Abstract: Plug-and-play functionality of sensors represents a new development in sensing technology and is of great significance for improving the performance of automatic control and testing systems. This paper mainly introduces the principle of plug-and-play sensors; based on this, a plug-and-play sensor measurement system based on Bluetooth technology is proposed and established. Keywords: Plug-and-play sensor, Bluetooth technology, DSP Introduction With the increasing automation, complexity, accuracy, and reliability requirements of measurement and control systems, the demands on sensor performance are also rising. However, certain shortcomings of traditional sensors have constrained this development, leading to the introduction of high-tech technologies represented by microprocessors. To reduce the time required for sensor configuration and the risks involved, IEEE 1451.4 recently provided a new standard for sensors. This standard establishes a universal method for enabling plug-and-play functionality in sensors—adding self-describing capabilities to analog interface sensors. Fieldbus technology is one of the hot topics in the field of automation today, and is hailed as the computer local area network of the automation field. Previously, fieldbuses generally used wired connections and specific bus protocols; however, the emergence of wireless networks has opened up new areas for the development of fieldbuses and improved their flexibility. Bluetooth technology is a short-range wireless digital communication standard designed to establish an open specification combining software and hardware, providing interoperable and cross-developable tools for all different devices. By utilizing the Bluetooth network, various test devices can be connected to form a measurement system network. Combining Bluetooth technology with plug-and-play sensors provides new ideas and approaches for improving and developing the performance of automatic control and testing systems. This paper aims to describe a plug-and-play sensor system based on Bluetooth technology, achieving plug-and-play functionality through identification, circuit conditioning, and Bluetooth wireless communication. 1. System Scheme The plug-and-play wireless networked sensor measurement system based on Bluetooth technology mainly consists of the following parts: a sensor module, an identification module, a signal conditioning circuit module, an A/D conversion module, a microprocessor module, a Bluetooth wireless transmission module, and a host computer module. The system structure is shown in Figure 1. The working process of this measurement system is as follows: The DSP reads the information from the identification module to identify the sensor currently connected to the system; the DSP configures the conditioning circuit appropriately based on the information from the identification module; the sensor output signal is sent to the DSP after A/D conversion; the DSP transmits the data to the host computer via the Bluetooth module. If a different sensor is replaced, simply resetting the DSP allows the system to reconfigure the circuitry according to the needs of the current sensor unit without manual intervention, thus achieving plug-and-play functionality. The identification module is a crucial component of plug-and-play sensors, providing self-description information. The IEEE 1451.4 standard defines a specification for this. This standard defines a hybrid interface that retains traditional analog signals while adding a low-cost digital interface to transmit the Sensor Electronic Data Sheet (TEDS) embedded in the sensor, enabling self-identification and self-description, as shown in Figure 2. The IEEE P1451.4 standard defines two types of hybrid interfaces: two-wire interfaces and multi-wire interfaces. Figure 1: System Structure Diagram 2: Plug-and-Play Sensor Model with Embedded TEDS Information Two-wire interface: for sensors operating under constant current excitation or with integrated piezoelectric circuits (ICP), such as accelerometers. Used to multiplex analog signals and digital TEDS signals on a single wire pair, as shown in Figure 3. Figure 3 shows the two-wire interface of the ICP sensor. Another interface mode for other types of sensors is to separate the analog and digital parts. While keeping the analog input/output of the sensor unchanged, the digital TEDS is added in parallel to the circuit. This essentially enables plug-and-play functionality for any type of sensor or actuator, including thermocouples, thermistors, and bridge sensors. Figure 4 shows the application of this hybrid interface in a bridge interface sensor. The digital part of the multi-wire hybrid interface applied to the bridge sensor in Figure 4 is based on the Maxim/Dallas 1Wire protocol. This is a very simple and low-cost master-slave serial communication protocol. This protocol requires only one master device (e.g., a data acquisition system) to power it and initialize each transmission of each node according to a specific timing sequence, and all communication for these operations is done on a single wire. The multi-wire hybrid interface has greater general applicability, so this paper will use this method to implement plug-and-play functionality for sensors and use Maxim/Dallas 1Wire devices to store standardized sensor electronic data sheets (TEDS). Unlike other plug-and-play smart sensor technologies, IEEE P1451.4 uniquely retains the analog output of the sensor. Therefore, IEEE P1451.4 sensors are compatible with systems containing traditional analog interfaces. Taking a sensor based on the bridge measurement principle as an example, a general conditioning circuit is designed, utilizing a sensitive resistor to sense changes in the measured quantity and convert them into voltage or current signals. To achieve plug-and-play functionality, the conditioning circuit of this system must have automatic adjustment capabilities. The lower-level machine primarily uses Motorola's DSP evaluation board DSP56311EVM as the basic device to establish a data acquisition and processing system. Upon system startup, it acquires sensor identification information and correctly configures the conditioning circuit by controlling various digital potentiometers and electronic switches to accurately process the sensor signals, thus achieving plug-and-play functionality. Finally, Bluetooth wireless network technology enables connection and digital communication between sensors. 2. Hardware Design of the System The plug-and-play sensor measurement system mainly consists of the following parts in terms of hardware design: The sensor unit includes a traditional analog sensor and identification module (TEDS), power supply unit, signal conditioning unit, A/D conversion and interface, as shown in Figure 5. Figure 5 Plug-and-play sensor system structure (1) The sensor unit adopts Honeywell's 24PCCFA6D silicon piezoresistive pressure sensor. The internal structure of this sensor is four resistors diffused on a silicon diaphragm. These four resistors are generally connected in a Wheatstone bridge. The identification module is composed of a low-cost memory chip, which stores the standardized sensor electronic data sheet (TEDS). The TEDS stores some important sensor information and parameters, which can perform self-identification and self-description. The author uses the DS2430A provided by Maxim/Dallas to store the TEDS information used to configure the sensor. (2) Power Supply Unit For the same Wheatstone bridge, different power supply methods result in different measurement effects. After comparison, the constant voltage source power supply is related to the resistance change caused by temperature; while the constant current source power supply, the output voltage is only related to the change caused by pressure on the bridge arm and the size and accuracy of the constant current source, and is not related to temperature. Therefore, a 2mA constant current source power supply matched with the sensor was adopted to achieve the minimum sensitivity temperature drift; however, when the constant current source is used to power the bridge, it will bring about the problem of excessive common mode signal output. Excessive common mode voltage may cause the operational amplifier in the amplifier circuit to malfunction. Therefore, a potentiometer VR2 to suppress common mode voltage was added to the constant current source circuit, as shown in Figure 6. Practice has proven that the current output of the improved constant current source circuit is stable and can easily adjust the common mode output of the sensor, so that the system works normally. Figure 6 Improved constant current source circuit (3) Signal conditioning unit The signal conditioning unit mainly realizes the acquisition and processing of signals. In addition to removing noise and interference, its function is more important: in order to realize the plug-and-play of the sensor, the parameters in the conditioning circuit should be automatically configured. In this system, the conditioning circuit is programmed to be controlled by multiple non-volatile adjustment potentiometers DS1804, such as adjusting the amplification factor and controlling the constant current source output. The NV calibration potentiometer DS1804 is a single-channel, non-volatile, 100-level digital potentiometer. The tap position is adjusted by three control pins: CS, INC and U/D. If necessary, the tap position can also be stored in EEPROM through the serial interface. In the hardware connection, the INC and U/D of all digital potentiometers are connected to PB4 and PB5 of the DSP respectively, and their strobe signal CS is connected to several other GPIO ports. The state of CS determines the digital potentiometer to be operated. (4) The output of the signal acquisition unit amplifier circuit is the pressure signal measured by the sensor. It is an analog signal and needs to be converted from analog to digital before being input into the DSP for processing. Based on the accuracy of the sensor itself and considering factors such as real-time performance, the Maxim MAX1065 analog-to-digital converter was finally selected. The main processes for controlling and acquiring data during A/D conversion are: starting the conversion, ending the conversion, and reading the data. The hardware connection of MAX1065 is shown in Figure 7. By connecting a 1μF capacitor and a 0.1μF capacitor in series between the REF and REFADJ pins and ground, the 4.096V reference voltage provided by the MAX1065 can be used to convert the analog signal without the need for an external reference voltage source, which simplifies the circuit design and reduces the cost. Figure 7 Hardware connection of MAX1065 (5) Connection design between front-end circuit and DSP As the core of the entire system, the DSP needs to make the final judgment and control on information from various aspects. Therefore, receiving signals and making judgments need to go through its interface. The main interfaces used are: external memory interface (PORT A), serial interface (SCI) and general purpose input/output interface (GPIO). The external memory interface is one of the characteristics of DSP. It can easily access various peripherals of the DSP and expand memory-mapped I/O ports; the address unit of the peripheral can be specified through the address allocator of PORTA; by accessing this address space, data reading and control of the peripheral can be realized. The Bluetooth module is connected to the DSP through the DSP's serial interface (SCI). The RS232 connection method of the Bluetooth module is selected according to the DSP interface. The DSP's serial interface (SCI) needs to be set to the RS232 serial port data transmission mode. The DSP56311 provides 34 bidirectional signal ports, which can be configured as GPIO (General Purpose Input/Output) signals or used as dedicated signals for peripheral devices. The DSP56311 does not provide dedicated GPIO signals; the default state is reached after reset. The aforementioned 34 signals constitute the GPIO. In the front-end circuit, the devices that need to be connected to the DSP's GPIO ports mainly include the 1Wire memory DS2430A, the A/D chip MAX1065, and several digital potentiometers DS1804. 3. System Software Design The test system, built upon the hardware architecture, requires software algorithms from a DSP and software design from a host computer. The DSP software algorithm needs to implement the following functions: reading standardized sensor electronic data sheets (TEDS), controlling and adjusting various digital potentiometers, acquiring and calculating sensor signals, and designing the interface control for the Bluetooth module. The host computer software is designed to control the main Bluetooth unit and display the final measurement results; the software flow is shown in Figure 8. Figure 8: Software Program Flow. In the system, the main function of the DS2430A is to provide the microprocessor with the TEDS stored within it. To achieve communication with the DS2430A, the core is to master the timing of the 1-Wire device signals. To ensure data integrity, the DS2430A has strict requirements for the communication protocol. The DS2430A's communication protocol mainly includes four signal types: initialization signals (including one reset pulse and one acknowledge pulse), write 0, write 1, and read data. Except for the acknowledge pulse, all these signals are issued by the bus control unit. Initialization Signals: An acknowledge pulse following a reset pulse indicates that the DS2430A is ready to receive ROM commands. The DSP first sends a (TX) reset pulse, then releases the bus and switches to receive (RX) mode. The 1Wire bus is pulled high through a pull-up resistor. After detecting the rising edge of the data pin, the DS2430A waits for tPDH before sending an acknowledge pulse. Read/Write Signals: All read/write sequences begin with the DSP pulling the data line low. The falling edge of the data line triggers an internal delay circuit in the DS2430A to synchronize it with the DSP. In the write sequence, the delay circuit determines when the DS2430A samples the data line. For the read sequence, if the data to be transmitted is "0", the delay circuit determines the length of time the DS2430A pulls the data line low after it has been set high by the DSP; if the data to be transmitted is "1", the DS2430A will not change the state of the data line during the read sequence. Conclusion This paper focuses on plug-and-play sensors, taking pressure sensors as an example. The system accurately identifies the connected pressure sensor by using the important sensor information and parameters stored in the TEDS table (manufacturer, model, and sensor serial number; most TEDS also describe the sensor's main characteristics, such as range, sensitivity, temperature coefficient, and electrical interface). Based on the information contained in the identification module, the system accurately configures the front-end circuitry. The goal of the "plug-and-play" sensor program is to create an open sensor standard, enabling system integrators and end users to automatically set up measurements and control the system. Users can download TEDS binary files or virtual TEDS to their systems, giving existing sensors "plug-and-play" functionality. Another important aspect of this research is the application of wireless communication technology to networked sensors, breaking through spatial limitations in signal connection. The expansion of wireless communication technology applications provides more new options for the measurement field. In industrial settings, short-range wireless connections have widespread application needs. Applying Bluetooth technology to industrial settings and using microwaves instead of infrared overcomes the shortcomings of infrared and reduces costs.