SENSORS & THERMOCOUPLES

The formula team’s normal suspension sensors are simple spring-loaded potentiometers. A reference voltage is supplied, and the position of the sensor’s inner collar varies the voltage according to the distance from the rest position. A poorly tuned suspension will bounce rapidly. A suspension that’s too tight won’t have enough give. We wired the four sensors to the 14-bit ADC (a MAX125) in order to get accurate position readings. The MAX125 has eight channels, four of which are hard-wired to a multiplexer. This gives four additional channels for future expansion and engine sensors.

The engine on the formula car already had temperature and RPM sensors. They were part of the MoTec system and relayed information via a varying voltage, which was perfect for use with the 14-bit ADC. Note that temperature measurements are handled by four K-type thermocouples connected to the SPI-type MAX6675 chips. Four chip selects easily give you access to the four temperatures in degrees Celsius through a formula in the datasheet.

The wheel speed sensors were an afterthought, but they worked the first time. Late one night, we remembered that we needed four wheel speeds. We had planned to use Hall effect sensors to sense notches or holes in the wheel rotor. The problem was getting the microcontroller to recognize every rotation and make use of the remaining pins on the controller. We devised a method to use four 4-bit counters and to select between the 8-bit values of the two front wheels or the two rear wheels. The four bit counters are repeatedly sampled and reset so every revolution of the wheels is counted accurately. Only nine pins are needed. An equation modeled how the 0.125-s sampling of the counters translates the 16 notches in the wheel to the actual speed of the car every second.

Luckily, the sensors worked well when we used them with the test setup consisting of an old CDR with cut metal washers spaced evenly on the disc, which was secured to a DC motor powered by a variable power supply. There was a solid linear reading in the VB program when a varying voltage was applied to the motor.