On a laser anglemeter for mobile robot navigation

Sammanfattning: A laser anglemeter for use in a navigation system for mobile robots has been developed. The anglemeter measures heading angles to beacons made of vertical stripes of retroreflective tape. The anglemeter uses an- optical system with a rotating mirror. It scans a laser beam which illuminates the beacons and receives pulses of reflected light from them. The pulses are amplified by a low-noise amplifier and then fed to a comparator. When the comparator trips it is assumed that the laser beam has hit a beacon. Then the angle is measured with a counter which is fed with pulses from an incremental encoder fixed on the axis of the mirror. A navigation system using the anglemeter has been developed. The system includes an algorithm that associates measured angles with beacon identities. The algorithm has been patented by the author. The navigation system has been implemented on a test vehicle. The optical and electronic parts of the anglemeter have been thoroughly analysed and simulated. The performance limits of the anglemeter are stated and design criteria for the optical and electronic parts are proposed. Measurements support the simulations and the theoretical analysis. Some results of the analysis of the optical part of the anglemeter are: A Gaussian expression describing the bidirectional reflection distribution function (BRDF) of the beacons is proposed. The shape and power of the received light pulses from the beacons are found to be determined by the (Gaussian) intensity distribution and the divergence of the laser beam, the scan velocity, the width of the beacons and their BRDFs. No signal is received from beacons at close range. At long range the laser beam diameter is much larger than the width of the beacons which causes the range-dependence of the width of the received pulses to disappear. The shape of the pulses is approximately Gaussian in this region due to the intensity distribution of the laser beam. The width of the pulses is determined by the laser beam divergence and the scan velocity. Optical misalignments in the anglemeter cause the dominating systematic error in measured angles. Another important error source is the range dependence of the received signal. Furthermore a non-circular cross-section of the laser beam is a potential source of large systematic errors while random errors due to electronic noise are surprisingly small. Some results of the analysis of the electronic part of the anglemeter axe: A low-noise photodiode-amplifier circuit with the photodiode in the feedback path has been developed. No noise generating resistor is needed to provide a DC-path to ground for signal and bias currents at the amplifier input. It is possible to design the amplifier as a matched filter for square shaped pulses. An implementation of the amplifier was found to have a NEP~ 3 fW/√Hz at frequencies below 10 kHz.