Introduction
As apart of the University of Toronto’s Formula Racing Team, we undertake the ambitious project of creating a formula-style racing car entirely from the ground up, to compete in global competitions at Formula SAE events. A key aspect of engineering a dependable vehicle lies in the meticulous gathering of data from vital systems throughout the car.
For the upcoming 2024 season, I was tasked with designing, manufacturing, and validating the PCBs that form the foundational elements of our data acquisition system—the Corner Modules. These PCBs are crafted to connect with a wide array of sensors and transmit their data through a CAN bus system. This allows for effective data logging and integration with the car's central rear controller for real-time usage and analysis.
Routing and Layer Stackup
The board uses a 4 layer 1oz stackup, with power on the inner layers and ground on the outer layers The goal here was to have all signals routed on the outer layers, while having ground planes as close as possible to signal traces to avoid EMI (electromagnetic interference), as well as act as a heat sink for certain components.
Signals such as CAN are routed with precision to maintain their differential characteristics, with careful separation from high-frequency digital signals to preserve signal integrity.
Power Regulation
The board is powered via 12V from a buck converter that steps down the car battery voltage, which is protected against reverse polarity in the event the voltage has been reversed. Linear regulators are used to step down the voltage from 12V to 5V, and then to 3.3V, to power each respective sensor and integrated circuit.
Signal Processing
A key consideration for the Corner Modules this year was maximizing signal integrity. Each sensor input is separated by their circuitry as follows: temperature (orange), strain gauges (purple), potentiometers and pressure (blue), flow rate (green), and extra miscellaneous sensors (yellow).
Strain Gauges
The Corner Module's strain gauge circuit uses a Wheatstone bridge and an adjustable op-amp to measure each of the seven strains accurately. Designed for compatibility with 3.3V strain gauges (120 ), it translates subtle voltage changes from strain into measurable signals. The circuit includes reference bridges for accurate readings and allows for the amplification of the signal to detect both positive and negative strain.
Analog Signals
Initially, the signals are filtered with a low pass filter sized for a cutoff frequency of around 100Hz, then fed through a buffer, and filtered again. Since these signals are all powered by 5V, they were stepped down to 3.3V through voltage division, which meant they had to be buffered and filtered again before being sent into the Teensy. Although each operational amplifier is dual input, we still needed two buffers per signal because each IC was powered with different voltages after the step down. There are 7 pairs of these conditioning buffers (2 signals per each pair) for a total of 14 different analog signals.
Flowrates
The board measures in flow rate as a series of high frequency pulses, pretty close to a digital signal. Rather than buffering and filtering a square wave, I opted for a compact level shifter to account for the differences in sensor power and the Teensy’s 3.3V logic requirement.
ESD Protection
ESD (or electrostatic discharge) protection was a high priority this year. Ensuring these TVS (transient voltage spike) diodes were routed as close as possible to the connectors minimized the length of the trace acting as an antenna, reducing the chance of induced transients and thus enhancing the overall robustness of the system against electrostatic shocks,
Teensy MCU
A Teensy 4.1 microcontroller serves as the brain of the operation, with built-in CAN functionality streamlining the communication setup. Signals are predominantly fed through analog inputs, with digital RX/TX handling specific communication tasks.
Flyback diodes safeguard the microcontroller from potential voltage spikes, ensuring stable operation.
The design allows for additional inputs on the corner modules by implementing multiplexing for temperature signals, optimizing the data acquisition strategy given the relatively slow variation in temperature readings.
Controller Area Network (CAN)
Once the signals have been processed and packaged into CAN format by the Teensy, they are received by the CAN transceiver before heading onto the CAN bus. A common mode choke, is used to filter and reduce EMI and common-mode noise present on the
CAN_H and CAN_L lines. A 60R termination resistors are used to match the 120 required to match the characteristic impedance in the CAN bus and prevent signal reflection.