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Figure 6.7. Ultra Cane® transducers and tactors

The left-hand image shows the handle from above with the handle-mounted overhead and forward transducers (and lobes for smaller sideways transducers not included here). The corresponding tactors are also visible.

Further details of the resulting commercialised product can be seen at www.ultracane.com.

6.7 Chapter Summary

The use of the long cane by visually impaired people as an obstacle detector is long standing. More recently the basic cane design has been equipped with laser or ultrasound transmitters and sensors and an interpretive human interface to improve its effectiveness, the objective being to allow safe travel by a visually impaired person.

This chapter reported a full and complete case study of the steps involved in developing an advanced technology obstacle avoidance cane that successfully used bat echolocation signal processing techniques and ultrasonic technology. The final cane design is now marketed worldwide as the UltraCane™.

The chapter began by reviewing the basic technological principles for ultrasonic waves and advanced signal processing methods. Thus, the scientific principles of the propagation, reflection and the collection of ultrasonic waves were presented. An extended presentation of bat sonar and the associated value of bat echolocation for obstacle detection followed.

The inspiration behind the first Batcane prototypes was to combine the use of ultrasonic technology with bat echolocation principles to obtain an efficiency gain in the ability of cane technology to detect obstacles. The chapter presented a detailed discussion of all the design and construction issues involved in creating and testing the first engineering prototypes. The use of vibrating buttons or tactors to indicate the location of an obstacle was an important feature of the cane user’s interface.

The prospect of producing the cane as a commercial product soon began to emerge from the phase of developing and testing a satisfactory prototype. Consequently, the final part of the chapter examines the issues involved in bringing the prototype to eventual commercialisation. These included determining which

features of the Batcane prototype should survive into the commercial UltraCane product.

Acknowledgement. The authors of this chapter have enjoyed participating in the design of the UltraCane, by Sound Foresight Ltd. See www.UltraCane.com for further information on its design and features. The authors also acknowledge the kind assistance provided by many organizations supporting visually impaired people in the UK, Germany, Canada and the USA and by the many volunteers who have tested prototype designs and enthusiastically provided encouragement and objective feedback. We also acknowledge the major product design contributions of Cambridge Consultants, Minima Design and Qinetiq at various stages.

Questions

Q.1 On detection angles

An experimental trial has determined that it is important to be able to detect a gap of 1 m between two objects (equivalent to detecting an open door) at a distance of 2 m. The transducers being used have a diameter of 32 mm. What is the minimum frequency of ultrasound that would give the angular resolution to detect the gap and not the door-frame?

As a first stage, use basic trigonometry to derive the angular width of the door at 2 m. Then rearrange Equation 6.4 to give the frequency that gives that angular beam pattern. A higher frequency would give a narrower beam. What is the problem associated with using higher frequencies?

Q.2 On detection distances

The user of a sonar device needs to detect a wall at 90◦ at a distance of 6 m. The transducers use a frequency of 40 kHz with an output level of 110 dB at 0.1 m and are sensitive enough to receive an echo at 32 dB SPL. Will the user be able to detect the wall?

First, find out the excess atmospheric attenuation at 40 kHz from Figure 6.1. Next, calculate the incident sound pressure at the wall using spherical spreading from Equation 6.6 and excess attenuation. Using the target strength of a planar target of −6 dB at 1 m, and spherical spreading and atmospheric attenuation on the return path, calculate the final echo strength received. Is it above the 32 dB SPL threshold of the receiver?

Projects

P.1 On maximum and minimum detection ranges

A useful exercise is to understand how parameters such as frequency, source level and the target strength of an object can affect detection distance. The best way of modelling these effects is to construct a simple spreadsheet which calculates the incident sound level at a target and the intensity of the reflected echo. To start, construct a column with target distances in increments of 0.1 m up to a maximum of 10 m. You will also need three cells to use to input the parameters of source level, atmospheric attenuation and target strength.

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Next, use the distance values to calculate the incident sound intensity at each of the target distances and place these in column 2 next to the distances. To do this, you will need to consider the effect of spherical spreading and atmospheric attenuation. The effect of spherical spreading can be derived from Equation 6.6, using the source level you have input which should be standardised to a distance of 1 m. The atmospheric attenuation term can be added in as a simple product of the distance and the attenuation factor for the frequency you wish to consider. This can be read from Figure 6.1.

Next, use the incident sound levels you have calculated in column 2 as the source level for the returning echo. Subtract the target strength, and then use the same principles of spherical spreading and atmospheric attenuation as you did to calculate column 3. You should now have three columns. Column 1 will give you the distance to the target, column 2 will give you the incident sound pressure at the target for each target distance, and column 3 will give you the returned echo intensity for a target at each distance. You can now explore how source level, frequency (and attenuation) and target strength affect target detection distance by changing these parameters. If you have set up your spreadsheet correctly, changing these parameters will change all the received and echo intensities. You may have to decide on an arbitrary cut-off in sensitivity for your receiver device, such at 20 dB SPL. Hence, to determine the maximum range that a signal can be detected, look down column 3 until you reach the received echo intensity of 20 dB SPL, and then read across the distance. As a check, for an emitted sound level of 90 dB SPL at 1 m, a target strength of −20 dB, and attenuation of 1.5 dB m−1, this should be around 6.1 m. For some starting parameters, the typical maximum output of a piezoelectric transducer is 110 dB SPL, with a maximum receiving sensitivity of 40 dB SPL at 40 kHz, where attenuation is 1.4 dB m−1. The typical target strength of an object such as a 10 cm diameter street signpost is approximately −30 dB. For planar targets such as walls, assume the target strength is −6 dB at 1 m, since the major effect is the spherical spreading loss back from the target to the receiver.

P.2 Building a bat detector

The ultrasonic calls of bats are normally so far above the range of human hearing that we never detect them. However, several bat detector designs are available which convert the inaudible echolocation calls of bats into audible outputs. Most of these detectors work on a heterodyning process whereby mixing two signals of frequency f1 and f2 produces two outputs, f1 + f2 and f1 − f2. If the bat call is f1, at around 40 kHz, then mixing it with another signal of 38 kHz produces an output of 40 + 38 = 78 kHz, which is also inaudible, and 40 − 38 = 2 kHz which is audible and can be heard via a loudspeaker. A 42kHz signal mixed with the bat call produces a frequency of −2 kHz, which is 2 kHz with a 180◦ phase shift but acoustically identical to a 2-kHz signal. The mixing signal is generated by an oscillator whose frequency can be selected using a simple rotary control. This allows the user to scan up and down the frequency range used by bats to identify the maximum and minimum frequency present, as well as different tonal signals depending on the duration