Plug and Play Sensor Based on Bluetooth Technology

With the increasing automation, complexity 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-tech represented by microprocessor. Recently, IEEE 1451.4 provides new standards for sensors in order to reduce the time it takes to configure them and the risks they face in the process. The Standard establishes a common method for making sensors plug-and-play capable, which adds self-describing capabilities to analog interface sensors.

Fieldbus technology is one of the hotspots of technology development in the field of automation and is known as computer local area network in the field of automation. In the past, the field bus used to be wired connection and used a certain bus protocol. The emergence of wireless network opens up a new field for the development of field bus and improves the flexibility of field bus. Bluetooth technology is a technical standard for close-range wireless digital communication. It aims to establish an open specification for the combination of hardware and software, providing interoperability and cross-development tools for all different devices. Using the scattering network of the Bluetooth system, each test device can be connected to form a measurement system network. Combining Bluetooth technology with Plug and Play sensor provides a new idea and way for the improvement and development of automatic control and test system performance.

The purpose of this paper is to introduce a Bluetooth-based Plug and Play sensor system, which implements the Plug and Play of sensors through identification, circuit adjustment and Bluetooth wireless communication.

　　1. System Scheme

The Plug and Play wireless network sensor measurement system based on Bluetooth technology consists of sensor module, recognition module, signal conditioning circuit module, A/D conversion module, microprocessor module, Bluetooth wireless transmission module and upper computer module. The system structure is shown in Figure 1.

The working process of the measurement system is as follows: The DSP reads the information of the identification module to identify the sensor currently connected to the system; DSP configures the conditioning circuit appropriately according to the information of the recognition module. The signal output from the sensor is A/D converted and sent to the DSP. DSP transmits data to PC through Bluetooth module. If different sensors are replaced, only reset the DSP, the system can configure the circuit again according to the needs of the current sensor unit, without manual interference, thus achieving the plug and play of the sensor.

The identification module is an important component of Plug and Play sensors, which provides self-describing information for sensors. The IEEE 1451.4 standard defines a specification for this. This standard defines a hybrid mode interface that retains the analog signals of traditional sensors and adds a low-cost digital interface to transmit a sensor spreadsheet (TEDS) embedded in the sensor for self-identification and self-description, as shown in Figure 2.

The IEEE P1451.4 standard defines two types of hybrid mode interfaces, two-line interfaces and multiline interfaces.

A two-wire interface operates under constant current excitation or a sensor integrated with a piezoelectric circuit (ICP), such as an acceleration sensor. To multiplex analog and digital TEDS signals on a single line pair, as shown in Figure 3.

Another interface mode for other types of sensors is to separate the analog part from the digital part. Digital TEDS is added to the circuit in parallel while the analog input/output of the sensor remains unchanged. This essentially allows plug-and-play of any type of sensor or actuator, including thermocouples, thermal resistors, bridge sensors, and so on. Figure 4 shows the application of this hybrid interface in the bridge interface sensor.

The digital part of the hybrid mode interface 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 primary device (for example, a data acquisition system) to power and initialize each transmission of each node according to a specific time sequence, and the communication for these operations is done on a single wire.

Multiline hybrid mode interfaces are more versatile, so this approach will be used to plug and play sensors and to store standardized sensor spreadsheets (TEDS) using a 1 Wire device provided by Maxim/Dallas. Compared with other smart sensor technologies, IEEE P1451.4 is unique in that it retains the analog output of the sensor. Therefore, the IEEE P1451.4 sensor can be compatible with systems containing traditional analog interfaces.

Taking the sensor based on the principle of bridge measurement as an example, a general regulating circuit is designed, which uses the sensitive resistance to sense the measured changes and convert them into voltage or current signals. In order to make the sensor plug and play, the regulating circuit part of the system must have the function of automatic adjustment. The lower computer mainly uses Motorola's DSP evaluation board DSP56311EVM as the basic device to set up a data acquisition and processing system. When the system starts, the sensor recognition information is collected, and the conditioning circuit is configured correctly by controlling the digital potentiometers and electronic switches to achieve the purpose of processing the sensor signal, thus achieving the plug and play of the sensor. The wireless network with Bluetooth technology enables the connection between sensors and digital communication.

　　2. Hardware design of the system

In terms of hardware design, Plug and Play sensor measurement system is mainly composed of the following components: sensor unit including traditional analog sensor and identification module (TEDS), power supply unit, signal conditioning unit, A/D conversion and interface, as shown in Fig. 5.

　　(1) Sensor unit

Honeywell's 24PCCFA6D silicon piezoresistive pressure sensor. The internal structure of the sensor is four resistors diffused on a silicon diaphragm. These four resistors are usually connected into a Whiston bridge. The recognition module consists of a low-cost memory chip that stores a standardized sensor spreadsheet (TEDS). TEDS stores some important sensor information and parameters for self-identification and self-description. The author uses DS2430A provided by Maxim/Dallas to store TEDS information for configuring sensors.

　　(2) Power supply unit

For the same Whiston bridge, different modes of power supply and different measurements can be made. By comparison, the constant voltage power supply is related to the change of resistance value caused by temperature. For a constant current source, the output voltage is only related to the amount of pressure change on the bridge arm and the size and size of the constant current source, but not to the temperature. Therefore, a 2 mA constant current power supply matching the sensor is used to achieve the minimum sensitivity temperature drift. However, when the bridge is powered by a constant current source, the common mode signal output will be too large. High common-mode voltage is likely to cause the amplifier in the amplifier circuit to not work properly. For this reason, a potential VR2 which suppresses common-mode voltage is added to the constant-current source circuit, as shown in Figure 6. The practice proves 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 implements signal collection and processing. Besides noise and interference removal, the more important point is that in order to achieve the Plug and Play of the sensor, the parameters in the conditioning circuit should be automatically configured. In this system, programmed control of the conditioning circuit, such as adjusting the amplification factor, is achieved through multiple non-volatile regulator potentiometers DS1804. Control constant current source output, etc. The NV calibration potentiometer DS1804 is a single, non-volatile, 100-level digital potentiometer. The tap position is adjusted by three control pins: CS, INC and U/D. Depending on your needs, you can also store the tap location in EEPROM through a serial interface.

In the hardware connection, connect the INC and U/D of all digital potentiometers to the PB4 and PB5 of the DSP, and connect their selective signal CS to several other GPIO ports. The status of the CS determines the current digital potentiometer to be operated.

　　(4) Signal acquisition unit

The output of the amplifier circuit is the pressure signal measured by the sensor. For the analog signal, the analog/digital conversion is needed and then input to the DSP for processing. Based on the sensor itself and considering the real-time and other factors, the MAX1065 A/D converter from Maxim Company was selected. The main processes of A/D conversion control and data collection are: start conversion, end of conversion and data reading.

The hardware connection of MAX1065 is shown in Figure 7. String 1 in between REF and REFADJ pins and ground respectively μ F and a 0.1 μ F's capacitance allows the analog signal to be converted using the 4.096V reference voltage provided inside MAX1065, eliminating the need for an external reference voltage source, simplifying the circuit design and reducing costs.

　　(5) Design of connection between front-end circuit and DSP

As the whole system, DSP needs to make final judgment and control on information from all aspects, so both receiving signal and sending judgment need to go through its interface. The main interfaces used are external memory interface (PORT A), serial interface (SCI), and general input-output interface (GPIO).

External memory interface is one of the features of DSP. It can easily access the peripherals of the DSP and expand the memory mapping I/O port. The address allocator of PORT A allows you to specify a peripheral address unit. By accessing this address space, you can read and control the external data.

The connection between the Bluetooth module and the DSP is achieved through the serial interface (SCI) of the DSP. Select RS232 connection mode of Bluetooth module according to the situation of DSP interface. The serial interface SCI of the DSP needs to be set to match the data transmission mode of the RS232 serial port.

DSP56311 provides 34 bidirectional signal ports and can be used as GPIO (General Purpose Input/Output) signal configuration or as a dedicated signal for peripheral devices. The DSP56311 does not provide a special GPIO signal and is reset to the default state. The 34 above signals are GPIO. In the front-end circuit, the main devices that need to be connected to the GPIO port of the DSP are 1 Wire memory DS2430A, A/D chip MAX1065, and several digital potentiometers DS1804.

　　3. System Software Design

The test system based on the design of hardware structure needs to be implemented by the software algorithm of DSP and the software design of PC. The software algorithm of DSP needs to realize the following functions: read the standardized sensor electronic data table (TEDS), control and adjust each digital potentiometer, collect and calculate the sensor signal, and design the interface control of the Bluetooth module. The upper computer software is designed to control the main Bluetooth unit and display the final measurement result. The software flow is shown in Figure 8.

In the system, the main function of DS2430A is to provide the microprocessor with TEDS stored inside it. To achieve communication with DS2430A, it is a problem to master the time sequence of signal transmission and reception of 1Wire device. To ensure data integrity, DS2430A has strict requirements for communication protocols. The communication protocol of DS2430A mainly consists of four signal types: initialization signal (including a reset pulse and an answer pulse), writing 0, writing 1, and reading data. In these signals, except for the response pulse, they are all sent by the bus control unit.

　　Initialization signal: An answer pulse from a reset pulse indicates that DS2430A is ready to receive ROM commands. The DSP first sends out a reset pulse (TX), then releases the bus to receive (RX) state. 1Wire bus is pulled up to high level through pull-up resistance. After DS2430A detects the rising edge of the data pin, it waits for tPDH to send out a response pulse.

　　Read and write signals:All read and write sequences start with the DSP pulling down the data line. The falling edge of the data line triggers a delay circuit inside the DS2430A to synchronize it with the DSP. In the write sequence, the delay circuit determines when DS2430A will sample the data lines. For the read sequence, if the data to be transmitted is "0", the delay circuit will determine how long DS2430A will pull down the data line that has been set high by DSP. If the data to be transmitted is "1", DS2430A will not change the state of the data line during the reading sequence.

The subprogram that controls the reading and writing time of DS2430A can be found on the website www.dpj.com.cn.

　　epilogue

This paper focuses on the Plug and Play sensor, which takes the pressure sensor as an example: the system accurately identifies the pressure sensor to which it is connected by the 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 factor, electrical interface, etc.). The front-end circuit is configured accurately according to the information contained in the identification module. The Plug and Play sensor program aims to create an open sensor standard that enables system integrators and end users to automatically set up measurements on sensors and automatically control the system. Users can download TEDS binaries or virtual TEDS to their systems to make the original sensor "plug and play".

Another important significance of this topic is that the wireless communication technology is applied to the network sensor, which makes the signal connection break through the space limitation. The extension of wireless communication technology provides more new choices in the field of measurement. In the industrial field, short-range wireless connection has a wide application requirement. Using Bluetooth technology in the industrial field, replacing infrared with microwave, not only overcomes the shortcomings of infrared, but also reduces the cost.

Reference:

[1].DSP56311EVM Datasheethttp://www.dzsc.com/datasheet/DSP56311EVM_2526484.html.
[2].24PCCFA6D Datasheethttp://www.dzsc.com/datasheet/24PCCFA6D_1055105.html.
[3].DS2430 Adatasheethttp://www.dzsc.com/datasheet/DS2430A_1056110.html.
[4]. DS1804 Datasheethttp://www.dzsc.com/datasheet/DS1804_1056042.html.
[5]. PB4 Datasheethttp://www.dzsc.com/datasheet/PB4_1202140.html.
[6]. PB5 Datasheethttp://www.dzsc.com/datasheet/PB5_1139269.html.
[7]. Maximdatasheethttp://www.dzsc.com/datasheet/Maxim_1062568.html.
[8].MAX1065 Datasheethttp://www.dzsc.com/datasheet/MAX1065_1089278.html.
[9].RS232 Datasheethttp://www.dzsc.com/datasheet/RS232_585128.html.
[10].DSP56311 Datasheethttp://www.dzsc.com/datasheet/DSP56311_979962.html.

Source:Xiang Xueqin