To shut the engine down if the driver is on the throttle and hard on the brakes at the same time, or if a throttle or brake sensor fails. Running electronic throttle control makes this circuit mandatory.
The Brake System Plausibility Device compares throttle position and brake pressure. Each signal passes through a voltage divider into a linear comparator. The comparator outputs feed OR gates: one detects either throttle position above 10%, the other detects either brake pressure above the hard-braking threshold (600 psi on the CR22 design). The two OR outputs feed an AND gate, so an implausibility only registers when both conditions are true. The AND output goes through a delay chip setting a 1 second delay, then into an SR latch. The latch drives MOSFETs which switch three automotive relays, wired normally open so the throttle body, fuel system and ignition operate normally and are cut on a fault. The latch is reset at startup. Separately, the circuit compares each throttle and brake sensor against a low voltage threshold to detect signal loss, and latches on that too. The subsystem also carries the brake light assembly, which is a team-designed and manufactured PCB in a housing.
Latch the fault. A momentary implausibility that clears itself would let the car keep running with a real problem, so the fault stays latched until the car is power-cycled. Design the enclosure with the board. Altium pushes the PCB layout to MCAD, which is how the BSPD case was designed alongside the board rather than after it. 1. Establish the rules requirement and the thresholds from real vehicle data (brake pressure at which hard braking is unambiguous, throttle position of 10%). 2. Schematic capture in Altium, split into the main connector, reset subcircuit, signal loss subcircuit, simultaneous brake and throttle subcircuit, and the fault and latching subcircuit. 3. Lay out the board, push to MCAD, design the case. 4. Manufacture and bench test each subcircuit. 5. Validate on the car by observing PDM outputs: output 1 (ignition), output 11 (injectors), output 5 (fuel pump) all drop for the fault duration.
- Meet the rules requirement so the car can run electronic throttle. - Be rules-legal in full. CR22's Rev 2 board was in the car but not fully rules legal, and the fully tested Rev 2 did not meet sensor loss rules; this has been the recurring problem to close out. - Detect both the implausibility case and sensor loss. - Be testable on a bench before it goes in the car.
- Rules compliance on sensor loss. CR22 BSPD Rev 2 was fully tested and did not meet the sensor loss rules requirement; it took to Rev 3 to address. - Iteration count. Rev 1 was manufactured August 2023, Rev 2 designed and manufactured January 2024, Rev 3 designed March 2024. Board turnaround is a real schedule risk. - An ETC Rules Exception question had to be submitted and accepted before the ETC Review Form was approved.
Altium Designer for schematic and PCB layout, with the ECAD-to-MCAD push used to design the enclosure. KiCad is available for simpler boards. Bench testing per subcircuit. On-car validation through MoTeC PDM output logging.