1 Introduction
Recently, Honeywell has launched the PPT series, PPTR series and PPTE series to realize networked intelligent precision pressure sensors. These sensors combine varistor sensors, A/D converters, microprocessors, memory (RAM, E 2PROM) and interface circuits to achieve high performance specifications and greatly facilitate users. These products can be widely used in industrial, environmental monitoring, automatic control, medical equipment and other fields.
2 PPT, PPTR series pressure sensor performance
(1) PPT series sensor adopts steel diaphragm with RS-232 interface and the sensor distance is less than 18m. It is suitable for measuring the pressure of various non-flammable and non-corrosive gases or liquids (ie gauge pressure) which change rapidly or slowly. Differential pressure and absolute pressure, measurement accuracy of up to ±0.05% (typical at full scale), while the past integrated pressure transmitter can only achieve an accuracy of ±0.1%. The PPTR series has an RS-485 interface with a transmission distance of several kilometers. It uses a stainless steel diaphragm to measure corrosive liquids or gases with a measurement accuracy of ±0.1%.
(2) It belongs to the network sensor. When the network is formed, the global address, group address and device identification number ID of each sensor can be determined, which can realize data exchange and resource sharing between sensors and between sensors and systems. Users can obtain data of any sensor through the network. The parameters of the sensor are set, and the set parameters are saved in the E2PROM.
(3) It can output the calibrated pressure digital quantity and analog quantity. It is a precision digital pressure sensor and an analog standard pressure sensor. The analog output voltage is continuously adjustable from 0 to 5V, which can be used as standard pressure. The signal source is used. The user can get the analog output without the host.
(4) Serial communication can be performed with the PC through the interface circuit, and up to 89 sensors can be attached to one PC. There are 7 baud rates to choose from for serial communication up to 28800 bit/s. The default baud rate after power-on is 9600 bit/s. The number format is 1 start bit, 8 data bits, and 1 stop bit.
(5) There are 12 pressure units to choose from, including International System of Units Pa (Pascal), Non-International System of Units P0 (atmospheric pressure), bar (bar), mmHg (mmHg), etc. The basic pressure potential is psi (lbs). / square inch). The range is from 1 psi to 500 psi (ie 6.8946 kPa to 3.4473 MPa) with a total of 10 specifications.
(6) The internal integrated temperature sensor is used to detect the temperature of the sensor and compensate the pressure. The temperature measurement error is less than 0.5 °C.
(7) The power supply voltage ranges from 5.5 to 30V, the operating current is 15 to 30 mA, and the operating temperature range is -40 to +85 °C.
3 Working principle
3.1 pin function
The physical dimensions of the PPT and PPTR series of intelligent pressure sensors are both 24.8mm × 62.2mm. It has two pressure ports P1 and P2. Take the PPT series as an example. The P1 port is suitable for non-flammable, non-corrosive liquid or gas, and the P2 port can only be connected to gas. The first leg of the 6-pin socket is the positive end of the RS-232 interface, and the second leg is the negative terminal. The third pin is the case ground (GND), the fourth pin is the DC power input (US), and the sixth pin is the analog voltage output (UO). A to F on the PPTR series sockets correspond to 1 to 6 feet in sequence, except that the A and B pins are the positive and negative ends of the RS-485 interface, respectively. In order to reduce the noise, a single point grounding is required for the analog output. The reference ground of the power ground and measuring instrument (such as 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. Mainly includes 8 parts: 1 pressure sensor; 2 temperature sensor; 316-bit A/D converter; 4 microprocessor (umP) and random access memory (RAM); 5 rewritable read-only memory (E2PROM); 6RS- 232 (or RS-485) serial interface; 712-bit D/A converter (DAC); 8 voltage regulator. Except for the specific serial number, all PPT and PPTR series are denoted by PPT, and are referred to as PPT units for short.
The core component of the PPT unit is a silicon piezoresistive sensor with pressure and temperature sensitive components. Digital signals representing temperature and pressure are sent to the mP for processing, and temperature compensated and pressure calibrated outputs are available from -40 to +85 °C. The output form is shown in Table 1. When measuring the rapidly changing pressure, the tracking input mode can be selected, and the sampling rate threshold is preset. When the measured pressure fluctuates within the threshold range, the sampling rate is automatically doubled. Once the pressure stabilizes, the normal sampling rate is restored. The PPT also features an idle count function that automatically skips 255 intermediate readings when measuring steady or slowly changing pressures, extending the time interval between outputs. In addition, it can also be set to the operating mode that is output 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 PPT, the integration time of A/D converter can be set within the range of 8ms~10s.
PPT can provide three levels of addressing. The lowest level addressing mode is the device identification number ID. This address level allows address assignment for any single PPT. The ID is allocated from 01 to 89. 00 is an empty address and is dedicated to PPTs with unspecified 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 E 2PROM), it is assigned as an empty address after power-on. The second-level addressing mode is group address, and the address range is 90-98, a total of 9 groups. With the ID command, each PPT can be assigned a group address, allowing the host to pass instructions to several PPTs with the same group address. The default value for group addresses is 90. The most advanced addressing mode is the global address, which is 99. The host can connect 9 groups of a total of 89 PPTs through the serial port.
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