Electronic Wheel Alignment
An electronic wheel‑alignment system for the Baja SAE USB off‑road vehicle prototypes (Universidad Simón Bolívar). It reads each wheel's camber with an accelerometer and the toe with ultrasonic sensors, then shows the mechanic what to adjust on a small handheld display. It is one of the sub‑systems of the wider Baja SAE USB electronics work — Divisiones de Electrónica y de Suspensión, August 2013.
Why
The team aligned its cars by hand: parallel lines chalked on the floor (or cones joined with string), then a tape measure to set each wheel parallel to them. There was no way to read the actual camber angle, or to check that the four wheels were symmetric. Commercial electronic aligners — infrared heads bolted to a bench, a PC doing the maths — are far too big and expensive for a student team.
The brief: cost under 3000 Bsf, everything inside 50×50×50 cm, a simple interface, and a written guide.
The car on the alignment jig.
System Architecture
Five modules:
| Module | Radio | Bus | Role |
|---|---|---|---|
| User interface | XBee (master) | — | 16×2 LCD, push‑buttons, µC. Shows the angles and a simple wheel diagram of which way to adjust. |
| Wheel 1 | XBee (slave) | I²C (master) | accelerometer + ultrasonic sensors + µC; bridges the wheel bus to the UI over XBee |
| Wheels 2–4 | — | I²C (slave) | accelerometer + ultrasonic sensors + µC |
On the car the four wheel modules share a single 3‑wire bus — Vdd, SDA, SCL (I²C, with pull‑ups) — with Wheel 1 as the I²C master. Wheel 1 also carries the XBee that talks wirelessly to the handheld UI module. (A CAN‑bus version was sketched first; the built system uses XBee + I²C.) Every module has its own 5 V and 3.3 V regulators and an ICSP header.
The on‑car I²C bus, and the module list.
Camber — the Accelerometer
An ADXL335 (±3 g, 3‑axis) is mounted parallel to the wheel. With the car stationary the only acceleration is gravity, so the sensor's X/Y/Z outputs give the wheel plane's tilt away from vertical. At a 3.3 V supply it sits at 0 g = 1.65 V with a 330 mV/g scale; read through a 10‑bit ADC:
a [m/s²] = ( reading_mV − 1650 ) / 330 × 9.80665Toe — the Ultrasonic Sensors
40 kHz ultrasonic emitter/detector pairs on the inner face of each tyre (the wave can't pass through the wheel). Time‑of‑flight × the speed of sound gives the distance to a reference point on another wheel; for a symmetric car the paired distances must match — D1 = D2, D3 = D4, D5 = D6. Ultrasonic was chosen over laser: cheaper, smaller, and no separate receiver structure to mount.
The measured distances between the wheels.
Mechanical Coupling
SolidWorks parts and assemblies attach the electronics to the car: a spring‑loaded clamp (pinza) that grips each tyre, an articulated link‑chain arm that references a longitudinal axis of the car, plus the tube‑axle parts and base plates.
The link‑chain coupling arm (SolidWorks).
Tests
The ADXL335 was bench‑tested in the datasheet's three reference orientations. The raw sensor showed a clear zero offset and scale error — on one axis it read about 1.3 g where 1 g was expected — so a per‑axis calibration is needed before the angle can be trusted. The ultrasonic driver circuits were prototyped in Multisim.
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Vehicles