218 6 Mobility AT: The Batcane (UltraCane)

design of bat-inspired assistive technology is binaural processing. Bats have one transmitter (their mouth) and two receivers (their ears). They are able to locate the position of an object in space by the echo delay, giving range; and by the relative intensity, phase and timing differences between the ears, giving the object vector.

By contrast, to sample multiple angles, sonar devices often use multiple transmitter and receiver pairs, each with a narrow angle of view. The decoding of multiple target vectors from a binaural receiver pair is obviously computationally complex, but is potentially of value in providing a more complex spatial map of the environment from a smaller number of transducers.

The main difference between sonar assistive devices and bat echolocation is the quantity of information extracted from the returning echo. Bats use the full range of fine resolution target information available to them in terms of spatial location of multiple targets and even target type and texture. Two issues currently prevent the incorporation of this information in an assistive technology device. First, many of the apparent pseudo-analogue, signal processing techniques used by bats to extract this information are unknown. It is of course possible to use conventional digital signal processing techniques to extract such information, such as crosscorrelation and Fourier analysis, but this is computationally intensive and cannot currently be realised in real-time on a small portable device. The second, and potentially more limiting problem, is how to relay the wealth of information to the user in a meaningful way that preserves the full information spectrum, while not otherwise limiting the user’s senses and ability to perceive their environment.

6.4 Design and Construction Issues

The design process embraced a range of key issues. There were based upon the specification of a set of outline requirements.

6.4.1 Outline Requirement Specification

The primary requirements for the aid were identified as:

•To exploit the confidence in the guide cane that users typically share. A further benefit of a system based upon a guide cane is that it clearly is able to provide a basic level of functionality in the event of failure of the technology for any reason.

•To design a sensory system that does not monopolize the hearing abilities of its user. Dependence upon acute hearing would limit the range of users who could benefit. For users who have a hearing capability it is important to allow this to be completely available to detect traffic and other hazards, and to simply allow conversation with others.

•To provide prioritized information on potential hazards and surrounding features to users within their ‘navigation space’, to enable them to move more confidently and effectively. A major benefit would be to extend the ‘effective

distance’ of the object detection capability relative to a standard cane, and to extend the ‘space’ to include potential hazards above and ahead.

•To exploit cognitive mapping to gain a capability that develops quickly with practice and also becomes subliminal to its user.

•To offer maximum convenience at a practical level: in terms of long battery life; of compact storage usability; and of flexibility of cane length and individual preference for cane tips.

•To provide a high quality design that is functional, pleasing and elegant to mature users, and appealing, stylish and desirable to younger users.

The primary performance requirements were translated into the twin core subsystems: the spatial sensing subsystem; and the user interface subsystem. These are integrated and controlled by a conventional embedded microprocessor. The cane should provide the functionality of a conventional guide cane and provide an elegant housing for power supply, processor, electronics, user-interface and sensor parts.

6.4.2 Ultrasonic Spatial Sensor Subsystem

A range of spatial sensing techniques were assessed in terms of range, coverage, and the availability and cost of sensor devices. The inspiration of the bat from nature shows ultrasound to be a powerful sensing capability that with appropriate processing can deliver high resolution spatial information to form the basis of assistive technology. The design process began with simple proof of principle trials. These explored the variety of the parameters of the ultrasound transducers, tactile feedback devices and their deployment.

6.4.3 Trial Prototype Spatial Sensor Arrangement

The resulting design employed a forward-looking ultrasound transducer, two transducers viewing respectively to the forward-left and forward-right, and importantly a further transducer looking upwards ahead of the user, to detect hazards that would otherwise threaten the head and neck area. The resulting electronic guide cane thus emulates the example of the bat through multiple transducers that detect targets from the ‘navigation space’ that the user may enter. Signal processing algorithms were needed to operate upon the ultrasonic data and perform a risk assessment whose results of potential hazards, in terms of criticality and distance, are available for delivery to the user.

In the early designs the transducers were located near the tip of the cane, except for the upward viewing transducer located on the handle. Figure 6.2 shows the schematic arrangement of the four transducers and their field of interrogation. A simple laboratory prototype based upon the above concept was designed at the University of Leeds. A short programme of feasibility study trials confirmed the potential of the concept.

To provide reliable detection of objects the transducers must interrogate the user space frequently and for efficiency share the pulse generation circuitry through a multiplexing system driven by an embedded processor. Received echoes from each transducer are also captured, encoded and passed to the processor for interpretation and integration with other transducer data to determine the hazard risk assessment.

6.4.4 Tactile User Interface Subsystem

The user interface needed to relay assessed hazard information is clearly a critical element. A number of inventions offer information to visually impaired users via audible signals, typically necessitating the wearing of headphones. Although this may be the most direct way to harness ultrasound echo information it is unacceptable as it denies this valuable sense to those having little or no vision. As noted above a core requirement is to retain unimpeded use of any hearing ability that a user may possess. This is obviously important in detecting surrounding moving hazards such as motor vehicles and people. Equally obviously, people who have a hearing impairment will be unable to use any aid that relies upon this to deliver guidance information. A multi-point tactile user interface has been designed to address this need. This has two key aspects: first, the design of the tactile transducers and their simple ergonomic matching to the human body; second, the cognitive mapping in the human brain that relays and interprets the tactile data to generate high level spatial awareness.

The use of a long cane in a sense allows its user to explore the world through an extended sense of ‘touch’—exploiting the human capability noted above. This extended concept of ‘touch’ was harnessed in the electronic cane to present processed information from the ultrasound transducers. The human hand is one of the richest parts of the body in terms of tactile sensitivity. Thus a tactile transducer, or tactor, able to present sensory information to the fingers and palm surfaces of the hand, offers a highly efficient route to the sensory perception parts of the brain. The left hand image of Figure 6.3 shows the tactors on the top surface of the electronic cane handle. These are linked via the processor to the upward and forward ultrasound transducers. The right hand image shows the tactors on the bottom surface that are linked similarly to the left and right transducers.

Figure 6.4 illustrates the corresponding thumb and fingers positions and the support provided for right and left handedness. Handedness influences the ease with which tactile sensations are associated with a particular direction. The right and left tactors are simply inverted to exploit this linkage.

The tactors inform the user of an object in the field of view of the corresponding ultrasound transducer through a vibration whose frequency increases as the user approaches. A non-linear relationship is found to be optimal. The use of a frequency mapping is preferred as this is much less prone to fatigue and loss of sensitivity. The vibration is of course intermittent and only occurs when a hazard is in view of a transducer.