Ultrasonic Altimeter

A lightweight ultrasonic ranging system that gives the blimps the ability to control their height to within a few millimetres with a maximum range of 2.5 metres has been designed. The transmitter bounces a 40 KHz ultrasonic pulse off the floor, which is picked up by the receiver to give a time of flight. The time-of-flight is then multiplied by the speed of sound and divided by two to give a distance measurement. For simplicity and to minimise processing power the first echo received is used to calculate the distance, i.e. the system does not check to see if it was a valid signal by counting the number of pulses received. The sensor has its own on-board processor (a PIC 16f873a) which enables height information to be updated at a given period (100ms) and stored in a buffer for use by the host computer. When a height is requested from the sensor the host reads the buffer and does not have to wait for a complete ping and echo cycle to complete. The sensor is fully programmable via the ICSP interface and RS232 port, this facilitates easy firmware upgrades.

Back of Height Sensor

This system works very well when used over a smooth flat surface but tends to give reduced range and accuracy when operating over a carpeted area. The accuracy and range are not only affected by the reflecting material but also the alignment of the receiver and transmitter. To gain maximum range and accuracy the transmitter is placed at least 50mm from and parallel to the receiver. The system uses small separate ultrasonic transducers for the receiver and transmitter to enable short-range measurements since a single transducer would require a blanking period to enable it to settle after a transmission. A time decaying threshold system is employed, which initially gives a high threshold to stop false triggers through conducted noise and then decays to a pre-set minimum allowing the weaker, more distant echoes to be registered.

Before the sensor was fitted to the Aerobot it was tested to find its minimum and maximum ranges, accuracy and linearity. The graph below shows the results of the test, the sensor output is in blue and the actual measured height value is in red (dotted).

Sensor output vs measured distance

From the graph it can be seen that the sensor has quite a good linear response although there is some error at the extremes of range. To account for these slight inaccuracies the firmware in the sensor was changed to give a modified output. With the modified sensor code the measured distance at the extremes of range were brought to within a few millimetres of the actual distance. The minimum range of the sensor is 23.5 cm, it is possible to alter the firmware to decrease this range but this will make the system susceptible to false echoes.

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