Electronic circuit help please
 

I am finalizing my plans to convert my car to electric power steering.

Belt driven pump will be replaced by a DC motor driven pump as part of this plan.
I am not thinking about a vehicle speed dependent variable speed/assist setup at this point. I will look into that as I work the wrinkles out of this setup I am working on.

The DC motor is 12V, 3300 Rpm, 3.5 Amp at no load.
It spins at 2850 Rpm, and draws 8.5-10 Amps under load.
Initial load spikes up to 60 Amps for a second or two.
Motor is designed for continuous duty, and the expected life is 11000 hours.
Pump pressure is 1232 psi (85 Bar). On par with what we have in our cars.

I do not want to run the motor full time. To this end, I have decided on certain parameters:

• It should of course come on only if the engine is running
• The pump should remain off if there is no steering input
• It should kick off over a certain RPM level
• And, it should also kick off once a certain vehicle speed is reached.

I thought about satisfying all these conditions with two relays.

The incredibly crude schematic I have come up with is as follows:

https://www.miataturbo.net/attachmen...ine=1366482092

Uploaded with ImageShack.us

ECU output will be from my Enhanced MSII, I will just use one of the programmable outputs for that.

I will also incorporate a "sensor block" on the pressure hose, and install a pressure switch on that block to provide a signal when the steering wheel is moved.

What I have described above is on the left side of the diagram.
There is a simpler approach using just a pressure switch on the left.

Please tell me if it will work, or if you have suggestions to simplify/improve on this idea, I would very much appreciate it.

I left the RPM and vehicle speed values blank on purpose. I have some ideas but, I welcome your input in that regard, as well.

Thank you

(A PWM controller is way above my head with my very limited knowledge of all things electronic, I will delve into that once I start grasping the concept to my satisfaction)

60A load for a second or two? I would drop the project immediately. I don't think the stock alternator can handle this, and even if it did, the engine will likely stall with that much of a load and a voltage drop.

Depower the rack, do a proper alignment and don't look back.

I've driving a miata with PS for the first time in like 5-6 years. it's ridiculous; WAY too easy.

I'd think he'd be ok if he runs a moderately sized cable cable (say 12ga or 2mm) from this system all the way back to the battery +, as well as a ground from the pump all the way back to the battery -. In other words, isolate the pump from the alternator as much as possible, keep its current off the main grounds, and use cable which is slightly thinner than you might otherwise to deliberately create a little bit of voltage drop to damp the startup of the motor.

That said, I'm not sure I can see how this pressure switch arrangement is going to work. Perhaps I'm not visualizing it correctly.

Also, if you're using VSS at the ECU to control the relay, then forget about RPM. Seems to me that would lead to the motor toggling as you shift from first to second to third, which would make the steering feel extremely weird.

I might also be tempted to install a solenoid-operated bypass valve across the motor which is open whenever the motor is off. That will lighten the steering load when the motor isn't running.

Rev, this pump is from a Renault Clio with a tiny engine. Renault Kangoo, Citroen Saxo and Peugeot 106 all use this pump, as well.
There are no reported voltage drop/stalling problems.

Also, a small and unscientific poll revealed a definite an element of regret associated with a dd Miata.

Just curious, how much would a large amplifier draw for instance?

I was actually thinking about running to and from the battery to feed the pump. That way the battery will indeed act as a buffer for the spikes.

BUT, I was thinking about using 4mm wires or better. Wouldn't you think 2mm wires would be taking a chance with the loads we are talking about?

Also, would running a capacitor on the B+ side help with the initial loads?

The more I think about how the electrical system would react to initial loads, the more I think the pressure switch would be a not so wise choice.
Incorporating a pressure switch to start the pump motor would mean starting it under load, in a very power hungry manner.
Exerting load on an already running pump would result in a milder initial power surge in comparison.


You have a point about the RPM input. Definitely not necessary. It would only complicate things.

A bypass valve would be a bit tough to integrate. It would actually be a plumbing nightmare with the pressure levels of the system I think.

On second thought, I really do not think two relays would be necessary.

I could just use a single 70 Amp relay, and use the ECU output (VSS only) on the "-" side, and ignition B+ on the other side to trigger the relay.

Pressure switch input would be detrimental per my above post in response to Joe.

Rev, I will just have to bump up the idle speed in line with voltage drop. Just like regular PS idle up protocol.

At 10 amps, 2mm wire is perfectly adequate.

At 60A, 2mm is definitely undersized. I spec'd this deliberately, to create some voltage drop across the line when the motor starts up. This will decrease the effect which it has on the rest of the vehicle's electrical system. Since it's only going to be happening for a short time, there is little danger of the wire overheating and starting a fire.



If you put it directly at the motor, perhaps. I doubt it would be necessary, though.



Yup.

I take it you want PS for parking lot speeds?

60A for a second or 2? If that is the current required to accelerate the motor shaft from a start (spoolup), that sounds too long. It probably is shorter than that.

Don't bother taking the wire all the way to the battery. Worst case you tap it from the starter motor. The starter motor sucks more than 60A when the oil is cold. Just make sure you fuse it properly. Find out how the OE's fuse it - the fuse (or breaker) has to be able to supply the spoolup current, and maybe blow after several seconds of >20A or so (to protect against a stalled motor). Or maybe they rely on the motor's built-in thermal cutout. The wiring's continuous current rating *always* has to be lower than the fuse's continuous rating.

Your ECU/setup has to cutout the PS motor during cranking. Probably have to leave it off for a few seconds after starting as well.

You can't tap it at the alternator, not because the alternator "only" puts out 60A, but because the line between the battery and alternator is fused at 60A (or 80A?). If the PS pump turns on at idle, and the alternator is putting out <60A, and there are a lot of other electrical loads, the fuse will have to pass >60A, and it will blow. It's not an issue otherwise, (i.e. not because the alternator "only" puts out 60A), because the alternator won't be damaged by the load. Alternators are inherently current limited, they will put out a certain max current, and any shortfall will be provided by the battery.

Check the inrush/startup current magnitude and duration; this will influence your system design.

If the inrush needs to be mitigated, Joe's idea of a skinny wire to mitigate it is a good one. You just have to choose the fuse very carefully. You don't want the wire to carry more than its rated current for more than several seconds.