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A portable mid-range localization system using infrared LEDs for visually impaired people
Infrared Physics & Technology 67 (2014) 583–589
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
A portable mid-range localization system using infrared LEDs for visually impaired people Suhyeon Park, In-Mook Choi ⇑, Sang-Soo Kim, Sung-Mok Kim Korea Research Institute of Standards and Science (KRISS), Daejeon 305-340, Republic of Korea
h i g h l i g h t s A portable sized guidance system for the visually impaired was developed. An ultrasound time-of-flight method and a differential infrared intensity method were realized. It is possible to use the infrared intensity method to generate a uniform signal field that exceeded 30 m. The measurement was independent of external conditions within a certain range. We made it possible to use this receiver in portable devices such as smartphones.
a r t i c l e
i n f o
Article history: Received 10 April 2014 Available online 1 November 2014 Keywords: Infrared Ultrasound Localization Positioning Pedestrian guidance Active beacon
a b s t r a c t A versatile indoor/outdoor pedestrian guidance system with good mobility is necessary in order to aid visually impaired pedestrians in indoor and outdoor environments. In this paper, distance estimation methods for portable wireless localization systems are verified. Two systems of a fixed active beacon and a receiver using an ultrasound time-of-flight method and a differential infrared intensity method are proposed. The infrared localization system was appropriate for the goal of this study. It was possible to use the infrared intensity method to generate a uniform signal field that exceeded 30 m. Valid distance estimations which were within 30 m of coverage indoors and within 20 m of coverage outdoors were made. Also, a pocket-sized receiver which can be adapted to a smartphone was found to be suitable for use as a portable device. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The emergence of smartphones and GPS-based localization technology has completely changed our everyday lives in only a few years. This change has also influenced the development of pedestrian guidance applications for the visually impaired. The current bottleneck is to realize versatile portable indoor localization, which is inevitable for practical pedestrian guidance, not only outdoors, but also, in public indoor facilities, such as subway stations, hospitals, shopping malls, and schools. Technical improvements of this indoor localization problem will be the most valuable contributions with regard to the resolution of the inequality of technology for visually impaired people. Attempts to utilize the recent developments pertaining to compact, light-weight, high-performance common mobile electric devices, such as smartphones, to create products for the disabled ⇑ Corresponding author. Tel.: +82 42 868 5117; fax: +82 42 868 5679. E-mail address: [email protected] (I.-M. Choi). http://dx.doi.org/10.1016/j.infrared.2014.09.023 1350-4495/Ó 2014 Elsevier B.V. All rights reserved.
are actively in progress. GPS-based personal guidance systems have been proposed and experimentally tested [1–5]. However, GPS, which uses satellite signals, is unsuitable in unfavorable weather conditions or in urban areas between skyscrapers. In particular, it is not feasible for indoor locations. As candidates that can be used in indoor environments, localizations methods that use electromagnetic waves such as WLAN, RFID and ZigBee, or mechanical waves such as ultrasound and audible sound, have been studied by many research groups [6–10]. However, prototypes for the indoor localization applications were often limited due to their size, weight, and/or cost. The performance levels with regard to coverage, accuracy, speed, and availability have not yet satisfied the required level for pedestrian guidance via a single method [11]. Recently, indoor optical wireless communication methods which use LEDs have become more common. As a result, indoor localization research is being inspired by the higher accessibility of LED sources [12]. In such a new area, localization methods using multiple visible LED lights have been introduced [13–16]. A similar
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outdoor environments. In Section 2, the concept and the goal of this work are elaborated. Sections 3 and 4 describe the details of the two localization systems. The first is an ultrasound time-offlight system with an infrared reference method, and the second is an infrared-only system. Each system is manufactured and characterized. In Section 5, conclusions will be presented with a comparison of the two methods in terms of the degree of signal field flexibility and mobility with handheld devices. 2. Portable pedestrian guidance system
Fig. 1. Concept of the mid-range infrared localization system.
localization method with one LED was reported as well [17]. Although there have been achievements in the field of LED localization technology, a complete system that can consider visually impaired people, which would require seamless operation in both indoor and outdoor environments, has yet to be realized to the best of our knowledge. On the other hand, portable handheld devices of a reasonable size and weight are another important aspect. The most accessible approach is to utilize the resource of smartphones, which show potential as a pedestrian guidance system for the visually impaired [5,9]. In this paper, we verify a fixed active-beacon wireless localization system for the visually impaired that works in both indoor and
Fig. 1 shows the concept of the guidance system. A beacon, representing a reference node, broadcasts signals to a wide coverage area such that a user holding a handheld receiver as a moving node will be informed of his/her location by means of the signal strength. A binary code can be extracted from the signal to identify the destination. Contrary to the common triangulation method, even if there is only one reference, the distance and orientation with regard to the reference can be obtained by sweeping the receiver horizontally. The performance aspects of the systems were designed and manufactured to aid a visually impaired person when that person is walking. Precision of less than 1 m at a range greater than 30 m from a destination or a target will be practical for large indoor facilities. 3. Ultrasonic time-of-flight method Initially, the localization system for the visually impaired included both ultrasound and infrared modules in an effort to measure relative distances and orientations with respect to a fixed reference. In this system, consisting of a beacon (reference node) and a receiver (moving node), the time-of-flight of ultrasound pulses were measured in reference to infrared pulses. The signals, initially synchronized at the beginning of the transmission, were measured with a time delay at the receiver due to the different speeds of the two waves. The distance between the beacon and the receiver can be calculated by multiplying the speed of sound (343.5 m/s
Fig. 2. Block diagrams of the reference node (beacon) and the moving node (receiver) of the ultrasound time-of-flight system.
S. Park et al. / Infrared Physics & Technology 67 (2014) 583–589
(a) Active beacon
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(b) Receiver
Fig. 3. Assembled devices of the ultrasound time-of-flight system.
(a) Distance: 3 m
(b) Distance: 10 m
Fig. 4. Received ultrasound and infrared signals; purple: received infrared signal and ID data, brown: delayed ultrasound signal in the ultrasound time-of-flight system.
at 20 °C) by the time delay of ultrasound, as the traveling time of light is negligible. 3.1. Design of an ultrasonic time-of-flight system As shown in the block diagram in Fig. 2, the beacon is made of infrared LEDs, ultrasound transducers, and driver circuits. It is capable of transmitting signals up to a distance of 12 m with one LED and to distances that exceed 30 m with three LEDs connected in parallel. The receiver consists of an ultrasound sensor, two infrared sensors (photodiodes), a geomagnetic sensor, and signal processing electronics. Two independent infrared sensors detect the ID code and the optical signal intensity separately. The complete system of the transmitter and receiver is shown in Fig. 3. Seven sets of ultrasound transducers are allocated at a spacing of 30° on the sides of the beacon as shown in Fig. 3(a), in order to cover 180° for installation on a wall. For the receiver, ultrasound and infrared sensors are located on the front side. The device can recognize its orientation using a geomagnetic sensor as well. The receiver is designed to be compact in order to provide comfort when holding and carrying it.
the beacon was 15 m for ultrasound and 30 m for infrared, as shown in Fig. 5. It was concluded that the signal field generated from a single transmitter was limited to only a half angle of 15° a maximum distance of 15 m. Coverage by ultrasound wave transmission could not be manipulated conveniently. Multiple ultrasound transducers caused unfavorable interference problems due to phase differences resulting from the path lengths, which were difficult to synchronize. In addition, the ultrasound receiving module itself is relatively large in size, impeding our pursuit of a pocket-sized device.
3.2. Results of the ultrasonic time-of-flight system The received ultrasound pulse signal and infrared signal at distances of 3.00 m and 10.00 m respectively are shown in Fig. 4(a) and (b). The time delays were measured and found to be 8.62 ms and 29.31 ms with calculated distances of 2.96 m and 10.07 m respectively. The ultrasound time-of-flight system shows distance errors of less than 2% over the measurable range, which is good enough for visually impaired pedestrians. The coverage area of the signal was also experimentally obtained to characterize the conditions of operation. The maximum detectable distance from
Fig. 5. Valid coverage of a pair of ultrasound and infrared transducers of the ultrasound time-of-flight system.
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Fig. 6. Active beacon of the infrared intensity system for generation of the signal field.
table device. Its measurable distance and its accuracy are suitable as a pedestrian guidance system for visually impaired people. The optical intensity of a spherical wave emitted from a point light source decays with the distance according to the inversesquare law. The exact relationship between the signal strength and the distance can be practically determined using reference data in the term of compensation coefficients measured in advance from an actual field test. When the signal field intensity requires further enhancement, concave mirrors or cylindrical lenses can be applied to make the characteristics of the field more efficient for two-dimensional mapping by a pedestrian guidance system. The signal can be modulated with a unique frequency; added noise from external infrared sources with different frequencies, which very significantly affects outdoor applications, can be mitigated by means of filtering or with a differential method.
4. Differential infrared intensity method
4.1. Design of a differential infrared intensity system
It was possible to use the previous system roughly to determine the distance between the beacon and the receiver using the infrared signal strength, even outside of the area of the ultrasound signal coverage. It is possible to remove all of the ultrasound parts from both the beacon and the receiver with an enhanced infrared system. The significant advantage of the infrared system is that LED signals do not cause an interference problem, unlike the ultrasound system. In this system, interference due to phase differences is negligible because the LED output spectrum exceeds more than tens of nm. In addition, the intensity and divergence of the infrared signal can be manipulated conveniently by optics such as lenses or mirrors. Among various infrared distance measurement methods, a distance–intensity relationship was utilized for the modified system. This system is beneficial because it has a simple principle and configuration, characteristics which are appropriate for a por-
An enhanced infrared localization system with a single simplified optical method was designed and manufactured. The system consists of a beacon and a receiver, as in the earlier case. Figs. 6 and 7 show an infrared-only beacon (reference node) and a receiver adapted for a smartphone (moving node). Three high-power LEDs in the beacon emit synchronized infrared signals containing binary codes with the ID information of the reference node. An LED operated at a drive current of 1 A emits an infrared signal with a wavelength of 940 nm wavelength at an optical power of 3.1 W. The full emission angle of more than 120° full enables the three LEDs to cover all directions around the beacon without any mechanical rotation mechanism. This device showed optimal efficiency at a duty ratio of 30%. A higher duty ratio only increased the power consumption without enhancing the device performance. The carrier frequency was set to 15.4 kHz considering the
(a) Receiver PCB
(b) Receiver adapted to a smartphone
Fig. 7. Receiver of the infrared intensity system.
Fig. 8. Block diagram of the reference node (beacon) and the moving node (receiver) of the infrared intensity system.
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(a) Signal from the logarithmic amplifier output
(b) Linearized signal
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natural light sources. The receiver design is focused on giving it a portable size comparable to that of a smartphone. Even with the optical filtering of the sensor, a relatively high DC offset was detected, which made outdoor usage impossible. A differential method of removing the DC noise from the original signal was used to avoid the effect of a slow infrared spectral component added from sunlight. After this method was applied, the signal strength received outdoors showed characteristics identical to those of signals received in an indoor environment. Fig. 8 provides block diagrams of the sensing and signal processing procedures. The beacon generates a signal field around it, and the receiver detects the strength of the received signal. The ID information of the reference node is also transferred via the signal. The receiver converts the current of the photodiode into a voltage signal with a logarithmic amplifier that has a wide dynamic range. If a linear amplifier were to be used, the signal would be saturated easily near the reference node or under strong sunlight. In such a case, the system may have short detection coverage, most likely only about 3 m6 m. The signal strength was quantized to digital data using the ADC (Analog-to-Digital Converters) of a MCU (Micro-Controller Unit). The data was sent to a smartphone or a PC for the conversion of the signal strength to the distance information and for the identification of the reference node information. The built-in Bluetooth module was used to transfer this data to the designated device. For the basic characterization of the system, in this case a sensor performance test, a DAQ board and a PC were used for the ADC and the data calculation. If the signal strength were to be scaled linearly, the DC noise could have been eliminated easily from the original signal. However, in this system the acquired digital data shows a non-linear characteristic due to the logarithmic amplifier. Consequently, post-processing is required for noise rejection with linearization of the data after it passes the amplifier. 4.2. Signal processing of the differential infrared intensity system Unless a noise-rejection method is applied, the system performance will be degraded due to the infrared component noise from sunlight in outdoor environments, whereas light from fluorescent lamps used for indoor illumination has almost no effect on the strength of the received signal. In an outdoor environment, the received signal is highly influenced by the DC component, as shown in Fig. 9(a). To remove this offset from the unnecessary light source, the data was linearized after logarithmic amplification and ADC processing. Fig. 9(b) shows that the data that was linearized using an exponential function, which is the inverse function of logarithmic amplification. It was possible to eliminate the offset so that there was no great difference in the signals between the indoor and outdoor conditions, as shown in Fig. 9(c). The final values of the data can be used to determine the distance between the receiver and the beacon, which can be calibrated afterwards.
(c) Offset compensated signal Fig. 9. Noise rejection process from the received infrared signal at a distance of 2 m using the infrared intensity system (a) signal from the logarithmic amplifier output, (b) linearized signal and (c) offset compensated signal.
response times of the system, including those of LEDs and photodiodes. The on–off modulation of the carrier conveys the ID data. The receiver acquires and synthesizes the received signal with two infrared sensors and signal processing electronics. A Bluetooth module was included in the electronics of the receiver to allow it to communicate with other portable devices, such as smartphones. The sensitive optical frequency of the infrared sensors ranges from 750 to 1100 nm, which can reduce external disturbances from
4.3. Signal field characteristics of the differential infrared intensity system The signal field around the beacon was examined in terms of the received signal strength with respect to the distance and angle. With one operating LED, the signal was measured at distant positions that were 10° apart from half angles of 0° to 120° with distances of 0.5 m, 1 m, 2 m, 5 m, and 10 m. The measured two-dimensional signal strength data was linearly interpolated to plot a set of lines of an equal strength, as shown in Fig. 10. The sum of the 120° rotated data sets shows the characteristics of the signal field from the three LEDs, as shown in the inset of Fig. 10, which can be expected to be obtained from a three-LED beacon. The standard deviation of the signal strength according to the
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Fig. 10. Signal field of an LED around the beacon of the infrared intensity system (inset: overlapped signal field for a 3-LED beacon).
Fig. 11. Signal receiving sensitivity according the relative orientation of the receiver of the infrared intensity system.
angle around the reference node is less than 5%. This device is not precise for accurate distance measurement, but is acceptable for the rough estimation required for a pedestrian guidance application. Using a quantitative comparison experiment in a narrow hallway and in an open area of a gym, it was found that the light reflected by walls did not greatly affect the shape and strength of the field. Fig. 11 presents the received signal strength according to the relative orientation of the receiver to the beacon, at the center line of the beacon signal field. The data was acquired using a procedure similar to the one used above. The infrared sensor built into the receiver responds to the signal sensitively within a half angle of 20°. The smaller the sensing angle is, the more precisely it can find the orientation to the beacon. The sensing angle can easily be adjusted in order to optimize it for the normal sweeping speed of the user holding the device. The maximum signal strength while a user sweeps the device provides distance and orientation information to the beacon.
4.4. Result of the differential infrared intensity system
Fig. 12. Signal strength characteristics of the infrared intensity system according to the distance in different environment (Error bars represent standard deviations).
The relationship between the distance and the received signal was characterized under different conditions at distances of 0.5 m, 1 m, 2 m, 5 m, 10 m, 20 m, and 30 m. The signal strength after post-processing decreased according to the distance. The results were independent of external conditions within a certain range, as shown in Fig. 12; however, due to the relatively high deviations in the outdoor settings, the distances were not distinguishable at distances of more than 20 m. The error bars of the graph show the standard deviations of the measurements repeated under the same conditions. The error levels indicate that the device can properly estimate distances within a 20 m range. Specifically, the distance information is very accurate, with repeatability of 5 cm up to 5 m and of 0.5 m up to 10 m. The valid coverage distances from the distance estimations were more than 30 m in the indoor environment and 20 m in the outdoor environment. As mentioned above, an improvement of the optical system or an increase in the number of LEDs can extend the valid coverage beyond 20 m outdoors and can enhance the distance resolution. For instance, if the transmitted optical output power doubles, the
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maximum distance will increase by about 40%, leading to an extended coverage range of 28 m. 5. Conclusion The simple principle of the differential intensity method can allow the device to work faster with less computational resources than those required by other, more complicated localization methods that demand complex calculations. Moreover, if the optical localization method is merged with a wireless optical communication system, there will be more of an opportunity to realize simultaneously infrared intensity-based localization, network access and pedestrian guidance. With the proposed portable pedestrian guidance system, a pedestrian with weak eyesight or a blind person can easily determine his or her current location, orientation, and actual distance to a destination. The user will be notified of real-time locations and path information while walking. The device can provide detailed information on the location through ID information from the infrared beacon signal or through an internet connection. This will improve the individual’s quality of life in terms of independence, privacy and dignity. A distance estimation method for a portable localization system for pedestrian guidance has been designed and realized. To choose the optimal method, we compared the performances of two systems using the ultrasound time-of-flight method and the differential infrared intensity method. It was possible to use the time-of-flight method with ultrasound and infrared at a range of 15 m with remarkable precision of less than a few cm. However, generating a uniform signal field with multiple transducers was difficult, due to interference problems caused by ultrasound waves. The relatively bulky ultrasound transducer parts were disadvantageous for use in portable devices. The differential method using the infrared intensity level was more appropriate because LEDs can generate a uniform signal field without any hindrances; such a field can extend to more than 30 m. Distance estimation was possible in a 30 m coverage range in indoor environments and in a 20 m coverage range in outdoor environments. Although this method does not determine accurate distances at a position farther than the outer limit of the range, the received signal can be used as orientation information for pedestrian guidance. In addition, the use of this method allows the receiver to be miniaturized, which makes it possible to use this receiver in portable devices. Conflict of interest There is no conflict of interest among authors.
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Acknowledgement This research was supported by the Converging Research Center Program through the Ministry of Science, ICT and Future Planning, Korea (2013K000328). References [1] Reginald G. Golledge, Roberta L. Klatzky, Jack M. Loomis, Jon Speigle, Jerome Tietz, A geographical information system for a GPS based personal guidance system, Int. J. Geogr. Inf. Sci. 12 (7) (1998) 727–749. [2] Hideo Mori, Shinji Kotani, Robotic travel aid for the blind: HARUNOBU-6, in: European Conference on Disability, Virtual Reality, and Assistive Technology, 1998, pp. 193–202. [3] Abdelsalam Helal, Steven Edwin Moore, Balaji Ramachandran, Drishti: an integrated navigation system for visually impaired and disabled, Int. Symp. Wearable Comput. (2001) 149–156. [4] Slim Kammoun, Marc J-M. Macé, Bernard Oriola, Christophe Jouffrais, Towards a geographic information system facilitating navigation of visually impaired users, Int. Conf. Comput. Help. People Special Needs (2012) 521–528. [5] Jeffrey R. Blum, Mathieu Bouchard, Jeremy R. Cooperstock, Spatialized audio environmental awareness for blind users with a smartphone, Mobile Networks Appl. 18 (3) (2013) 295–309. [6] Luis A. Guerrero, Francisco Vasquez, Sergio F. Ochoa, An indoor navigation system for the visually impaired, Sensors 12 (6) (2012) 8236–8258. [7] Saleh Alghamdi, Ron van Schyndel, Ahmed Alahmadi, Indoor navigational aid using active RFID and QR-code for sighted and blind people, Intell. Sens. Sens. Networks Inf. Process. (2013) 18–22. [8] Diego López-de-Ipiña, Tania Lorido, Unai López, Indoor navigation and product recognition for blind people assisted shopping, Ambient Assisted Living (2011) 33–40. [9] Sharly Joana Halder, Joon-Goo Park, Wooju Kim, Adaptive filtering for indoor localization using zigbee RSSI and LQI measurement, IEEE Trans. Syst. Man Cybernetics (2011) 305–324. [10] Atri Mandal, Cristina V. Lopes, Tony Givargis, Amir Haghighat, Raja Jurdak, Pierre Baldi, Beep: 3D indoor positioning using audible sound, Consumer Commun. Network. Conf. (2005) 348–353. [11] Rainer Mautz, Indoor positioning technologies, ETH Zürich, Department of Civil, Environmental and Geomatic Engineering, Institute of Geodesy and Photogrammetry, 2012. [12] Joseph M. Kahn, John R. Barry, Wireless infrared communications, Proc. IEEE 85 (2) (1997) 265–298. [13] Baris Tas, Nihat Altiparmak, Ali Saman Tosun, Low cost indoor location management system using infrared leds and wii remote controller, Int. Perform. Comput. Commun. Conf. (2009) 280–288. [14] Soo-Yong Jung, Chang-Soo Park, Lighting LEDs based indoor positioning system using received signal strength ratio, Three Dimensional Syst. Appl. (2013). [15] Mohammad Shaifur Rahman, Md Mejbaul Haque, Ki-Doo Kim, Indoor positioning by LED visible light communication and image sensors, Int. J. Electr. Comput. Eng. 1 (2) (2011) 161–170. [16] Anna Maria Vegni, Mauro Biagi, An indoor localization algorithm in a smallcell LED-based lighting system, Int. Conf. Indoor Position. Indoor Navig. (2012). [17] Keita Atsuumi, Masafumi Hashimoto, Manabu Sano, Optical azimuth sensor for indoor mobile robot navigation, Int. Conf. Comput. Eng. Syst. (2008) 381– 386.
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
A portable mid-range localization system using infrared LEDs for visually impaired people Suhyeon Park, In-Mook Choi ⇑, Sang-Soo Kim, Sung-Mok Kim Korea Research Institute of Standards and Science (KRISS), Daejeon 305-340, Republic of Korea
h i g h l i g h t s A portable sized guidance system for the visually impaired was developed. An ultrasound time-of-flight method and a differential infrared intensity method were realized. It is possible to use the infrared intensity method to generate a uniform signal field that exceeded 30 m. The measurement was independent of external conditions within a certain range. We made it possible to use this receiver in portable devices such as smartphones.
a r t i c l e
i n f o
Article history: Received 10 April 2014 Available online 1 November 2014 Keywords: Infrared Ultrasound Localization Positioning Pedestrian guidance Active beacon
a b s t r a c t A versatile indoor/outdoor pedestrian guidance system with good mobility is necessary in order to aid visually impaired pedestrians in indoor and outdoor environments. In this paper, distance estimation methods for portable wireless localization systems are verified. Two systems of a fixed active beacon and a receiver using an ultrasound time-of-flight method and a differential infrared intensity method are proposed. The infrared localization system was appropriate for the goal of this study. It was possible to use the infrared intensity method to generate a uniform signal field that exceeded 30 m. Valid distance estimations which were within 30 m of coverage indoors and within 20 m of coverage outdoors were made. Also, a pocket-sized receiver which can be adapted to a smartphone was found to be suitable for use as a portable device. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The emergence of smartphones and GPS-based localization technology has completely changed our everyday lives in only a few years. This change has also influenced the development of pedestrian guidance applications for the visually impaired. The current bottleneck is to realize versatile portable indoor localization, which is inevitable for practical pedestrian guidance, not only outdoors, but also, in public indoor facilities, such as subway stations, hospitals, shopping malls, and schools. Technical improvements of this indoor localization problem will be the most valuable contributions with regard to the resolution of the inequality of technology for visually impaired people. Attempts to utilize the recent developments pertaining to compact, light-weight, high-performance common mobile electric devices, such as smartphones, to create products for the disabled ⇑ Corresponding author. Tel.: +82 42 868 5117; fax: +82 42 868 5679. E-mail address: [email protected] (I.-M. Choi). http://dx.doi.org/10.1016/j.infrared.2014.09.023 1350-4495/Ó 2014 Elsevier B.V. All rights reserved.
are actively in progress. GPS-based personal guidance systems have been proposed and experimentally tested [1–5]. However, GPS, which uses satellite signals, is unsuitable in unfavorable weather conditions or in urban areas between skyscrapers. In particular, it is not feasible for indoor locations. As candidates that can be used in indoor environments, localizations methods that use electromagnetic waves such as WLAN, RFID and ZigBee, or mechanical waves such as ultrasound and audible sound, have been studied by many research groups [6–10]. However, prototypes for the indoor localization applications were often limited due to their size, weight, and/or cost. The performance levels with regard to coverage, accuracy, speed, and availability have not yet satisfied the required level for pedestrian guidance via a single method [11]. Recently, indoor optical wireless communication methods which use LEDs have become more common. As a result, indoor localization research is being inspired by the higher accessibility of LED sources [12]. In such a new area, localization methods using multiple visible LED lights have been introduced [13–16]. A similar
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outdoor environments. In Section 2, the concept and the goal of this work are elaborated. Sections 3 and 4 describe the details of the two localization systems. The first is an ultrasound time-offlight system with an infrared reference method, and the second is an infrared-only system. Each system is manufactured and characterized. In Section 5, conclusions will be presented with a comparison of the two methods in terms of the degree of signal field flexibility and mobility with handheld devices. 2. Portable pedestrian guidance system
Fig. 1. Concept of the mid-range infrared localization system.
localization method with one LED was reported as well [17]. Although there have been achievements in the field of LED localization technology, a complete system that can consider visually impaired people, which would require seamless operation in both indoor and outdoor environments, has yet to be realized to the best of our knowledge. On the other hand, portable handheld devices of a reasonable size and weight are another important aspect. The most accessible approach is to utilize the resource of smartphones, which show potential as a pedestrian guidance system for the visually impaired [5,9]. In this paper, we verify a fixed active-beacon wireless localization system for the visually impaired that works in both indoor and
Fig. 1 shows the concept of the guidance system. A beacon, representing a reference node, broadcasts signals to a wide coverage area such that a user holding a handheld receiver as a moving node will be informed of his/her location by means of the signal strength. A binary code can be extracted from the signal to identify the destination. Contrary to the common triangulation method, even if there is only one reference, the distance and orientation with regard to the reference can be obtained by sweeping the receiver horizontally. The performance aspects of the systems were designed and manufactured to aid a visually impaired person when that person is walking. Precision of less than 1 m at a range greater than 30 m from a destination or a target will be practical for large indoor facilities. 3. Ultrasonic time-of-flight method Initially, the localization system for the visually impaired included both ultrasound and infrared modules in an effort to measure relative distances and orientations with respect to a fixed reference. In this system, consisting of a beacon (reference node) and a receiver (moving node), the time-of-flight of ultrasound pulses were measured in reference to infrared pulses. The signals, initially synchronized at the beginning of the transmission, were measured with a time delay at the receiver due to the different speeds of the two waves. The distance between the beacon and the receiver can be calculated by multiplying the speed of sound (343.5 m/s
Fig. 2. Block diagrams of the reference node (beacon) and the moving node (receiver) of the ultrasound time-of-flight system.
S. Park et al. / Infrared Physics & Technology 67 (2014) 583–589
(a) Active beacon
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(b) Receiver
Fig. 3. Assembled devices of the ultrasound time-of-flight system.
(a) Distance: 3 m
(b) Distance: 10 m
Fig. 4. Received ultrasound and infrared signals; purple: received infrared signal and ID data, brown: delayed ultrasound signal in the ultrasound time-of-flight system.
at 20 °C) by the time delay of ultrasound, as the traveling time of light is negligible. 3.1. Design of an ultrasonic time-of-flight system As shown in the block diagram in Fig. 2, the beacon is made of infrared LEDs, ultrasound transducers, and driver circuits. It is capable of transmitting signals up to a distance of 12 m with one LED and to distances that exceed 30 m with three LEDs connected in parallel. The receiver consists of an ultrasound sensor, two infrared sensors (photodiodes), a geomagnetic sensor, and signal processing electronics. Two independent infrared sensors detect the ID code and the optical signal intensity separately. The complete system of the transmitter and receiver is shown in Fig. 3. Seven sets of ultrasound transducers are allocated at a spacing of 30° on the sides of the beacon as shown in Fig. 3(a), in order to cover 180° for installation on a wall. For the receiver, ultrasound and infrared sensors are located on the front side. The device can recognize its orientation using a geomagnetic sensor as well. The receiver is designed to be compact in order to provide comfort when holding and carrying it.
the beacon was 15 m for ultrasound and 30 m for infrared, as shown in Fig. 5. It was concluded that the signal field generated from a single transmitter was limited to only a half angle of 15° a maximum distance of 15 m. Coverage by ultrasound wave transmission could not be manipulated conveniently. Multiple ultrasound transducers caused unfavorable interference problems due to phase differences resulting from the path lengths, which were difficult to synchronize. In addition, the ultrasound receiving module itself is relatively large in size, impeding our pursuit of a pocket-sized device.
3.2. Results of the ultrasonic time-of-flight system The received ultrasound pulse signal and infrared signal at distances of 3.00 m and 10.00 m respectively are shown in Fig. 4(a) and (b). The time delays were measured and found to be 8.62 ms and 29.31 ms with calculated distances of 2.96 m and 10.07 m respectively. The ultrasound time-of-flight system shows distance errors of less than 2% over the measurable range, which is good enough for visually impaired pedestrians. The coverage area of the signal was also experimentally obtained to characterize the conditions of operation. The maximum detectable distance from
Fig. 5. Valid coverage of a pair of ultrasound and infrared transducers of the ultrasound time-of-flight system.
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Fig. 6. Active beacon of the infrared intensity system for generation of the signal field.
table device. Its measurable distance and its accuracy are suitable as a pedestrian guidance system for visually impaired people. The optical intensity of a spherical wave emitted from a point light source decays with the distance according to the inversesquare law. The exact relationship between the signal strength and the distance can be practically determined using reference data in the term of compensation coefficients measured in advance from an actual field test. When the signal field intensity requires further enhancement, concave mirrors or cylindrical lenses can be applied to make the characteristics of the field more efficient for two-dimensional mapping by a pedestrian guidance system. The signal can be modulated with a unique frequency; added noise from external infrared sources with different frequencies, which very significantly affects outdoor applications, can be mitigated by means of filtering or with a differential method.
4. Differential infrared intensity method
4.1. Design of a differential infrared intensity system
It was possible to use the previous system roughly to determine the distance between the beacon and the receiver using the infrared signal strength, even outside of the area of the ultrasound signal coverage. It is possible to remove all of the ultrasound parts from both the beacon and the receiver with an enhanced infrared system. The significant advantage of the infrared system is that LED signals do not cause an interference problem, unlike the ultrasound system. In this system, interference due to phase differences is negligible because the LED output spectrum exceeds more than tens of nm. In addition, the intensity and divergence of the infrared signal can be manipulated conveniently by optics such as lenses or mirrors. Among various infrared distance measurement methods, a distance–intensity relationship was utilized for the modified system. This system is beneficial because it has a simple principle and configuration, characteristics which are appropriate for a por-
An enhanced infrared localization system with a single simplified optical method was designed and manufactured. The system consists of a beacon and a receiver, as in the earlier case. Figs. 6 and 7 show an infrared-only beacon (reference node) and a receiver adapted for a smartphone (moving node). Three high-power LEDs in the beacon emit synchronized infrared signals containing binary codes with the ID information of the reference node. An LED operated at a drive current of 1 A emits an infrared signal with a wavelength of 940 nm wavelength at an optical power of 3.1 W. The full emission angle of more than 120° full enables the three LEDs to cover all directions around the beacon without any mechanical rotation mechanism. This device showed optimal efficiency at a duty ratio of 30%. A higher duty ratio only increased the power consumption without enhancing the device performance. The carrier frequency was set to 15.4 kHz considering the
(a) Receiver PCB
(b) Receiver adapted to a smartphone
Fig. 7. Receiver of the infrared intensity system.
Fig. 8. Block diagram of the reference node (beacon) and the moving node (receiver) of the infrared intensity system.
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(a) Signal from the logarithmic amplifier output
(b) Linearized signal
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natural light sources. The receiver design is focused on giving it a portable size comparable to that of a smartphone. Even with the optical filtering of the sensor, a relatively high DC offset was detected, which made outdoor usage impossible. A differential method of removing the DC noise from the original signal was used to avoid the effect of a slow infrared spectral component added from sunlight. After this method was applied, the signal strength received outdoors showed characteristics identical to those of signals received in an indoor environment. Fig. 8 provides block diagrams of the sensing and signal processing procedures. The beacon generates a signal field around it, and the receiver detects the strength of the received signal. The ID information of the reference node is also transferred via the signal. The receiver converts the current of the photodiode into a voltage signal with a logarithmic amplifier that has a wide dynamic range. If a linear amplifier were to be used, the signal would be saturated easily near the reference node or under strong sunlight. In such a case, the system may have short detection coverage, most likely only about 3 m6 m. The signal strength was quantized to digital data using the ADC (Analog-to-Digital Converters) of a MCU (Micro-Controller Unit). The data was sent to a smartphone or a PC for the conversion of the signal strength to the distance information and for the identification of the reference node information. The built-in Bluetooth module was used to transfer this data to the designated device. For the basic characterization of the system, in this case a sensor performance test, a DAQ board and a PC were used for the ADC and the data calculation. If the signal strength were to be scaled linearly, the DC noise could have been eliminated easily from the original signal. However, in this system the acquired digital data shows a non-linear characteristic due to the logarithmic amplifier. Consequently, post-processing is required for noise rejection with linearization of the data after it passes the amplifier. 4.2. Signal processing of the differential infrared intensity system Unless a noise-rejection method is applied, the system performance will be degraded due to the infrared component noise from sunlight in outdoor environments, whereas light from fluorescent lamps used for indoor illumination has almost no effect on the strength of the received signal. In an outdoor environment, the received signal is highly influenced by the DC component, as shown in Fig. 9(a). To remove this offset from the unnecessary light source, the data was linearized after logarithmic amplification and ADC processing. Fig. 9(b) shows that the data that was linearized using an exponential function, which is the inverse function of logarithmic amplification. It was possible to eliminate the offset so that there was no great difference in the signals between the indoor and outdoor conditions, as shown in Fig. 9(c). The final values of the data can be used to determine the distance between the receiver and the beacon, which can be calibrated afterwards.
(c) Offset compensated signal Fig. 9. Noise rejection process from the received infrared signal at a distance of 2 m using the infrared intensity system (a) signal from the logarithmic amplifier output, (b) linearized signal and (c) offset compensated signal.
response times of the system, including those of LEDs and photodiodes. The on–off modulation of the carrier conveys the ID data. The receiver acquires and synthesizes the received signal with two infrared sensors and signal processing electronics. A Bluetooth module was included in the electronics of the receiver to allow it to communicate with other portable devices, such as smartphones. The sensitive optical frequency of the infrared sensors ranges from 750 to 1100 nm, which can reduce external disturbances from
4.3. Signal field characteristics of the differential infrared intensity system The signal field around the beacon was examined in terms of the received signal strength with respect to the distance and angle. With one operating LED, the signal was measured at distant positions that were 10° apart from half angles of 0° to 120° with distances of 0.5 m, 1 m, 2 m, 5 m, and 10 m. The measured two-dimensional signal strength data was linearly interpolated to plot a set of lines of an equal strength, as shown in Fig. 10. The sum of the 120° rotated data sets shows the characteristics of the signal field from the three LEDs, as shown in the inset of Fig. 10, which can be expected to be obtained from a three-LED beacon. The standard deviation of the signal strength according to the
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Fig. 10. Signal field of an LED around the beacon of the infrared intensity system (inset: overlapped signal field for a 3-LED beacon).
Fig. 11. Signal receiving sensitivity according the relative orientation of the receiver of the infrared intensity system.
angle around the reference node is less than 5%. This device is not precise for accurate distance measurement, but is acceptable for the rough estimation required for a pedestrian guidance application. Using a quantitative comparison experiment in a narrow hallway and in an open area of a gym, it was found that the light reflected by walls did not greatly affect the shape and strength of the field. Fig. 11 presents the received signal strength according to the relative orientation of the receiver to the beacon, at the center line of the beacon signal field. The data was acquired using a procedure similar to the one used above. The infrared sensor built into the receiver responds to the signal sensitively within a half angle of 20°. The smaller the sensing angle is, the more precisely it can find the orientation to the beacon. The sensing angle can easily be adjusted in order to optimize it for the normal sweeping speed of the user holding the device. The maximum signal strength while a user sweeps the device provides distance and orientation information to the beacon.
4.4. Result of the differential infrared intensity system
Fig. 12. Signal strength characteristics of the infrared intensity system according to the distance in different environment (Error bars represent standard deviations).
The relationship between the distance and the received signal was characterized under different conditions at distances of 0.5 m, 1 m, 2 m, 5 m, 10 m, 20 m, and 30 m. The signal strength after post-processing decreased according to the distance. The results were independent of external conditions within a certain range, as shown in Fig. 12; however, due to the relatively high deviations in the outdoor settings, the distances were not distinguishable at distances of more than 20 m. The error bars of the graph show the standard deviations of the measurements repeated under the same conditions. The error levels indicate that the device can properly estimate distances within a 20 m range. Specifically, the distance information is very accurate, with repeatability of 5 cm up to 5 m and of 0.5 m up to 10 m. The valid coverage distances from the distance estimations were more than 30 m in the indoor environment and 20 m in the outdoor environment. As mentioned above, an improvement of the optical system or an increase in the number of LEDs can extend the valid coverage beyond 20 m outdoors and can enhance the distance resolution. For instance, if the transmitted optical output power doubles, the
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maximum distance will increase by about 40%, leading to an extended coverage range of 28 m. 5. Conclusion The simple principle of the differential intensity method can allow the device to work faster with less computational resources than those required by other, more complicated localization methods that demand complex calculations. Moreover, if the optical localization method is merged with a wireless optical communication system, there will be more of an opportunity to realize simultaneously infrared intensity-based localization, network access and pedestrian guidance. With the proposed portable pedestrian guidance system, a pedestrian with weak eyesight or a blind person can easily determine his or her current location, orientation, and actual distance to a destination. The user will be notified of real-time locations and path information while walking. The device can provide detailed information on the location through ID information from the infrared beacon signal or through an internet connection. This will improve the individual’s quality of life in terms of independence, privacy and dignity. A distance estimation method for a portable localization system for pedestrian guidance has been designed and realized. To choose the optimal method, we compared the performances of two systems using the ultrasound time-of-flight method and the differential infrared intensity method. It was possible to use the time-of-flight method with ultrasound and infrared at a range of 15 m with remarkable precision of less than a few cm. However, generating a uniform signal field with multiple transducers was difficult, due to interference problems caused by ultrasound waves. The relatively bulky ultrasound transducer parts were disadvantageous for use in portable devices. The differential method using the infrared intensity level was more appropriate because LEDs can generate a uniform signal field without any hindrances; such a field can extend to more than 30 m. Distance estimation was possible in a 30 m coverage range in indoor environments and in a 20 m coverage range in outdoor environments. Although this method does not determine accurate distances at a position farther than the outer limit of the range, the received signal can be used as orientation information for pedestrian guidance. In addition, the use of this method allows the receiver to be miniaturized, which makes it possible to use this receiver in portable devices. Conflict of interest There is no conflict of interest among authors.
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