Pid Loop Operation - Hitachi L100 Instruction Manual

PID Loop Operation

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In standard operation, the inverter uses a reference source selected by parameter A01 for
the output frequency, which may be a fixed value (F01), a variable set by the front panel
potentiometer, or value from an analog input (voltage or current). To enable PID opera-
tion, set A71 = 01. This causes the inverter to calculate the target frequency, or setpoint.
A calculated target frequency can have a lot of advantages. It lets the inverter adjust the
motor speed to optimize some other process of interest, while saving energy as well.
Refer to the figure below. The motor has some eventual effect on a process of interest. To
control the external process, the inverter must monitor the process variable. This requires
wiring a sensor to either the analog input terminal O (voltage) or terminal OI (current).
Setpoint Error
Calculation
SP
PV
When enabled, the PID loop calculates the ideal output frequency to minimize the loop
error. This means we no longer command the inverter to run at a particular frequency, but
we specify the ideal value for the process variable. That ideal value is called the setpoint,
and is specified in the units of the external process variable. For a pump application it
may be gallons/minute, or it could be air velocity or temperature for an HVAC unit.
Parameter A75 is a scale factor that relates the external process variable units to motor
frequency. The figure below is a more detailed diagram of the PID function.
Setpoint
Standard
(Target)
setting
Scale factor
F01
reciprocal
Multi-speed
1/A75
settings
A20 to A35
Potentiometer
on keypad
V/I input
select
[AT]
Voltage
O
A GND
L
OI
A76
Current
PID V/I
input select
Freq.
PID
Inverter
Frequency
source select
A01
Error
SP
PV
Process Variable
(Feedback)
Analog input scaling
A12
A11
A15 A13 A14
L100 Inverter
External
Motor
Process
Process Variable (PV)
Sensor
Scale factor
A75
P gain
A72
Frequency
I gain
A73
D gain
A74
Scale factor Monitor
A75
4–27
F01
setting
D04