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What is a hydraulic control unit?

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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Bertrandt AG

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RWTH Aachen, WKM

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Volkswagen AG

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Mahle Behr GmbH & Co. KG

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Burkhard Goeschel Consultancy

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Schaeffler AG

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Robert Bosch GmbH

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MAN Truck & Bus AG

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Ford-Forschungszentrum Aachen GmbH

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Audi AG

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Continental Teves AG & Co. oHG

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Valeo Schalter und Sensoren GmbH

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ZF Friedrichshafen AG

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Edag GmbH & Co. KGaA

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Hochschule München

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Adam Opel AG

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Daimler AG

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Wabco GmbH

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