I'm thinking pump speed could be correlated to pressure if they have a flow restriction at the steering actuator. The system is always "leaking", in other words.
To maintain constant pressure, steering wheel torque/speed(derived from position) would need to be an input and the pump speed would need to map to those inputs to derive a pressure ("assist"). More pump displacement (speed) means more assist. More operator steering speed means the pump needs to increase its displacement to keep assist constant - if that's what's desired. The third dimension in the map is vehicle speed which increases or decreases the assist.
I seriously doubt you actually set the pressure as Brian suggested. You have to constantly read steering position and torque, and vehiclecspeed, and 3D map pump speed to it all is my thinking.
A dry pump would go into fail as a motor current sense being too small - implicit broken hose fault which is likely why it stops. You need a wet loop with a flow control valve (fairly cheap hydraulics piece) to simulate it on the bench, if my hypothesis is correct.
I think running it in the car, only mapped to vehicle speed will be kinda sucky and open loop with a constant pump motor speed command will also suck.
Need the steering wheel torque and position sensors and vehicle speed as a 3D map to pump speed to get it right.
The thing is the pump doesn't actually stop. For those first five seconds, the pump is responding to the command and changing RPM in correlation to byte changes. After those five seconds, it stops responding to the byte changes but keeps whatever RPM it was previously running (and becomes much quieter on the network) and keeps on running at that RPM.
It's worth noting that this car was originally equipped with normal hydro steering setup, thus at idle (open loop constant pump speed essentially) the steering performed just fine. Thus my hypothesis is that if I were to run this pump at a constant speed as well (mimicking a belt driven pump) it'd perform similarly to the original setup. From what I've read, though, the failsafe speed (that 50% - 70% people have mentioned) might not be high enough for very quick turns, hence is why I would like to gain control over the speed. I can add a sensor to my steering column and when the sensor sees me turning the wheel very quickly it'll bump up the pump speed accordingly.
Otherwise, this post basically sums up my assumptions about this unit!
Thanks.
Of course that's not how a steering system works - it's never just short-circuited. The pump provides a regulated pressure to a control valve at the end of the steering column shaft. When the driver turns the wheel, part of the valve moves against a torsion spring (that tries to keep the valve centred) and so the harder you turn the more the valve opens to route fluid to one side of the steering rack (or box). So if you're not turning the wheel, the only fluid flowing is through the pressure regulator (which is presumably the bypass type, short-circuiting fluid from the pump outlet to the inlet), right at the pump.
Fluid volumetric flow rate (not pressure) is proportional to pump speed; the pump only needs to turn fast enough to move the fluid used to move the rack (zero if not turning the wheels) plus the regulator bypass flow. As a result, maintaining constant pressure (constant amount of boost) means a varying pump speed - very slow when not turning and much faster when spinning the wheel to park. It would make absolutely no sense for a control unit somewhere else commanding that speed, since it is pressure that needs to respond to vehicle speed and steering angle.
As a result, you could test the system with the fluid routed through a simple manual valve: closed is driving straight, partially open is turning the steering wheel slowly, fully open is turning the steering wheel quickly.
The varying speed is actually the original purpose of electro-hydraulic steering: rather than an engine-driven pump turning a something proportional to engine speed all of the time, regardless of what is actually needed, an electrically-driven pump only needs to turn as fast as required to meet steering needs, saving power on average. Ironically, the highest fluid flow demand is during parking maneouvers at very slow speed when the engine is nearly idling, and the lowest fluid flow demand is on a straight highway or while accelerating in a straight line when the engine is turning fast; directly belted to the engine was always a really unfortunate way to drive a steering assist pump.
Brian, you still seem to be under the impression that I'm saying the message I mentioned directly controls the speed.
This is not the case. I am not saying this command directly controls the speed. I'm saying that when I change the contents of that message, the pump speed changes.
I am not saying this is a speed control command. This is a cause-effect statement. When I change that byte, the speed is changed in a predictable and repeatable manner.
This message
could be:
1. Steering velocity (how fast the steering wheel is turning)
2. Vehicle speed (how fast the car is moving)
3. Steering torque (how hard the user is turning the wheel)
4. A direct speed command (yes, it could also be what you seem to think I'm saying it is)
5. Any number of other variables the pump cares about.
It may then use that data to calculate a final RPM (or current draw, or duty cycle, or etc etc).
I do not know what it
actually is. It's proprietary, so I can't know what it actually is without more effort that doesn't seem to justify the end result. Existing documentation on these pumps indicates that these pumps listen for steering velocity and vehicle speed, and do some magic proprietary math to calculate how fast to spin the pump motor. The post I quoted above this by remy_martian sums it up pretty well.
Also, I agree with remy_martian in that I don't believe this unit cares about pressure at all. It's likely not operating in any sort of closed loop, though I haven't seen inside yet so I can't see if it has a pressure sensor or not. I only think this simply because it doesn't
need to care about pressure. No existing belt driven pumps care about pressure at all, and this unit seems to be designed to directly replace those. It's probably only varying the RPM at all to conserve energy and reduce waste heat, and is
maybe basing that RPM based on steering velocity and vehicle speed. Supposing there is some sort of 3D map that takes both
steering velocity and
vehicle speed as an input and produces
pump RPM as an output, you'd just have to tune that map to always be fast enough so you can steer comfortably.
I should make clear that all of this is just a broad assumption. This is a closed source proprietary device. We can't know exactly how it works. Hence is why I'm interested in replacing it's brain with something that I can make work however I'd like.