Power steering - What systems are people using?

[anonymous-user]

GavinPearson said:

Max - you need to consider the fact that a hydraulic pas rack will have a torsion shaft which transmits torque to the pinion and operates the spool valve that controls the assistance. If the assistance is removed, the torsion bar transmits torque, but it is unlikely to be sized for infinite life unassisted. If you were to reuse the rack with epas, you would want to use a far thicker version that was sized to have infinite life.

Indeed, but i wouldn't be reusing the existing hydraulic rack!

(also, the torsion bar has mech stops to prevent over rotation, and i'd imagine these are well within the elastic limit of the material, so high cycle fatigue wouldn't be a massive issue (i guess it could be eventually). However, the reason to ditch the std "hydraulic" torsion bar is that the electronic ones (dual serial synchro-resolver) have a much higher torsional stiffness as the electronics can add the necessary "gain" to the torque signal data.
This means less twist betwixt handwheel and pinion which is of course good for steering linearity and phase margin!



Current plan is to modify existing hydraulic power pack to increase flow (1'm waiting for some machined pump housings to do that). if that proves to be insufficient, i'm going to ditch the hydraulics completely, get a custom rack made using the motor/drive/ballscrew of one of the OEM EPAS racks.

[annodomini2]

Nope you use a column drive EPS rack which has no torque valve and is effectively the same as an unassisted rack.

[anonymous-user]

But you've still got to get "handwheel torque" information from somewhere, so the system will need to have a torsion bar in it somewhere. It's pretty irrelevant if that is in the rack pinion or up in the column! (As previously mentioned i can't fit a column assist unit due to lack of space (even if i wanted to, which i don't (mass too high up!!) ;-)

[annodomini2]

It's inside the column unit.

[anonymous-user]

annodomini2 said:

It's inside the column unit.

Indeed, i realise that. But you still need to have a torsion bar of some description in the system, even if that is the actual column itself, using strain gauges etc (not used for OEM due to poor signal to noise ratio and unreliability factors)