The DomMeter is a car performance meter that measures acceleration to compute values important to car enthusiasts. Specifically, the DomMeter calculates the 0-60mph time, 0-30mph time, 0-100mph time, quater mile, eighth mile time and braking time, the max acceleration during that interval, distance travelled in feet and max horse power generated (after drag, roll losses and drive train losses). This is all accomplished by sampling an analog accelerometer’s output and performing computations on the value of acceleration. Integrating accerlation yields speed. Integrating speed yields distance.
HIGH LEVEL DESIGN
We share a passion for performance cars. Our holy grail of a performance car is a sub 5sec 0-60mph split. Additionally, Dominik works on the FSAE racecar here at Cornell and desires a way to measure performance. There are commercial products that do the same thing and even more, but this project gave us the opportunity to have our own, at a cost of only our labor.
There are basically 5 components to the DomMeter : The accelerometer, the lcd display, the analog amplifier, regulator, and the MCU.
We used a Kionix single axis accelerometer. The output at 0g’s was already centered at almost exactly 2.5 (2.48) V. There was a 0.4g swing in either direction (-1g = 2.1V, 1g = 2.9V). The output was incredibly linear in that range. Since we wanted maximum resolution from the ADC, we needed a way to get the output as close as possible between 0-5V instead of 2.1-2.9V.
We also tried an ADXL220, dual axis accerlometer from Analog Devices. This one supported a PWM output; however, for our application, the Kionix was better in terms of performance and use of use.
The analog amp was a subtractor/amplifier curcuit built using a 7111 opamp and several resistors. Basically the amp centered the output, and amplified that signal such that the resolution was 5x or 2.0V/g.
The lcd display and pushdowns act as the user interface to the DomMeter. The combination allows the user to change settings, enter values and toggle modes.
- 7111 OpAmp
- LCD 16×2 display
- ATMEL Mega32
- Kionix Accerlometer – KXB00-L50 (bandwidth 250Hz @ 3dB)
For more detail: Vehicle Performance Meter