Patent Publication Number: US-2022236116-A1

Title: Daisy chain two-wire sensor measurement system and method

Description:
CROSS REFERENCE TO THE RELATED APPLICATIONS 
     This application is the national phase entry of International Application No. PCT/CN2021/101173, filed on Jun. 21, 2021, which is based upon and claims priority to Chinese Patent Application No. 202010576139.6, filed on Jun. 22, 2020, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to the field of communication technologies, and more particularly, to a daisy chain two-wire sensor measurement system and method. 
     BACKGROUND 
     Most of prior sensor networks adopt a bus topology, such as the RS485 network technology. Each node has two communication wires, and further has two power wires when being supplied by using a bus, and each node has an independent address. This type of network supports two-way communication, data uploading, and delivery of a control command, and is suitable for a general measurement and control system. However, in this type of network, the bus includes four wires, causing complex cabling and high wire and hardware costs. Each node needs an independent address, so the protocol is complex. Moreover, the system installation and debugging are inconvenient. 
     Maxim company&#39;s temperature measurement chip DS18B20, which is widely used in the market, adopts a one-wire sensor bus. In addition to two power wires, only one communication bus is required. All chips are connected by the three same wires to complete data communication, and each chip has a unique independent identification (ID) number. In this way, the cabling is simplified to a certain extent. However, the three wires are still required, and are distinguished by the ID number, while the ID number needs to be read during installation and deployment, resulting in inconvenient replacement. 
     As a common communication topology, a daisy chain network can be applied to a sensor network. In the daisy chain network, each node has two pairs of transceivers, and is connected to two communication wires. Each communication wire is connected to two nodes, and the nodes are connected to each other hand in hand. Data of a node is forwarded by an adjacent node. Therefore, the daisy chain network does not need an address. However, the node in the daisy chain network still needs an independent power wire, and thus needs two power wires and two communication interfaces. If one wire is adopted for communication, each node also needs to be connected to four external wires, and data transmission from an end node to a master node requires a lot of forwarding operations. As a result, data may be easily lost, and it is unsuitable to construct a large-scale network. 
     In some battery management applications, such as a container-specific lithium battery energy storage system, there are often hundreds of packs of batteries connected in series, and each pack of batteries is composed of many lithium batteries connected in parallel. For battery safety, it is necessary to measure temperature of these batteries. In the prior solution, lots of sensors and communication buses are required, resulting in complex connection and making it difficult to perform effective management. In view of this, the present invention provides a daisy chain two-wire sensor measurement system and method. 
     SUMMARY 
     In order to solve the problems in the prior art, the present invention provides a daisy chain two-wire sensor measurement system and method. 
     The present invention is implemented by the following technical solutions. 
     According to a first aspect, the present invention provides a daisy chain two-wire sensor measurement system, including: 
     one or more slave modules, connected to one or more sensors, and configured to detect a sensor signal and modulate the detected sensor signal into a corresponding current signal; 
     a master module, successively cascaded to a first slave module, an upper-stage slave module, and a lower-stage slave module in the one or more slave modules through a daisy chain wire to form a daisy chain sensor network, and interconnected with an upper computer through a communication wire; and 
     the daisy chain wire, configured to supply power and transmit the current signal obtained by the slave modules through modulation, such that the master module obtains and then decodes the current signal on the daisy chain wire to obtain corresponding sensor data. 
     Further, the master module includes a power interface, a master lower switch, a master lower interface, a signal demodulation circuit, a master voltage stabilizing circuit, a master controller, and a communication interface. 
     Further, the power interface is connected to an external power supply and the master voltage stabilizing circuit. The master voltage stabilizing circuit is connected to the master controller, and configured to stabilize an input supply voltage and output a stabilized supply voltage to the master controller and a peripheral circuit of the master controller. 
     Further, the master lower switch is connected to the master controller and connected in series on the daisy chain wire, and controls conduction of a slave upper interface of the first slave module through the master lower interface. 
     Further, the signal demodulation module is connected to the master controller, and configured to decode the current signal on the daisy chain wire to obtain the corresponding sensor data. 
     Further, the communication interface is connected to the upper computer, and transmits the sensor data to the upper computer. 
     Further, the slave module includes a slave upper interface, a slave lower interface, a slave voltage stabilizing circuit, a signal modulation circuit, a slave controller, and a slave lower switch. The slave module further includes an anti-reverse connection circuit to prevent a circuit of the slave module from being damaged when the daisy chain wire is connected reversely. 
     Further, the slave upper interface of the lower-stage slave module is connected to the slave lower interface of the upper-stage slave module, or the slave upper interface of the first slave module is connected to the master lower interface of the master module. 
     A slave lower bus switch is connected between the slave upper interface and the slave lower interface to control conduction of the lower-stage slave module; 
     The slave voltage stabilizing circuit is connected between the slave upper interface and the slave controller, configured to stabilize a voltage input by a daisy chain bus and output the stabilized voltage to the slave controller and a peripheral circuit of the slave controller for power supply, and further configured to shield interference to the slave controller caused by signal transmission when the slave module transmits data. 
     The signal modulation circuit is connected between the slave upper interface and the slave controller, and configured to modulate the sensor signal into the corresponding current signal and transmit the current signal to the master module through the daisy chain wire. 
     Further, the signal modulation circuit uses a light-emitting diode to perform signal modulation, and displays a working status of the slave module during data transmission. 
     Further, the slave module includes an internal sensor and an external sensor. a slave controller is connected to the internal sensor and the external sensor to detect the internal sensor and the external sensor. The internal sensor is configured to obtain an internal parameter of the slave module. The external sensor includes but is not limited to one or more of a temperature sensor, an ultrasonic sensor, an acceleration sensor, a gas smoke sensor, a humidity sensor, a vibration sensor, a photosensitive sensor, or a Hall switch sensor. 
     According to a second aspect, the present invention provides a daisy chain two-wire sensor measurement method, where the measurement method uses the daisy chain two-wire sensor measurement system described in the first aspect, and includes the following steps: 
     S 1 : initializing the system, activating sampling the sensor signal, and encoding the collected sensor signal; 
     S 2 : modulating the sensor signal into the corresponding current signal, and sending the current signal to the daisy chain wire; 
     S 3 : decoding the current signal in S 2  to obtain the corresponding sensor data; and 
     S 4 : determining, based on the sensor data, whether a faulty slave module or abnormal sensor data occurs. 
     Further, in the method, data transmission and fault location of a single slave module include the following time periods: 
     T 0  represents a time period in which a bus is turned off, and is controlled by a master module, where in this time period, the master module turns off a master lower switch, and all slave modules are in a power-off state; 
     T 1  represents a time period for synchronization, where in this time period, the master module already turns on the master lower switch, and the slave modules are powered on successively; a duration of T 1  is determined by the slave modules; the slave modules prepare for data transmission after being powered on and completing sensor sampling within T 1 , where an upper limit of the duration of T 1  is T 1 max; and 
     T 2  represents a time period for data transmission, where in this time period, the slave modules complete collection of the sensor data, process and transmit the data, where an upper limit of a duration of T 2  is T 2 max. 
     Further, in the method, data transmission and fault location of N slave modules include the following steps: 
     S 1 : turning on the master lower switch by the master module, terminating T 0 , and activating timing for T 1 , where n is set to 1; 
     S 2 : if the master module does not receive any data within T 1 max, determining that n slave modules fail to perform reading, and jumping to SE; 
     S 3 : activating timing for T 2  after the master module receives data, terminating all data, and waiting for completion of data transmission; 
     S 4 : if T 2 &gt;T 2 max, determining that a data error occurs on an N th  slave module, and jumping to SE; 
     S 5 : if a check error exists in the data, jumping to SE; if a fault indicating that a sensor is open-circuited or short-circuited exists in the data, jumping to S 6 ; 
     S 6 : setting n to n+1, and activating timing for T 1 ; 
     S 7  jumping to S 2 ; and 
     SE: if n=N+1, indicating that data of all the slave modules is received and data transmission is completed, turning off the master lower switch by the master module; if n&lt;N+1, determining that a communication fault occurs on an n th  slave module. 
     The present invention has the following advantages. 
     In the present invention, the master module and the plurality of slave sensor detection modules are successively connected in series through the power wire to form the daisy chain sensor monitoring network, thereby realizing easy expansion. A wire is shared for power supply and communication, in other words, only the power wire is employed to supply power for the master module and the plurality of slave sensor detection modules and transmit data, thereby simplifying on-site cabling. The signal is transmitted in a current mode, such that an anti-interference capability is strengthened and no address encoding is required. In addition, the present invention also has advantages of low cost and low power consumption. 
     In the present invention, a special daisy chain structure is adopted. Data of the lower-stage slave sensor detection module is directly sent to the master module without being forwarded by an upper-stage slave sensor detection module. Each slave sensor detection module is only configured for sampling, data transmission, and controlling the lower bus switch, thereby imposing a low requirement for the slave controller and facilitating implementation at a low cost. 
     In the present invention, the two-wire bus is adopted, and the cabling is convenient. This technical solution can also be used for other sensor monitoring system. Each slave sensor detection module can also be connected to a plurality of external sensors, and one data transmission channel is shared, thereby further reducing the system cost and simplifying the device installation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To describe the technical solutions in the embodiments of the present invention or in the prior art more clearly, the following briefly describes the drawings required for describing the embodiments or the prior art. Apparently, the drawings in the following description show merely some embodiments of the present invention, and those having ordinary skill in the art may still obtains other drawings from these drawings without creative efforts. 
         FIG. 1  is a schematic diagram of the structure of a daisy chain two-wire sensor measurement system according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of the structure of a slave module according to an embodiment of the present invention; 
         FIG. 3  is a circuit diagram of the slave module according to an embodiment of the present invention; 
         FIG. 4  is a schematic diagram of the structure of a master module according to an embodiment of the present invention; 
         FIG. 5  is a circuit diagram of the master module according to an embodiment of the present invention; 
         FIG. 6  is a flow chart of a daisy chain two-wire temperature measurement method according to an embodiment of the present invention; 
         FIG. 7  is a schematic diagram of a circuit structure used for battery temperature measurement according to an embodiment of the present invention; and 
         FIG. 8  is a timing diagram of a two-wire daisy chain sensor network system according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In order to make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are a part, rather than all, of the embodiments of the present invention. All other embodiments obtained by those having ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention. 
     Embodiment 1 
     As shown in  FIG. 1 , this embodiment provides a daisy chain two-wire sensor measurement system, including: 
     one or more slave modules, connected to one or more sensors, and configured to detect a sensor signal and modulate the detected sensor signal into a corresponding current signal; 
     a master module, successively cascaded to a first slave module, an upper-stage slave module, and a lower-stage slave module in the one or more slave modules through a daisy chain wire to form a daisy chain sensor network, and interconnected with an upper computer through a communication wire; and 
     the daisy chain wire, connected between the master module and the first slave module or between the upper-stage slave module and the lower-stage slave module, and configured to supply power and transmit the current signal obtained by the slave modules through modulation, such that the master module obtains and then decodes the current signal on the daisy chain wire to obtain corresponding sensor data. 
     In this embodiment, the master module includes a power interface, a master lower switch, a master lower interface, a signal demodulation circuit, a master voltage stabilizing circuit, a master controller, and a communication interface. 
     In this embodiment, the power interface is connected to an external power supply and the master voltage stabilizing circuit. The master voltage stabilizing circuit is connected to the master controller, and configured to stabilize an input supply voltage and output the stabilized supply voltage to the master controller and a peripheral circuit of the master controller. 
     In this embodiment, the master lower switch is connected to the master controller and connected in series on the daisy chain wire, and controls conduction of a slave upper interface of the first slave module through the master lower interface. 
     In this embodiment, the signal demodulation module is connected to the master controller, and configured to decode the current signal on the daisy chain wire to obtain the corresponding sensor data. 
     In this embodiment, the communication interface is connected to the upper computer, and transmits the sensor data to the upper computer. 
     In this embodiment, the slave module includes a slave upper interface, a slave lower interface, an anti-reverse connection circuit, a slave voltage stabilizing circuit, a signal modulation circuit, a slave controller, and a slave lower switch. 
     The slave upper interface of the lower-stage slave module is connected to the slave lower interface of the upper-stage slave module, or the slave upper interface of the first slave module is connected to the master lower interface of the master module. 
     A slave lower bus switch is connected between the slave upper interface and the slave lower interface to control conduction of the lower-stage slave module. 
     The slave voltage stabilizing circuit is connected between the slave upper interface and the slave controller, configured to stabilize a voltage input by a daisy chain bus and output the stabilized voltage to the slave controller and a peripheral circuit of the slave controller for power supply, and further configured to shield interference to the slave controller caused by signal transmission when the slave module transmits data. 
     The signal modulation circuit is connected between the slave upper interface and the slave controller, and configured to modulate the sensor signal into the corresponding current signal and transmit the current signal to the master module through the daisy chain wire. 
     In this embodiment, the signal modulation circuit uses a light-emitting diode to perform signal modulation, and displays a working status of the slave module during data transmission. During data transmission, the light-emitting diode blinks. 
     In this embodiment, the slave module includes an internal sensor and an external sensor. a slave controller is connected to the internal sensor and the external sensor to detect the internal sensor and the external sensor, and the internal sensor is configured to obtain an internal parameter of the slave module. 
     In this embodiment, the external sensor includes one or more of a temperature sensor, an ultrasonic sensor, an acceleration sensor, a gas smoke sensor, a humidity sensor, a vibration sensor, a photosensitive sensor, or a Hall switch sensor. 
     In this embodiment, the connection is simple. Only two wires are used to connect the master module and the first slave module, and connect the upper-stage slave module and the lower-stage slave module. 
     In this embodiment, data of any slave module is directly sent to the master module without being forwarded by an intermediate node, and the data transmission adopts the current signal, which is suitable for a long-distance and multi-node network system. 
     In this embodiment, the slave module does not need address encoding. No address field is needed in a protocol. It is convenient for installation, deployment, debugging, maintenance, and replacement. 
     In this embodiment, only a voltage stabilizing circuit, an analog-digital (AD) sampling circuit, a metal-oxide-semiconductor (MOS) switch, and a plurality of resistors are required in the circuit of the slave module, and there is no large capacitor and inducer. This makes it easy to realize an integrated circuit and further reduce a marginal cost. 
     In this embodiment, each node only works when it needs to transmit data, and is in a power down or sleep mode at other time. Power consumption of the module in the sleep mode is about 10 uA. Even for a network including 100 nodes, the average power consumption of the network is only 1 mA. 
     Embodiment 2 
     This embodiment provides a daisy chain topology. One-way communication in which a slave temperature detection module sends data to a master module is performed, and the slave temperature detection module is connected to a plurality of temperature sensors, which are usually distributed in an environment in which the temperature sensors are adjacent to each other. 
     For example, the temperature sensors are attached to different parts of an identical battery pack, so the temperature difference is small. Considering that the master module spends a lot of time in receiving a large amount of temperature sensor data, which affects the average data refreshing speed, this embodiment proposes a data compression algorithm. 
     In this embodiment, in order to further reduce the cost, volume and fault rate of the system, a slave controller of the slave temperature detection module may employ a low-precision oscillator as a clock source. 
     For example, an RC oscillator inside a chip is employed. For this purpose, this embodiment proposes a data transmission protocol that can tolerate a large clock error. 
     First, abnormality of the temperature sensor is screened out. If the temperature sensor is open-circuited, Tn=Tmax; if the temperature sensor is short-circuited, Tn=0. A value in the range (0, Tmax) is a measured value of the temperature sensor. For example, if Tn=1, corresponding temperature is −40° C.; if Tn=160, corresponding temperature is 120° C. 
     The minimum value Tmin of normal N temperature values of the sensors that are collected by the slave controller is calculated as a reference value, and Tn′=Tn−Tmin is used as a temperature difference. Taking four temperature sensors as an example, a corresponding transmission protocol consists of the following fields: 
     1. Measurement interval field. The measurement interval field specifies time for measuring a measured value of each temperature sensor after the slave temperature detection module is powered on, and a certain time interval maintained to divide a data waveform of data transmission of an upper-stage slave temperature detection module  12 . 
     2. Clock synchronization field. The slave temperature detection module uses its own clock as a reference to send 1010 in sequence to inform the master module of a clock frequency used by the slave temperature detection module. Based on the interval time of receiving the two numbers “1”, the master module determines the clock reference used by the slave temperature detection module. The width of a clock bit in this field is set as W. 
     3. Temperature reference value field. A binary code of Tmin is sent bit by bit at the interval of W If all the temperature sensors are short-circuited, then Tmin=00000000b, and a temperature difference field is empty. If all the temperature sensors are open-circuited, then Tmin=11111111b, and the temperature difference field is empty. If some temperature sensors are open-circuited, and the other temperature sensors are short-circuited, in other words, there is no normal temperature sensor, then Tmin=00000000b, and the temperature difference field is not empty. 
     4. Temperature difference field. N temperature differences Tn′ are sent in sequence according to the following encoding rule: if the temperature difference is less than 8, then 10b+TN′3 is sent (TN′3 indicates a 3-bit binary number); if the temperature difference is equal to 0, then 1100b is sent; if the temperature sensor is open-circuited, then 1110b is sent; if the temperature sensor is short-circuited, then 1101b is sent; if the temperature difference is greater than or equal to 8, then 01b+Tn′8 is sent. 
     5. Check field. A check result is calculated through parity check or in other check manner and sent. 
     In this embodiment, the clock synchronization field, the temperature reference value field, the temperature difference field, and the check field are collectively referred to as a data transmission field. 
     The protocol and the encoding rule in this embodiment define the fault types and codes of the temperature sensors, thereby facilitating debugging. There will not be 0000000000b in a code stream, so loss of synchronization will not occur easily. There will not be 1111111111b in the code stream, so loss of synchronization will not occur easily. It takes short time to transmit a fault code, such that a sensor fault has little impact on overall communication. The protocol implicitly specifies the clock used by the slave module, and thus can be used for low-precision clock system. Taking four temperature sensors as an example, in most cases, the protocol can save transmission time of data of 8-12 bits. Even if there is a large temperature difference, accurate transmission can still be realized. 
     Embodiment 3 
     In order to further reduce data transmission time, improve a data refreshing rate (for example, in a large energy storage system in which there are two hundred battery packs and each battery pack has ten single batteries connected in parallel, it takes a lot of time to transmit thousands of temperature values), discover an abnormal battery temperature as soon as possible, and improve safety, this embodiment employs a circuit structure shown in  FIG. 7 . 
     In  FIG. 7 , R 21 , R 24  and Q 22  constitute one signal modulation module  28 , and R 21   a , R 24   a  and Q 22   a  constitute another signal modulation module  28 . The two signal modulation modules  28  are connected in parallel and controlled by slave controllers  21  respectively. When on-resistances of Q 22  and Q 22   a  are ignored, assuming that R 21 &gt;R 21   a , the following four cases are available: 
     1. Q 22  and Q 22   a  are turned off, the total output current of the signal modulation modules  28  is 0 and denoted as I 0 , and the voltage of R 11  is 0. 
     2. Q 22   a  is turned off, Q 22  is turned on, and the total output current of the signal modulation modules  28  is equal to the current on R 21  and denoted as I 1 . 
     3. Q 22   a  is turned on, Q 22  is turned off, and the total output current of the signal modulation modules  28  is equal to the current on R 21   a  and denoted as I 2 . 
     4. Q 22   a  is turned on, Q 22  is turned on, and the total output current of the signal modulation modules  28  is equal to the sum of the current on R 21   a  and the current on R 21  and denoted as I 3 . 
     In this embodiment, since R 21 &gt;R 21   a , I 0 &lt;I 1 &lt;I 2 &lt;I 3 . By selecting appropriate resistance values of R 21  and R 21   a , four distinct voltages can be obtained on R 11  of a master module (as shown in  FIG. 5 ). By means of this modulation circuit, four-symbol encoding can be realized, for example, I 0  corresponds to 00b, I 1  corresponds to 01b, I 2  corresponds to 10b, and I 3  corresponds to 11b. This type of symbol encoding can greatly improve the speed of data transmission. 
     The modulation circuit in this embodiment is simple, can be easily realized, and has low cost. The symbol encoding greatly saves the transmission time. In addition, symbols differ from each other greatly, so it is not easy to cause a bit error. 
     This embodiment employs a four-symbol transmission protocol and an encoding rule as follows. 
     1. Measurement interval field. 
     2. Clock synchronization field. The slave temperature detection module  12  uses its own bit clock W as a reference to send I 3 , I 2 , I 1  and I 0  in sequence to inform the master module  11  of a clock frequency and a symbol level that are used by the slave temperature detection module  12 . 
     3. Temperature reference value field. A quaternary code of Tmin is sent bit by bit at the interval of W If all the temperature sensors are short-circuited, then Tmin=0000q (quaternary system), and a temperature difference field is empty. If all the temperature sensors are open-circuited, then Tmin=3333q (quaternary system), and the temperature difference field is empty. If some temperature sensors are open-circuited, and the other temperature sensors are short-circuited, in other words, there is no normal temperature sensor, then Tmin=0000q, and the temperature difference field is not empty. 
     4. Temperature difference field. N temperature differences Tn′ are sent in sequence according to the following encoding rule: if the temperature difference is less than 16, then 2q+Tn′2q is sent (Tn′2q indicates a 2-bit quaternary number); if the temperature difference is equal to 0, then 30q is sent; if the temperature sensor is open-circuited, then 32q is sent; if the temperature sensor is short-circuited, then 31q is sent; if the temperature difference is greater than or equal to 16, then 1q+Tn′4q is sent. 
     5. Check field. A check result is calculated through parity check or in other check manner and sent. 
     The protocol in this embodiment adopts symbol encoding, thereby achieving high transmission efficiency. There will be neither 00000q nor 33333q in a code stream, so loss of synchronization will not occur easily, which has a lower requirement for clock precision. The synchronization field completes transmission of bit timing and bit encoding synchronously. 
     Embodiment 4 
     Taking temperature measurement as an example, this embodiment provides a daisy chain two-wire temperature measurement system, including: a master module configured to obtain a temperature signal, and a plurality of slave temperature detection modules configured to detect the temperature signal. The master module and the plurality of slave temperature detection modules are successively connected in series through a power wire to form a daisy chain temperature monitoring network. The slave temperature detection module modulates the detected temperature signal into a corresponding current signal and transmits the current signal through the power wire. The master module decodes the current signal on the power wire to obtain corresponding temperature data. 
     In this embodiment, each of the plurality of slave temperature detection modules includes an upper temperature transmission interface and a lower temperature transmission interface that are connected to each other. The upper temperature transmission interface of a lower-stage slave temperature detection module is connected to the lower temperature transmission interface of an upper-stage slave temperature detection module through the power wire to directly transmit the temperature signal to the master module, without requiring the upper-stage slave temperature detection module to process the temperature signal. 
     The concept of a daisy chain in this embodiment refers to a wreath formed by connecting many daisies in series. In this embodiment, the daisy chain means that the plurality of slave temperature detection modules are successively connected in series, including a first-stage slave temperature detection module, a second-stage slave temperature detection module, . . . , and so on, and transmit the detected temperature signals to the master module. 
     The greatest advantage of the daisy chain structure in this embodiment is that a plurality of devices can be connected by using the limited signal transmission wire to share an identical service, which do not have bus competition, blocking or other problems. When a new slave temperature detection module needs to be added, it only needs to be connected to the tail of the daisy chain structure. Therefore, the daisy chain structure can be easily expanded. 
     In the prior art, a power wire is employed for power supply and a communication wire is employed for communication. When there are many slave temperature detection modules, wire complexity is increased and it is prone to occur a wire fault. 
     In this embodiment, the slave temperature detection module encodes the temperature signal to change a wire current, such that the master module decodes the current signal on the power wire to obtain the corresponding temperature data, thereby realizing temperature signal transmission. Two power wires are used to supply power and transmit the temperature signal at the same time, such that no communication wire for separate communication is needed, and no address encoding is needed. The signal is transmitted in the current mode, which strengthen the anti-interference capability. 
     When the slave temperature detection module encodes the temperature signal, a unique corresponding number is added. When decoding the current signal, the master module can determine a source of the temperature signal based on the number. When the temperature signal is abnormal, the faulty slave temperature detection module can be determined. 
     In this embodiment, as shown in  FIGS. 2 and 4 , the master module includes a power interface and a lower temperature transmission interface, and each of the plurality of slave temperature detection modules includes the upper temperature transmission interface and the lower temperature transmission interface. 
     The power interface is connected to a power supply, the lower temperature transmission interface is connected to the upper temperature transmission interface of the first-stage slave temperature detection module through the power wire, and the upper temperature transmission interface of the lower-stage slave temperature detection module is connected to the lower temperature transmission interface of the upper-stage slave temperature detection module through the power wire. A connection mode based on the daisy chain facilitates expansion and requires no communication interface of the master module. 
     A direct current (DC) voltage is input to the power interface. A recommended voltage of an input power supply is 12 V In some embodiments, an anti-reverse connection diode may be designed to prevent the input power supply from being connected reversely, or a full-bridge rectifier circuit may be designed to ignore the polarity of the power supply, such that the power supply can work no matter whether it is connected forwardly or reversely. 
     In this embodiment, each of the plurality of slave temperature detection modules includes: a temperature sensor, configured to obtain the temperature signal; and a slave lower bus switch, connected between the upper temperature transmission interface and the lower temperature transmission interface to control conduction of the lower-stage slave temperature detection module. 
     A slave controller connected to the temperature sensor, the slave lower bus switch and the lower temperature transmission interface is configured to control the temperature sensor, the slave lower bus switch and the lower temperature transmission interface. 
     A signal modulation module connected between the upper temperature transmission interface and the slave controller is configured to modulate the temperature signal into the corresponding current signal. 
     In this embodiment, each of the plurality of slave temperature detection modules further includes: a slave voltage stabilizing circuit, connected between the upper temperature transmission interface and the lower temperature transmission interface to stabilize an input supply voltage and output the stabilized supply voltage. When the voltage changes greatly, the output voltage is stabilized to provide a stable DC power supply for the slave controller. 
       FIG. 3  is a circuit diagram of the slave temperature detection module. In  FIG. 3 , MCU_S represents the slave controller, and LDO represents the slave voltage stabilizing circuit. The resistor R 24 , the switch transistor Q 22 , the capacitor C 21 , and the resistor R 21  constitute the signal modulation module. R 5  and RT constitute a temperature sensor circuit. The resistor R 22 , the capacitor C 22 , the switch transistor Q 21 , the resistor R 23 , and the switch transistor Q 22  constitute the slave lower bus switch. BUS+0 and BUS− constitute the upper temperature transmission interface, and BUS+1 and BUS− constitute the lower temperature transmission interface. 
     After there is a voltage on an upper temperature transmission bus, the slave controller is powered on to be started. After the slave controller is stable for a period of time, temperature sampling is activated. After the slave temperature detection module is powered on, voltages of the capacitor C 22  and the capacitor C 21  remain 0 V. The pin DO of the slave controller outputs a high level to turn on the switch transistor Q 22 . The voltage of the capacitor C 22  cannot change suddenly, so the switch transistor Q 21  remains off. The voltage of the capacitor C 21  cannot change suddenly, and the resistor R 21  instantaneously bears a voltage of the upper temperature transmission interface, resulting in a large modulated current. 
     Then, the pin DO of the slave controller outputs a low level, and the switch transistor Q 22  is turned off, or at least the turn-on time of the switch transistor Q 22  is not long. The switch transistor Q 21  remains off. During turn-on and turn-off of the switch transistor Q 22 , a pulse current is generated on the upper bus. Since lower bus switches of all upper-stage slave temperature detection modules are turned on, temperature information can be transmitted to the upper temperature transmission bus by using the pulse current, and a demodulation circuit of the master controller can directly restore the modulated temperature data. 
     After the temperature data is completely transmitted, the pin DO of the slave controller remains in an output state of the high level, the current of the resistor R 21  is gradually reduced to zero, the voltage of the capacitor C 22  is gradually increased, so that the switch transistor Q 21  is turned on, and the lower-stage slave temperature detection module is started. Meanwhile, the current-stage slave temperature detection module can enter a low-power sleep state. 
     In this embodiment, the master module includes: a signal demodulation module, configured to decode the current signal on the power wire to obtain the corresponding temperature data; and a master controller, connected to the signal demodulation module and the lower temperature transmission interface to control conduction of the lower temperature transmission interface. 
     In this embodiment, a master lower bus switch is connected between the lower temperature transmission interface and the master controller. In addition, the sampling resistor R is connected in series between the lower temperature transmission interface and the master lower bus switch, and the signal demodulation module is connected in parallel to the sampling resistor R and connected to the master controller. 
     In this embodiment, the signal demodulation module includes the sampling resistor R that is connected in series on the power wire and connected in parallel to the signal demodulation module, and the signal demodulation module samples and demodulates the voltage on the sampling resistor. 
     When the current on the power wire changes, the voltage on the sampling resistor R also changes, and the signal demodulation module performs decoding based on the voltage change to obtain the corresponding temperature data. 
     In this embodiment, the master module further includes a master lower bus switch connected to the power interface and the lower temperature transmission interface to control conduction of the slave temperature detection module. 
     In this embodiment, the master module further includes a master voltage stabilizing circuit connected to the power interface and the master controller and configured to stabilize an input supply voltage and output the stabilized supply voltage. 
     When the supply voltage changes greatly, an output voltage is stabilized to provide a stable DC power supply for the master controller. 
     In this embodiment, to facilitate storage and analysis of the temperature data, the master module further includes a communication interface connected to the master controller and an upper computer to transmit the temperature data to the upper computer. 
     In this embodiment,  FIG. 5  is a circuit diagram of the master module. In  FIG. 5 , MCU_M represents the master controller. The comparator COMP, the resistor R 11 , the resistor R 12 , and the diode D 11  constitute a simple demodulation circuit. The resistor R 10  and the switch transistor Q 11  constitute a lower bus switch. After the switch transistor Q 11  is controlled by the master controller to be turned on, a voltage is generated on the sampling resistor R 11 . After a current is generated on a modulation circuit of the slave module, the current also flows through the resistor R 11  to generate a voltage. The demodulation circuit can compare values of the voltages to restore the temperature data signal. 
     Embodiment 5 
     As shown in  FIG. 6 , this embodiment provides a daisy chain two-wire temperature measurement method, including the following steps: 
     S 1 : A slave temperature detection module activates temperature sampling and encodes a temperature signal. 
     S 2 : The slave temperature detection module modulates the temperature signal into a corresponding current signal through a signal modulation module, and sends the current signal to a power wire. 
     S 3 : A signal demodulation module decodes the current signal to obtain corresponding temperature data. 
     S 4 : A master controller determines, based on the temperature data, whether a faulty slave temperature detection module or an abnormal temperature occurs. 
     In this embodiment, a master module receives an instruction from a communication interface of an upper computer, and starts a temperature measurement process. The master controller turns on a lower bus switch and prepares for receiving data. 
     In this embodiment, after being powered on, the slave temperature detection module starts to work, activates temperature sampling, and encodes the temperature signal. The slave temperature detection module sends the temperature signal to the power wire through the signal modulation module. The temperature signal passing through the power wire is converted into a change of a resistance voltage by a signal demodulation unit of the master module, and then is converted into a serial signal by a demodulation circuit to be received by the master controller of the master module. 
     In this embodiment, the slave temperature detection module turns on a lower bus switch, and activates a sleep mode after the bus is connected to a lower-stage slave temperature detection module, such that the current-stage slave temperature detection module is in a low-power sleep state. 
     The above steps are repeated until data transmission of all slave temperature detection modules is completed. The master controller turns off the lower bus switch, analyzes and processes all temperature data, and determines whether a faulty slave temperature detection module or an abnormal temperature occurs. 
     In this embodiment, the master module transmits the collected temperature data and fault information to the upper computer through the communication interface of the upper computer. So far, one temperature data collection process is completed. In the whole measurement process, the slave modules are powered on successively, and enter the sleep mode after transmitting data to be in a low-power state. After the measurement is completed, the master controller turns off the lower bus switch, and all the slave temperature detection modules are powered off to reduce power consumption. Therefore, the system has low power consumption in the whole measurement process. 
     Embodiment 6 
     This embodiment provides a fault location method.  FIG. 8  is a timing diagram of a two-wire daisy chain sensor system. 
     T 0  represents a time period in which a bus is turned off, and is controlled by a master module. In this time period, the master module turns off a master lower switch, and all slave modules are in a power-off state. 
     T 1  represents a time period for synchronization. In this time period, the master module already turns on the master lower switch, and the slave modules are powered on successively. Duration of T 1  is determined by the slave modules. The slave modules prepare for data transmission after being powered on and completing sensor sampling within T 1 . The upper limit of the duration of T 1  is T 1 max. 
     T 2  represents a time period for data transmission. In this time period, the slave modules complete collection of the sensor data, process and transmit the data. The upper limit of the duration of T 2  is T 2 max. 
     This embodiment provides a data transmission and fault location method of the two-wire daisy chain sensor system with N slave modules, including the following steps: 
     S 1 : turning on the master lower switch by the master module, terminating T 0 , and activating timing for T 1 , where n is set to 1. 
     S 2 : if the master module does not receive any data within T 1 max, determining that n slave modules fail to perform reading, and jumping to SE. 
     S 3 : activating timing for T 2  after the master module receives data, terminating all data, and waiting for completion of data transmission. 
     S 4 : if T 2 &gt;T 2 max, determining that a data error occurs on an N th  slave module, and jumping to SE. 
     S 5 : if a check error exists in the data, jumping to SE; if a fault indicating that a sensor is open-circuited or short-circuited exists in the data, jumping to S 6 . 
     S 6 : setting n to n+1, and activating timing for T 1 . 
     S 7  jumping to S 2 . 
     SE: if n=N+1, indicating that data of all the slave modules is received and data transmission is completed, turning off the master lower switch by the master module; if n&lt;N+1, determining that a communication fault occurs on an n th  slave module. 
     In conclusion, according to the present invention, the master module and the plurality of slave sensor detection modules are successively connected in series through the power wire to form the daisy chain sensor monitoring network, thereby realizing easy expansion. A wire is shared for power supply and communication, in other words, only the power wire is employed to supply power for the master module and the plurality of slave sensor detection modules and transmit data, thereby simplifying on-site cabling. The signal is transmitted in a current mode, such that an anti-interference capability is strengthened and no address encoding is required. In addition, the present invention also has advantages of low cost and low power consumption. 
     In the present invention, a special daisy chain structure is adopted. Data of the lower-stage slave sensor detection module is directly sent to the master module without being forwarded by an upper-stage slave sensor detection module. Each slave sensor detection module is only configured for sampling, data transmission, and controlling the lower bus switch, thereby imposing a low requirement for the slave controller and facilitating implementation at a low cost. 
     In the present invention, the two-wire bus is adopted, and the cabling is convenient. This technical solution can also be used for other sensor monitoring system. Each slave sensor detection module can also be connected to a plurality of external sensors, and one data transmission channel is shared, thereby further reducing the system cost and simplifying the device installation. 
     The above embodiments are only used to explain the technical solutions of the present invention, and are not intended to limit the present invention. Although the present invention is described in detail with reference to the above embodiments, those having ordinary skill in the art should understand that they can still modify the technical solutions described in the above embodiments, or make equivalent substitutions on some technical features therein, without making the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.