“With the continuous improvement of the automation, complexity, accuracy and reliability requirements of the measurement and control system, people have higher and higher requirements for sensor performance; however, some shortcomings of the traditional sensor itself restrict this development, so people have introduced High and new technology represented by microprocessor. In order to reduce the time spent on sensor configuration and the risks faced in the process, IEEE1451.4 recently provided a new standard for sensors. This standard establishes a universal method for making sensors have plug-and-play functionality-adding self-describing functions to analog interface sensors.
With the continuous improvement of the automation, complexity, accuracy and reliability requirements of the measurement and control system, people have higher and higher requirements for sensor performance; however, some shortcomings of the traditional sensor itself restrict this development, so people have introduced High and new technology represented by microprocessor. In order to reduce the time spent on sensor configuration and the risks faced in the process, IEEE1451.4 recently provided a new standard for sensors. This standard establishes a universal method for making sensors have plug-and-play functionality-adding self-describing functions to analog interface sensors.
Fieldbus technology is one of the hotspots of technology development in the field of automation today, and is known as the computer local area network in the field of automation. In the past, fieldbuses generally used wired connections and adopted certain bus protocols; the emergence of wireless networks opened up new areas for the development of fieldbuses and improved the flexibility of fieldbuses. Bluetooth technology is a technical standard for short-distance wireless digital communication. It aims to establish a public specification that combines software and hardware to provide interoperable and cross-development tools for all different devices. Use the scatter network of the Bluetooth system to connect each test device to form a measurement system network. Combining Bluetooth technology with plug-and-play sensors provides new ideas and approaches for the improvement and development of automatic control and test system performance.
This article aims to describe a plug-and-play sensor system based on Bluetooth technology, which realizes the plug-and-play of the sensor through identification, circuit conditioning and Bluetooth wireless communication.
1 System solution
The plug-and-play wireless networked sensor measurement system based on Bluetooth technology is mainly composed of the following parts: sensor module, identification module, signal conditioning circuit module, A/D conversion module, microprocessor module, Bluetooth wireless transmission module and Host computer module. The system structure is shown in Figure 1.
The working process of the measurement system is as follows: DSP reads the information of the identification module to identify the sensor currently connected to the system; DSP appropriately configures the conditioning circuit according to the information of the identification module; the signal output by the sensor is sent after A/D conversion Enter DSP; DSP transmits data to the host computer through the Bluetooth module. If you change a different sensor, you only need to reset the DSP, and the system can configure the circuit again according to the needs of the current sensor unit without manual intervention, thus realizing the plug and play of the sensor.
The identification module is an important part of the plug-and-play sensor, which provides self-descriptive information for the sensor. The IEEE1451.4 standard defines a specification for this. This standard defines a mixed-mode interface. While retaining the analog signal of the traditional sensor, it also adds a low-cost digital interface to transmit the sensor Electronic data sheet (TEDS) embedded in the sensor to achieve The functions of self-identification and self-description are shown in Figure 2.
The IEEEP1451.4 standard defines two types of mixed-mode interfaces, a two-wire interface and a multi-wire interface.
Two-wire interface, working under constant current excitation, or integrated piezoelectric circuit (ICP) sensors, such as acceleration sensors. It is used to realize the multiplexing of analog signal and digitized TEDS signal on a single wire pair, as shown in Figure 3.
Another interface mode for other types of sensors is to separate the analog part from the digital part. On the basis that the analog input/output of the sensor remains unchanged, the digital TEDS is added to the circuit in parallel. In this way, plug-and-play of any form of sensor or exciter can be realized, including thermocouples, thermistors, and bridge sensors.
The digital part of the mixed-mode interface is based on the 1Wire protocol of Maxim/Dallas. This is a very concise, low-cost master-slave serial communication protocol. This protocol only requires a master device (for example, a data acquisition system) to supply power and initialize each transmission of each node according to a specific time sequence, and the communication of these operations is completed on a wire.
The multi-wire mixed mode interface has more general versatility, so this article will use this method to realize the plug and play of the sensor, and use the 1Wire device provided by Maxim/Dallas to store the standardized sensor electronic data sheet (TEDS). Compared with the plug and play of other smart sensor technologies, IEEEP1451.4 is unique in that it retains the sensor’s analog output. Therefore, the IEEEP1451.4 sensor is compatible with systems that include traditional analog interfaces.
Take the sensor based on the bridge measurement principle as an example, design a general conditioning circuit, and use a sensitive resistor to sense the measured change and convert it into a voltage or current signal. In order to realize the plug and play of the sensor, the conditioning circuit part of this system must have an automatic adjustment function. The lower computer mainly uses Motorola’s DSP evaluation board DSP56311EVM as the basic device to establish a data acquisition and processing system. When the system is started, the sensor identification information is collected, and the conditioning circuit is correctly configured by controlling each digital potentiometer and electronic switch to achieve the purpose of accurately processing the sensor signal, so as to realize the plug and play of the sensor. Finally, the connection and digital communication between sensors are realized through the wireless network of Bluetooth technology.
2 System hardware design
In terms of hardware design, the plug-and-play sensor measurement system is mainly composed of the following parts: The sensor unit includes the traditional analog sensor and identification module (TEDS), power supply unit, signal conditioning unit, A/D conversion and interface, as shown in Figure 5. Shown.
(1) Sensor unit
Use Honeywell’s 24PCCFA6D silicon piezoresistive pressure sensor. The internal structure of the sensor is to diffuse four resistors on a silicon diaphragm. These four resistors are generally connected to a Wheatstone bridge. The identification module is composed of a low-cost memory chip with a standardized sensor electronic data sheet (TEDS) stored inside. Some important sensor information and parameters are stored in TEDS, which can be self-identified and self-described. 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, the power supply method is different, and the measurement effect is different. 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 in the bridge arm caused by the pressure and the size and accuracy of the constant current source, and has nothing to do with the temperature. Therefore, a 2mA constant current source matched with the sensor is used for power supply to achieve the minimum temperature drift of sensitivity; but when a constant current source is used to power the bridge, it will cause the problem of excessive common-mode signal output. Too high common-mode voltage is likely to cause the operational amplifier in the amplifying circuit to fail to work normally. For this reason, a potentiometer VR2 that suppresses the common-mode voltage is added to the constant current source circuit, as shown in Figure 6. Practice has proved that the current output of the improved constant current source circuit is stable, and the common mode output of the sensor can be easily adjusted to make the system work normally.
(3) Signal conditioning unit
The signal conditioning unit mainly realizes signal acquisition and processing. 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, through multiple non-volatile adjusting potentiometer DS1804, to realize the programmed control of the conditioning circuit, such as adjusting the magnification. Control the output of constant current source, etc. The NV calibration potentiometer DS1804 is a single-channel, non-volatile, 100-level digital potentiometer. The wiper position is adjusted by 3 control pins: CS, INC and U/D. If necessary, the wiper position can also be stored in the EEPROM through the serial interface.
In the hardware connection, connect the INC and U/D of all digital potentiometers to PB4 and PB5 of the DSP, and connect their strobe signals CS to several other GPIO ports. The status of CS determines the current requirements. Operated digital potentiometer.
(4) Signal acquisition unit
The output of the amplifier circuit is the pressure signal measured by the sensor, which is an analog signal, which needs to be converted from analog to digital, and then input to the DSP for processing. According to the accuracy of the sensor itself, taking into account real-time and other factors, the MAX1065 analog-to-digital converter of Maxim was finally selected. The main processes of A/D conversion control and data acquisition are: start conversion, conversion end and data reading.
The hardware connection situation of MAX1065 is shown as in Fig. 7. Between the REF and REFADJ pins and ground, connect a capacitor of 1μF and a capacitor of 0.1μF respectively, and then you can use the 4.096V reference voltage provided inside the MAX1065 to convert the analog signal, without the need for an external reference voltage Source, simplifies the design of the circuit and reduces the cost.
(5) Connection design of front-end circuit and DSP
As the core of the whole system, DSP needs to make final judgment and control on information from various aspects, so both receiving signals and sending judgments need to go through its interface. The main interfaces used are: external memory interface (PORTA), serial interface (SCI) and general-purpose input and output interface (GPIO).
The connection between the Bluetooth module and the DSP is realized through the serial interface (SCI) of the DSP. According to the DSP interface, select the RS232 connection mode of the Bluetooth module. Need to set the serial interface SCI of DSP to conform to the data transmission mode of RS232 serial port.
DSP56311 provides 34 bidirectional signal ports, which can be used as GPIO (General Purpose Input/Output) signal configuration or as special signals for peripheral devices. DSP56311 does not provide a special GPIO signal, and it is in the default state after reset. The above 34 signals are GPIO. In the front-end circuit, the devices that need to be connected to the GPIO port of the DSP mainly include the 1Wire memory DS2430A, the A/D chip MAX1065, and several digital potentiometers DS1804.
3 System software design
The test system built based on the design of the hardware structure needs to be realized through the software algorithm of the DSP and the software design of the upper computer. The DSP software algorithm needs to realize the following functions: read the standardized sensor electronic data sheet (TEDS), control and adjust each digital potentiometer, sensor signal acquisition and calculation, and design the interface control of the Bluetooth module. The upper computer software is designed to achieve the control of the main Bluetooth unit and Display the final measurement results. The software flow is shown in Figure 8.
In the system, the main function of DS2430A is to provide the microprocessor with TEDS stored in it. To realize the communication with DS2430A, the core is to master the signal transmission and reception timing of the 1Wire device. In order to ensure the integrity of the data, DS2430A has very strict requirements on the communication protocol. The communication protocol of DS2430A mainly includes four signal types: initialization signal (including 1 reset pulse and 1 response pulse), write 0, write 1, and read data. Among these signals, except for the response pulse, they are all issued by the bus control unit.
Initialization signal: a response pulse sent after a reset pulse, indicating that the DS2430A is ready to receive ROM commands. DSP first sends out (TX) a reset pulse, then releases the bus, and switches to receiving (RX) state. The 1Wire bus is pulled up to a high level through a pull-up resistor. After DS2430A detects the rising edge of the data pin, it waits for tPDH and sends out an acknowledge pulse.
Read and write signals: All read and write sequences are started by DSP pulling down the data line. The falling edge of the data line will trigger a delay circuit inside the DS2430A to synchronize it with the DSP. In the write sequence, the delay circuit will determine when the DS2430A will sample the data line. For the read sequence, if the data to be transmitted is “0”, the delay circuit will determine the length of time that the DS2430A will pull the data line low by the DSP; if the data to be transmitted is “1”, the DS2430A will not be in the read sequence. Change the status of the data line.
This article focuses on plug-and-play sensors using pressure sensors as an example: the system uses important sensor information and parameters stored in the TEDS table (manufacturer, model, and serial number of the sensor. Most TEDS also describes the main characteristics of the sensor. , Such as range, sensitivity, temperature coefficient, electrical interface, etc.), accurately identify the pressure sensor connected to it, and accurately configure the front-end circuit according to the information contained in the identification module. The goal of the “plug and play” sensor program is to create an open sensor standard that enables system integrators and end users to automatically set and measure sensors and automatically control the system. Users can download TEDS binary files or virtual TEDS to their system, so that the original sensor has a “plug and play” function.
Another important significance of this subject lies in the application of wireless communication technology to networked sensors, which enables signal connections to break through the limitations of space. The expansion of the application of wireless communication technology has provided more new options for the measurement field. In industrial sites, short-range wireless connections have a wide range of application requirements. Applying Bluetooth technology to industrial sites and using microwaves to replace infrared have overcome the shortcomings of infrared and reduce costs.