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Steer Calibration

The RC cars used in real-world experiments use a low-level steering controller to track the desired steer angle reference that is given by CRS. The goal is for the steering system to act like a servo motor would.

Steering Hardware

A potentiometer is installed in the gear of the steer system of the car. The voltage \(U\) measured at the potentiometer is read by an analog-to-digital converter on the PCB of the cars.

Steer Angle Calculation

The steer angle of the car is locally estimated by the firmware by using a steer map: this is (currently) a linear function of the potentiometer voltage.

  • The neutral position of the steer is called the trim position \(U_0\), expressed in ADC units.
  • The slope is the derivative of the steer angle with respect to voltage changes, \(s = \Delta \delta / \Delta U\).

At any given moment, the car estimates the current steer angle as \(\delta = f(U) = (U - U_0) \cdot s\).

Physical limitations

The wheels of the car have a maximum turn position, which is called the steer range \(\hat \delta\), such that \(\delta \in [-\hat\delta, +\hat \delta]\).

  • The two voltages that correspond to the wheel positions in their leftmost and rightmost positions are called the left limit and right limit and also expressed in ADC units, mV or similar.

Calibration Procedure

live_steer

Trim voltage

  • The trim voltage can be calibrated using the car's configuration interface. Place the car on the floor and start the trimming: by driving the yaw rate to zero, the neutral steer position is found and saved automatically.

Slope

  • Usually, the slope \(s\) does not change from experiment to experiment. It has to be determined once and is saved on the car PCB.
  • If the slope should be adjusted, measure the physical steering range \(\hat \delta\) by hand and enter it in the configuration interface.
  • Then, move the wheels by hand from side to side and watch the live ADC voltage display in the interface. Enter the left and right physical limits in the corresponding fields.
  • Press "Calculate slope" to calculate the slope. Or, alternatively, calculate it manually.

Live Display

A live display of the current steer estimation can also be found on the web interface. Use it to judge the fidelity of the calibration.

Known Calibration Values

For some of the car models, there already exist "good" values to use for the steer map.

In general, the small cars have ~27° of steer range and the buggies have ~19°.

Car Model Steer Range Slope
Ben 27deg 1.182532
Christian 27deg 1.013417
Daniel 27deg 1.278803