Steering angle, function

W46.80-1103-09

W46.80-1167-09

General

When the steering function is activated, the auxiliary steering axle can steer actively and the position of the steered wheels is maintained. By shifting the steering point on the steering line, the curve radius is reduced and the maneuverability of the vehicle is improved.

This is, however, only possible at low vehicle speeds of up to 45 km/h. At vehicle speeds higher than 45 km/h, steering of the auxiliary steering axle would impair driving stability.

Function

The electrohydraulic auxiliary steering (EHZ) control unit (A72) continuously evaluates the input signals from the front axle steering angle sensor (B96), from the auxiliary steering axle steering angle sensor (B97) as well as the CAN message on vehicle speed (information available via the connection to the vehicle CAN (CAN 1)). From this information, the electrohydraulic auxiliary steering (EHZ) control unit (A72) calculates the corresponding specified values for the steering angle and steering direction of the auxiliary steering axle. If the vehicle speed is lower than 25 km/h, the steering angle is set irrespective of speed, whereby the steering angle at the front axle must be greater than 4°. If the vehicle speed is between 25 km/h and 45 km/h, the steering angle is continuously reduced to the value "zero" as the vehicle speed increases. As soon as the vehicle speed is greater than 45 km/h, no steering angle is set.

If the driver presses the axle centering button (S123), the electrohydraulic auxiliary steering (EHZ) control unit (A72) receives a corresponding CAN message via the connection to the vehicle CAN (CAN 1). In this case, no steering angle is possible.

Depending on the continuously recalculated specified values for the steering angle, the electrohydraulic auxiliary steering (EHZ) control unit (A72) actuates the valves of the hydraulic control block (5) in such a way that the volumetric flow rates in the hydraulic system are regulated so that the steering cylinder (7) moves to the switching position necessary to trigger a steering operation. Here, the valves perform different tasks:

 the monitoring valve (Y75) is a proportional valve which regulates the pressure in the steering circuit

 the control valve (Y76) is a proportional valve which controls the volumetric flow rates in the steering circuit

 the release valve (Y77) separates or connects the two hydraulic steering circuits

 the centering circuit pressure control valve (Y97) regulates the pressure in the centering circuit

 the common valve (Y78) regulates the volumetric flow rate in the centering circuit

"Steer to right" function

For the wheels of the auxiliary steering axle to steer to the right, the main piston rod (7.1) of the steering cylinder (7) must move to the "main piston rod extended" position.

The electrohydraulic auxiliary steering (EHZ) control unit (A72) actuates the release valve (Y77) so that the volumetric flow fed from the Flowcontrol (4) via the connection (PX) on the hydraulic control block (5) flows via the check valve (CVX) to the control valve (Y76). Both steering circuits are now separated from each other. At the same time, the electrohydraulic auxiliary steering (EHZ) control unit (A72) actuates the control valve (Y76) in such a way that this valve switches to switching position 4. The volumetric flow now flows via the control valve (Y76) directly to the connection (X2) on the hydraulic control block (5) to which the connection (C2) of the steering cylinder (7) is connected. The hydraulic fluid flows via the connection (C2) into the steering cylinder (7). This causes the main piston rod (7.1) to move toward the "main piston rod extended" position. The piston of the main piston rod (7.1) pushes against the floating piston (7.2) so that this piston also moves.

Owing to the displacement of the floating piston (7.2), the volume of the front centering chamber (7.5) decreases and the hydraulic fluid flows via the connection (Y1) into the centering circuit. The out-flowing volume of hydraulic fluid is distributed as follows:

 During the phase in which the main piston rod (7.1) moves toward the "main piston rod extended" position and the volume of the rear centering chamber (7.4) increases, hydraulic fluid flows via the connection (Y2) into the rear centering chamber (7.4).

 If the volume of the rear centering chamber (7.4) can no longer increase, the hydraulic fluid flows off via the actuated common valve (Y78) into the hydraulic accumulator (6).

Via the connection (C1) on the steering cylinder (7), hydraulic fluid flows out of the steering cylinder (7) and back into the hydraulic control block (5). There the hydraulic fluid can flow via the actuated control valve (Y76) into the return line of the hydraulic control block (5) and back into the power steering fluid reservoir (3). Through the connection (T), hydraulic fluid is drawn via the return line out of the power steering fluid reservoir (3) and into the steering cylinder (7).

The electrohydraulic auxiliary steering (EHZ) control unit (A72) uses the centering circuit pressure sensor (PSY) (B94) to detect the pressure of the hydraulic fluid in the centering circuit, the steering circuit pressure sensor (PSX) (B95) to detect the pressure of the hydraulic fluid in the steering circuit, and the auxiliary steering axle temperature sensor (TSX) (B131) to detect the temperature of the hydraulic fluid.

The centering circuit pressure control valve (Y97) is actuated if the pressure in the centering circuit needs to be increased or reduced. In the case of a pressure increase, the hydraulic fluid present at the connection (PX) is fed directly to the centering circuit via the centering circuit pressure control valve (Y97). In the case of a pressure reduction, the hydraulic fluid flows via the centering circuit pressure control valve (Y97) and monitoring valve (Y75) out of the centering circuit and then via the connection (TX) and return line to the power steering fluid reservoir (3).

"Steer to left" function

For the wheels of the auxiliary steering axle to steer to the left, the main piston rod (7.1) of the steering cylinder (7) must move to the "main piston rod retracted" position.

The function corresponds to the "steer to right" function:

The common valve (Y76) is actuated such that it moves to switching position "2". In this switching position, the volumetric flows are controlled in such a way that the conditions with regard to the direction of the volumetric flows at the connections (X1, X2, C1 and C2) are reversed relative to the "steer to right" function.