Air suspensiion control with ADL3

[JulianEdgar]

I want to use my ADL3 to control ride height of a car with air suspension.

Just taking one end of the car as an example: the rear suspension has interconnected air springs so needs only one control system. Ride height is measured with a pot i.e. a voltage signal is available that can be correlated with ride height. Increasing height is achieved by triggering a relay; decreasing height is achieved by triggering a second relay.

The way I am thinking of doing it is to use two User Conditions, set up where the pot value is compared with a preset value. If the value is lower than the preset value, the relay for 'up' is turned on. If the value is higher than the preset value, the relay for 'down' is turned on. I can achieve hysteresis by keeping a small difference between the preset values for 'up' and 'down' triggers.

For three channel height control (two front, one rear) I then use six User Conditions and six outputs that drive the six relays.

Is there a better way to do it?

[adrian]

Potentially that might work but there are so many unknowns with controlling a physical system simply by turning relays on and off. How fast is this supposed to work? Is it a static setting ie the car is stationary and then the control is turned off when it is driving? Do you have some method of adjusting the aim height such as a pot in the car?

I think that you will have to have big hysteresis values and filter the input channel quite heavily to get this to work but without know how the whole system works it is a bit hard to tell.

The other option is to use the PID control in the dash. It may be a fair bit more complicated to set up but it will give you a better output. If the relays are standard mechanical relays you would probably have to replace them with solid state relays if you were to go down this path.

[JulianEdgar]

Like most factory cars, I am using a restriction in the airlines so that the height control changes relatively slowly eg 20 seconds to move 50mm.

Levelling would happen both when the car is both stopped and moving. I would set the preferred height in the User Conditions ie if height value reported by the pot is value is less than X, then raise car (where X = preferred height).

Yes I would filter the input.

I am not sure that PID control in the ADL3 can output on/off signals?

[adrian]

If it is working that slowly then you are probably fine just using User Conditions.

You can set the limits for the PID control to what ever you want but in your case because you would want to control two separate outputs from one PID function you would need to have two User Conditions as well.

[JulianEdgar]

Just if anyone refers to this in the future, I ended up going with a PIC-based controller (Stemsel from eLabtronics) running custom software they developed.

Runs two solenoids for front, and two solenoids for the back. Front and rear suspension height inputs. On-dash driver height select control.

Uses different height averaging logic depending on if the car is moving or stationary, and whether or not a solenoid is running. Have it working well on rear suspension - front will follow. Module is only about US$50.