# Strategies and Methods for the Fault-Tolerant Function Development of Multi-Domain Systems +
## 6 Product Development Examples
### Accommodation of Fault \"Incomplete Engagement of Gear\" in a Race Car Gear System  
The final example concerns the development of a gear system for a driverless formula student race car with a combustion engine. The given use case consisted of a race car which still had a combustion engine but could be used for autonomous driving. In the formula student competition, usually, powerful motorcycle combustion engines are used; for engines such as the Honda CBR 600 (Honda Motor Co., Ltd., Tokio, Japan), automated gear systems are not available. Due to the characteristics of such engines, alternations of the transmission ratio are necessary for competitive performance and smooth clutch operation [4]. It was therefore decided to develop an automated version of the manual sequential gear system of the motorcycle (which is integrated in the housing on the engine). The central element of this kind of gear system is a shift drum; in the manual version, the driver can rotate this shift drum through a certain angle using the foot lever. It was decided that the automated version would mimic this movement by means of an electrical motor. In the early phases of the system development process, it was decided to use an electrical motor assembly available on the market which is equipped with an incremental encoder using hall sensors. This incremental encoder allows obtaining the exact angular position of the shaft of the electrical motor, which is connected to the shift drum via a long shaft and a pair of spur gears. Consequently, it is theoretically possible to determine the angular position of the shift drum and to realise a control cycle for this angular position. This angular position is linked to the engagement of a certain gear, which is the objective of the gear system automation. In a nutshell: the central ECU for autonomous driving would determine the most sensible gear for a certain speed, for certain radii of track curves, and even for forecasted manoeuvers. The gear system ECU will then determine a certain angle the shift drum should achieve for the selected gear and a certain angle that the electrical motor should achieve. However, incomplete engagement of the gear occurred; this is often the result of inconvenient angular positions of the gear wheels in the gear system (tooth-on-tooth) which prevents smooth application of a gear. It was therefore decided to obtain direct information concerning the angular position of the shift drum using an absolute sensor (in the electrical motor, only an incremental sensor is present; to know the angular position, knowledge of an initial state is required) based on electrical resistance (potentiometer). A third, mechanical system is present to assure the engagement of gears: the shift drum is joined with a system consisting of a star-like element and a roller. A spring presses this roller into the indentations of the star which conforms to certain angular positions of the shift-drum corresponding to the engagement of a gear.  
Through this, three redundant, functionally independent countermeasures for the fault \"incomplete engagement of gear\" are present, these are summarised in Figure 12.  
Figure 12: Accommodation of the fault “incomplete engagement of gear” in a race car gear system.  
The same example of the safe shifting of a motorcycle gear was demonstrated in industry. This operation might be faulty and the gear might not switch properly due to production tolerances, wear, or weak actuation by the driver. It can be answered by a mechanical solution (more precise gears), by additional sensors or actuators, or by software functions changing the engine torque in a supporting way. The latter has been proven in a sample project as the most stable solution.  
The functional accommodation and the implementation are summarised in Table 7.