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More and more functions in the car are now being electrically operated and controlled by actuators or servomotors. Modern braking systems, however, still use a hydraulic control unit.

Hydraulic control unit ensures stable vehicle dynamics


In spite of ongoing advances in the electrification of numerous functions in the car, a few mechanical components are still necessary. The ABS (anti-locking brake system) driver assistance function, for example, still requires the use of a hydraulic control unit. This allows the brake pressure to be applied and adapted to each wheel brake independently of the driver. In this way, the control program stored in the ECU of the ABS in modern ABS systems prevents the wheels from locking. This enables car manufacturers to significantly improve steerability and directional stability during emergency braking.


The hydraulic control unit plays a key role in this. In addition to an ABS valve system, it also has a shut-off valve for each brake circuit which enables the brake pressure generated by the driver when braking to be maintained in at least one brake circuit. When the brake is released, this brake pressure is reduced in a controlled manner. A hydraulic pump and four additional hydraulic valves are sufficient to generate controlled hydraulic pressure at each wheel brake.


Electronic Stability Program


The introduction of the Electronic Stability Program (ESP) further increased the complexity of modern vehicle brake systems. In addition to a more complex hydraulic control unit, the system usually also requires twelve hydraulic valves for a vehicle with two brake circuits. While eight hydraulic valves are assigned to the ABS control circuit, the other four hydraulic valves are distributed across the two brake circuits. A hydraulic valve for each brake circuit controls the pressure between the main brake cylinder and the hydraulic pump, while a further hydraulic valve controls the pressure between the main brake cylinder and the ABS system.


The most important technical components of a hydraulic control unit for an anti-locking brake system are:



Control strategy of modern ESP systems


Finding the suitable control strategy for the activation of the ABS and ESP vehicle dynamics control systems requires qualified data. Only then can the ESP system calculate suitable measures to prevent skidding or a loss of steering control. For example, the wheel speed sensor continuously provides information on the speed of each individual wheel. A special sensor for lateral acceleration (yaw rate sensor) reports to the ECU if it detects a risk of losing lateral control, while the wheel speed sensor detects whether the vehicle is at risk of skidding. Today, both functions are installed in one sensor element.


Further sensors continuously monitor the brake pressure and the steering motion of the vehicle. A steering angle sensor, the only one of the sensors to do this, sends its data directly to the ECU via the CAN bus. In the ECU, the data are combined and special programs calculate the intended steering direction and the intended driving behaviour of the vehicle. These are then compared with the actual state. If the actual value deviates from the intended value, a control intervention is sent to the hydraulic control unit.


Bleeding an ABS system


The brake control systems in modern vehicles are equipped with numerous measures to make them fail-safe (functional safety). Faults are reported via special diagnosis programs of the on-board power system. For example, the diagnosis programs might report that the brake pads are excessively worn or that a wheel speed sensor is faulty. The system diagnosis also monitors whether the brake fluid level is too low. If there is insufficient brake fluid in the system due to a leak in the brake pipes, it may be necessary in certain cases to bleed the entire brake system, in which case the ABS unit must first of all be completely refilled with brake fluid.


Only then can the brake circuits be bled in a specified order, usually starting with the rear brake circuit. First, the high-pressure pump must generate sufficient pressure in the pressure accumulator, with the bleed nipple opened. This only requires switching on the ignition. To prevent air from accidentally entering the brake circuit again, brake fluid is continuously added until the pump switches off by itself, having achieved the correct operating pressure. This procedure is then repeated for all brake circuits.

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Dr. Alexander Heintzel
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For 118 years, ATZ - Automobiltechnische Zeitschrift has been presenting cutting-edge solution concepts in automotive development and the very latest information for the everyday work of engineers relating to every aspect of the complete vehicle – whether it is the chassis or body, lighting technology or NVH, packaging or thermal management.

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Scientific advisory board

The scientific advisory board of ATZworldwide is made up of industry experts who work for leading companies and research institutions. By sharing ideas on a regular basis with the editorial team, the board members help to maintain the high qualitly of the magazine's content. The board provides the editorial team with first-hand information about the latest development trends and offers advice and constructive criticism.

Dipl.-Ing. Dietmar Bichler
Bertrandt AG

Dipl.-Ing. Kurt Blumenröder
IAV GmbH

Dr.-Ing. Joachim Damasky
VDA/FAT

Prof. Dr.-Ing. Lutz Eckstein
RWTH Aachen, WKM

Dr.-Ing. Ulrich Eichhorn
Volkswagen AG

Dr. rer. nat. Andreas Eilemann
Mahle Behr GmbH & Co. KG

Dipl.-Ing. Klaus Fröhlich
BMW AG

Prof. Dr.-Ing. Burkhard Göschel
Burkhard Goeschel Consultancy

Prof. Dr.-Ing. Peter Gutzmer
Schaeffler AG

Dr.-Ing. Markus Heyn
Robert Bosch GmbH

Dr.-Ing. Carsten Intra
MAN Truck & Bus AG

Prof. Dr.-Ing. Pim van der Jagt
Ford-Forschungszentrum Aachen GmbH

Dr.-Ing. Stefan Knirsch
Audi AG

Dipl.-Ing. Ralph Lauxmann
Continental Teves AG & Co. oHG

Dipl.-Ing. (BA) Joachim Mathes
Valeo Schalter und Sensoren GmbH

Dr.-Ing. Harald Naunheimer
ZF Friedrichshafen AG

Dipl.-Ing. Jörg Ohlsen
Edag GmbH & Co. KGaA

Prof. Dr. Dipl.-Ing. Peter Pfeffer
Hochschule München

Prof. Dr.-Ing. Rodolfo Schöneburg
VDI-FVT

Dipl.-Ing. (FH) Dipl.-Wirtsch.-Ing. (FH) Wolfgang Schwenk
Adam Opel AG

Dr. Michael Steiner
Dr. Ing. h.c. F. Porsche AG

Prof. Dr.-Ing. Thomas Weber
Daimler AG

Dr.-Ing. Christian Wiehen
Wabco GmbH

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