Steering angle, function

W46.80-1032-79

W46.80-1031-06

When the steering is turned, the front axle steering angle sensor (B96) on the steering shaft of the steering gear measures the current steering angle and the steering direction. At the same time, the auxiliary steering axle steering angle sensor (B97) detects the current steering direction and the current steering angle of the auxiliary steering axle.

These measurement values are processed by the electrohydraulic auxiliary steering (EHZ) control unit (A72), together with the vehicle speed. This gives the specified steering angle and steering direction of the auxiliary steering axle.

The hydraulic control system reacts depending on the calculated steering direction and the deviating angle. The monitoring valve (Y75) in the Flowcontrol (4) and the control valve (5.1) in the hydraulic control block (5) are actuated. The volumetric flow rate is fed to the appropriate cylinder chamber of the steering cylinder (7) according to the steer direction. The maximum pressure is limited by the monitoring valve (Y75).

The steering cylinder (7) actuates the steering linkage of the auxiliary steering axle. If the actual steering angle matches the specified steering angle, the maximum pressure drops. The force generated in the steering cylinder (7) by the pressure is the same as the counteracting external forces.

If the steering gear (1) is actuated again or the measurement values are outside the tolerance range, regulation becomes active again.

If the vehicle speed increases to above 25 km/h, steering angle reduction becomes active, i.e. the higher the vehicle speed is, the smaller the steering angle of the auxiliary steering axle will be in relation to the steering angle of the front axle. If the vehicle speed is above 45 km/h, steering angle suppression becomes active. This means that regardless of how far the front axle is steered, the auxiliary steering axle will remain in the straight-ahead position. The steering cylinder (7) is hydraulically centered and held in position.

Control sequence

The electrohydraulic auxiliary steering (EHZ) control unit (A72) activates the following valves when the steering is turned:

 monitoring valve (Y75) in the Flowcontrol (4) in order to increase or limit the maximum pressure in the steering system

 the release valve (5.2) in the hydraulic control block (5) is closed in order to interrupt the shorted circuit of the connections (C1 and C2) at the steering cylinder (7)

 the common valve (5.7) in the hydraulic control block (5) is opened so that volume equalization can take place between the centering side of the steering cylinder and the hydraulic pressure reservoir (6)

 the control valve (5.1) in the hydraulic control block (5) is moved to switch position 4 or 2 according to the steering angle (left and right)

Depending on the position of the control valve, the hydraulic fluid now flows into the connection (C1 or C2) of the steering cylinder (7) and causes the steering movement.

The connection (C1 or C2) to which no pressure is applied, is connected to the return flow by the control valve (5.1), thereby enabling volume equalization in the respective chamber of the steering cylinder (7).

After the programmed steering angle of the auxiliary steering has been reached, the control valve (5.1) remains in switch position 2 or 4 and the maximum pressure is reduced (20 to 30 bar).

When the steering is turned, a change in pressure takes place in the centering circuit of the hydraulic control block (5) owing to the difference in volume in the centering chambers of the steering cylinder (7). The hydraulic fluid flows through the connections (Y) and into the centering circuit, and is fed into the hydraulic pressure reservoir (6), which leads to an increase in pressure of up to 40 bar. As a result, a maximum pressure of 60 to 80 bar builds up in the centering circuit.