Secondary instrument design of flow meter based on vortex flow sensor AT89C51/LV51 microcontroller

1 Introduction

With the continuous improvement of the automation level of modern industry, in many cases it is necessary to monitor multiple flow points intensively. For example, in the process of oil water injection extraction, in order to maintain the mining effect, protect the underground environment and subsequently analyze the water injection data, the water injection volume needs to be monitored. , smart instruments are gradually used in oilfield systems to measure the oil production of oil wells. The accuracy of measuring instruments directly affects the economic benefits of an enterprise. At present, my country's flow meter started late, with a low starting point and is still relatively backward. The flow meter developed using a single-chip microcomputer is very suitable as a secondary instrument and will greatly promote oilfield automation.

2. System principle

In terms of the overall structure, the flow meter mainly consists of a vortex flow sensor, a preamplifier, and a counting display unit. The vortex flow sensor is mainly composed of a vortex generator, a vortex frequency detector and a housing. Its function is to generate stable and strong vortices, detect the frequency of the vortex and output an electrical signal proportional to the vortex frequency. The vortex generator is an important component of the vortex flowmeter. In order to generate a strong and stable vortex and have a constant Strouhal number within a wide pipe Reynolds number range to ensure the linear output of the meter, the vortex generator There must be a certain proportion of shape, structure and size, which is determined through practice.

Generally speaking, since the signal output by the detection element is relatively weak and contains noise, the preamplifier of various detection methods should also include amplification, filtering, shaping and other parts, which together are also called primary instruments.

The counting display unit is also called the secondary instrument of the flow meter. It is generally composed of a single-chip microcomputer system to form a flow display instrument. It counts the signals output by the converter to realize the functions of recording, storing, calculating and setting the flow rate. In addition, the LCD display must be controlled to display information such as the measured instantaneous flow rate and total flow rate. Some flow meters also have communication interfaces for data communication with remote computers to achieve dynamic monitoring.

Instruments developed with microcontrollers have multiple functions. The intelligent flow indicator whose nonlinear error is adjusted by software is a secondary instrument matched with various pulse output primary flow measurement instruments. It has the following characteristics:

●Using microprocessing technology, high reliability;

●Using micro-power consumption devices, it consumes less power and is suitable for battery power supply;

●Can display cumulative flow and instantaneous flow at the same time;

●Using intelligent multi-stage flow software compensation technology, high measurement accuracy;

●Can store traffic.

Its technical properties are described below.

The input signal is the frequency input signal of the primary instrument, which is connected to the flow sensor. After detection and amplification, the flow rate of the measured medium is obtained. The signal frequency is a pulse signal of 0MHz~1000Hz. The length of the sensor connecting cable is less than 1,000m, the wire capacitance is less than 0.1μF, and the inductance is less than 10mH. The intelligent flow indicator (secondary instrument) is placed in the measurement room, and each measurement room can accommodate 30 indicators.

There are two types of output signals: the cumulative flow display is an 8-digit floating point type, starting with 5 digits after the decimal point (0.000 00m3), and finally displaying 3 digits after the decimal point (0 000.000m3), indicating the cumulative flow after the instrument starts working; instantaneous The flow display is a 4-digit floating point type. It starts with 2 digits after the decimal point (00.00m3) and finally displays 1 digit after the decimal point (000.Om3), indicating the current instantaneous flow rate.

Anti-interference: 10kW~17kW motor rotates, and the display does not change.

Accuracy: About 3% without correction, ±0.01% after correction.

Flow correction range: Compensation correction is carried out in 8 sections within the entire process range.

Set the compensation coefficient bit of accumulated flow: up to 8-bit integer.

Power supply: 4V~5V DC.

Total power consumption: <1.0mW.

With LB undervoltage indication.

Ambient temperature range: -20℃～70℃.

Relative humidity: «85%.

Flameproof enclosure.

3. Hardware composition

The core of the hardware is the AT89C51/LV51 microcontroller produced by ATMEL. It is a low-power, low-voltage, high-performance 8-bit microcontroller. There is a 4K-byte Flash programmable and erasable read-only memory on the chip. , using CMOS technology and ATMEL's high-density non-volatile memory technology, its output pins and instruction system are compatible with the MSC-5l microcontroller.

The memory used to store traffic is ATMEL's AT24C16 series, which is an electrically erasable, programmable read-only memory with 2K bytes of Flash. Microcontroller. The RXD port and TXD port are respectively connected with its SCI. Connected to SDA for read and write operations.

When measuring, use a 1s hardware reset circuit to connect. The RE-SET terminal enables the microcontroller to reset regularly. The microcontroller works in power-saving mode and waits for wake-up after each input, accumulation, and display processing, and the working power consumption is very small. When setting the compensation coefficient, the reset pin is connected normally.

After the input pulse is divided by frequency and latched by 74HC393, it is sent to the P2 port of the microcontroller. After being processed by the program in the microcontroller, the output display signal is sent to the P1 port (4 bits of data, 4 bits of selection signal). 4 bits of the data are sent to MC14543 (BCD latch/7-segment decoding/driver). There are 12 MC14543s, which respectively control the display data of the liquid crystal block connected to the 8-bit cumulative flow rate and the 4-bit instantaneous flow rate. 4 The bit selection signal is sent to the CD4514 type decoder (4-bit latch, 4-16 line decoder) to select the MC14543 type circuit that receives the data and the decimal point of the floating display.

The hardware is divided into two parts: the control part processes the flow signal and obtains the display data of cumulative flow and instantaneous flow, as shown in Figure 1; the display part sends the display data to the LCD screen, as shown in Figure 2. These two parts are connected through the 10-pin socket J1.

4. Software design

The main program block diagram is shown in Figure 3.

5. Usage effects and improvements

This intelligent flow indicator has been used for on-site measurement for more than 5 years. It is reliable in operation, accurate in measurement, very cost-effective, and easy to expand its functions. Functions such as flow alarm and flow control have been added. When the flow rate exceeds the set alarm value, a voice alarm can be issued; when the flow control parameters are set, the flow rate can be automatically adjusted. The instrument can also be combined with the industrial configuration software MCGS or KingView to serve as a slave computer to collect flow data and send it to the configuration server for processing.