JPH01166299A - Two-wire differential pressure transmitter - Google Patents

Abstract

PURPOSE:To easily reduce expense and to easily hold the measuring accuracy of a transmitter by executing a zero point calibration under a state in which a differential pressure transmitter is fitted to a plant. CONSTITUTION:Valves 2-4 are provided between a digital differential pressure transmitter 5 and a process line 1. These valves are connected so that they can open and close by a microcomputer, the valves 2-4 are controlled from the microcomputer so that all of them can close, and a pressure sensor at this time is read. The microcomputer makes a multiplexer MPX into a differential pressure sensor input mode after closing a signal to close the valves 2-4. After that, the microcomputer takes the signal of a differential pressure sensor in itself. The taken-in value is written in an EEPROM. After the writing, the valves 2-4 are opened. The correction of the differential pressure sensor can be obtained by a prescribed calculating direction from the zero point value written in the EEPROM. Thus, the zero point calibration of the differential pressure sensor can be attained.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for calibrating a digital differential pressure transmitter,
In particular, by controlling the three valves installed at the input end of a conventional transmitter with a digital differential pressure transmitter, a digital differential pressure transmitter is suitable for performing high-precision calibration without removing the transmitter from the process line. Regarding transmitters.

[Conventional technology]

Conventional calibration of differential pressure transmitters involves removing the differential pressure transmitter from the plant and transporting it to the location where the calibration equipment is located.
This had to be done with the plant stopped, which required a great deal of effort.

[Problem that the invention seeks to solve]

The present invention makes it possible to perform zero point calibration with the differential pressure transmitter installed in the plant, which was performed by removing it from the plant in conventional differential pressure transmitters, thereby reducing the cost required for one calibration and the difference The purpose of the above is to provide a method for improving the accuracy of a pressure transmitter. 6 [Means for solving the problem] The above purpose is to provide a method for improving the accuracy of a pressure transmitter. The three valves that control the can be controlled with a digital differential pressure transmitter,
The three valves are controlled by a microcomputer to 0N10F.
FL, calibrate the zero point of the differential pressure transmitter by measuring the signals of each sensor at that time. Valve is turned on by electric signal
An electronically controlled valve capable of 10FF is used, and the microcomputer is provided with a control signal output terminal for the electronically controlled valve. The three valves are a first valve that connects two pressure ends of the differential pressure transmitter, and a second and third valve that are connected in series to each pressure input.

[Effect]

When the second and third valves are closed using a digital differential pressure transmitter with a built-in microcomputer, and then the valve of cylinder 1 is closed, the differential pressure applied to the differential pressure sensor becomes zero, and the value of the differential pressure sensor at that time is calculated. The amount of offset of the differential pressure sensor can be measured. The offset amount is compared with the value measured at the factory, and is reflected in the arithmetic processing method used when transmitting subsequent differential pressure data using a two-wire system, thereby improving the measurement accuracy of the differential pressure transmitter. In addition, the zero point calibration of the differential pressure transmitter, which was previously performed when the differential pressure transmitter was removed from the plant, can be performed with the differential pressure transmitter installed in the plant, thereby reducing the cost required for calibration.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
The figure shows a state in which a digital differential pressure transmitter employing the present invention is installed in a process.

Three valves are placed in the middle of the piping that leads the process pressure to the differential pressure transmitter.The first valve connects the two piping, and the second and third valves are installed between the transmitter and the line. . The digital differential pressure transmitter is connected to a receiving device through two wires, and transmits differential pressure measurement data by controlling the current supplied from the receiving device to 4 to 20 mA. The communicator 6 is connected in parallel with the examination device and communicates with the digital differential pressure transmitter to diagnose, calibrate, change the operating pressure range, etc. of the transmitter. The three valves are connected so that they can be controlled on and off by a digital differential pressure transmitter.

FIG. 2 shows the configuration of the digital differential pressure transmitter, and FIG. 3 shows a flowchart showing the operation of the transmitter.

The MPU (hereinafter referred to as a microcomputer) converts information on differential pressure, temperature, and static pressure in the process into electrical signals at the pressure receiving section, and converts them into digital values using an analog/digital converter (hereinafter referred to as the A/D converter). get it. Here, an example of a method for obtaining differential pressure information will be explained. The pressure receiving section, which converts the physical quantities of the process into electrical signals, is equipped with a composite sensor that can measure differential pressure, temperature, and static pressure (absolute pressure). The microcomputer gives a signal to the MPX (multiplexer) to select a differential pressure sensor from among the composite sensors to be connected to a programmable gain amplifier (referred to as PGA). By the way, the output of the sensor also changes depending on the differential pressure applied to the sensor.

The resolution of the A/D converter is constant. The PGA is provided in order to use a method of amplifying the sensor signal with an appropriate gain in order to make the most of the ability of the A/D converter. The microcomputer sets the gain of the PGA to an appropriate value, generally the highest gain that gives t'l rr to the most significant bit of the A/D converter and that does not overflow. Next, A
/Gives a conversion start signal to the D converter. When a conversion end signal is obtained from the A/D converter, the A/D conversion result is written into a predetermined measurement differential pressure signal area in the RAM. In this way, differential pressure information is captured. Temperature and static pressure information are also taken in using a similar method. Here, the obtained information is processed according to the procedure stored in the microcontroller in advance and converted into digital/digital data.
Analog converter (hereinafter referred to as

D/A converter), voltage-current converter (hereinafter referred to as V
-1 conversion device).

Here, the characteristics of the differential pressure, temperature, and static pressure sensor of the pressure receiving part are E
The output signal of each sensor is stored in an EFROM (electrical write/erase memory), and after being digitized by an A/D converter, the output signal of each sensor is referred to the contents of the EEPROM, converted to each physical quantity, and further converted into a working pressure range. The output current is converted into an output current according to the output current, and transmitted to the receiving device via a D/A converter and a V-1 converter. The zero point data of the differential pressure sensor is also stored in the EEFROM. When a pressure sensor is used for a long time, the zero point changes. Therefore, it is very necessary for the transmitter to periodically calibrate the zero point. Therefore, conventionally, the transmitter was removed from the process and calibrated when the process was stopped.

The present invention realizes this zero point calibration using a transmitter with a built-in microcomputer without removing the transmitter from the process. Three valves as shown in FIG. 1 are provided between the transmitter and the process, the method of which will be described below. The valves are connected to a microcomputer so that they can be opened and closed, and the microcomputer controls all three valves to close. At this time, the differential pressure sensor is read. An example of the control program is shown in FIG. 4. After the microcomputer outputs signals to close the three valves, it puts the MPX into differential pressure sensor input mode. The rest is to import the signal of the differential pressure sensor into the microcontroller in the same way as a normal program, and the value imported at this time is stored in the EEFROM (
Electrical write/electrical erase FROM) After writing, open the three valves.

One method for starting the program is as shown in FIG. 2, in which a starting signal is sent from a communicator and received by the microcomputer via the V/1 conversion circuit and communication circuit in the transmitter. When the microcomputer receives the activation signal, it temporarily suspends the normally executed program shown in FIG. 3, executes the program, and after finishing, executes the program shown in FIG. 3 again. The differential pressure signal DP output from the D/A converter is calculated by the following formula.

DP=DP1+δP ・・・・・・(1
) DPI is the differential pressure signal output value obtained by a predetermined procedure from the signals of the differential pressure sensor, temperature sensor, and static pressure sensor, and δP is the zero point correction value of the differential pressure sensor, as shown in Figure 3. It is calculated using a predetermined calculation method from the zero point value written in the EEFROM by the program. This allows zero point calibration of the differential pressure sensor.

〔Effect of the invention〕

With the method of the present invention, the differential pressure transmitter can be used to perform zero point calibration by removing the transmitter in the past, but can be done by issuing commands from a communicator or the like without removing it from the process. Measurement accuracy can be easily maintained.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing an example of the use of a digital differential pressure transmitter which is an embodiment of the present invention, Fig. 2 is a block diagram of a digital transmitter with a built-in microcomputer, and Fig. 3 is a diagram showing the operation of the transmitter. The flowchart, FIG. 4, is a diagram showing the control program. 1... Process line, 2... First valve, 3...
- Second valve, 4... Third valve, 5... Digital differential pressure transmitter, 6... Communicator, 7... Receiving device. 1st I ¥1 Ka 2 Mouth Figure 3

Claims (1)

[Claims] 1. A two-wire measurement, calculation, and communication device that performs measurement and calculation processing on process variables and communicates data corresponding to the process variables as changes in current from a supplied power source, and controls the supply of process variables to the device. A two-wire differential pressure transmitter, characterized in that the measurement calculation communication device controls a process variable supply control device and calibrates the measurement calculation communication device.