JP5442215B2 - Ultrasonic distance measurement system - Google Patents

Description

  The present invention relates to a configuration of a distance measurement system using ultrasonic waves, and particularly to a system that dynamically measures a distance to a moving object with low power consumption that can be introduced into a future sensor network or the like.

Conventionally, a method called a pulse echo method has been used for distance measurement using ultrasonic waves. The pulse echo method is to cut out an ultrasonic wave of a certain frequency (for example, 40 kHz in the case of air use) into a very narrow pulse shape, measure it, and measure the time until it hits an object and returns. , To find the distance. This technology is often used for passenger car back sonar.
Junsuke Kuroki, “Electronic suspension of an automobile using ultrasonic road sonar,” October 1998, Ultrasonic TECHNO (Nikkan Kogyo Publishing), paragraphs 62-66.

  We will focus on homes and offices and assume that a sensor network that has begun to attract attention recently will realize a more comfortable, safe and healthy living environment. Considerations are being made to introduce a monitoring function for grasping the dynamic state of the resident together with various sensors in the sensor node of the sensor network. The most common device for such a monitor is a CCD camera, but the sensor node requires characteristics with very low power consumption, for example, a single battery is required to operate for several years. Therefore, it is impossible to introduce a CCD camera into the sensor node.

  In a conventional pulse echo method, a sensor using ultrasonic waves transmits pulsed ultrasonic waves and monitors reflected waves from an object. By combining a plurality of transmitting transducers and a plurality of receiving transducers, it is possible to obtain information regarding the position of a reflecting object, particularly an object in a dynamic state. However, when the conventional pulse echo method is introduced into a general house or the like, it is necessary to narrow the pulse width in order to obtain a certain degree of distance resolution with respect to the reflecting object. When the pulse width is narrowed, the transmission power decreases. Therefore, the transmission power is kept constant by increasing the amplitude of the pulse. The pulse voltage of the final transmission stage that drives the transmission transducer may reach several tens of volts (V). Therefore, such a circuit cannot be introduced into a low power consumption device that needs to operate for several years with a single battery like a sensor node of a sensor network. In general, an ultrasonic sensor using the pulse echo method requires many operations such as amplification of the received ultrasonic wave, envelope detection, and differential processing on the receiving side. Is virtually impossible.

The conventional ultrasonic pulse echo method cannot be applied to the sensor node because of power consumption or the like. In the present invention, an ultrasonic pulse is not used as in the prior art, but an increase in power consumption due to handling a large-amplitude pulse is achieved by using an ultrasonic continuous wave (that is, a CW wave). A configuration that does not occur is proposed. The frequency from the transmitting transducer, the frequency components f (1) that n divides the period from f ₁ to _{f 2, f (2),} ···, and transmits the CW wave with f (n + 1) Similarly, the receiving transducer receives a CW wave having these frequency components. The relative amplitude and phase of the received CW wave with respect to the transmitted CW wave are stored as data for each frequency component. The impulse response between the transmitting transducer and the receiving transducer is derived by performing a discrete Fourier transform using these data. Since the impulse response shows the time delay between transmission and reception, distance information is obtained by multiplying the time delay by the ultrasonic velocity.

  This proposal does not use a pulse having a large amplitude as described above. Further, in the case of a sensor network, the receiving transducer can send raw amplitude and phase data to the center node via the network without processing the received CW signal. Data sent from each sensor node is processed in a batch at the center node. That is, the distance of the measurement object with respect to each node is obtained by performing discrete Fourier transform or the like. An object map or the like can be formed based on such distance information. In general, the center node is operated by a separate power source unlike the sensor node, and the power consumed by various processes does not become a problem. Further, each sensor node transmits or receives a CW ultrasonic wave having a small amplitude, and only sends data related to the amplitude and phase of the received signal to the center node, and can operate with extremely low power consumption.

  As described above, according to the present invention, it becomes possible to introduce a distance measurement sensor using an ultrasonic wave to a sensor node that requires particularly low power consumption characteristics in a future sensor network, for example, It can be applied to collecting dynamic information of elderly residents, monitoring behavior of people in public facilities such as hospitals, and crime prevention.

  The best mode for carrying out the present invention will be described below. In sensor networks, standardized 2.4 GHz ZigBee is often used as a wireless medium. As shown in a simplified explanatory diagram in FIG. 1, one form of sensor network is an environment in which a large number of sensor nodes are arranged in a living environment, and mainly related to life such as temperature, humidity, and brightness. It senses parameters and performs control to realize a comfortable living environment based on such data. As one of the functions of such a sensor network, it is required to incorporate functions such as a patient behavior monitor in a public facility such as a hospital, a remote monitor of a resident in an elderly house, and a crime prevention function.

The proposed ultrasonic distance measuring system meets the above requirements by including an ultrasonic transmission / reception transducer as a part of the sensor node as shown in FIG. Measure the distance from each sensor node to the object, create a 2D or 3D object map based on the data, and infer changes in human movement and environment from the temporal change of the map It can be done. It also guarantees the most important low power consumption required by the sensor node. Measurement data is aggregated to the center node via a wireless medium network such as ZigBee. The center node performs necessary signal processing and the like collectively. As a result, both low power consumption and aggregation of necessary data to the center node are achieved. It is possible to construct a sensor network system capable of various controls based on data at every place.

  Embodiments of the present invention will be specifically described below with reference to the drawings.

  FIG. 1 is a configuration showing a specific first embodiment of ultrasonic measurement according to the present invention. In the sensor network, in addition to information on the temperature, humidity, illuminance, etc. in the house, a large number of sensor nodes installed in the house, etc. will collect dynamic information on the residents in the future. That is, many sensor nodes also have a function of a transducer that transmits and receives ultrasonic waves, sequentially transmitting ultrasonic waves from the nodes, and receiving reflected ultrasonic waves from the object at its own node or other nodes. If the distance between the object and the transmission node and the reception node can be accurately measured, a two-dimensional or three-dimensional map can be formed by associating these values among a plurality of nodes. Dynamic information such as human movements can also be obtained from changes in the map over time. Such a measurement system is highly expected for future monitoring of elderly homes and hospitals, or for applications such as crime prevention. In particular, the Sense Network is a new concept that has been proposed with the advance of mobile communications, and will become widespread in the future. Introducing an ultrasonic position measurement system as part of such a network is also a very natural requirement. In general, ZigBee using a frequency in the 2.4 GHz band is the most promising wireless medium for sensor networks. Therefore, as shown in FIG. 1, the data measured at each sensor node can be aggregated to the center node via ZigBee, where signal processing and the like can be performed collectively. Naturally, Bluetooth, wireless LAN, WiMAX or a mobile phone can also be used as a wireless medium.

Next, problems when using a conventional ultrasonic measurement method as an ultrasonic measurement system will be described. FIG. 2 shows a pulse echo method that is an ultrasonic measurement method that has been most often used conventionally. As shown in FIG. 2A, it is assumed that a signal having a short time width (pulse form) is transmitted from a transmission transducer with ultrasonic waves and received by a reception transducer. When there is a reflected wave (echo) from an object, as shown in Fig. 2 (b), the time determined by the distance to the object (l ₁ between the transmitting transducer and the object and l ₁ 'between the receiving transducer and the object) An ultrasonic pulse is received with a delay of (l ₁ + l ₁ ') / v (assuming the ultrasonic velocity is v). The problem here is that the received pulse is attenuated compared to the transmitted pulse, so the amplitude is small, but the rising and falling portions of the pulse have a tail, as shown in FIG. That is, the actual reception pulse is considerably spread on the time axis than the transmission pulse. When there are a plurality of reflecting objects as shown in FIG. 3 (a), a plurality of corresponding reflected waves are received as shown in FIG. 3 (b). If these waves have a sufficient difference in the distance between the transmitting / receiving transducer (sensor node) and a plurality of objects, the corresponding reflected waves are also separated on the time axis, and each can be identified. However, in actual applications, there are very many reflecting objects, and it is often difficult to separate many reflected waves from the reflecting objects into corresponding objects. The conventional pulse echo method generally employs a method of narrowing the pulse width in order to separate and identify many reflected waves (that is, improve the distance resolution). That is, by narrowing the transmission pulse as much as possible, the width of the reception pulse is also narrowed, and as a result, the distance resolution is improved. However, if the pulse is narrowed, the transmission power of the pulse itself is reduced. Therefore, in order to maintain the same transmission power, it is necessary to increase the amplitude of the pulse. In practical applications, the pulse width can reach tens of volts (V). A circuit that generates or amplifies such a large-amplitude ultrasonic pulse consumes a large amount of power. On the other hand, in a sensor network, each sensor node may be required to operate for several years with a single battery, and thus requires operation with very low power consumption. Therefore, it is difficult to introduce a conventional ultrasonic measurement method into a sensor network or the like.

  As a conventional pulse echo method, there is a so-called code spreading method as shown in FIG. This method has the advantage that the pulse width can be increased, and unlike the pulse echo method, it is not necessary to increase the amplitude along with the narrowing of the pulse. As shown in FIG. 4A, the phase of the transmission ultrasonic wave is changed according to a spreading code (Barker code, Gold code, m-sequence, etc.). That is, the phase is 0 with respect to 0 of the spread code and the phase is π with respect to 1 of the spread code. On the receiving side, the correlation between the spread code and the received wave is taken. That is, the convolution integration of the spread code and the received wave is performed in real time. When both match, a large output is obtained. As shown in FIG. 4B, this is a small value because the correlation output becomes a cross-correlation output even if the spread code and the received wave are slightly shifted on the time axis. When the spread code and the received wave exactly match on the time axis, the correlation output becomes an autocorrelation output, so that a large output can be obtained. The larger the ratio between the autocorrelation output value and the cross-correlation output value, for example, the greater the signal-to-noise ratio (S / N) value, and the better the measurement accuracy. However, this depends on the length of the spreading code. When a spreading code having a very long code length is used, even if there are reflected waves from a plurality of reflecting objects as shown in FIG. 5 (a), the corresponding reflected waves are shown in FIG. 5 (b). The distance resolution is improved compared to the pulse echo method. However, if the spreading code length is increased, the correlation between the spreading code and the received wave, that is, convolution integration becomes complicated, and a very high-speed microprocessor is required for real-time processing. Since a high-speed microprocessor consumes a large amount of power, it is difficult to introduce it into a sensor node of a sensor network, as in the case of the conventional pulse echo method.

The present invention solves the problems of the conventional ultrasonic measurement method described above. It is possible to introduce ultrasonic measurement to a system that requires operation with very low power consumption such as a sensor node of a sensor network. Further, in terms of distance resolution, it is possible to realize performance that exceeds the conventional pulse echo method. A case where there is one reflecting object as shown in FIG. In the measurement method of the present invention, the transmitted wave is not an ultrasonic pulse or a code-spread ultrasonic wave but a continuous wave having a constant frequency. As a transmission frequency, as shown in FIG. 6B, a frequency band between f ₁ and f ₂ is considered. f ₂ −f ₁ is divided into n, and the frequency f (i) = f ₁ + (f ₂ −f ₁ ) / n × (i−1), i = 1 from the lowest frequency in the band to n + 1 , ..., define n + 1. FIG. 7A schematically shows an actual ultrasonic waveform in the case of f (1) = f ₁ and f (n + 1) = f ₂ . As compared to the transmitted ultrasound of f (1), ..., f (n + 1) frequency, the received ultrasound has a smaller amplitude due to attenuation due to propagation and reflection, as schematically shown in FIG. 7 (b). Become. The phase increases in proportion to the distance between the transmitting / receiving transducer and the object, but repeats with 2π as the period. Therefore, the phase delay of the received wave with respect to the transmitted wave is a value between 0 and 2π. When the above relationship is shown in the figure, the frequency and phase of the received wave are represented in FIG. Here, as shown in FIG. 6B, the phase of the transmission wave is assumed to be uniform with respect to the frequency because it is a reference phase. FIG. 6 (c) is actually expressed by taking the horizontal axis as a discrete frequency obtained by dividing the frequency band of f ₂ −f ₁ by n, but generally n is a sufficiently large value. ), The horizontal axis is treated as an approximately continuous frequency. From FIG. 6 (c), it can be seen that the amplitude of the received wave is smaller than that of the transmitted wave of FIG. 6 (b) and the value of the phase delay is increased. Here, the relative amplitude of each amplitude and phase of the received wave in FIG. 6 (c) with respect to the transmitted wave in FIG. 6 (b) is standardized in amplitude, but the phase is interpreted as FIG. 6 (c) itself. I know it ’s good.

The processing using the data in FIG. 6 (c) will be described. The data in FIG. 6 (c) is assumed to be a relative value normalized by each value of the transmission wave in FIG. 6 (b). Inverse Fourier transform is performed using the relative amplitude and relative phase corresponding to the n + 1 frequencies in FIG. 6 (c). Strictly speaking, since the inverse Fourier transform is a transform for a continuous function, an approximate transform is performed in the case of n + 1 frequencies as shown in FIG. However, it is well known mathematically that an almost accurate conversion value can be obtained by setting the value of n + 1 to a sufficiently large value. Here, further consider the case where f ₁ is zero and f ₂ is infinite. In this case, the inverse Fourier transform of FIG. 6C becomes an approximate delta function as shown in FIG. This can also be seen from the fact that a signal distributed with a uniform amplitude on all frequency axes is a delta function as a function on the time axis that is in a relationship of inverse Fourier transform. In this case, the phase of FIG. 6C corresponds to the shift amount on the time axis of the approximate delta function of FIG. 8A, that is, the time delay (l ₁ + l ₁ ′) / v. The same can be derived from FIG. 6 (c). If the phase in FIG. 6C is Φ, the group delay is given by τ _g = −δΦ / δω. In the ultrasonic propagation in the space as discussed here, the propagation speed is not dispersive depending on the frequency, and the group velocity and the phase velocity are equal. Therefore, the group delay τ _g is the wave delay, that is, the time delay of FIG. Equivalent to (l ₁ + l ₁ ') / v. Therefore, if the phase characteristic of FIG. 6C is given, the time delay of FIG. 8A is also uniquely determined. That is, in FIG. 6 (a), assuming that the transmitting transducer is an input and the receiving transducer is an output, FIG. 6 (c) is an amplitude and phase characteristic of a transfer function between input and output, and FIG. 8 (a) is an impulse response thereof. I can do it. When the time delay of the impulse response is known, the distance to the reflecting object is derived by multiplying the value by the speed v.

Next, let us consider a case where f ₁ to f ₂ in FIG. In this case, as shown in FIG. 8B, the Fourier response subjected to Fourier transform spreads slightly on the time axis and has a function form close to a sinc function (hence sinx / x). However, the time delay until the maximum value of the approximate sinc function (the center of the main lobe) is given by (l ₁ + l ₁ ') / v, and this value is the same as in the case of the approximate delta function of FIG. . That is, if f ₂ −f ₁ is finite but sufficiently large, the time delay (l ₁ + l ₁ ′) / v can be determined with high accuracy. Further, by multiplying the value by the velocity v, the distance to the reflecting object can be measured with high distance resolution.

  The above is the basic principle of the present invention. That is, as shown in FIG. 7 (a), for each transmission wave, a continuous wave whose frequency does not change with time is used, and relative values of amplitude and phase between transmission and reception for these continuous waves are obtained, and these The inverse Fourier transform is performed using the above data. Thus, an impulse response between transmission and reception is obtained, and the distance to the reflecting object is measured from the time delay. The important point here is not to transmit a pulse with a short time width as in the conventional pulse echo method, but to transmit a wave whose phase reverses with time as in the code diffusion method. However, it is a point that transmits a continuous wave having a constant frequency with respect to time. Therefore, the power consumption on the transmission side can be extremely reduced. On the receiving side, a relative value between the received wave and the transmitted wave, that is, a relative amplitude and a relative phase are obtained. The reference amplitude and phase of the transmitted wave, when each sensor node is connected via a sensor network, etc., can be transmitted to other sensor nodes related to the transmission of ultrasonic waves by wireless means such as ZigBee. Send the information (in the case of a wired network, it may be sent by wire). Each sensor node obtains the relative amplitude and relative phase of the received wave and the transmitted wave, and sends the information to the center node via the sensor network or the like. Since the center node is generally operated by a separate power source, power consumption is not a problem. Based on the relative amplitude and relative phase from each sensor node, inverse Fourier transform is performed collectively. As a result, an impulse response between the sensor and the node is obtained. That is, the distance between each sensor node and the reflecting object can be obtained.

FIG. 8C shows a characteristic example of the second embodiment of the ultrasonic measurement according to the present invention. As described in the first embodiment, the impulse response when f ₂ −f ₁ in FIG. 6C is finite is an approximate sinc function as shown in FIG. 8B. Further, when f ₂ −f ₁ is small, an approximate sinc function spread further on the time axis as shown in FIG. 8C. There is a mathematically related relationship between the spread on the time axis and f ₂ −f ₁ in FIG. As shown in FIG. 6C, in the case of an ideal rectangular frequency characteristic having a component only in f ₂ −f ₁ , the width of the central portion (main lobe) of the approximate sinc function in FIG. The interval between the front and rear zero points) Δt is given by Δt = 2 / (f ₂ −f ₁ ). Therefore, as f ₂ −f ₁ is larger, Δt is smaller, that is, the distance resolution is improved. However, a general ultrasonic transmission / reception transducer does not have a rectangular frequency characteristic as shown in FIG. 6C with respect to the frequency, and the amplitude characteristic attenuates from the center frequency to both sides. The phase characteristic does not necessarily have a linear constant value as shown in FIG. 6B outside the small amplitude. From various examination results including experiments, it was found that in a system using an actual transducer for ultrasonic transmission / reception, the value of f ₂ −f ₁ should be set to 50 to 120% of the 3 dB frequency bandwidth of the transducer. . This setting is another feature of the present invention, which effectively utilizes the bandwidth of the transducer to improve the distance resolution, and deteriorates the measurement accuracy especially in the vicinity of the outside of the band due to the phase disturbance. Can be avoided.

FIG. 9 shows a characteristic example of the third embodiment of ultrasonic measurement according to the present invention. FIG. 9 is a characteristic example of the present invention for a case where a plurality of reflecting objects exist as shown in FIG. By making f ₂ −f ₁ large to some extent and narrowing the width of the approximate sinc function, distance resolution can be improved, and reflected waves from a plurality of equivalent reflective objects can be separated. What is important here is to determine n of n + 1 frequencies in FIG. 6 (c). As described in the first embodiment, the inverse Fourier transform is not performed on a continuous function but on a function described by n + 1 pieces of data. Inverse Fourier transform using such discrete data, generally I FFT (Inverse
By using a discrete Fourier inverse transform algorithm called (Fast Fourier Transform), it is possible to achieve very simple and high speed.

If the frequency interval obtained by dividing f ₂ −f ₁ into n in FIG. 6C is Δf, Δf = (f ₂ −f ₁ ) / n. In IFFT, aliasing due to discretization becomes a problem. Therefore, if the maximum frequency of I FFT operation is F, F must be at least twice the maximum frequency to be handled. That is, F ≧ 2f ₂ is set. Therefore, the number N of IFFT discrete frequencies is N = F / Δf. Further, when the IFFT calculation is performed, T = 1 / Δf, where T is the maximum value on the time axis (that is, the value at which the time is repeated from zero again when this time is exceeded).

The following are other features of the present invention. In general, the increase in N is related to the calculation scale of I FFT and is considered to increase the calculation time. However, the calculation time is hardly affected by applying a dedicated algorithm. Therefore, there is no need to consider the magnitude of N. n is related to the number of transmissions and receptions because the number of transmission waves is given by n + 1. Therefore, it is better to set n as small as possible as long as the requirements such as measurement accuracy can be satisfied. In FIG. 9, an object 1 having a relatively small time delay (l ₁ + l ₁ ′) / v and an object 2 having a relatively large time delay (l ₂ + l ₂ ′) / v are considered separately. That is, since the object 1 is close, it is necessary to measure the distance with high accuracy, and since the object 2 is separated, it is considered that the distance can be measured with a certain degree of accuracy. For example, a short distance and a long distance are considered separately with a boundary at the midpoint between the objects 1 and 2. Let To be the equivalent time at the boundary. For short distance measurement, it is preferable from the viewpoint of distance resolution that f ₂ −f ₁ is set as large as the bandwidth characteristics of the transducer allow. Δf is obtained from Δf = 1 / To. n is obtained from n = (f ₂ −f ₁ ) / Δf, and the maximum frequency F of I FFT is determined as F≈2f ₂ (actually any value of F ≧ 2f ₂ may be used). The number N of discrete frequencies of I FFT is defined as N = F / Δf. In such a setting, since f ₂ −f ₁ is large, a good distance resolution can be obtained. Furthermore, since To is a midpoint between objects and is set to be small to some extent, Δf does not become an extremely small value. Therefore, n is not an extremely large value. That is, measurement with high distance resolution is possible with a relatively small number of transmission waves.

On the other hand, for far distance measurement, first, a maximum distance far from the object 2, that is, an equivalent maximum value T on the time axis is determined. Next, Δf is obtained from Δf = 1 / T. This value is a small value. Although n is determined from n = (f ₂ −f ₁ ) / Δf, since Δf is small, n is large, that is, the number of transmission waves may increase. However, when the distance is large to some extent, generally a very high distance resolution is not required. Therefore, in this case, since f ₂ −f ₁ can also be reduced, the value of n can be set to a value that is not much different from that at the time of short distance measurement. The following procedure is similar to that of the short distance measurement, the maximum frequency F of the I FFT is defined as F ≒ 2f _2. The number N of discrete frequencies of IFFT is N = F / Δf, but N is large because Δf is small. However, this point has almost no influence on the calculation time and the like by applying the dedicated algorithm for I FFT as described above.

  As described above, in the present invention, by considering the object at a short distance and the object at a long distance separately, n is large especially for an object at a long distance, that is, the number of transmitted waves is very large. It can be avoided. As a result, an object at a short distance and an object at a long distance can be measured with substantially the same number of steps, and the dynamic range of the measurement range can be expanded simultaneously with simplification of the process.

Sensor network configuration Illustration of the conventional pulse echo method Explanatory drawing when there are two reflective objects in the conventional pulse echo method Illustration of the principle of a conventional code spreading method Explanatory drawing of measurement method using conventional code spreading method Illustration of the principle of the measurement method according to the present invention Illustration of the principle of the measurement method according to the present invention Illustration of the principle of the measurement method according to the present invention Illustration of the principle of the measurement method according to the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Sensor node of sensor network 2 ... Center node of sensor network 3 ... Data transmission path by ZigBee etc. 4 ... Transmission / reception path of ultrasonic wave 5 ... Reflective object 6 ...・ Transmission transducer 7 ・ ・ ・ Reception transducer

Claims (3)

In a system for measuring a distance by transmitting an ultrasonic wave with a transmitting transducer and receiving the ultrasonic wave with a receiving transducer, the transmitted ultrasonic wave has a plurality of continuous waves whose frequency does not change with time for each transmitted wave. The frequency of each wave corresponds to each frequency of the inverse discrete Fourier transform, and the discrete Fourier inverse is applied to the relative amplitude and relative phase , which are relative values of the amplitude and phase of the received ultrasonic wave with respect to the transmitted ultrasonic wave of each frequency. ultrasonic distance measurement system, characterized by determining the distance by multiplying the ultrasonic speed of sound in the time delay of the impulse is obtained by applying a transform response. 2. The distance measuring system according to claim 1, wherein the transmission / reception transducer exists in a sensor node of the sensor network, and an ultrasonic wave transmitted from a transmission transducer in the own sensor node is transmitted in the own sensor node or another sensor node. received by the receiving transducer, determined the relative amplitude and relative phase for each of said transmission ultrasonic waves of the respective received ultrasound, Ri send a its these values to the center nodes operating at different supply more to radio, Se in printer node based on the relative amplitude and relative phase from each sensor node, by performing a discrete Fourier inverse transform collectively, characterized by the Turkey seek distance of the reflecting object and each sensor node Ultrasonic distance measurement system. In the distance measuring system according to claim 2, to create a map indicating the distance of the reflecting object and the sensor node, and wherein the inferring temporal change in the reflected object movement or environment than a change in the map Ultrasonic distance measurement system.

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