PID-Control with Mitsubishi Inverter

[DWIM 1]

Hello everyone,

 

I need some help with configuration of a Mitsu FR-D720 with a water pump. 

Pressure target: 6 bar

Sensor: 0..25bar , 4..20mA, connected to Terminal 4 of Inverter. Yes, the sensor range is suboptimal, I know.

Pump: can make 12 bar, the pump should stop working when the 6 bar target is reached.

Target: I want to use parameter 133 to set the pressure to 6 bar (no external adjustment, just set and forget) . I calculate 100%/25bar*6bar = 24%. So I set parameter 133 to 24.

- PID action selection (Param 128) : "20" (PID reverse action)

- RH-Terminal (Param 182) set to "14" (PID control valid terminal)

- Terminal 4 input selection (Param 267)  set to "0" (Terminal 4 input 4 to 20mA)

Question:
What I do not understand are the parameters 904,905:

904    Terminal 4 frequency setting bias frequency    0 to 400Hz    0.01Hz    0    0        
(904)    Terminal 4 frequency setting bias    0 to 300%    0.1%    20    20        
905    Terminal 4 frequency setting gain frequency    0 to 400Hz    0.01Hz    50    50        
(905)    Terminal 4 frequency setting gain    0 to 300%    0.1%    100    100        

I have not understood the concept/relationship between frequency/percentage units of the parameters and pressure/mA on the input.
Why do I have to or can enter a frequency? This relationship is unknown/dynamic.
Do I have to do enter here some scaling (gain/bias)? The pump should run at full speed (50 Hz) with 4mA and should stop at 4mA+(20mA-4mA)/25bar*6bar=7.84mA . This might be done by the reverse PID setting in PARAM 128?
If I do the scaling on the input, then I have another range for parameter 133? It would be nice, the value "60" is 6 bar and "40" is 4 bar in parameter 133.
It would be nice, if someone can give some insights. I would like to understand these parameters rather than try and error to have a half baked solution.

Have a nice day
DWIM