Wednesday, January 13, 2016

EE Module Design Review: Wheel Velocity Sensor

On Tuesday, all the EE's gathered for the first module design review.

Electronics architecture (depicted on whiteboard)

The Opel's electronics architecture is designed to be modular. It consists of a number of modules where each module is designated a specific high level function, such as logging or sensing. The Opel's Vehicle Controller Area Network (VCAN) bus facilitates communication of data between the modules. Over the past semester, members of the EE crew have been working out the implementation details of their assigned modules. While each module is derived from the same generic design, members must specify the code and any auxiliary electronics required for their specific module.

Dev board, which shows the generic PCB layout

The module design review on Tuesday was for the Wheel Velocity Sensor (WVS). The purpose of the WVS is to measure wheel velocity and relay this information back to the Driver Interface (DI) module over VCAN. Separate WVS modules exist for the right and left wheels. Should the car enter a prolonged drift, the DI would be able to sense the condition from WVS messages and respond appropriately.

The design review began with an overview of the WVS operating principle. Essentially, the WVS measures motor speed from the frequency of the sinusoidal back-EMF waveform. Designers Allison (sophomore) and Helmuth (freshman) took us on a deep dive into the underlying analog signal conditioner schematic. The circuit uses a comparator with added hysteresis to transform the motor back-EMF waveform into a square wave. On the embedded end, the LPC microcontroller computes the wheel velocity from pulse period and broadcasts this data over VCAN.

WVS schematic

Next, the designers walked us through the PCB layout. The WVS specific analog circuitry is located in the middle of the right half plane of the PCB model below.

Wheel Velocity Sensor PCB

The design review was very successful. Designers did a great job presenting, and members were able to walk away with a better understanding of the WVS's inner workings. We were able to evaluate design choices, change component values, and catch typos. Among other things, we decided that 12V logic would probably not be suitable for our 5V tolerant microcontrollers.

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