Figure 5.10. Assistive technology system block diagram – GuideCane

formance of the obstacle avoidance algorithm and to achieve safe travelling at fast walking speeds through cluttered and unknown environments, mobile robot obstacle avoidance algorithms and methods are used.

5.5 Obstacle Avoidance AT: Canes

There are three different types of canes:

1.Symbol canes

2.Long canes

3.Technology canes

Symbol canes

The symbol cane is a lightweight cane of length 1.0–1.5 m. It is slim, cylindrical, white and often made of aluminium. This cane is simply an indicator of the fact that the cane-bearer has poor sight. It is a useful signifier to other pedestrians, drivers, and people in public and commercial settings that its bearer is visually impaired. The symbol cane is not designed to be used as a tactile detector of obstacles.

A more robust version of the symbol cane is available and often called a guide cane or mobility cane. Guides canes are also indicator canes but have a more rugged construction to allow them to survive urban wear and tear longer than the usual slim symbol cane.

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5.5.1 Long Canes

The long cane is the standard mechanical mobility device used by many visually impaired and blind people. It should be selected to stand to the height of the breastbone of the user. The cane is of robust construction with a purposelydesigned handle-grip and tip. The tip of the cane can be either a roller ball or just a plain blunt tip. With a roller ball, the ball remains in contact with the ground and the cane is “bounced” from side to side, to sense any obstacles. The shank of the cane transmits tactile obstacle detection information to the user. If the cane has a plain tip then the cane is “bounced” from side to side so as to make contact with the ground at the two outermost points of its trajectory. The resulting tactile information on obstacle locations is transmitted by the shank of the cane to the user. The long cane has for a long time been considered to be the most effective and efficient mobility aid devised for safe and independent travel for blind people. It will be interesting to see whether the more recently developed electronic travel aids, such as the ultracane and the teletact, displace it from this ranking.

The long cane is a simple, robust, reliable and inexpensive device that provides immediate tactile information about the path of travel in front of the user by scanning. It also gives some information about the surface and the condition of the ground ahead, as well as some sound information (echolocation cues) about the path ahead. Users can relate the sound and tactile information to their memories of previous trips over the same route. This provides limited but possibly important information about navigation or orientation on longer journeys. Due to its robust construction, the long cane can be used in poor weather. It requires no maintenance and the replacement accessories are small, inexpensive and only required infrequently. The long cane will give warnings of drop-offs and objects ahead and thereby provide protection for the lower part of the body from collisions. However, it is unable to provide information about overhead obstacles to prevent collisions to the upper part of the body.

The long cane is instantly recognised by sighted pedestrians, who tend to move out of the way and give the user a clear-path. However, this means that the long cane overtly labels the user as blind and some visually impaired and blind people might prefer to avoid this labelling. Other disadvantages include that the long cane can be quite tiring to use over a long distance. Arm fatigue is common and the double touch cane technique is more tiring than the roller ball method. The long cane often snags holes and cracks in poorly finished pavements and this usually slows the pedestrian’s walking pace considerably. One example of obstacle snagging occurs when negotiating an open pedestrian area, such as a public square, where the long cane can easily slide under a bench that is across the direction of travel. Learning to use the long cane requires special training because incorrect use of the cane can be dangerous to both the user and possibly to others. The length of the training period depends on the mode of use of the cane and the individual’s capabilities and perseverance. Figure 5.11 shows a block diagram for the long cane system.

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Figure 5.13. Assistive technology system block diagram – obstacle detection technology

The following two main technologies are used in the obstacle detection component of these modern technology canes:

•Infrared emission and detection systems, which are generally referred to as “laser canes” (Light Amplification by Stimulated Emission of Radiation), use coherent beams of infrared radiation concentrated on a single frequency.

•Ultrasound emission and detection systems. In Chapter 6, a full presentation of the development of the UltraCane™, which uses ultrasound, is given by the developers of this particular advanced technology cane.

A brief review of both “laser” cane and ultrasound systems is presented below.

Laser cane systems

The use of light in an obstacle detection system dates back to 1943 when incandescent light was first used in prototype systems. Today, infrared lasers are generally used. For example, a laser cane might use miniature solid-state gallium-arsenide room temperature injection lasers. These lasers emit 0.2 μs wide pulses of infrared light (930 nm) with a repetition rate of 40–80 Hz. Photosensitive receivers (photodiodes) are used as the detectors. The beams emitted are narrow, at 2.5 cm wide and directed within the semi-plane corresponding to the direction of travel. Obstacles can be located quite accurately because the emitted beams are quite narrow and the angular resolution of the detection system is small.

Figure 5.14. Laser cane – beam geometry

The degree of complexity of a particular obstacle detection system is related to its arrangement of detection beams and receivers. Three beam directions are often described, but different versions of the canes may carry technology for fewer beam directions. These directions are (see Figure 5.14):

•Upward beam. This beam is designed to detect objects at head level, typically at

1.80m height.

•Forward beam. This beam detects obstacles in the travel path directly ahead. A typical setup would detect objects up to a height of 0.6 m at a distance of 1.5–3.6 m from the cane tip.

•Downward beam. This beam detects any down steps in the forward-path. Typically, the resolution is such that down steps of height of at least 15 cm at about

0.9m from the cane tip can be detected.

A beam to detect obstacles at the side, as well as any narrowing of the path, would be useful, but, currently available laser canes do not have a sideways beam.

The actual detection principle is Cranberg’s principle of optical triangulation. The device emits pulses of infrared light, which are detected by photodiodes located behind the receiving lens if reflected from an object in the travel path. The angle made by the diffusely reflected infrared ray passing through the receiving lens is an indication of the distance to the object. This geometry is shown in Figure 5.15.

Ultrasound cane systems

The other main approach is the of use of ultrasound, i.e. sound waves at frequencies greater than 20 kHz as the detection medium. Whilst optical systems are based on angular geometry, the ultrasonic system calculates the obstacle’s distance from the elapsed time.

Polaroid makes one of the most frequently used ultrasonic sensors. This sensor emits a short pulse of ultrasound. If an object is located in the path of the ultrasound beam, then a portion of the beam will be reflected back to the sonar that has switched to microphone mode immediately after transmitting the ultrasound

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Figure 5.15. Cranberg’s principle of optical triangulation

beam. When an echo is received at the sonar a measurement of the elapsed time can be used to compute the distance of the object from the emitting source. The velocity of ultrasound in air at 0 ◦C is constant at 331 m s−1 and increases at approximately 0.6 m s−1 per ◦C rise in temperature; thus the required distance is easily calculated.

Polaroid sonars have a maximum range of 10 m and an accuracy of 0.5% of the distance measured; thus at a distance of 5 m, the accuracy is ±0.025 m. The beams are fairly narrow, since ultrasound waves from the Polaroid sonar propagate with a cone-shaped profile of 30◦ spread. Ultrasonic technology has advanced significantly in recent years and sophisticated signal processing algorithms are used in a device like the Ultracane, which is described fully in Chapter 6.

A key difference between laser and ultrasound systems is their effectiveness in detecting plate-glass and transparent plastic. Many urban environments include doors and shop windows made of plate-glass and transparent plastic is a common material for bus shelters. Transparent surfaces will generally reflect ultrasound, but transmit light. Therefore, infrared-based systems may not be able to detect and indicate the presence of a glass door or transparent bus shelter, whereas an ultrasound system will.

Information transfer in obstacle detection systems

This involves the human–technology interface through which information about obstacles or obstacle-free paths is relayed to the end-user. The interface has two important features, the information to be communicated to the end-user, and the means of communicating this information.

Both audio and tactile interfaces, as well as a combination of the two can be used to transfer information to the user. A tactile interface usually comprises vibrating buttons or pins. The audio interface usually comprises tones of different pitch, though speech and musical sounds are other possibilities. These sounds or speech should preferably be conveyed through a single earphone, so as not to impede perception of other environmental sounds and not to add to existing noise pollution.

Other important issues include how much and what types of information should be conveyed to the user. In the case of infrared and ultrasound technology canes,