Signal Evaluation Subsystem Clause Samples
Signal Evaluation Subsystem. For the pilot deployment report we selected the signal evaluation subsystem. Besides the signal evaluation subsystem, the cruise control system consists of the velocity control subsystem which calculates an acceleration demand based on the current vehicle speed and the stored target speed and the dis- play subsystem which is responsible for displaying the status of the cruise control system to the driver and to other control units. The signal evalu- ation subsystem is responsible for evaluating the signals generated by the control elements of the operating lever and by the pedals. Based on the evaluation of the signals the signal evaluation subsystem changes the inter- nal state of the cruise control system and sets or deletes the target speed. Contrary to the velocity control subsystem the signal evaluation subsystem only consists of discrete behaviour and thus is especially amenable for formal modelling with Event-B. As there exists a large number of operating lever variants we decided to choose one operating lever variant for pilot deployment which only provides the following control elements:
Signal Evaluation Subsystem. 11 • •
1. Disambiguation by the low-level hardware driver: The hardware driver is able to disambiguate single button presses from holding a specific button. Thus the ACC and DEC functions can be disambiguated.
2. Disambiguation by the signal evaluation subsystem: The signal evalu- ation subsystem of the cruise control disambiguates signals generated by the low-level hardware driver based on the internal cruise control state. The signal evaluation subsystem distinguishes between the following in- ternal states of the cruise control system: As shown in Table 2.1, for each mode there exists a number of different states which record the history of input signals which have been evaluated. It is important to note that state transitions within one of the three modes do not change the external behaviour of the cruise control. For example, if the cruise control transitions from state OFF to state STANDBY the invariant of the NO CONTROL mode still holds. This means, that the vehicle speed is still only controlled by the accelerator pedal position. If the cruise control transitions from the state STANDBY to the state CRUISE which indirectly also involves switching modes from NO CONTROL to CONTROL, then the externally visible behaviour of the cruise control is changing, i.e., after the state switch the vehicle speed is controlled by the cruise control and not anymore by the accelerator pedal position. Mode State Description NO CONTROL UBAT OFF Ignition is off and engine not running INIT Ignition is ON and cruise control is be- ing intialized OFF Ignition is ON, cruise control has been initialized and is switched OFF ERROR An irreversible error has occurred STANDBY Cruise control has been switched ON R ERROR A reversible error has occurred CONTROL CRUISE Cruise control is maintaining the target speed RESUME The target speed is approached from above or from below ACONTROL ACC Cruise control is accelerating the car DEC Cruise control is decelerating the car RAMP DOWN Cruise control is being switched off