How to set zero position and how to set relief valve in hybrid (hydraulic + electric) steering system?

There are at least two "zeros" involved:

The column torque sensor, which must give no or a balanced output with no driver force applied to the steering wheel. In a purely hydraulic system that is a mechanical valve but with an electric system it will have a torque transducer of some sort as well or instead.

The steering angle sensor, for road wheel angle.

The electrical sensors would have calibration values such as offset and gain correction factors stored in the relevant ECU, which are applied to the transducer signals as part of the software processing.

Original calibration is done at the factory, recalibration eg. after replacing parts can be done using an appropriate scan tool, from the appropriate ECU service procedures.


Hydraulic power steering has a pressure relief valve on the pump output, so if the power cylinder reaches the end of its travel with torque still being applied, the hydraulic pressure is limited to a safe value rather than bursting pipes or cylinder seals.

You can often hear noise from the relief valve when you hold power steering on full lock while manoeuvring, on hydraulic power steering systems.

Can you explain the arrangement?

I've seen the purely hydraulic power steering systems where there is no electrical components at all. ( ) Cars up until about 2000 usually had systems that worked on that principle, whether on rack or steering box layouts.

Later, the engine driven pump was replaced with an electric pump, basically to save power and to ease installation, but the control was still hydraulic.

Most recently, (starting with the MG-F if I understand it correctly) cars have a purely electric power steering where the motor is geared directly to the steering, and is energised as called for by the torque sensor. Obviously systems with driver aids like self parking etc. use those.

I'm not understanding how the truck system works.

How does relief valve know power cylinder has reached end of it's travel?

>> Can you explain the arrangement?
On front left, the steering gear is connected to the Pitman Arm. Pitman arm is connected to the drag link. Drag link is connected through something to left front road wheel.

Tie rod is mechanically connected from around drag link to front right road wheel.

When the hand wheel is turned left to full lock, Pitman arm makes about 55 deg angle with the drag link.
When the hand wheel is turned right to full lock, Pitman arm makes about 125 deg angle with the drag link.

There is an inlet steering pipe connected to steering gear from the pump.
There is outlet steering pipe from the steering gear to the fluid reservoir.

There is a pipe connected from fluid reservoir to the pump.

The Torque Angle Sensor (TAS) is somewhere on the gear. It is connected to steering ECU TAS connector.

Steering ECU also contains power, ground, and CAN connectors. Power is fed directly to steering ECU from vehicle battery. Or vehicle battery to some relay control circuit to ECU power connector.

Lower steering shaft is connected to steering gear. Steering gear is connected to electric motor. Electric motor is connected to steering ECU.

Some of the questions I have are as follows:
1. Are relief valve, control valve, cylinder, and piston inside the steering gear?
2. Is the steering gear output torque input to the system that is being controlled by the steering controller? And is the road wheel angle output of the system that is being controlled?
3. Is the steering gear output torque connected to the Pitman Arm?

jani12 said:

How does relief valve know power cylinder has reached end of it's travel?

Click to expand...

It's a pressure relief valve. It does not "know" anything.

If you kept a control valve open to a hydraulic cylinder after it has reached a mechanical limit, without a pressure relief valve or pressure regulator in the system, the flow stops and the pressure would try to become infinite; things would burst or break...

A pressure relief valve is just something like a poppet valve with a calibrated spring holding it closed, so if the pressure reaches a certain point, the valve is forced open against the spring until the pressure drops enough so the spring overrides it again.

It's the fluid equivalent of a Zener diode in electronics.

They are made in thousands of different forms for various types of fluid systems, but the principals are the same:


Re. the second question: The relief valve could be in the pump, allowing fluid from the output back to the input if the pressure gets too high.

Or it could be in the steering gear, bypassing that when there is excess pressure.
Or they could each have one, for reliability & to minimise pressure surges in the pipework.

If it's a steering box rather than rack & pinion style, the hydraulic actuation could be a short straight cylinder, a curved cylinder or a hydraulic motor.


Some Heavy vehicles may have the hydraulic cylinder separate, eg. either fitted crossways between the chassis and trackrod or chassis and wheel arms; sometimes one each side.



If it's an all-in-one box, then all torque is applied via the output shaft from that & Pitman arm.

I'd need a good diagram of the system to follow how the electrical / electric motor part is integrated as there are numerous possibilities.

jani12 said:

>> Can you explain the arrangement?

There is an inlet steering pipe connected to steering gear from the pump.
There is outlet steering pipe from the steering gear to the fluid reservoir.
There is a pipe connected from fluid reservoir to the pump.

The Torque Angle Sensor (TAS) is somewhere on the gear. It is connected to steering ECU TAS connector.

Lower steering shaft is connected to steering gear. Steering gear is connected to electric motor. Electric motor is connected to steering ECU.

Click to expand...

A diagram would help, but as far as I can tell the electric motor's torque is multiplied by the hydraulic power steering.

The manufacturers wanted to have electric control, but didn't want to have a large enough electric motor to turn the steering on its own, so they kept the hydraulic power steering and put the motor on the driver's side of that.