FAQ

Ultrasonic Rangers FAQ

Q. What is the accuracy of the ranging?

A. We quote 3-4cm. Its normally better than this, however so many factors affect

accuracy that we won't specify anything better than this. The speed of sound in air is

approx. 346m/S at 24 degrees C. At 40KHz the wavelength is 8.65mm. The sonar's detect

the echo by listening for the returning wavefronts. This echo has an attack/decay

envelope, which means it builds up to a peak then fades away. Depending on which

wavefront is the 1st to be strong enough to be detected, which could be the 1st, 2nd or

even 3rd, the result can jitter by this much. Another effect which limits accuracy is a

phasing effect where the echo is not coming from a point source. Take a wall for

example, the ping will bounce off the wall and return to the sonar. The wall is large,

however, and there will be reflections from a large area, with reflections from the outside

being slightly behind the central reflection. It is the sum of all reflections which the

sensor sees which can be either strengthened or weakened by phasing effects. If the echo

is weakened then it may be the following wavefront which is detected - resulting in

8.65mm of jitter. It is possible to see changes of distance as small as mm but then get cm

of jitter.

Q. How can I narrow the beam width?

A. The beam pattern is conical with the width of the beam being a function of the surface

area, frequency and type of transducers and is fixed. The beam patterns of the

transducers used on rangers, taken from the manufacturers data sheets, are shown below.

beam widths are taken at the -6dB points. There is an interesting article by Harold Carey

on reducing the beam width here

Note that, despite the manufacturers spec' of 55° for both the SRF04/08 transducers and

the SRF02 transducer, the SRF02 is slightly narrower. This is noticeable in practice as

well.

SRF235 15° SRF04/08 55°

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Ultrasonic Rangers FAQ Q. What is the accuracy of the ranging? A. We quote 3-4cm. Its normally better than this, however so many factors affect accuracy that we won't specify anything better than this. The speed of sound in air is approx. 346m/S at 24 degrees C. At 40KHz the wavelength is 8.65mm. The sonar's detect the echo by listening for the returning wavefronts. This echo has an attack/decay envelope, which means it builds up to a peak then fades away.
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SRF10 72° SRF01/02 55° Q. What are the units on the vertical axis in the beam pattern diagram? A. Units are dB, taken from the manufacturers data sheet at: http://www.robotelectronics.co.uk/datasheets/t400s16.pdf Q. What distance above the floor should the sonar be mounted? A. If you can mount the SRF04/8 12in/300mm above the floor, that should be OK. If you mount them lower, you may need to point them upwards slightly to avoid reflections from the carpet pile or ridges in a concrete floor. Q.
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"ping" resulting in a false readings. Fire them sequentially 65mS apart A. Yes! We do this all the time on our test robot, firing 8 SRF08's at the same time. They are facing outwards and fitted around a 15inch diameter circle. The gain is set to minimum and they are fired using the I2C general call at address 0, and read individually at their set addresses. Under these circumstances there is no direct interference. A.