Using Pid Control For Constant Flow / Pressure Applications - gefran VDI100 Series Instruction Manual

Using PID Control for Constant Flow / Pressure Applications
9.
9.1. What is PID Control?
The PID function in the inverter 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).
Target
Value
10-00
(Feedback
Gain)
Feedback
10-04
Value
10-01
The amplitude of the error can be adjusted with the Proportional Gain parameter 10-05 and is directly related to
the output of the PID controller, so the larger gain the larger the output correction.
Example 1:
Gain = 1.0
Set-Point = 80%
Feedback = 78%
Error = Set-point - Feedback = 2%
Control Error = Gain x Error = 2%
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 10-06. 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 (00-14) and deceleration (00-15) times are adjusted to achieve
optimum performance for a particular application.
For typical fan and pump applications a Proportional Gain (10-05) of 2.0 and an Integral Time (10-06) of
5.0 sec is recommended.
Code Parameter Name / Range
10-03 PID control mode
xxx0b: PID disable
xxx1b: PID enable
xx0xb: PID positive characteristic
xx1xb: PID negative characteristic
326
(Bias)
10-09
10-03=xx0xb
(PID output gam)
+
×1
+
10-24
-1
10-03=xx1xb
PID Output
+
-
PID Input
(Deviation)
10-03=x0xxb
(D)
+
10-07
+
10-03=x1xxb
10-03=3,4,7,8
±200% Limit +
10-25=0
+
10-03=1,2,5,6
10-25=1
-109%
PID=OFF
1. 10-03=0 (PID Disabled)
2. during JOG mode
3. multi - function digital input
(03-00 – 03-07 setting = 29)
10-14
100%
(P)
(I)
10-05 10-06
-
100%
(I-Limit)
(D)
10-03=1,3,5,7
G23-04 10-07
10-03=2,4,6,8
Example 2:
Gain = 2.0
Set-Point = 80%
Feedback = 78%
Error = Set-point - Feedback = 2%
Control Error = Gain x Error = 4%
PID=OFF
+109%
+
+ Frequency
PID=0N
Reference
(Fref)
+109%
10-23
100%
+
+
10-10
+ (Primary
-100%
delay)
(PID Limit)
Integral Reset
(using Multi-function
Digital Input)
VDI100 • Instruction manual