The monitoring of the environment is implemented in the Concept Car using different sensors. In the following, all the sensors are described.
Wheel speed sensors
The rotation speed of the wheels is measured with simple optical sensors, where 26 black stripes in the inside of the wheel are passing a photo sensor. A photo diode (CNY 70) detects the passing of the stripes, and the interval between two stripes passing is measured with a resolution of 0.5µs. Every 20ms the wheel speed of all four sensors is written to the CAN-Bus. Depending on the current wheel speed, one of the following conditions holds:
- The wheel speed is so low that no black stripe has passed within the last 20ms (may happen if speed is lower than 692.3077°/s or 0.725m/s): The value 0xffffffff is sent via CAN.
- During 20ms exactly one black stripe has passed. In this case the period since the last stripe has passed is sent in the unit of 0.5µs. The last passing of a stripe may be several 20ms intervals ago, internally 24 bits are counted, resulting in a maximum interval of 0xffffff * 0.5µs = 8.39s. Values above that will be truncated.
- During 20ms multiple stripes pass. In this case the average value is taken.
The sensory data from the wheel speed sensors is to be considered as extremely noisy, and should not be used by any system without applying adequate filtering algorithms, as described in [Mit09].
On the front side of the car there are two ultrasonic distance sensors. They are triggered in alternating order every 65ms to measure the distance to the next obstacle in front of the car. Their range is 15cm to 6m, with a resolution of 1cm. For further details consult the documentation, accessible under [RHW09].
The acceleration of the Concept Car is measured with the ADIS16006 sensors. The dual-axis accelerometer ADIS16006 is capable of measuring -5g to +5g at a resolution of 1,9mg at 60Hz measurement rate. The maximum measurement range is ±8g. It has a built-in temperature sensor to mask out the temperature drift of the measurement results. For further details consult the documentation [Ana07].
The vehicle rotation along the yaw axis is measured using a rotation sensor of the type ADIS16100. The dynamic range of the yaw rate sensor ADIS16100 is 300°/s at a resolution of 0.244°/s. This sensor also provides temperature information for separating out the temperature drift. Further documentation can be found under [Ana09].
Radio Receiver Decoder
The Radio Receiver outputs a pulse width modulation (PWM) signal. The PWM signal from the radio receiver’s channels must be decoded to be usable inside the system. Therefore the length of the pulses (duty cycle is 5% to 10%) is measured and normalized, with 5% being -100 to 10% being +100. The resolution is 0.01%. Radio receiver signals are sent over the CAN-Bus whenever a PWM-pulse with a valid duty cycle has been measured, so under typical driving conditions every 20ms for each channel.