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[Debs.]

I can see how cars with 'speed sensitive' power-assisted steering could simply be RPM modulated (through PAS pump output-pressure changes)....but how might it be genuinely 'road speed' modulated?

Is it`s operation; electronic, or hydraulic? :-/

[TheBoy]

ecu controlled, so must be electronic, though the PAS itself is obviously hydraulic

[Kevin Wood]

There's a valve in the steering box that is modulated by the Servotronic ECU, which itself has a speed input from the ABS ECU. I'm guessing it's a bypass valve that just bleeds off some of the pressure from the pump.

On the omega, at any rate. Newer cars are going over to electric assistance, which can obviously be controlled by varying the current to the motor.

Kevin

[Andy B]

..... Newer cars are going over to electric assistance, which can obviously be controlled by varying the current to the motor.

Kevin

Do bigger new cars use just a motor for assistance? SWBMO's Smart use just a motor & I know the little Fiat Siecento is the same.

[Marks DTM Calib]

..... Newer cars are going over to electric assistance, which can obviously be controlled by varying the current to the motor.

Kevin

Do bigger new cars use just a motor for assistance? SWBMO's Smart use just a motor & I know the little Fiat Siecento is the same.

Corsa sized cars (which incldues Corsa C onwards) have a column mounted motor for assistance.

Larger units tend to be electro-hydraulic and have an electric powered hydraulic pump rather than an engine driven one.

[Amigo]

I've never driven fast enough to know if mine works!

[hotel21]

I've never driven fast enough to know if mine works!

Watch the way you turn your head, Pinnochio.  You'll have someones eye out......
 

[Debs.]

I've never driven fast enough to know if mine works!

  You yikkle yiar!

[2woody]

there's five systems that I know of :-

1 - standard power steering. Hydraulic and not speed-sensitive

2 - the Omega / Carlton one, which uses a simple solenoid to bleed off some of the assistance at speed. Generally speaking, the faster you go, the more gets bled off. The solenoid is a PWM-type, which works in the same way as a turbo boost pressure control solenoid. IMO this system is absolutely horrible.

3 - EPAS, or electric. Here, there is no hydraulic system and an electric motor reacts to steer input speed. ok for smaller cars, but the assistance has a slight delay. You can always tell a car that's got EPAS from the steering feedback. Easy to make speed-dependant.

4 - electro-hydraulic. Similar to hydraulic, but with an electric motor driving the pump instead of the engine to avoid power losses. easy to make speed-dependant.

5 - late-type servotronic (ZF). In this case, the steering rack is a normal power type, but there's a separate hydraulic rotary actuator working against the pinion valve, giving a "profiled" feelback force. A bit "artificial", but easily the best system out there.

for reference, the vehicle speed signal generated by the ABS (as usual these days) is a "frequency" signal, simply on-off at a frequency proportional to road speed. This is easily provided to the steering box, where the solenoid simply switches the hydraulics on and off with a frequenct also proportional to road speed.

[Marks DTM Calib]

2 - the Omega / Carlton one, which uses a simple solenoid to bleed off some of the assistance at speed. Generally speaking, the faster you go, the more gets bled off. The solenoid is a PWM-type, which works in the same way as a turbo boost pressure control solenoid. IMO this system is absolutely horrible.

Indeed but, works VERY well when applied to non-rack applications such as on the Omega.

I ahev a few pics and working description of the Omega setup soemwhere, I will ahve to dig it out.

for reference, the vehicle speed signal generated by the ABS (as usual these days) is a "frequency" signal, simply on-off at a frequency proportional to road speed. This is easily provided to the steering box, where the solenoid simply switches the hydraulics on and off with a frequenct also proportional to road speed.

Indeed but, not in the case of the Omega, its actually put through a processor which in turn applies a map from which the modulation is derived and, via Tech2, you can also select 1 of 3 different maps for use 

[Andy B]

2 - the Omega / Carlton one, which uses a simple solenoid to bleed off some of the assistance at speed. Generally speaking, the faster you go, the more gets bled off. The solenoid is a PWM-type, which works in the same way as a turbo boost pressure control solenoid. IMO this system is absolutely horrible.

Indeed but, works VERY well when applied to non-rack applications such as on the Omega.

I ahev a few pics and working description of the Omega setup soemwhere, I will ahve to dig it out.

for reference, the vehicle speed signal generated by the ABS (as usual these days) is a "frequency" signal, simply on-off at a frequency proportional to road speed. This is easily provided to the steering box, where the solenoid simply switches the hydraulics on and off with a frequenct also proportional to road speed.

Indeed but, not in the case of the Omega, its actually put through a processor which in turn applies a map from which the modulation is derived and, via Tech2, you can also select 1 of 3 different maps for use 

Love - 15       

[Debs.]

Thankyou for the very interesting replies, chaps.....`most informative!