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electric/hydraulic power steering - need help with controller

Deyeme (Automotive) (OP)
28 Jun 07 11:20
Hi,
  I have a hobbiest-built electric vehicle.  He installed a Subaru 12v power steering pump, switched with ignition (and a manual shut-off near the steering wheel).  What I would like to do is get greater control over the steering assist to improve mileage.

The goal is to hook up a controller to the PS pump with an input from the vehicle speed sensor (+5vdc square wave) so that when the vehicle is stopped, the motor is going 100%.  As vehicle speed increases, the pump slows proportionally to vehicle speed.  I can find controllers out there for battle bot motors and such, but nowhere can I find described this one simple closed-loop operation.

Any help?

patprimmer (Publican)
28 Jun 07 17:59
Yes

Please don't double post.

It wastes our time and diminishes the evolution of the thought process in each thread.

Power steering systems are complex. Just changing line pressure can have unexpected results and might require recalibration of the torsion bar and spool valves.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

Deyeme (Automotive) (OP)
28 Jun 07 19:18
My intention in the similar post in the other forum was to encourage responses from a wider variety of engineering minds.  People sometimes get stuck in their own boxes of thought, especially with something so specific as automotive power steering.

In fact, your response is evidence to this effect.  My question was in regard to tying a vehicle speed sensor to a motor controller.  Your response was that power steering systems are complex (apparently too complex for me, as you offered no expansion beyond this statement), and then unexpected results, and torsion bars and spool valves.

I am not trying to imitate the precise original equipment feel.  This car is far from original.  Rather, I am trying to reduce the needless work the power steering pump is doing when power assist is not required, in order to increase fuel economy.  I don't expect the response to be perfectly linear or as complex and precise as the Toyota MR2 or the Saturn Ion, but what I can't do is just install a switch or trigger of some kind that just turns it on and off.  That WOULD produce unexpected results- like losing power steering when you least expect it!  All I'm trying to do is find something that can turn a square wave pulse from the VSS into a PWM for the pump.
GregLocock (Automotive)
28 Jun 07 20:15
Since it is a site policy not to cross post you don't get to decide.

Assuming your PS pump's motor is brushed DC then

You need a power amp with a voltage output. I am no electrician, but the word MOSFET springs to mind.

Then you need a voltage input, proportional to vehicle speed. If you have ABS you can take one of the wheel speed sensors, to give you a squarewave. Then  use an F to V circuit to convert that to a DC voltage proportional to speed. Then drive the power amp with the difference between that and a refernce value.

The above is not the most elegant of approaches, but it breaks the problem down into steps that can be tested seperately.

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

Yawing (Mechanical)
8 Aug 07 1:29
TWR make a variable speed controlled electric motor-hydraulic assisted steering rack. but i believe it is a two stage thing not infinitely variable.

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