Hello 

I have problems with controlling the concrete level in a hopper on a casting head.

Concrete Input in the hopper is from a sliding hydraulic gate.

Output of the hopper is from on/off gates. 

So when the ouput gates open, the sliding gate should fill the hopper to the setpoint.  

The sliding gate is controlled by a hydraulic cylinder with in-cylinder positioner. The cylinder is controlled by a 3/4 valve (3 steps plus, minus and closed)

I am trying to use the PID_3step controlller, however it seems not to work. 

It seems work fine in the beggining, but after some time the level increases above setpoint, and the controller opens the gate, even though the input is well above the setpoint. 

The setpoint is 200 kg. 

The input is the concrete level in the hopper in kg, the level is measured with a ultrasonic sonsor and scaled based on the form of the hopper. 

I don´t understand why the controller sets the output_UP even though the input is above the setpoint. 

Is it because this controller can´t be used for a integrating process? 

Hope somebody has experience with this? 

BR Martin

Hello BR Martin,

have you already tried the characteristic linearization? A better control quality can be achieved through the linearization of the characteristic. Further information and an example can be found in the AWB:

PID Control with Characteristic Linearization
https://support.industry.siemens.com/cs/ww/en/view/109783795

Thanks for the input. I ended up with not using the PID_3Step.

Instead I used a normal PID where the output was used as setpoint for the in-cylinder positioner feedback 0-100%. 

Then the hydraulic valve signals was activated by on/off signals, greather than or lower than. 

Open True:=SP>feedback (Valve opens until the positioner is above SP)

Close True:=SP+5<feedback (Valve closes until the positioner is below SP+5)

Best regards 
Martin