Controlling 12V proportional solenoids

[cmcraeslo]

Hey guys.

This is my first post to this forum

I would like to drive a proportional hydraulic valve in automotive application. Valve comes from used BMW SMG unit. I have a cylinder that has a position element on it (LVDT) so I can do closed loop control PWM.

I was thinking of controlling this with a uC and some mosfet, but the problem here (I think) is that voltage is not constant. Do I need constant current driver for such thing then?

Now, I have very little information about the valve, so some testing will have to be done. What I do know that max output is 2 amps. This is copied from from official BMW's document (there are actually two, so this is the reason they're talking in plural)

The two proportional directional valves have three positions: pressure reduction, pressure
holding, pressure increase.
The proportional directional valves are set in the pressure reduction position when there is
no gear shifting or clutch operation in progress. In this condition the two cylinders (clutch
cylinder and selector angle cylinder) are connected to the expansion tank.
During clutch operation or main selector shaft rotation the valve is placed in the pressure
increase position. In this position the valve connects the cylinder to the hydraulic pressure
accumulator. At this stage the solenoid will consume approximately 1.1 - 2 amps. The valve
is fully open up to 2 amps.
When the clutch is to be disengaged or the main selector shaft is to remain in the activated
position, the valve switches to the pressure holding position. With the valve in this position
the flow of hydraulic oil to the cylinder is interrupted and the piston of the activated
cylinder remains in a pressurized mode.

I was looking very hard for the past 2 days to find a solution that would fit this, but haven't really been successful. There are so many options that I find everything very confusing.

Surely, this is a common problem?

Any help would be appreciated.

Thanks!

[Zero999]

I don't see why you'd need constant current. If you've got positional feedback, then the duty cycle of the PWM can be adjusted appropriately.

[joeqsmith]

Do you have the hardware designed? 

Normally, I would use PWM with a flyback diode.  Too low a freq, the valve will hammer on/off, a few KHz maybe you hear it.   Some dither may be good and prevent the piston from coming to a rest.  Software may need to train for the valve. 

Once you have the mechanics done, I would just run the valve open loop for starts with a function generator and plot out what it's doing. 

[Andreas]

Hello,

usually PWM is used for proportional solenoids.
You can do a pre-compensation of the duty cycle for the battery voltage.
For the slip-stick effect you will either need a dithering or a relative large PWM ripple current.
Sometimes avoiding of mechanical resonance frequencies of PWM frequency over the whole operation range is necessary.

The position feedback might be too slow for a stable controller.
Sometimes there is a additional pre-control of the PWM to give a fast response time.
The sensor is then only for fine adjustment.
If the sensor fails you have also a backup strategy for the system.

With best regards

Andreas

[bte2]

One of the biggest issues with mobile hydraulics is that every part of things change. Coils are warm. Coils are cold. Oil is warm. Oil is cold. Oil is old. Oil is new. Battery voltage is high. Battery voltage is low. Temperature is cold. Temperature is freezing. Temperature is freezing with old nasty oil. Filter hasn't ever been changed.

Then you get into the dynamics of the machine you are trying to control. Temperature, lubrication, structural soundness, and precision all come into play. If you are just trying to bang a cylinder to a stop, that's easy. If you are trying to swing a 100' boom 80 feet from the centerline without tossing the guy out of the basket, that's something else. In those cases, the difference between hot and cold outside matter more than you might be aware.

Even if you get it dialed in on a warm summer morning, that setup might be wholly inappropriate on a cold winter night, or with a different operator, or with a different hydraulic oil.

This is simply my opinion, but if you wish to be successful, you need to consider the whole package.

Another thing, there is a valve from a company called Apitech that runs at a very low frequency (like 30 Hz), and their technique does have some merit. You can see the levers pulsing in sync with the control signal.

In all cases, you want to run any coil at the manufacturer-intended voltage and frequency. Too low and it comes out as heat. Too high and it's hard to control predictably.

Good luck!

[bte2]

By the way, hi everyone! I have lurked for awhile but that was my first post. Sorry for the novel.