Speed-sensitive power steering (SPS), function - GF46.50-P-0001LEH
MODEL 221.095 /195
General function requirements
- Circuit 61 ON
Speed-Sensitive power steering (SPS), general
The steering wheel's rotary motion is converted into horizontal movement through the steering gear of the rack-and-pinion steering.
The steering gear of the rack-and-pinion steering has a variable gear ratio.
The gear ratio increases continuously from the center and reaches its maximum value at a steering wheel angle of 90 Degrees.
The steering assistance is regulated by the rack-and-pinion steering gear itself.
It takes place hydraulically through an electrically driven power steering pump, the electrohydraulic power steering (A91/1).
The Manual effort required to turn the steering wheel is increased from vehicle standstill up to a speed of v = 100 km/h in accordance with a given characteristic.
To cool the power steering fluid an oil cooler in the form of a coiled pipe is installed immediately upstream of the A/C system's condenser.
The hydraulic reaction assembly is adapted to the respective requirements via the SPS [PML] solenoid valve (Y10). The electronic control takes place by the regenerative braking system control unit (N30/6).
- Speed-sensitive power steering turn operation function sequence
- SPS [PML] electrically controlled reaction function sequence
- Rack-and-pinion steering straight-ahead driving function sequence
The SPS [PML] system is comprised of the following partial functions:
Speed-sensitive power steering turn operation function sequence
If the steering wheel is turned, then the drive pinion is supported on the toothed rack. The rotary valve is twisted against the pilot bushing surrounding it via the stabilizer bar and this changes the position of the control grooves in relation to one another. The pressure oil flow now reaches the right working cylinder (left wheel lock) or the left working cylinder (right wheel lock) and causes the axial movement of the gear rack to the right or left. The oil from the unpressurized working cylinder is displaced. It flows back into the power steering pump reservoir via the open non-return grooves of the pilot bushing.
The steering effort assistance is regulated depending on the vehicle speed. Control takes place via the SPS solenoid valve which is actuated by the regenerative braking system control unit speed-dependent.
Shown on left wheel lock
The speed-sensitive power steering is a rack-and-pinion steering. Turning the steering wheel moves the steering column which is connected to the rotary valve in the rotary valve/pilot bushing housing.
The rotary valve is comprised of the rotary valve which has six control grooves with ground control edges on its casing surface and the pilot bushing likewise with six control grooves which the stabilizer bar is pinned on one side with the rotary valve and on the other side with the drive pinion. It holds the rotary slide in the neutral position when the steering wheel is not turned. If the steering wheel is turned, then the drive pinion is supported on the toothed rack. The rotary valve is twisted against the pilot bushing surrounding it via the stabilizer bar and this changes the position of the control grooves in relation to one another. The oil flow is now fed under pressure into the right working cylinder (left steer) or into the left working cylinder (right steer) and causes the axial movement of the toothed rack to the right or left.
The oil from the unpressurized working cylinder is displaced. It flows back into the power steering pump reservoir via the open non-return grooves of the pilot bushing.
The speed-sensitive power steering (SPS) progressively increases the force of the hand to be applied to the steering wheel from a vehicle standstill up to speed of approx. v = 100 km/h. The hydraulic reaction assembly is adapted to the respective requirements via the SPS [PML] solenoid valve.
The electronic control is integrated in the regenerative braking system control unit.
- Function of vehicle speed v = 0 km/h
- Function at vehicle speed v > 0 km/h and ≤ 100 km/h
- Function with vehicle speed v > 100 km/h
The function speed-sensitive power steering, steering angle is comprised of the following partial function:
Shown on left steer (Vehicle Speed v = 0 km/h)
The SPS [PML] solenoid valve is energized to a maximum and as a result closed against the force of the integrated pressure spring. The pressure ratios in the reaction chambers are identical since the supply of oil is stopped. No force is applied to the reaction balls, no additional moment arises on the surfaces of the rotary valve.
The steering is light, as the steering torque applied by the driver, consisting of overcoming the force of the torsion bar, is very low. Via a constant throttle bore the oil flow is passed on virtually unpressurized to the reservoir of the electrohydraulic power steering.
Shown on steering left (Vehicle Speed v > 0 km/h and < 100 km/h)
As the vehicle speed increases the flowing of the SPS [PML] solenoid valve decreases continuously, this is opened by the force of the integrated compression spring. The controlled pressure is lead to the reaction balls via the regulating valve which causes the pressure applied to the surfaces of the rotary valve to increase.
The force on the torsion bar is increased further as a result and the driver has to operate the steering wheel with a higher steering torque. The constant constriction orifice reduces the regulated pressure from the SPS solenoid valve. This is then passed on as a reverse flow virtually unpressurized to the reservoir of the electrohydraulic power steering.
Shown on steering left (Vehicle Speed v > 100 km/h)
The SPS [PML] solenoid valve is energized to the minimum extent. As a result the highest possible pressure is led from the SPS [PML] solenoid valve to the reaction balls via the control valve which causes the pressure applied to the surface of the rotary valve to increase to a maximum.
The constant constriction orifice reduces the regulated pressure from the SPS solenoid valve. This is then passed on as a reverse flow virtually unpressurized to the reservoir of the electrohydraulic power steering.
SPS [PML] electrically controlled reaction function sequence
The electronic control of the speed-sensitive power steering is integrated in the regenerative braking system control unit. A certain manual force is set at the steering wheel according to the speed of the vehicle. As vehicle speed increases, the manual effort required to turn the steering wheel increases at the steering wheel (up to v = 100km/h)
The vehicle speed determined by the regenerative braking system control unit is assigned a specified current in a performance map.This is then set in the SPS [PML] solenoid valve and influences the oil flow (hydraulic reaction assembly) within the rack-and-pinion steering gear and thus the manual force required to turn the steering wheel.
The actuation of the SPS[PML] solenoid valve is pulsed. The current is regulated by pulse-width modulation of these pulses.
Rack-and-pinion steering straight-ahead driving function sequence
When driving straight ahead (steering wheel in center position) the rotary valve of the rotary slide shift valve is in the neutral position. The control grooves of the rotary valve and pilot bushing are in a position relative to one another so that the oil flow, delivered by the electro-hydraulic power steering, is led directly back to the reservoir via the return. The working cylinders are therefore unpressurized.
The stabilizer bar is pinned on one side with the rotary valve and on the other side with the drive pinion.
| Speed-sensitive power steering (SPS[PML]), location of components | GF46.50-P-0001-01LEH | ||
| Electrical function schematic for the electrohydraulic speed-sensitive power steering | PE46.50-P-2051-97SAA | ||
| Speed-sensitive power steering (SPS [PML]), block diagram | GF46.50-P-0001-02LEH | ||
| Electrohydraulic power steering, component description | A91/1 | GF46.30-P-2310LEH | |
| Component description for regenerative braking system control unit | N30/6 | GF42.22-P-1100LEH | |