JP6445486B2 - Sensor terminal - Google Patents

Description

  The present invention relates to a sensor terminal that measures physical quantities such as acceleration, angular acceleration, and electrocardiographic waveform and wirelessly transmits sensor data.

  In an IoT (Internet of Things) society in which all devices are connected to the Internet, various sensors are connected to the network, collecting a large amount of data and analyzing the data to extract useful information for humans. Expected. Specifically, the use of data that requires real-time analysis to some extent, such as acceleration, angular acceleration, and electrocardiographic waveform, has been expanded based on recent advances in wireless technology.

  Among these, short-range wireless communication technology for transmitting data from a sensor terminal to a terminal located at a short distance has come to be used by the advent of BLE (Bluetooth (registered trademark) Low Energy). In this case, in order to transmit data to a network such as a cloud, it is necessary to relay the data in some form and connect to the network. For example, in the example of FIG. 10, the sensor relay device 101 transmits data from the sensor terminals 100-1 to 100 -N to the server device 102 via the network 103. However, unlike the short-range wireless communication between the sensor terminals 100-1 to 100 -N and the sensor relay device 101, the middle / long-range wireless communication between the sensor relay devices 101 or the middle / long-range wireless communication between the sensor relay device 101 and the network 103. Generally have problems such as low throughput, high cost, and high power consumption. Therefore, some processing needs to be performed by the sensor relay device 101.

  Under such circumstances, an increase in the amount of data when the above-mentioned various real-time data is collected and transmitted wirelessly by a sensor terminal is a big issue, and currently, lossy compression and run-time such as MPEG (Moving Picture Experts Group) are in progress. Lossless compression represented by length codes and the like is used (see Non-Patent Document 1).

"Knowledge Base Knowledge Forest Group 2-5-5 Chapter 11 International Standard", IEICE, 2013, Internet <http://www.ieice-hbkb.org/files/02/02gun_05hen_11.pdf>

  However, when data compression is performed using the technique disclosed in Non-Patent Document 1, there is a problem that the calculation is complicated because the compression method is based on abundant hardware resources such as a PC. In addition, in this data compression technique, a certain amount of data length is required for calculation, and thus there is a problem that there is a high possibility of losing real-time properties.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a sensor terminal capable of realizing data compression that can be performed even by a low-performance microcomputer and realizing efficient real-time transmission. And

The sensor terminal of the present invention includes a sensor circuit for measuring a physical quantity, an AD converter for converting an analog signal output from the sensor circuit into digital data, and delaying the digital data output from the AD converter by one sample. A sample delay element, a subtraction circuit for calculating difference data between the digital data output from the AD converter and the digital data of one sample before output from the one sample delay element, and the difference data and the sign are associated with each other A compression dictionary that is stored in advance, an encoding unit that performs encoding by replacing difference data output from the subtraction circuit with a corresponding code stored in the compression dictionary, and a code output from the encoding unit And an RF circuit that converts the signal into an RF signal and wirelessly transmits the signal.

The sensor terminal of the present invention includes a sensor circuit for measuring a physical quantity, an AD converter for converting an analog signal output from the sensor circuit into digital data, and delaying the digital data output from the AD converter by one sample. A 1-sample delay element to be performed, a subtracting circuit for calculating difference data between the digital data output from the AD converter and the 1-sample previous digital data output from the 1-sample delay element, and a measurement start time of the sensor terminal The initial setting timer counter that measures the elapsed time of the data, and the frequency of appearance of the difference data output from the subtraction circuit when the output of the initial setting timer counter indicates that it is during a predetermined initial setting period Appearance frequency memory to be stored every time, and the pressure for storing the correspondence between the difference data and the sign Dictionary and a dictionary creation unit for setting the compression dictionary to assign a short code to occurrence frequent order of the initial setting period difference data stored in the appearance frequency memory during, during pre Symbol initialization period The difference data output from the subtraction circuit is output as it is, and after the output of the initial setting timer counter indicates the end of the initial setting period, the difference data output from the subtraction circuit is stored in the compression dictionary. an encoding unit for performing encoding by replacing the corresponding code, before Symbol default duration wirelessly transmits the converted differential data output from the encoding unit into an RF signal, the end of the initialization period Thereafter, an RF circuit that converts a code output from the encoding unit into an RF signal and wirelessly transmits the RF signal is provided .

The sensor terminal of the present invention includes a sensor circuit for measuring a physical quantity, an AD converter for converting an analog signal output from the sensor circuit into digital data, and delaying the digital data output from the AD converter by one sample. 1 sample delay element to be processed, a subtraction circuit for calculating difference data between the digital data output from the AD converter and the digital data of one sample before output from the 1 sample delay element, and the difference data and the sign A compression dictionary that is stored in advance, an appearance frequency memory that stores the appearance frequency of the difference data output from the subtraction circuit for each data, and a series that stores in advance the maximum value of the number of retained difference data in the appearance frequency memory The number of difference data stored in the retention number initial setting unit and the appearance frequency memory is the sequence retention When the maximum value preset in the initial setting unit is reached, the oldest data stored in the appearance frequency memory is erased, and the appearance frequency of the difference data according to the stored contents of the appearance frequency memory after data erasure A dictionary creation unit that updates the compression dictionary so as to assign a short code in descending order, and a code that performs coding by replacing the difference data output from the subtraction circuit with a corresponding code stored in the compression dictionary And an RF circuit that converts a code output from the encoding unit into an RF signal and wirelessly transmits the RF signal .

  Further, in one configuration example of the sensor terminal of the present invention, when the detection signal indicating that the packet storing the code output from the encoding unit is full is output from the RF circuit, A reset signal generation unit that outputs a reset signal for resetting the one-sample delay element to an initial state is provided.

  According to the present invention, by providing a one-sample delay element and a subtracting circuit, efficient real-time transmission can be realized, and data compression that can be performed even with a low-performance microcomputer can be realized.

  In the present invention, data can be compressed and transmitted by further providing a compression dictionary and an encoding unit.

  Further, in the present invention, by providing an initial setting timer counter, an appearance frequency memory, a compression dictionary, a dictionary creation unit, and an encoding unit, it is possible to set a compression dictionary according to a subject to be measured by initial setting. Therefore, better data compression becomes possible.

  Further, in the present invention, by providing the compression dictionary, the appearance frequency memory, the sequence retention number initial setting unit, the dictionary creation unit, and the encoding unit, the compression dictionary can be dynamically updated, which is better. It is possible to achieve compression performance.

  Further, in the present invention, by providing the reset signal generation unit, it is possible to avoid the problem that the restoration error continuously occurs on the receiving side, and it is possible to perform data transmission accurately.

It is a histogram of the difference value of sensor data. It is a block diagram which shows the structure of the sensor terminal which concerns on the 1st Embodiment of this invention. It is a block diagram which shows the structure of the sensor terminal which concerns on the 2nd Embodiment of this invention. It is a block diagram which shows the structure of the sensor terminal which concerns on the 3rd Embodiment of this invention. It is a block diagram which shows the structure of the sensor terminal which concerns on the 4th Embodiment of this invention. It is a block diagram which shows the structure of the sensor terminal which concerns on the 5th Embodiment of this invention. It is a figure which shows the state by which the data were stored in the payload of the packet in the 5th Embodiment of this invention. It is a block diagram which shows the structure of the sensor terminal which concerns on the 6th Embodiment of this invention. It is a block diagram which shows the structure of the sensor terminal which concerns on the 7th Embodiment of this invention. It is a block diagram which shows the structure of the conventional sensing system.

[First Embodiment]
In solving the above-described problems, the sensor terminal which is the subject of the present invention focuses on sensing physical quantities related to the natural world and the human body. As a feature of physical quantities related to the natural world and the human body, there is a point that these physical quantities have little change because of their constancy. Therefore, when sampling the data of these physical quantities and taking the primary difference, the appearance frequency of the difference draws a histogram having a spread centering on the difference value 0 as shown in FIG. 0 indicates that there is no difference. If there is a difference, it is interpreted that the amount of information is small. When the amount of information is small, an allocation with a small amount of information (bit) represented by “1” and “0” is sufficient. Therefore, the present invention proposes an encoding method that can be calculated even by a low-performance microcomputer that is built in real time and has a large amount of calculation based on the difference calculation of sensor data.

  FIG. 2 is a block diagram showing the configuration of the sensor terminal according to the first embodiment of the present invention. The sensor terminal includes, for example, a sensor circuit 1 that measures a physical quantity such as acceleration, angular acceleration, and an electrocardiogram waveform, and an analog front end circuit that performs processing such as amplification and noise removal of an analog signal output from the sensor circuit 1 (Analog Front End (hereinafter referred to as AFE) 2, an analog to digital converter (hereinafter referred to as ADC) 3 that converts an analog signal output from AFE 2 into digital data, and digital data output from ADC 3 for one sample (sample of ADC 3) 1 sample delay element 4 delayed by one clock cycle), difference data between digital data (sensor data) output from ADC 3 and digital data of one sample before output from 1 sample delay element 4 are calculated. The subtraction circuit 5 and the difference data output from the subtraction circuit 5 are converted into an RF (Radio Frequency) signal. An RF circuit 6 that converts and transmits wirelessly is configured.

Since the sensor circuit 1, the AFE 2, the ADC 3, and the RF circuit 6 are conventionally provided in the sensor terminal, an additional configuration necessary in the present embodiment is a one-sample delay element 4 configured by a D-flip flop array. And only the subtracting circuit 5.
If certain data D _N, 1 sample data before the D _N-1, the difference data D _N -D _N-1 is less than D _N, also D _N at the receiving side, D _N-1 and D _N - Needless to say, it can be restored from _DN-1 .

Since natural physical quantities and human body data have the characteristic that the amount of change is very small as described above, efficient transmission is possible according to this embodiment.
The configuration of the present embodiment is not dependent on what radio system the RF circuit 6 employs, and can be applied regardless of what radio system is employed.
In the present invention, the mounting of the AFE 2 is not an indispensable constituent element. For example, the sensor circuit 1 may include the AFE.

[Second Embodiment]
Next, a second embodiment of the present invention will be described. FIG. 3 is a block diagram showing a configuration of a sensor terminal according to the second embodiment of the present invention, and the same reference numerals are given to the same configurations as those in FIG. The sensor terminal according to the present embodiment has a fixed circuit that stores the sensor circuit 1, the AFE 2, the ADC 3, the one-sample delay element 4, the subtracting circuit 5, the RF circuit 6, the difference data, and the code in advance in association with each other. The type compression dictionary 7 is provided between the subtraction circuit 5 and the RF circuit 6 and includes an encoding unit 8 that encodes difference data by referring to the fixed type compression dictionary 7.

In the fixed compression dictionary 7 according to the present embodiment, a variable-length code whose code length becomes shorter as the output data (difference data) of the subtraction circuit 5 is closer to 0 is associated in advance with a possible value of the difference data. It is registered.
The encoding unit 8 performs encoding by replacing the difference data output from the subtraction circuit 5 with a corresponding code registered in advance in the fixed compression dictionary 7. The RF circuit 6 of the present embodiment converts the code output from the encoding unit 8 into an RF signal and wirelessly transmits it.

  According to the configuration of the present embodiment, since a short code is assigned to data close to the difference value 0, the code length can be shortened as a whole. That is, data can be compressed. The most suitable method for the compression method according to the present embodiment is Huffman code encoding, and the Huffman code is also a compact code. Decoding is possible without doing so. This characteristic is particularly effective for data that requires real-time characteristics, and is suitable for transmission of sensor data that is clearly associated with an event such as an electrocardiographic waveform and acceleration. In addition, the present embodiment can be realized with a small amount of hardware resources because the memory for storing the fixed compression dictionary 7 and the encoding unit 8 need only be implemented in addition to the configuration of the first embodiment. Is possible.

[Third Embodiment]
Next, a third embodiment of the present invention will be described. FIG. 4 is a block diagram showing the configuration of a sensor terminal according to the third embodiment of the present invention. The same components as those in FIGS. 2 and 3 are given the same reference numerals. The sensor terminal of the present embodiment has a predetermined initial setting period from the measurement start time of the sensor circuit 1, the AFE 2, the ADC 3, the one sample delay element 4, the subtraction circuit 5, the RF circuit 6, and the sensor terminal. Initial setting timer counter 9 for measuring the time until elapse, switches 10 and 11 controlled by the output of initial setting timer counter 9, and output data (difference data) of subtraction circuit 5 during the initial setting period Appearance frequency memory 12 that stores appearance frequency for each data, initial setting type compression dictionary 13 for storing difference data and code in association with each other, subtraction circuit 5 and RF circuit 6 are provided between The difference data output from the subtraction circuit 5 is output as it is during the setting period, and the difference data is encoded by referring to the initial setting compression dictionary 13 after the end of the initial setting period. An encoding unit 14 that performs, and a dictionary preparing section 15 for setting an initial setting type compression dictionary 13 to assign a short code during the initial setting period to a higher order of occurrence frequency of the difference data.

In the present embodiment, unlike the fixed compression dictionary 7 of the second embodiment, the correspondence between the difference data and the code is not registered in the initial setting compression dictionary 13.
The initial setting timer counter 9 of the present embodiment measures the elapsed time from the measurement start time of the sensor terminal.

  The switches 10 and 11 are controlled by the output of the initial setting timer counter 9 and operate complementarily. Specifically, the switch 10 is turned on when the output of the initial setting timer counter 9 indicates that it is in the initial setting period, and the output of the initial setting timer counter 9 has ended the initial setting period (timeout ) Is turned off. Conversely, the switch 11 is turned off when the output of the initial setting timer counter 9 indicates that the initial setting period is in progress, and when the output of the initial setting timer counter 9 indicates that the initial setting period has ended. Turned on.

The appearance frequency memory 12 stores the appearance frequency of the output data (difference data) of the subtraction circuit 5 input via the switch 10 during the initial setting period for each data.
The dictionary creation unit 15 sets the initial setting type compression dictionary 13 so that short codes (for example, Huffman codes) are assigned in descending order of appearance frequency of the difference data stored in the appearance frequency memory 12 during the initial setting period. In this way, the correspondence between the difference data and the code is registered in the compression dictionary 13.

  Since the encoding unit 14 cannot refer to the initial setting compression dictionary 13 by turning off the switch 11 during the initial setting period, it outputs the difference data output from the subtraction circuit 5 to the RF circuit 6 as it is, and the initial setting period ends. When the switch 11 is turned on, encoding is performed by replacing the difference data output from the subtraction circuit 5 with the corresponding code registered in the initial setting type compression dictionary 13. The RF circuit 6 of the present embodiment converts the difference data output from the encoding unit 14 into an RF signal during the initial setting period and wirelessly transmits it, and is output from the encoding unit 14 after the end of the initial setting period. The code is converted into an RF signal and transmitted wirelessly.

  According to the present embodiment, the compression dictionary can be initialized for each target data to be compressed, so that a compression dictionary suitable for the data type can be provided. In particular, when collecting human body data such as an electrocardiogram, it is difficult to have an optimal fixed compression dictionary from the beginning because there are individual differences in the contact impedance between the human body and electrodes and the signal voltage obtained. There is a feature. On the other hand, according to the present embodiment, it is possible to set a compression dictionary according to the subject to be measured by the initial setting, and therefore better data compression than the second embodiment. Is possible.

  In this embodiment, during the initial setting period, the difference data is converted into an RF signal and wirelessly transmitted. As described in the first embodiment, the sensor data can be easily restored from the difference data on the receiving side. In addition, after the initial setting period, the code is converted into an RF signal and wirelessly transmitted. However, as described in the second embodiment, the differential data can be easily decoded from the code on the receiving side.

[Fourth Embodiment]
Next, a fourth embodiment of the present invention will be described. FIG. 5 is a block diagram showing a configuration of a sensor terminal according to the fourth embodiment of the present invention. The same components as those in FIGS. The sensor terminal of the present embodiment has a maximum value of the number of difference data held in the sensor circuit 1, the AFE 2, the ADC 3, the one sample delay element 4, the subtraction circuit 5, the RF circuit 6, and the appearance frequency memory. A sequence retention number initial setting unit 16 to be stored in advance, an appearance frequency memory 17 to store the appearance frequency of the difference data for each data, a dynamic variable compression dictionary 18 to store the difference data and the code in association with each other, The encoding unit 19 that encodes the difference data by referring to the dynamic variable compression dictionary 18 and the number of difference data stored in the appearance frequency memory 17 are preset in the sequence holding number initial setting unit 16. When the maximum value is reached, the oldest data stored in the appearance frequency memory 17 is deleted, and the dictionary creation unit 20 is updated to update the dynamic variable compression dictionary 18.

  In the present embodiment, as in the fixed compression dictionary 7 of the first embodiment, the correspondence between the difference data and the code is registered in advance in the dynamic variable compression dictionary 18. However, in the present embodiment, the dynamic variable compression dictionary 18 is dynamically updated.

Specifically, the maximum value of the number of difference data that can be stored in the appearance frequency memory 17 is preset in the series retention number initial setting unit 16.
The dictionary creation unit 20 constantly updates the dynamic variable compression dictionary 18 so as to assign a short code (for example, a Huffman code) in order of appearance frequency of the difference data stored in the appearance frequency memory 17, but the appearance frequency memory When the number of difference data stored in 17 reaches the maximum value preset in the sequence retention number initial setting unit 16, the oldest difference data stored in the appearance frequency memory 17 and its appearance frequency are deleted. .

Since the appearance frequency of the difference data is changed by deleting the data stored in the appearance frequency memory 17, the dictionary creation unit 20 stores the difference data according to the stored contents of the appearance frequency memory 17 after the data deletion. The dynamic variable compression dictionary 18 is updated so that short codes are assigned in order of appearance frequency.
The encoding unit 19 performs encoding by replacing the difference data output from the subtraction circuit 5 with a corresponding code registered in the dynamic variable compression dictionary 18. The RF circuit 6 converts the code output from the encoding unit 19 into an RF signal and wirelessly transmits it.

  According to the present embodiment, it is possible to dynamically update the compression dictionary, so that it is possible to achieve better compression performance than the second and third embodiments.

[Fifth Embodiment]
Next, a fifth embodiment of the present invention will be described. FIG. 6 is a block diagram showing the configuration of a sensor terminal according to the fifth embodiment of the present invention. The same components as those in FIGS. The sensor terminal according to the present embodiment is obtained by adding a reset signal generator 21 to the sensor terminal according to the second embodiment shown in FIG.

  When, for example, BLE is adopted as the short-range wireless communication standard of the RF circuit 6 of the second embodiment, there is a limit on the length of a packet transmitted and received by wireless communication. As described above, when the packet length is limited, the RF circuit 6 fills the payload of the packet storing the code output from the encoding unit 8 and becomes unable to store the code. A packet full detection signal is output.

The reset signal generation unit 21 of the present embodiment outputs a reset signal to the one-sample delay element 4 when the packet full detection signal is output from the RF circuit 6.
By the output of this reset signal, the one sample delay element 4 is reset to the initial state, and the output of the one sample delay element 4 becomes zero. That is, since the difference calculation of the output data (sensor data) of the ADC 3 by the 1-sample delay element 4 and the subtraction circuit 5 becomes invalid once the 1-sample delay element 4 is reset, the beginning of the payload of the packet immediately after the packet Full detection signal is output Stores the reference data, that is, the code obtained by encoding the sensor data as it is without performing the difference calculation.

  Since the difference calculation of the sensor data by the one-sample delay element 4 and the subtraction circuit 5 is resumed in the sampling period after the reference data, a code obtained by encoding the difference data is stored after the reference data of the packet. FIG. 7 shows a state in which data is stored in the payload of the packet in this embodiment. As described above, the reference data 200 is stored at the beginning of the payload, and the encoded data 201 is stored after the reference data 200. Since the length of the encoded data 201 varies depending on the difference data, the number of encoded data 201 after the reference data 200 depends on the length of the payload and the difference data.

  In the first to fourth embodiments, the reference data is stored only at the beginning of the payload of the first packet at the start of measurement by the sensor terminal. For this reason, once a bit error occurs on the communication path, the sensor data cannot be correctly restored on the receiving side that receives the RF signal from the sensor terminal. There is a problem that a restoration error occurs.

  In contrast, in this embodiment, when a packet error occurs, a sensor data restoration error occurs for the packet and data loss occurs. However, if the next packet is error-free, the payload of this packet Since the sensor data can be correctly restored using the reference data stored at the head of the data, it is possible to avoid the problem of differential encoding in which restoration errors occur continuously, and to accurately transmit the data Can do. The present embodiment is particularly suitable for a wireless communication standard such as BLE in which the maximum packet size is set low.

[Sixth Embodiment]
Next, a sixth embodiment of the present invention will be described. FIG. 8 is a block diagram showing the configuration of a sensor terminal according to the sixth embodiment of the present invention. The same components as those in FIGS. The sensor terminal according to the present embodiment is obtained by adding a reset signal generator 21 to the sensor terminal according to the third embodiment shown in FIG.

The operations of the RF circuit 6, the reset signal generator 21, and the one-sample delay element 4 are as described in the fifth embodiment.
As described in the third embodiment, since encoding is not performed during the initial setting period, in this embodiment, sensor data is directly stored as reference data at the beginning of the payload of the packet during the initial setting period. The difference data is stored after the reference data. In addition, after the end of the initial setting period, encoded data of sensor data is stored as reference data at the beginning of the payload, and encoded data of difference data is stored after the reference data.

  Thus, in this embodiment, in addition to the effect of the third embodiment, it is possible to obtain an effect that it is possible to accurately transmit data with a limited packet length.

[Seventh Embodiment]
Next, a seventh embodiment of the present invention will be described. FIG. 9 is a block diagram showing a configuration of a sensor terminal according to the seventh embodiment of the present invention. The same reference numerals are given to the same configurations as those in FIGS. The sensor terminal according to the present embodiment is obtained by adding a reset signal generator 21 to the sensor terminal according to the fourth embodiment shown in FIG.

The operations of the RF circuit 6, the reset signal generator 21, and the one-sample delay element 4 are as described in the fifth embodiment.
Thus, in this embodiment, in addition to the effect of the fourth embodiment, it is possible to obtain an effect that it is possible to accurately transmit data with a limited packet length.

  Among the configurations described in the first to seventh embodiments, at least the compression dictionaries 7, 13, 18, the encoding units 8, 14, 19, the initial setting timer counter 9, the switches 10, 11, the appearance frequency memory 12, 17, dictionary creation units 15 and 20, sequence retention number initial setting unit 16, reset signal generation unit 21, and part of the RF circuit 6 include a CPU (Central Processing Unit), a computer having a storage device and an interface, It can be realized by a program that controls hardware resources. The CPU executes the processes described in the first to seventh embodiments in accordance with a program stored in the storage device.

  The present invention can be applied to a sensing system that collects sensor data from a sensor terminal.

  DESCRIPTION OF SYMBOLS 1 ... Sensor circuit, 2 ... Analog front end circuit, 3 ... AD converter, 4 ... 1 sample delay element, 5 ... Subtraction circuit, 6 ... RF circuit, 7 ... Fixed compression dictionary, 8, 14, 19 ... Encoding part , 9 ... Initial setting timer counter, 10, 11 ... Switch, 12, 17 ... Appearance frequency memory, 13 ... Initial setting type compression dictionary, 15, 20 ... Dictionary creation unit, 16 ... Sequence retention number initial setting unit, 18 ... Dynamic Variable compression dictionary, 21... Reset signal generator.

Claims (6)

A sensor circuit for measuring physical quantities;
An AD converter that converts an analog signal output from the sensor circuit into digital data;
A one-sample delay element that delays the digital data output from the AD converter by one sample;
A subtraction circuit for calculating difference data between the digital data output from the AD converter and the digital data of one sample before output from the one-sample delay element;
A compression dictionary that stores difference data and a code in association with each other;
An encoding unit that performs encoding by replacing the difference data output from the subtraction circuit with a corresponding code stored in the compression dictionary;
An RF circuit that converts a code output from the encoding unit into an RF signal and wirelessly transmits the sensor terminal. A sensor circuit for measuring physical quantities;
An AD converter that converts an analog signal output from the sensor circuit into digital data;
A one-sample delay element that delays the digital data output from the AD converter by one sample;
A subtraction circuit for calculating difference data between the digital data output from the AD converter and the digital data of one sample before output from the one-sample delay element;
An initial setting timer counter for measuring the elapsed time from the measurement start time of the sensor terminal;
An appearance frequency memory for storing, for each data, an appearance frequency of difference data output from the subtraction circuit when the output of the initial setting timer counter indicates that it is during a predetermined initial setting period;
A compression dictionary for storing the correspondence between the difference data and the code;
A dictionary creation unit that sets the compression dictionary so as to assign a short code to the order of appearance frequency of the difference data stored in the appearance frequency memory during the initial setting period;
The difference data output from the subtraction circuit is output as it is during the initial setting period, and the difference data output from the subtraction circuit is output after the output of the initial setting timer counter indicates the end of the initial setting period. An encoding unit that performs encoding by replacing the corresponding code stored in the compression dictionary;
During the initial setting period, the difference data output from the encoding unit is converted into an RF signal and wirelessly transmitted. After the initial setting period, the code output from the encoding unit is converted into an RF signal. And an RF circuit for wireless transmission . A sensor circuit for measuring physical quantities;
An AD converter that converts an analog signal output from the sensor circuit into digital data;
A one-sample delay element that delays the digital data output from the AD converter by one sample;
A subtraction circuit for calculating difference data between the digital data output from the AD converter and the digital data of one sample before output from the one-sample delay element;
A compression dictionary that stores difference data and a code in association with each other;
An appearance frequency memory for storing the appearance frequency of difference data output from the subtraction circuit for each data;
A series retention number initial setting unit for preliminarily storing a maximum value of the difference data retention number of the appearance frequency memory;
When the number of difference data stored in the appearance frequency memory reaches the maximum value preset in the sequence retention number initial setting unit, the oldest data stored in the appearance frequency memory is deleted, and data is deleted. A dictionary creation unit that updates the compression dictionary so as to assign a short code in order of high appearance frequency of difference data according to the storage content of the later appearance frequency memory;
An encoding unit that performs encoding by replacing the difference data output from the subtraction circuit with a corresponding code stored in the compression dictionary;
An RF circuit that converts a code output from the encoding unit into an RF signal and wirelessly transmits the sensor terminal. The sensor terminal according to claim 1 ,
Further, when a detection signal indicating that the packet storing the code output from the encoding unit is full is output from the RF circuit, a reset signal for resetting the one-sample delay element to an initial state is provided. A sensor terminal comprising a reset signal generator for outputting. The sensor terminal according to claim 2 , wherein
Further, when a detection signal indicating that the differential data or code storing the code output from the encoding unit is full is output from the RF circuit, the one-sample delay element is reset to an initial state. A sensor terminal comprising a reset signal generator for outputting a reset signal. The sensor terminal according to claim 3 , wherein
Further, when a detection signal indicating that the packet storing the code output from the encoding unit is full is output from the RF circuit, a reset signal for resetting the one-sample delay element to an initial state is provided. A sensor terminal comprising a reset signal generator for outputting.

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