Electric power steering

[mielke]

Do electric power steering units use inverters? I’ve seen some commercial vehicle ePS unit’s talking about integrated inverters, but I was assuming ePS units operate a DCDC motor, so is there ever a need to convert to AC? I assume if it’s operating at a different voltage there may need a converter to match voltage levels, but is there ever an application that uses inverters in ePS? Thanks!

 

[TugboatEng]

BLDC motors are 3 phase AC motors with a built in inverter. With low cost solid state controls there is zero reason to run a true DC motor except in the case of series wound motors which are also controllable with solid state electronics but have brushes which reduce durability.

 

[BrianPetersen]

That application would use a permanent-magnet servo motor with an integrated servo drive. To call it an "inverter" understates its function. It needs fine control of both torque and speed right down to zero speed; that's what servo drives do. If the motor is spinning (because you're turning the steering wheel) then the waveform being fed to the motor will look like AC synchronised to motor speed, but the drive is controlling the waveform so as to produce the proper torque. If the motor is not spinning because you are driving in a constant radius around a corner, the motor will still be fed current on the appropriate phase(s) to provide the power-steering assistance. If the motor is not spinning because you're driving in a straight line, it may still be fed current on the appropriate phase so as to produce torque (e.g. for cross-wind compensation so that the driver doesn't have to fight this).

 

[enginesrus]

Can the systems over power the driver inputs?

 

[GregLocock]

Depends on the spec. Some systems can give 220 Nm of steering wheel torque, which is far beyond what a typical human can exert on a steering wheel. I imagine the same is true of many powered systems in general.

Cheers

Greg Locock


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[BrianPetersen]

The same is true of ALL power steering systems.

 

[IRstuff]

which is far beyond what a typical human can exert on a steering wheel.

That's kind of the point of power steering, isn't it? It's intended to make it easier to turn the steering wheel; I recall the first time I drove a car which didn't have power steering; it was tolerable at speed, but turning at low speed took a lot of effort, but I guess it built up my arm and shoulder muscles.

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[3DDave]

I had a couple of cars without power steering - the first had an offset from the pivot to the center of tread of about 8 inches - very easy to turn as there was nearly no scrub. I could grab a tire and backdrive the steering wheel with the tires on the ground.

 

[GregLocock]

You'd probably find that you could stall the epas on a small car if you were reasonably strong. The smallest I can quickly find is 32 Nm, which is far less than the design strength of the steering system (200 Nm typically). FSAE drivers were measured and most could achieve 50 Nm even though the steering wheel diameter is small. Most rack based EPAS have higher capacity than column based, but comparisons are then confused by the gear ratio of the rack to the column.

Cheers

Greg Locock


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[hydtools]

0ld 'armstrong' steering before power steering.

Ted