this error to maintain a constant process variable by regulating the output frequency
(motor speed).
PID Command
J03
Drive
Section
Control
Target
J04
J05
+
+
+
+
-
Proportional Gain
(0.000 - 30.000)
Integral Time
(0.1 - 3600.0 Sec.)
Derivative Time
(0.00 - 600.00 Sec.)
Feedback Filter
PID Control Block Diagram
F01 Frequency
Command 1
Frequency
Command
PID Control
Cancellation
(E01
– E07 = 20)
ON
OFF
*Terminal
12
Feedback
Transducer
Terminal 12
E61 = 5
E61, E62 or E63 = 5
Feedback Signal Select
Normal
Operation
Dancer
Control
3
1
0
J01
PID Control
J06
(0.0 - 900.0 Sec.)
Selected by
J02 (0 Default)
0: Keypad
(Default)
Terminals:
1: 12- 0-10VDC
1: V2
– 0-10VDC
1: C1
– 0-20mA
3: Up/Down
4: Communication
* The setpoint and feedback sources
cannot be the same terminals.
Inverse
Operation
2
Normal
Operation
PID OFF
*
Terminal C1
E62 = 5
Terminal V2
E63 = 5
*
PID Feedback Signal
The amplitude of the error can be adjusted with the Proportional Gain parameter J03 and is directly related to the
output of the PID controller, so the larger gain the larger the output correction.
Example 1:
Example 2:
Gain = 1.0
Gain = 2.0
Set-Point = 80%
Set-Point = 80%
Feedback = 78%
Feedback = 78%
Error = Set-point - Feedback = 2%
Error = Set-point - Feedback = 2%
Control Error = Gain x Error = 2%
Control Error = Gain x Error = 4%
Please note that an excessive gain can make the system unstable and oscillation may occur.
The response time of the system can be adjusted with the Integral Gain set by parameter J04. Increasing the Integral
Time will make the system less responsive and decreasing the Integral Gain Time will increase response but may
result in instability of the total system.
Slowing the system down too much may be unsatisfactory for the process. The end result is that these two
parameters in conjunction with the acceleration (F07) and deceleration (F08) times (see chapter 8.2) are adjusted to
achieve optimum performance for a particular application.
For typical fan and pump applications a Proportional Gain (J03) of 2.0 and an Integral Time (J04) of 5.0 sec is
recommended.