A two-dimensional multi-target ultrasonic positioning system and positioning method

Abstract

The present invention relates to a two-dimensional multi-target ultrasonic positioning system and positioning method, and belongs to the field of ultrasonic pen positioning. The transmitting device includes an infrared transmitting module and two ultrasonic transmitters that can emit ultrasonic waves of the same frequency. The receiving device includes multiple ultrasonic pens, and each ultrasonic pen is equipped with a corresponding infrared receiver and ultrasonic receiver. The transmitting end works in the form of time division multiplexing, which can accommodate multiple ultrasonic pens working at the same time; the DOA method is adopted, and a positioning technology based on a synchronous signal is used. The infrared signal is used as the time reference to obtain the time value of the ultrasonic pen receiving the infrared signal and the ultrasonic signal. The Bluetooth communication technology is used to wirelessly transmit data. The coordinates of the receiving module in the two-dimensional plane are obtained by solving geometric equations to realize the positioning of multiple ultrasonic pens. The advantages of the present invention are that it supports multiple pens working at the same time, the ultrasonic pen can be added to the work at any time, and it is easy to implement. It is suitable for the application of information collection of flat-screen TVs.

Description

Technical Field

[0001] The present invention relates to an ultrasonic pen positioning method, and in particular to an ultrasonic pen positioning method for a television screen using infrared signals as a time reference. The method can enable multiple pens to write on the screen simultaneously, thereby being better applied to interactive touch screen functions and more intuitive use of flat-screen televisions. Background Art

[0002] With the rise and development of virtual reality technology, people's demand for the experience of human-computer interaction products is increasing. The interactive screens currently on the market all use single-pen writing, and there have been some introductions to dual-pen writing functions. The Chinese patent "A method for realizing dual-pen writing based on an electronic whiteboard" (application number 201210338872.X) provides a method for realizing dual-pen writing based on an electronic whiteboard, wherein two cameras are used to detect the status of the corresponding stylus respectively, and the image acquisition module collects the image data of each camera, and stores the image data into the SDRAM chip through the image storage module. At the same time, the coordinates of the light spot left by the stylus are calculated by the image processing module, and the coordinates of the light spot are displayed by the PC to realize dual-pen writing. However, this type of method requires a specific camera to locate the stylus, which is relatively expensive and is affected by factors such as lighting and skin color; the Chinese patent "Multi-target three-dimensional ultrasonic positioning system" (201320612723.8) proposes a multi-target three-dimensional ultrasonic positioning system. The 3D ultrasonic positioning system uses infrared signals as a reference and achieves multi-target differentiation through frequency division multiplexing of infrared and ultrasonic signals. It uses a combination of a notch filter circuit and a digital filter to separate the ultrasonic positioning signals of different targets. However, this method cannot distinguish between two ultrasonic pens emitting ultrasonic signals with the same waveform and frequency. The patented "Ultrasonic Writing Tablet with Flat Structure" (CN201010264728.7) includes a handwriting ultrasonic pen, at least three ultrasonic receivers mounted at different positions on the edge of an isotropic hard writing base, as well as matching signal receiving and amplifying processing circuits and data calculation and processing units, a calibration channel for ultrasonic signal emission time, and an element that can be installed in the ultrasonic pen to detect writing pressure. The ultrasonic pen includes an ultrasonic transmitter mechanically coupled to a pen tip made of a hard elastic material. The pen tip can also use a variable amplitude rod structure, but it cannot support multiple pens working simultaneously.

[0003] The Chinese patent application "An Electromagnetic Induction Display Device for Simultaneous Multi-Pen Writing" (201420151401.2) proposes an electromagnetic induction display device for simultaneous multi-pen writing. This device uses a receiving coil to receive electromagnetic wave frequency signals emitted by different electromagnetic pens. After passing through an analog switch, an analog frequency-selective amplifier circuit, and an A / D converter circuit, the electromagnetic signals are converted into digital signals and sent to a processor for processing. The processor uses existing computational methods to process the handwriting of the different electromagnetic pens and then transmits the writing trajectory signals via a computer to a projector, achieving multi-pen writing. However, this method requires the use of a dedicated electronic pen, and the electromagnetic waves also pose a certain risk of radiation to the human body.

[0004] The Chinese patent "A dual-pen writing method for an electronic whiteboard based on ultrasound" (CN201710678439.3) uses an ultrasonic pen as the transmitter of infrared signals and ultrasonic signals, fixes the receiving end on the device, and uses the different pulse widths of infrared signals as the time reference to distinguish the two pens. By receiving the infrared signal emitted by the other pen, it determines whether it is in the working state, and then combines the position area status instructions obtained by wireless transmission to realize its own working logic timing, and presses the pen tip to start working. The difference of this patent is that the ultrasonic pen is used as the receiving end, and the infrared and ultrasonic transmitting ends are fixed on the screen. The transmitting end works in the form of time division multiplexing and can support the simultaneous use of more than two pens. The disadvantage is that the transmitting elements arranged on the edge of the screen take up a lot of space.

[0005] The patent "Ultrasonic handwriting input detection device for handheld or portable devices" (CN00133388.7) also uses an ultrasonic pen as the transmitting end, and the receiving end is installed in the housing of the portable device, and only supports single-pen writing. Summary of the Invention

[0006] The present invention provides a two-dimensional multi-target ultrasonic positioning system and positioning method. The system utilizes the DOA principle, uses infrared signals as synchronization signals, uses ultrasonic pens as receiving ends, and operates in a time-division multiplexing mode at the transmitting end. After a series of calculations, the two-dimensional coordinates of multiple pens are acquired. Ultrasonic waves are used to complete positioning, which is not affected by electromagnetic signals and supports multiple pens working simultaneously.

[0007] The technical solution adopted by the present invention is to include a transmitting device and a receiving device, wherein the transmitting device includes a first ultrasonic transmitting module, a second ultrasonic transmitting module, an infrared transmitting module, a processor, and a Bluetooth communication module host. The positions of the two ultrasonic transmitting modules are known, and the material is PVDF piezoelectric film, so that the ultrasonic transmission angle is 180°, and the transmitted ultrasonic signals have the same waveform and frequency. KHz ultrasonic wave, two ultrasonic transmitting modules are installed on the edge of the LCD screen; the infrared transmitting module, the first ultrasonic transmitting module, the second ultrasonic transmitting module and the Bluetooth communication module host are connected to the processor;

[0008] The receiving device includes n ultrasonic pens, Each ultrasonic pen includes an ultrasonic receiving module, an infrared receiving module, a microprocessor, a clock module, and a Bluetooth communication module slave. The ultrasonic receiving module includes two ultrasonic sensors installed at the bottom of the ultrasonic pen. From the cross-section, the angle between the installation position and the center of the circle is ninety degrees. When in use, it is facing the transmitting device. The ultrasonic receiving module, infrared receiving module, clock module, and Bluetooth communication module slave are connected to the microprocessor.

[0009] The positioning method using a two-dimensional multi-target ultrasonic positioning system includes the following steps:

[0010] (a) The receiving module is installed on n ultrasonic pens, and the coordinates are set as , … ;

[0011] (b) Using the time difference of arrival (TDOA) principle, the infrared signal is used as the time reference, because the propagation speed of the infrared signal is much faster than that of the ultrasonic signal;

[0012] (c) Positioning is performed by the time difference between the infrared signal and the ultrasonic signal received by the ultrasonic pen. The timer in the clock module starts counting from 0 with the time when the ultrasonic pen n receives the infrared signal as the time reference, and gets the time value when the ultrasonic receiving module receives the ultrasonic wave. , and continue timing until the ultrasonic receiving module receives the ultrasonic wave again to get the time value , the two time values ​​are transmitted from the machine to the screen processor via the Bluetooth communication module;

[0013] (d) The position coordinates of the two ultrasonic transmitting modules are known and are ; The coordinates of the ultrasonic pen are set to , is the system's duty cycle, is the speed of ultrasonic wave propagation;

[0014] (e) It is The distance between the ultrasonic pen and the first ultrasonic transmitting module, is the distance between the nth ultrasonic pen and the second ultrasonic transmitting module. The two distances are as follows:

[0015]

[0016] The solution is:

[0017]

[0018] As described above, the coordinates of multiple ultrasonic pens are acquired in two-dimensional space, and two-dimensional multi-target positioning is achieved.

[0019] The system working cycle described in step d of the present invention When there is an ultrasonic pen with the point of landing farthest from the first ultrasonic transmitting module on the screen, the time required from the first ultrasonic transmitting module starting to work to the ultrasonic pen receiving the ultrasonic signal is set to When there is an ultrasonic pen with the landing point farthest from the second ultrasonic transmitting module on the screen, the time required from the second ultrasonic transmitting module starting to work to the ultrasonic pen receiving the ultrasonic signal is set to ; , with the two ultrasonic transmitting ends as the center of the circle, The intersection area of ​​the two circles with radius is the writing area, is the ultrasonic wave propagation speed, The value of can make the screen within the writing area, so that the ultrasonic pen can complete positioning no matter where it writes on the screen.

[0020] Working cycle of the present invention As the writing screen area changes, When the area of ​​a circle with radius needs to change, Also changes accordingly.

[0021] The two ultrasonic emission modules in step d of the present invention adopt a time division multiplexing method to accommodate multiple ultrasonic pens working simultaneously without affecting each other. The specific method is as follows: the infrared emission module and the first ultrasonic emission module start working at the same time, The second ultrasonic transmitting module starts working until the end of the cycle, and the ultrasonic receiving module of the ultrasonic pen Only the first ultrasonic signal can be received before Then only the second ultrasonic signal can be received.

[0022] When any ultrasonic pen of the present invention starts working, it starts working when it receives the infrared signal emitted by the infrared emission module. Therefore, there are two situations when the ultrasonic pen starts working:

[0023] (1) The ultrasonic pen starts working just when the infrared pulse is emitted. There is no delay at this time and it works normally.

[0024] (2) When the ultrasonic pen is put into operation, it misses the rising edge of the infrared pulse or misses the entire infrared pulse. It is considered that the complete infrared pulse is not received and a delay is required. Waiting for the next cycle of infrared pulse to start working, .

[0025] The advantages of the present invention are that it adopts the TDOA principle, uses a positioning technology based on synchronous signals, takes infrared signals as the time reference, obtains the time value when the ultrasonic pen receives the infrared signal and the ultrasonic signal, utilizes Bluetooth communication technology to wirelessly transmit data, and obtains the x-axis and y-axis coordinates of the receiving module in the two-dimensional plane by solving geometric equations to realize the positioning of multiple ultrasonic pens. The present invention supports multiple pens working at the same time, and ultrasonic pens can be put into work at any time. It has the advantages of simple structure and easy implementation, and is suitable for information collection of flat-screen TVs. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a structural schematic diagram of the television screen of the present invention;

[0027] Figure 2 It is a structural schematic diagram of the ultrasonic pen of the present invention;

[0028] Figure 3 This is a diagram showing the location of the ultrasonic receiving module of the present invention;

[0029] Figure 4 This is a diagram of the maximum working area of ​​the ultrasonic pen of the present invention;

[0030] Figure 5 This is a schematic diagram of the ultrasonic pen positioning principle of the present invention;

[0031] Figure 6 This is a timing diagram of the ultrasonic pen working of the present invention;

[0032] Figure 7 It is a GAP broadcasting workflow diagram of the present invention;

[0033] Figure 8 It is a workflow diagram of the present invention. DETAILED DESCRIPTION

[0034] It includes a transmitting device and a receiving device, wherein the transmitting device includes a first ultrasonic transmitting module, a second ultrasonic transmitting module, an infrared transmitting module, a processor, and a Bluetooth communication module host. The positions of the two ultrasonic transmitting modules are known, and the material is PVDF piezoelectric film, so that the ultrasonic transmission angle is 180°, and the transmitted ultrasonic signals have the same waveform and frequency. KHz ultrasonic wave, two ultrasonic transmitting modules are installed on the edge of the LCD screen; the infrared transmitting module, the first ultrasonic transmitting module, the second ultrasonic transmitting module and the Bluetooth communication module host are connected to the processor;

[0035] The receiving device includes n ultrasonic pens, Each ultrasonic pen includes an ultrasonic receiving module, an infrared receiving module, a microprocessor, a clock module, and a Bluetooth communication module slave. The ultrasonic receiving module includes two ultrasonic sensors installed at the bottom of the ultrasonic pen. From the cross-section, the angle between the installation position and the center of the circle is ninety degrees. When in use, it is facing the transmitting device. The ultrasonic receiving module, infrared receiving module, clock module, and Bluetooth communication module slave are connected to the microprocessor.

[0036] The positioning method using a two-dimensional multi-target ultrasonic positioning system includes the following steps:

[0037] (a) The receiving module is installed on n ultrasonic pens, and the coordinates are set as , … ;

[0038] (b) Using the time difference of arrival (TDOA) principle, the infrared signal is used as the time reference, because the propagation speed of the infrared signal is much faster than that of the ultrasonic signal;

[0039] (c) Positioning is performed by the time difference between the infrared signal and the ultrasonic signal received by the ultrasonic pen. The timer in the clock module starts counting from 0 with the time when the ultrasonic pen n receives the infrared signal as the time reference, and gets the time value when the ultrasonic receiving module receives the ultrasonic wave. , and continue timing until the ultrasonic receiving module receives the ultrasonic wave again to get the time value , the two time values ​​are transmitted from the machine to the screen processor via the Bluetooth communication module;

[0040] (d) The position coordinates of the two ultrasonic transmitting modules are known and are ; The coordinates of the ultrasonic pen are set to , is the system's duty cycle, is the speed of ultrasonic wave propagation;

[0041] (e) It is The distance between the ultrasonic pen and the first ultrasonic transmitting module, is the distance between the nth ultrasonic pen and the second ultrasonic transmitting module. The two distances are as follows:

[0042]

[0043] The solution is:

[0044]

[0045] As described above, the coordinates of multiple ultrasonic pens are acquired in two-dimensional space, and two-dimensional multi-target positioning is achieved.

[0046] The system working cycle described in step d of the present invention When there is an ultrasonic pen with the point of landing farthest from the first ultrasonic transmitting module on the screen, the time required from the first ultrasonic transmitting module starting to work to the ultrasonic pen receiving the ultrasonic signal is set to When there is an ultrasonic pen with the landing point farthest from the second ultrasonic transmitting module on the screen, the time required from the second ultrasonic transmitting module starting to work to the ultrasonic pen receiving the ultrasonic signal is set to ; , with the two ultrasonic transmitting ends as the center of the circle, The intersection area of ​​the two circles with radius is the writing area, is the ultrasonic wave propagation speed, The value of can make the screen within the writing area, so that the ultrasonic pen can complete positioning no matter where it writes on the screen.

[0047] Working cycle of the present invention As the writing screen area changes, When the area of ​​a circle with radius needs to change, Also changes accordingly.

[0048] The two ultrasonic emission modules in step d of the present invention adopt a time division multiplexing method to accommodate multiple ultrasonic pens working simultaneously without affecting each other. The specific method is as follows: the infrared emission module and the first ultrasonic emission module start working at the same time, The second ultrasonic transmitting module starts working until the end of the cycle, and the ultrasonic receiving module of the ultrasonic pen Only the first ultrasonic signal can be received before Then only the second ultrasonic signal can be received.

[0049] When any ultrasonic pen of the present invention starts working, it starts working when it receives the infrared signal emitted by the infrared emission module. Therefore, there are two situations when the ultrasonic pen starts working:

[0050] (1) The ultrasonic pen starts working just when the infrared pulse is emitted. There is no delay at this time and it works normally.

[0051] (2) When the ultrasonic pen is put into operation, it misses the rising edge of the infrared pulse or misses the entire infrared pulse. It is considered that the complete infrared pulse is not received and a delay is required. Waiting for the next cycle of infrared pulse to start working, .

[0052] In order to enable those skilled in the art to better understand the present invention, the present invention is further described in detail below with reference to the accompanying drawings.

[0053] Figure 1 、 23 is a structural diagram of the TV screen and ultrasonic pen required by the present invention, including a TV 100 and multiple ultrasonic pens 200. The integrated module circuit board 101 20cm*3cm above the TV mainly includes a first ultrasonic transmitting module 102, a second ultrasonic transmitting module 103, an infrared transmitting module 104, a processor 105, and a Bluetooth communication module host 106. In this actual system, the ultrasonic signal frequency emitted by the ultrasonic transmitting module is given as KHz, the infrared signal pulse transmitted is ;The TV screen area is determined by actual needs; Figure 2 The ultrasonic pen 200 mainly includes a button 201, an infrared receiving module 202, a microprocessor 203, an ultrasonic receiving module 204, a pen tip 205, and a Bluetooth communication module slave 206. The ultrasonic receiving module includes two ultrasonic receivers, and the position relationship is shown in the cross section. Figure 3 .

[0054] Defining the duty cycle of a two-dimensional multi-target ultrasonic positioning system : When the first ultrasonic transmitting module is working, there is an ultrasonic pen 108 with the landing point farthest from it. After the ultrasonic pen receives the ultrasonic wave, the second ultrasonic module starts working immediately. At this time, there is also an ultrasonic pen 107 with the landing point farthest from the second ultrasonic module. In this case, the time length from the infrared transmitting module starting to work to the ultrasonic pen receiving the second ultrasonic signal is defined as , like Figure 4 As shown, any receiving end is composed of The working area within the circle with a radius of is the writing area. The value of can make the screen within the writing area, so that the ultrasonic pen can complete positioning no matter where it writes on the screen.

[0055] like Figure 5 As shown, the position coordinates of the two ultrasonic transmitting modules 304 and 305 are known and are When the ultrasonic pen is working on the screen, the points of the points are shown as 301, 302, and 303. The unknown coordinates are set as , … .

[0056] The ultrasonic transmitting module works in the form of time division multiplexing, and the infrared transmitting module 406 and the first ultrasonic transmitting module 404 start working at the same time. The second ultrasonic transmitting module 405 starts working. The timer starts from 0 with the moment when the infrared receiving module 201 of the ultrasonic pen n receives the infrared signal as the time reference, and gets the time value when the ultrasonic receiving module 204 receives the ultrasonic wave emitted by the first ultrasonic transmitting module 404. , continue to count until the ultrasonic receiving module 204 receives the ultrasonic wave emitted by the second ultrasonic transmitting module 405 to obtain the time value The position coordinates of the two ultrasonic transmitting modules 404 and 405 are known and are , v is the speed of ultrasonic propagation, and the calculation method is as follows:

[0057] The nth ultrasonic pen The location information is as follows:

[0058]

[0059] The solution is

[0060]

[0061] like Figure 6 As shown, 501 and 502 are infrared pulses emitted by the infrared emission module, 503 is the time when the infrared emission module and the first ultrasonic emission module start working at the same time, and 504 is the time when the second ultrasonic module starts working.

[0062] When any ultrasonic pen starts working, it starts working when it receives the infrared signal emitted by the infrared emission module. Therefore, there are two situations when the ultrasonic pen starts working:

[0063] (1) The ultrasonic pen 503 starts working just when the infrared pulse is emitted. There is no delay at this time and it works normally.

[0064] (2) When the ultrasonic pen is put into operation, it misses the rising edge of the infrared pulse or misses the entire infrared pulse. It is considered that the complete infrared pulse is not received and a delay is required. Waiting for the next cycle of infrared pulse to start working, .

[0065] After the infrared transmitting module and the first ultrasonic transmitting module start working at the same time, the plurality of ultrasonic pens successively receive the ultrasonic wave 505 emitted by the first ultrasonic transmitting module until the infrared transmitting module starts working 503 After a certain time, the second ultrasonic transmitting module starts to work 504, and the plurality of ultrasonic pens successively receive the ultrasonic waves 506 emitted by the second ultrasonic transmitting module.

[0066] When the ultrasonic pen transmits the time value data of the received ultrasonic signal, it adopts the form of one host and multiple slaves. The method of multiple ultrasonic pens transmitting data simultaneously through the Bluetooth communication module is as follows:

[0067] Adopting GAP (Generic Access Profile) access specification, it supports one master and multiple slaves data transmission. The host 106 is installed in the Bluetooth communication module at the transmitter end, and each ultrasonic pen is equipped with a slave 206. The maximum number of slave connections supported is 20.

[0068] This access specification ensures that different Bluetooth chips can discover each other and establish connections, thereby exchanging more data; Figure 7 As shown in the GAP broadcast workflow diagram, the data transmission process between the host and the slave in GAP is as follows:

[0069] a. The slave device needs to continuously broadcast to let the host know its existence.

[0070] b. The host sends a scan request.

[0071] c. The slave device responds with a scan reply and transmits data.

[0072] In GAP communication, the slave is the GAP server and the master is the GAP client. The master initiates a scan request to the slave, and the slave recommends a connection interval to the master. The master will try to reconnect at each connection interval to check if there is any new data.

[0073] Figure 8 As shown in workflow diagram 700, the ultrasonic pen is initialized 702, and the infrared transmitter module and the first ultrasonic transmitter module simultaneously begin operating 703. The ultrasonic pen detects the presence of infrared radiation. If infrared radiation is detected, it determines whether it is the system's infrared signal to eliminate interference 704. If it is the first infrared signal, a timer starts counting. Upon receiving an ultrasonic signal, the Bluetooth communication module transmits the current time value from the host to the host 705. The timer continues counting, and the second ultrasonic transmitter module begins operating 706. The ultrasonic pen continues detecting ultrasonic signals. Upon receiving an ultrasonic signal, the Bluetooth communication module transmits the current time value from the host to the host, and the timer is reset 707. The process returns to 703 to continue detecting. Simultaneously, the processor calculates the coordinates of the ultrasonic pen's landing point based on the obtained time value, completing positioning 708.

[0074] Some negligible errors may occur during the working process as follows:

[0075] There is an error between the position where the ultrasonic pen starts working and the actual point where the pen is placed. The writing speed of the ultrasonic pen on the screen is estimated. , the error distance is less than , calculated based on the screen area Generally, it is between 1-20ms, and the error distance is about a few millimeters and can be ignored. The ultrasonic pen is constantly moving, and there will be errors between the positioning and the actual position. The time interval from the ultrasonic pen receiving the ultrasonic wave emitted by the first ultrasonic transmitting module to the processor receiving the data and calculating the end is During this period, the ultrasonic pen is moving, and the error is less than , Less than , can be ignored.

[0076] The above description is directed to specific embodiments. In actual implementation, other variations and modifications may be made to the described embodiments to achieve more operations or more convenient writing. Therefore, these descriptions should be understood as merely illustrating various principles and not limiting the present invention.

Claims

1. A two-dimensional multi-target ultrasonic positioning method, characterized in that: The following steps are involved: (a) The receiving module is installed on n ultrasonic pens, n ≥ 3, and the coordinates are set as P1(x1, y1), P2(x2, y2)…P n (x n ,y n ); (b) Using the time difference of arrival (TDOA) principle, the infrared signal is used as the time reference, because the propagation speed of the infrared signal is much faster than that of the ultrasonic signal; (c) Positioning is performed by the time difference between the infrared signal and the ultrasonic signal received by the ultrasonic pen. The timer in the clock module starts timing from 0 with the moment when the ultrasonic pen n receives the infrared signal as the time reference, and obtains the time value t when the ultrasonic receiving module receives the ultrasonic wave. 1n , and continue timing until the ultrasonic receiving module receives the ultrasonic wave again to obtain the time value t 2n , the two time values ​​are transmitted from the machine to the screen processor via the Bluetooth communication module; (d) The position coordinates of the two ultrasonic transmitting modules are known and are O1(0,0) and O2(a,0). The coordinates of the ultrasonic pen are set to P n (x n ,y n ), T is the working cycle of the system, and v is the speed of ultrasonic propagation; (e), O1P n is the distance between the nth ultrasonic pen and the first ultrasonic transmitting module, O2P n is the distance between the nth ultrasonic pen and the second ultrasonic transmitting module. The two distances are as follows: O1P n =t 1n ×v The solution is: As described above, the coordinates of multiple ultrasonic pens are acquired in two-dimensional space, and two-dimensional multi-target positioning is achieved.

2. A two-dimensional multi-target ultrasonic positioning method according to claim 1, characterized in that: The system working cycle T described in step d is: when there is an ultrasonic pen with the landing point farthest from the first ultrasonic transmitting module on the screen, the time required from the first ultrasonic transmitting module starting to work to the ultrasonic pen receiving the ultrasonic signal is set to T1; when there is an ultrasonic pen with the landing point farthest from the second ultrasonic transmitting module on the screen, the time required from the second ultrasonic transmitting module starting to work to the ultrasonic pen receiving the ultrasonic signal is set to T2; T=2max(T1, T2), with the two ultrasonic transmitting ends as the center of the circle, The intersection area of ​​the two circles with a radius of is the writing area, v is the ultrasonic wave propagation speed, and the value of T can make the screen within the writing area, so that the ultrasonic pen can complete positioning no matter where it writes on the screen.

3. A two-dimensional multi-target ultrasonic positioning method according to claim 2, characterized in that: The duty cycle T changes with the writing screen area. When the area of ​​the circle with radius needs to change, T also changes accordingly.

4. The two-dimensional multi-target ultrasonic positioning method according to claim 1, characterized in that: The two ultrasonic emission modules in step d adopt a time division multiplexing method, which can accommodate multiple ultrasonic pens working at the same time without affecting each other. The specific method is as follows: the infrared emission module and the first ultrasonic emission module start working at the same time, The second ultrasonic transmitting module starts working until the end of the cycle, and the ultrasonic receiving module of the ultrasonic pen Only the first ultrasonic signal can be received before Then only the second ultrasonic signal can be received.

5. The two-dimensional multi-target ultrasonic positioning method according to claim 4, characterized in that: When any ultrasonic pen starts working, it starts working when it receives the infrared signal emitted by the infrared emission module. Therefore, there are two situations when the ultrasonic pen starts working: (1) The ultrasonic pen starts working just when the infrared pulse is emitted. There is no delay at this time and it works normally. (2) When the ultrasonic pen is put into operation, it misses the rising edge of the infrared pulse or misses the entire infrared pulse. It is considered that the complete infrared pulse is not received and it needs to delay m to wait for the next cycle of infrared pulse to start working. <T。 6. A system for implementing a two-dimensional multi-target ultrasonic positioning method according to any one of claims 1 to 5, characterized in that: The system comprises a transmitting device and a receiving device, wherein the transmitting device comprises a first ultrasonic transmitting module, a second ultrasonic transmitting module, an infrared transmitting module, a processor, and a Bluetooth communication module host. The positions of the two ultrasonic transmitting modules are known, and the material is PVDF piezoelectric film, so that the ultrasonic transmission angle is 180 degrees, and the transmitted ultrasonic signals are ultrasonic waves with the same waveform and a frequency of f KHz. The two ultrasonic transmitting modules are installed on the edge of the liquid crystal screen; the infrared transmitting module, the first ultrasonic transmitting module, the second ultrasonic transmitting module, and the Bluetooth communication module host are connected to the processor; The receiving device includes n ultrasonic pens, each of which includes an ultrasonic receiving module, an infrared receiving module, a microprocessor, a clock module, and a Bluetooth communication module slave. The ultrasonic receiving module includes two ultrasonic sensors, which are installed at the bottom of the ultrasonic pen. From a cross-section, the angle between the installation position and the center of the circle is ninety degrees. When in use, it is pointed towards the transmitting device. The ultrasonic receiving module, infrared receiving module, clock module, and Bluetooth communication module slave are connected to the microprocessor.

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