Embedded Files
ABS
Bosch now uses exclusively Hall sensor (active) wheel speed sensors because they react to the smallest changes in magnetic field and therefore allow greater air gaps compared to passive wheel-speed sensors.
Hydraulic Modulator:
The modulator is connected to solenoid valves with two hydraulic connections and two valve positions (2/2):
inlet valve between master cylinder and brake <-- controls pressure application
outlet valve between brake and return pump <-- controls pressure release
"Maintain pressure" scenario:
As the degree of tyre slip increases, the solenoid valves are switched into position so that inlet valve is closed (the outlet valve also): thus any increase in master cylinder pressure does not lead to pressure increase in each brake line
"Pressure release" scenario:
As the degree of tyre slip approaches the limit, the outlet valve opens to allow the return pump to draw fluid from the relevant brake corners
Influence of Lateral Acceleration:
If the vehicle is braked hard in a corner when lateral acceleration is high then the ABS cuts in sooner. The system for example may allow a maximum slip of 10% in order to obtain a friction coefficient of 0.35 which benefits a lateral force coefficient of 0.8
Initial braking phase simplified:
The road-surface frictional torque lags slightly behind the braking torque by a time delay (T) if the braking is within the stable (linear) zone of the friction vs brake slip. After 130ms approximately, the maximum mu is reached and from then on the braking torque starts to exceed the friction torque (from 130ms to 240ms). The ABS exploits the opposing characteristics of friction vs slip in the stable and unstable zones.
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The ECU takeas signals from a pair of diagonally opposed wheels and calculates the reference speed from them. During the ABS control sequence the ECU provides the reference speed based on the speed at the start of the control sequence and reduces it at a linear rate.
If 1st or 2nd gear is engaged when the brakes are applied, the engine acts on the driven wheels and substantially increases their mass moment of inertia. This causes their acceleration/deceleration response to slow down. The initial lag is also affected.
Control with high mu:
- During the initial phase of braking the brake pressure at the wheel and the rate of wheel deceleration rise.
- At the end of phase 1, the wheel deceleration passes the set threshold level (-a)
- As a result the relevant solenoid valve switches to "maintain pressure" setting
- At this point the reference speed is snapshotted and then reduced according to a defined linear gradient
- A slip switching threshold (lambda) is created as a line parallel to the reducing v_ref
- When the wheel speed drops below the lambda threshold the solenoid switches to the "reduce pressure" setting
- When the deceleration rate crosses the -a level another pressure-maintenance phase follows
- The wheel acceleration increases sufficiently to also cross the +a threshold and +A so brake pressure is increased
- and then kept constant for some time
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The ideal method of inhibiting yaw-moment buildup involves a compromise between good steering characteristics and a suitably short braking distance and is calibrated by Bosch with the OEM for each specific vehicle model.
Automatic brake functions:
- HBA Hydraulic Brake Assist - detects emergency braking situations and builds up pressure to the lock limit
- HHC Hill Hold Control
- HDC Hill Decent Control
- HFC Hydraulic Fading Compensation
- HRB Hydraulic Rear Wheel Boost
- BDW Brake Disc Wiping
EBD Electronic Braking-force Distribution:
Legal requirements demand that the braking system of road vehicles is designed to ensure at least 0.83g of deceleration and stable behaviour.
The controller continuously calculates the slip difference between front and rear axle wheels and can close the pressure inlet valve for any rear wheel if necessary.
HRB on the other hand was introduced because many drivers do not increase the pedal force at the start of ABS control even though the situation would require it. So the boost increases the pressure to the rear wheels until they also reach lock-up pressure.
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SBC
Sensotronic Brake Control is the Bosch Brake-By-Wire system.
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TCS
The Traction Control System is divided in Drive Axle Speed Controller & Transversal Differential Lock Controller
On a split mu surface, the wheel on low mu has a brake force applied to stop it spinning. The differential can then transfer this force to the other side, thus allowing more overall traction.
The transversal differential lock controller is designed as a PI-controller that contains additional non-linear elements.
The TCS employs a relatively large time constant to describe the dynamic response of the propshaft speed which is affected by inertia of shaft, engine, transmission and driven wheels.
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EDTC
The Engine Drag-torque Controller can "gently" accelerate the engine if engine inertia is causing wheels to lose traction.
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ESP
The Electronic Stability Programme governs the two status parameters:
- yaw velocity
- side slip angle
while calculating the Yaw Moment required to make the "actual" state parameters converge with the "desired" ones.
The maximum potential lateral acceleration and yaw velocity are determined for each vehicle on steady-state skid-pad testing. The yaw response is stored in the program in the form of a single-track model.
The ESP applies specific braking interventions such as inner rear wheel to counteract understeer or outer front wheel during oversteer.
But it can also accelerate the driven wheels.
During a lane change with emergency braking on mu = 0.15 the ESP reduces yaw velocity and side slip angle to levels controllable by the driver. Essentially it lowers the complexity of the steering process.
For example in an extended steering sequence (like a country road "snake" pass), the vehicle without ESP will generate consistent lateral acceleration but the yaw velocity will start to increase. This can be considered like an onset of "fish-tailing".
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Closed Loop Control
The ESP essentially controls the vehicle on-limit dynamics for:
- longitudinal velocity
- lateral velocity
- yaw velocity
It does this by calculating how the vehicle should respond to driver demand vs how it is actually responding, and then applies actuators to minimise the difference. Vehicle response is adopted as an element within the control loop.
Controller hierarchy:
Level 1 ESP - "monitor" takes wsp, gLat, pBrake, vYaw to estimate side-slip angle; executes desired yaw moment by controlling ABS & TCS
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Adaptive Cruise Control
The ability for the driver to follow slower vehicles is perceived to be a benefit and relief, to avoid concentration fatigue. The system also keeps the appropriate distances.
It is linked with ESP, for example to work safely and reliably in cornering situations, and uses a radar sensor for detecting other vehicles.
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Active Steering
For Active Steering, the mechanical components include a special feature of a differential gearbox within the steering column. An electric motor fitted with a worm can turn a worm gear. At low speeds the effective steering angle at the wheels is greater than the angle set on the steering wheel but at high speed it is smaller (where the motor subtracts some amount).
The Bosch system uses two processors that monitor eachother: one activates the servomotor and the other calculates the control angle.
By combining active steering (ASC) and wheel slip control (ABS) optimum driving stabilisation is attained.
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