Principle and application of intelligent precision pressure sensor based on network

Recently, Honeywell of the United States has launched the PPT series, PPTR series and PPTE series of intelligent precision pressure sensors that can be networked. These sensors integrate piezoresistive sensors, A/D converters, microprocessors, memories (RAM, E2PROM) and interface circuits, which not only achieve high performance indicators, but also greatly facilitate users. These products can be widely used in the fields of industry, environmental monitoring, automatic control, medical equipment, etc.

2 PPT, PPTR series pressure sensor

Performance (1) PPT series sensors use steel diaphragms and have RS-232 interfaces. The sensor distance is no more than 18m. They are suitable for measuring the pressure (i.e. gauge pressure), differential pressure and absolute pressure of various non-flammable, non-corrosive gases or liquids that change rapidly or slowly. The measurement accuracy is as high as ±0.05% (typical value at full scale), while the previous integrated pressure transmitters can only achieve a maximum accuracy of ±0.1%. PPTR series products have RS-485 interfaces and can reach a transmission distance of several kilometers. They use stainless steel diaphragms and can measure corrosive liquids or gases with a measurement accuracy of ±0.1%.

(2) It is a network sensor. When forming a network, it can determine the global address, group address and device identification number ID address of each sensor, and can realize data exchange and resource sharing between sensors and between sensors and systems. Users can obtain data from any sensor through the network and set the parameters of the sensor. The set parameters are saved in the E2PROM.

(3) It can output calibrated digital and analog pressure. It is both a precision digital pressure sensor and an analog standard pressure sensor. The analog output voltage is continuously adjustable within the range of 0 to 5V and can be used as a standard pressure signal source. Users can obtain analog output without a host.

(4) It can communicate with a PC via an interface circuit. A maximum of 89 sensors can be connected to a PC. There are 7 baud rates to choose from during serial communication, up to 28800 bit/s. The default baud rate after power-on is 9600 bit/s. The digital format is 1 start bit, 8 data bits, and 1 stop bit.

(5) There are 12 pressure units to choose from, including the international unit Pa (Pascal), non-international unit P0 (atmospheric pressure), bar (bar), mmHg (millimeters of mercury), etc. The basic pressure potential is psi (pounds per square inch). The range is from 1psi to 500psi (i.e. 6.8946kPa ~ 3.4473MPa), with a total of 10 specifications.

(6) The internal integrated temperature sensor is used to detect the sensor temperature and compensate the pressure. The temperature measurement error is less than 0.5℃.

(7) The power supply voltage range is 5.5~30V, the operating current is 15~30mA, and the operating temperature range is -40~+85℃.

3 Working Principle

3.1 Pin Function
The dimensions of the PPT and PPTR series intelligent pressure sensors are both 24.8mm×62.2mm. It has two pressure ports, P1 and P2. Taking the PPT series as an example, the P1 port is suitable for non-flammable, non-corrosive liquids or gases, and the P2 port can only be connected to gases. The first pin on the 6-pin socket is the positive end of the RS-232 interface, and the second pin is the negative end. The third pin is the shell ground terminal (GND), the fourth pin is the DC power input terminal (US), and the sixth pin is the analog voltage output terminal (UO). A to F on the PPTR series socket correspond to pins 1 to 6 in turn, and the only difference is that pins A and B are the positive and negative ends of the RS-485 interface, respectively. In order to reduce noise, single-point grounding is required for analog output. The power ground and the reference ground of the measuring instrument (such as a digital voltmeter) should be directly connected to the signal ground of the sensor.

3.2 Working Principle
The internal block diagram of the PPT intelligent pressure sensor is shown in Figure 1. It mainly includes 8 parts: ① pressure sensor; ② temperature sensor; ③ 16-bit A/D converter; ④ microprocessor (umP) and random access memory (RAM); ⑤ electrically erasable read-only memory (E2PROM); ⑥ RS-232 (or RS-485) serial interface; ⑦ 12-bit D/A converter (DAC); ⑧ voltage regulator. In the following text, except for the specific serial number, all PPT and PPTR series are represented by PPT and referred to as PPT units.

The core component of the PPT unit is a silicon piezoresistive sensor, which contains components that are sensitive to pressure and temperature. The digital signals representing temperature and pressure are sent to the mP for processing, and the output after temperature compensation and pressure calibration can be obtained in the range of -40 to +85°C. The output form is shown in Table 1. When measuring rapidly changing pressure, the tracking input mode can be selected, and the threshold of the sampling rate can be pre-set. When the measured pressure fluctuates within the threshold range, the sampling rate is automatically doubled. Once the pressure tends to stabilize, the normal sampling rate is restored. The PPT also has an idle counting function. When measuring stable or slowly changing pressure, it can automatically skip 255 intermediate readings and extend the time interval between two outputs. In addition, it can also be set to a working mode in which the output is only when the pressure exceeds the specified value or when the host queries. In order to adapt to different environments and improve the anti-interference ability of the PPT, the integration time of the A/D converter can be set in the range of 8ms to 10s.

PPT can provide three levels of addressing. The lowest level of addressing is the device identification number ID. This address level allows address allocation for any single PPT, and the ID allocation range is 01 to 89. 00 is a blank address, which is dedicated to PPTs with unassigned addresses. Therefore, a host can be equipped with up to 89 PPTs. If a PPT is not assigned an ID address (or the ID is not stored in the E2PROM), it will be assigned an empty address after power-on. The second level of addressing is the group address, with an address range of 90 to 98, for a total of 9 groups. Through the ID instruction, each PPT can be assigned to a group address, allowing the host to send instructions to several PPTs with the same group address. The default value of the group address is 90. The highest level of addressing is the global address, which is 99. The host can connect 9 groups of 89 PPTs in total through the serial port.

4 Application of Intelligent Pressure Sensor

4.1 Configuration of PPT analog output
Using a single PPT can replace the traditional analog pressure sensor. Its biggest advantage is that it can achieve high precision without calibration. The wiring of PPT analog output and measuring instrument is shown in Figure 2. Users can read the exact value of pressure through a digital voltmeter (DVM) or use an analog voltmeter (V) to observe the pressure change process and trend. After the PPT is set up, it can also receive valve control signals from the control processor while transmitting pressure data to achieve automatic pressure regulation, which is very useful for pressure measurement and control systems. Valve control data can be independent of pressure data.

4.2 Transmission and recording of remote analog pressure signals
The analog signal of PPT can be directly sent to the recorder to record the pressure waveform, but it is easily affected by line interference and environmental noise when transmitting analog signals over long distances, and it will also cause signal attenuation. To solve the above problems, a PPT can be added to the terminal as shown in Figure 3. First, PPT1 sends the pressure data, then transmits it remotely to PPT2, and then the analog output of PPT2 is connected to the recorder. This method is suitable for RS-485 interface and the transmission distance can reach several kilometers. If a PPT series sensor with RS-232 interface is used, a driver and repeater need to be added. Another advantage of this solution is the fast transmission rate. When the baud rate is selected as 28800bit/s, the delay time caused by data transmission does not exceed 2ms.

4.3 PPT and host wiring
The PPT and host wiring is shown in Figure 4. D9 and D25 in the figure represent the 9-pin and 25-pin sockets on the host, respectively.

4.4 RS-485 multi-point network
The RS-485 network takes the host as the starting point and the farthest end from the host as the end point. It adopts a multi-point network structure, also known as a star network structure. This network not only has a long transmission distance, but also can increase and decrease the number of PPTs without disconnecting the network. RS-485 can only connect up to 32 PPT units, which can be expanded to 89 PPT units using a repeater. At the beginning and end of RS-485, a 120Ω resistor needs to be connected in parallel as a matching load.

An RS-485 multipoint network with 6 PPT units is shown in Figure 5. In this network, the ID addresses of the PPT units can be arranged in any order. The following example introduces the process of transmitting global addresses and assigning group addresses:

(1) First, send the global command *99WE and *99S=00001234, which will make the PPT unit with ID number #00001234 specify its own ID number before the next command and be ready to receive new commands. Then send the command *99WE, *99ID=02, *02WE and *02SP=ALL to complete the address allocation of the device ID. Repeat the above process on the RS-485 network to complete the ID address allocation;

(2) Assigning group addresses: Once the device ID is set, the group ID can be assigned. Each PPT unit in the same group must have a subaddress starting with 01. The subaddress will tell each PPT unit the order in which to respond to the group address instruction. To set the group address of device ID = 02 to 91, subaddress 01, you can send the following command: *02WE, *02ID = 9101, *02WE, *02SP = ALL. When the first instruction is sent to group 91, the unit with ID = 02 will be the first to respond.