In: Working Papers of the Eighth International Workshop on Principles of Diagnosis (DX-97), Mont Saint Michel, France, 1997 Fault Isolation in the Hydraulic Circuit of an ABS: A Real-World Reference Problem for Diagnosis Martin Sachenbacher Robert Bosch GmbH Department FV/SLN Robert-Bosch-Str. 2 D-70442 Stuttgart Germany Peter Struss Technical University of Munich Department of Computer Science Orleansstr. 34 D-81667 Munich Germany {sachenba, struss}@informatik.tu-muenchen.de http://wwwradig.informatik.tu-muenchen.de/research/qreason/ Abstract We present requirements and success criteria for the real- world problem of (off-board) diagnosis of the hydraulic circuit of an anti-lock braking system. The primary problems to be addressed are quite fundamental: First, it is practically impossible to predict the dynamic behavior of this controlled automotive system as it depends on a number of unknown context conditions. Second, there are no direct measurements of the actual behavior available. Instead, the only available input are a number of temporally unspecified and inherently vague symptoms such as "vehicle is yawing to the right". Both problem dimensions might be considered a real challenge for model-based prediction and consistency checking techniques. A number of diagnostic scenarios for the system have been selected based on existing failure mode and effects analysis (FMEA) documents. Their practical relevance is confirmed by experts in the domain. Description of the Application Domain Purpose The anti-lock braking system (ABS) [Bosch 95, 96] is a safety add-on to the regular braking system of a car. Its purpose is to prevent the wheels from locking up and, thus, to maintain steerability and stability of the car during braking. Structure and Components An ABS is composed of an electronic control unit (ECU), wheel speed sensors and hydraulic pressure modulators. The hydraulic part consists of two symmetric subsystems, each one operating on a pair of (typically diagonally opposite) wheels. As shown in Figure 1, the hydraulic circuit of each diagonal comprises • four valves, • two brake cylinders, • a return pump element, • an accumulator chamber, • a damper with throttle. The pump elements of the two diagonals share one common drive motor. The hydraulic circuit is connected to the master cylinder that transforms a force acting on the brake pedal into increased pressure. To ensure that the pressure in the brake cylinders is never higher than the actual pressure in the master cylinder, the inlet valves have built-in non-return valves. damper pump element accumulator with (weak) spring to front right / rear left wheel master cylinder hydraulic unit brake pedal rear right wheel brake cylinders front left wheel secondary circuit primary circuit outlet valve throttle valve in closed position valve in open position magnetic coil inlet valve with non-return valve Figure 1: Hydraulic circuit of the ABS (diagonal distribution pattern)