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1 Introduction

Globally, 285 million people are estimated to be visually impaired: 39 million are blind and 246 have low vision [1]. Outdoor mobility tasks are the main issues for visually impaired subjects, influencing their social and professional life [2, 3].

Main assistive devices for these subjects are: sighted guides, white or long canes and dog guides [4, 5]. In particular, the most diffused is the cane, which can only provide alerts for obstacles found in front of the user’s feet (<1 m), without any protection against obstacles on the upper part of the body and without information regarding their speed, volume and distances [5]. In order to overcome such limitations, specific electronic travel devices have been proposed, named Electronic Travel Aids (ETAs) [6, 7], devised for detection of objects along the user’s pathway. The international guidelines for ETAs [6] require the following:

  1. (1)

    detection of obstacles in the travel path from ground level to head height for the full body width;

  2. (2)

    travel surface information including textures and discontinuities;

  3. (3)

    detection of objects bordering the travel path for shore lining and projection;

  4. (4)

    distant object and cardinal direction information for projection of a straight line;

  5. (5)

    landmark location and identification information;

  6. (6)

    information enabling self-familiarization and mental mapping of an environment.

Today, there is a wide range of navigation systems and tools available for visually impaired individuals [7, 8]. Most of the proposed ETAs are based on the transmission of an energy wave and the detection of echoes from objects present on the user pathway and a great number of them use ultrasonic emitter/receiver transducers [7, 8].

There are also some commercial products available on the market for the mobility of the blind and visually impaired (ultrasonic sonars, laser range sensors; infrared and photodiodes sensors, etc.). They have been proposed with the aim to solve the problem of individuate shoulder-width openings [9].

All of these systems require a mental effort to identify the obstacle with respect to the user’s position and/or direction. Despite recent improvements, these devices still present some drawbacks such as restricted functionalities, relatively high cost, and limited acceptability by the users. In general, the recognized limitations of the actual ETAs (commercial systems or prototypes) based on ultrasonic are the following [6–10]: limited useful range, difficulties of operating on highly reflective surfaces (smooth surfaces) with a low incidence angle (<40°), and when detecting small openings due to the aperture of the emission cone of ultrasonic waves. Optical ETAs, which do not suffer from these limitations due to their shorter wavelength, however suffer other difficulties such as high sensitivity to ambient natural light or dependence on the optical characteristics of the obstacle surface (transparency or mirror-like reflection). It is generally agreed that, currently, no available ETA incorporates all the required features to a satisfactory extent [6–10].

It is interesting to note that, in the literature, there is a lack of studies that consider electromagnetic (EM) radiation as the physical quantity able to deliver information on obstacle presence for visually impaired users.

The main aim of this paper is to present the characteristics of EM assistive technologies for visually impaired users and present some works carried out by the authors mainly addressed to investigate the capability of novel EM systems designed to support visually impaired and blind user mobility.

At first an obstacle detection system was proposed and set up to carry out a preliminary experimental analysis to investigate the possibility of adopting EM waves for ETAs and to provide useful information for the design of an EM sensor to aid visually impaired users during mobility tasks, possibly allowing them to walk safely and independently [11].

On the base of the encouraging results obtained in the preliminary study [11], an innovative sensing method, based on EM pulses, was designed and will be presented together in the following with some experimental results achieved in obstacles detection. The proposed approach accomplishes most of the operative requirements of ETAs for visually impaired subjects and can provide additional information (height form the ground, distance and position of the obstacle) on obstacles respect to the available assistive technologies currently used.

The third example can be considered an indirect and unexpected result of the experimental activity concerning the “EM cane” previously mentioned. The blind person that we involved in our activity to test the system is a world champion of marathon for disabled people. For this reason, the idea was designing a system able to make the athlete free of running independently from his guide, at least during training sessions.

In this contribution, the authors want to underline that the satisfactory results obtained demonstrate how EM technology is capable of providing some helpful aids for visually impaired people. Nevertheless, the full development of a device to insert on the market at reasonable costs requires the joint effort of different scientific expertise and a multidisciplinary approach deeply involving the visually impaired user community. Therefore, the paper finally presents some hints for discussion to the automatic control community.

2 A Preliminary Experimental Study

The aim of any obstacle detection system is to allow a visually impaired person to speed up mobility mainly by getting information on the surrounding complex environment. Our system will explore a defined volume in front of the partially sighted user. The scenario we refer to is a 3-D region in front of a walking person, where there may be some obstacles at different heights, which can be very dangerous for blind people [6]. In a realistic context, several kinds of situations can be a serious threat, e.g., open windows, public telephones (that are large but attached to a slender pole), low branches, or rears of trucks, and for these cases, the cane is not able to help the person; some examples of possibly dangerous obstacles are shown in Fig. 1.

Fig. 1
figure 1

Examples of obstacles potentially dangerous for visually impaired mobility not detectable by the cane

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The system has to give information on the presence, location, and, possibly, the nature of the obstacles immediately in front of the subject, exploring in elevation a region from ground-to-head level and in azimuth an area corresponding to the subject’s body; this explored volume has also been determined as significant for blind users in [11]. The minimum distance or range over which this information is needed is a comfortable stopping distance at normal walking speed [6–10, 12]. The volume explored in this study is a parallelepiped in front of the subject of 3 m of length, 1.5 m of width, and 2 m of height (Fig. 2).

Fig. 2
figure 2

Volume explored by the proposed system

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Such dimensions are a compromise between the necessity, in a real system, to give sufficient information to the user and the need to limit meaningless alarms. For a quick identification of obstacle location, we have considered three different sub volumes of the scenario:

  1. (1)

    leg zone: volume in front of the subject in contact with the walking surface;

  2. (2)

    trunk zone: volume in front of the subject’s trunk;

  3. (3)

    head zone: volume in front of the subject’s head.

The system was realized using laboratory instrumentation and some tests were performed considering dangerous obstacles similar to those reported in Fig. 1 [11]. Figure 3, where the horizontal axis has been converted in distance, clearly shows how the EM pulse is able to detect, discriminate different obstacles and even locate their position at the right distance.

Fig. 3
figure 3

Signals measured in cluttered scenarios. a Indoor corridor without obstacles and c the same corridor with the obstacles. b Outdoor scenario with metal poles and chain and d same as b with a subject behind

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3 The Electromagnetic Cane

The experimental set-up realizing the EM obstacle monitoring system is described in detail in [11]. The EM system consists of a control unit, a Tx/Rx unit and a radiating element. Each part has to be set up and/or properly designed according to the requirements of the scenario. The radiated signal is a short pulse whose echo will be used to extract information of the object. In detail, the obstacle is illuminated using an antenna and a Vectorial Network Analyzer (VNA) has been used to measure the reflection coefficient at the antenna input. The VNA provides a time domain pulse with duration of 0.4 ns (calculated at 50 % of the pulse amplitude), corresponding to 12 cm spatial resolution. Actually the pulse is reconstructed from the VNA that generates a continuous wave signal with a frequency sweeping from 1 to 6 GHz as explained in [11]. The system is intrinsically immune to the external disturbances, because in our system, the reflected and transmitted signal are always synchronized.

Figure 4 shows the portable prototype of the EM obstacle detection system realized with a homemade helical antenna (matched from 4 to 6 GHz; half power beam of about 35.9° in the plane φ = 0° and 36.9° in the plane φ = 90°, realizing a suitable beam able to scan the volume depicted in Fig. 2), a portable VNA and a laptop for signal processing and acoustic warning. After a first stage of laboratory measurements to optimize the set-up parameters, different tests have been carried out asking to a blind volunteer to walk toward a sound source using the EM prototype and holding the antenna connected to the VNA in one hand and wearing a backpack containing the laptop [13]. The test carried out confirmed the effectiveness of the system and highlighted important suggestions for future developments [14].

Fig. 4
figure 4

Photos of the EM obstacle detection sensor. The homemade antenna and the VNA (left) the system in use (right)

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4 Athlete Guiding System for Marathon Race

Although the view plays an extremely important rule in almost every sport, many people with visual disabilities participate in high performance, competitive and/or recreational sports, but no electronic devices have been conceived yet to improve their autonomy during sports. In fact, looking at the marathon race, the most widely used technique by blind athletes is to run linked to a sighted guide by means of a non-stretchable tether tied around the wrists or held between fingers. For these reasons, the a different EM system has been thought to support and guide blind athletes while training or running a marathon race [15].

As an overview, the system has to draw an invisible track that should be followed by the runner by means of the generation of two “EM walls” due to the radiation patterns of high directional transmitting antennas, as depicted in Fig. 5. To make sure that the athlete always remains inside the virtual hallway, it is necessary to generate a warning whenever he is getting closer to the borderlines so as to encourage the runner to move toward the central position. To this aim, the whole system proposed consists of transmitting and receiving subsystems. The transmitting subsystem, to be placed on a trolley or a vehicle running in front of the user, includes: two radiating elements that generate the two EM walls and two amplitude (AM) modulators at two different frequencies in order to discriminate between left and right boundary. On the other side, the receiving system, to be worn by the user, is composed of a receiving antenna, an AM demodulator, two band-pass filters, a Micro Controller Unit for thresholds settings and further signal processing and two vibration transducers to communicate a warning signal to the blind athlete.

Fig. 5
figure 5

Transmitting system placed on a car and representation of the virtual hallway (left); image of the receiving unit worn by the athlete (right)

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An important issue is the design of the transmitting and receiving antennas, working at about 10.4 GHz. Since to generate an EM wall the radiation pattern should be narrow on the horizontal plane and wide on the vertical one (fan-shaped), a slot antenna has been chosen as radiating element.

As concerns the receiving antenna, it has been designed in order to have a lightweight and wearable device with a radiation characteristic similar (but less restrictive) to those of the transmitting antennas. A microstrip array antenna of four elements has been designed and realized. The combination of the transmitting and receiving radiation patterns leads to different levels of the demodulated signal. As the position of the runner changes with regard to the transmitting antennas, the magnitude of the received signal changes accordingly and it can be used to drive the warning signals.

5 Discussions

The encouraging results reported in this paper and mentioned above seem to be potentially useful for the scope of assisting visually impaired users in their daily life.

The EM obstacle detection as well as the EM system for runners can contribute to reduce the daily limitations that visually impaired users have to undergo, improving their quality of life. Nevertheless, it is worth noting that they still are laboratory prototypes and consequently far to be ready for the market. To reach this aim, a series of improvements of the existing prototypes are necessary in order to address the following aspects: portability, communication with the user (user-interface), real-time data processing, etc. These aspects, common to all ETAs [6], are requiring the joint effort of different expertise and the very important contribution from the visually impaired user-community.

An important aspect that needs to be addressed is the realization of a computationally efficient algorithm for a real-time inversion of radar data in order to greatly improve the knowledge of the surrounding. A lot of information is stored into the reflected echo signal, but a powerful signal-processing algorithm is necessary to be integrated within traditional radar techniques in order to extract significant parameters in real time. More specifically, from the radar signal, suitable parameters related to the geometrical and physical characteristics of obstacles could be obtained and inverted to determine distance, velocity, dielectric properties and dimensions of the detected objects.

Another important issue to be addressed is the user interface that allows the visually impaired user to control the system and to receive information about the detected obstacles according to strategies meeting the needs of visual impairments. Pushbuttons/mini joysticks could be integrated in the cane or in a device worn or held in one hand; by means of them, the user would be able to control the system activating functioning modalities (enabling/disabling obstacle detection, disabling vibration feedback, getting information about the battery status, etc.). The output user interface could be based on vibration feedback, audio messages and sound alerts, and could operate according to proper strategies to avoid overcharging the sense of hearing, widely used by visually impaired people to get information on environments. The possibility of interconnection between the user interface device and a smart-phone could offer new capability to the user. It is worthwhile to underline the importance of using the experience of visually impaired users for this issue.

Finally, portability aspects have been recently been addressed for what concern the possibility to miniaturize the radiating element in order to easily install it on the VI user cane. Further improvement of the system portability could be obtained substituting the PC with an embedded electronic board providing the data processing and output signals for the user.

Many other opportunities may arise joining EM technology and assistive robotics. Autonomous mobile robots can be adapted to work as assistive robots, because their capabilities to navigate in unstructured environments and react to changes in it [16]. Now it is an easy task mounting a radio beacon on the robot with the aim of realizing an automated system able to guide a visually impaired subject in a complex and indoor environment as an airport, a station or a hospital. A small and lightweight device could receive the beacon signal providing in real time direction and distance of the robot, allowing the user to follow the robot along the desired path.

6 Conclusions

The capability of EM devices working in the radio frequency range to be used in the realization of ETAs for visually impaired people has been highlighted. Two practical examples, an EM cane for obstacle detection to warrant a safe walking and a system for autonomous running of a marathon athlete, have been shown and realized using laboratory instrumentations.

However, the main aim of this contribution is to provide hints for discussion, because technology is mature enough to allow design of small, lightweight, and cheap devices, but the reliability of the system depends on the joint interdisciplinary work of scientist with different expertise.