Design of digital vortex flowmeter based on PIC32

• research background and meaning

1. Overview of vortex flowmeter

When a non-streamlined vortex generator is placed in the fluid, the fluid will be separated alternately on both sides of the generator and release two strings of regularly staggered vortices. Within a certain range, the vortex separation frequency is proportional to the flow rate. The flow meter that measures flow based on this principle is called a vortex flowmeter. The vortex flowmeter has the advantages of small pressure loss, wide range, high accuracy, simple and firm structure, easy maintenance, low installation cost, long service life, and wide application range. Therefore, it is widely used in the flow measurement of industrial pipeline medium fluids.

2. Research status of vortex flowmeter

Although vortex flowmeters have many advantages and are widely used in industrial production, there are still some problems with existing vortex flowmeters, which affect their further promotion and application. For example, the vortex flowmeter has a high lower limit range, poor resistance to mechanical vibration interference, and inconvenient online parameter modification. Therefore, solving the problems of small flow measurement and anti-interference of vortex flowmeters is of great significance to improving the measurement accuracy and reliability of existing vortex flowmeters. At the same time, optimizing the operation interface of the vortex flowmeter can greatly facilitate the use of users.

3. The significance of vortex flowmeter research

As mentioned above, vortex flowmeters have many advantages, but also many problems that need to be solved. Our research will focus on solving the three problems of small flow measurement, anti-interference and optimizing the operation interface. By introducing temperature and pressure compensation, digital signal processing, and USB data transmission, we can solve the bottleneck problem that has long hindered the development and application of vortex flowmeters, further improve the reliability, accuracy and practicality of vortex flowmeters, and promote the application of vortex flowmeters in industry.

• Features of the research project

2.1 Innovation

⑴ The current vortex flowmeter does not consider the impact of environmental factors such as temperature and pressure on flow measurement, and the measurement accuracy cannot be improved. Our research will add temperature and pressure compensation functions to make the measurement process unaffected by environmental factors and the results more accurate and reliable;

(2) Since the current vortex flowmeter uses counting to calculate flow velocity, when the flow velocity is low, the counting pulse signal weakens, resulting in inaccurate measurement results of small flow. Our research will use the method of software and hardware synthesis filtering to extract and amplify the weak low flow velocity signal to achieve accurate measurement of low flow velocity;

⑶ The current vortex flowmeter uses table lookup and linear fitting to calculate the flow rate. Due to the existence of fitting errors, the measurement results have large errors. Our research will use analytical calculation formulas and add temperature and pressure compensation to achieve accurate calculation of the flow rate;

⑷ The current vortex flowmeter is difficult to eliminate the vibration noise inside the pipeline, which causes large measurement errors. Our research will apply the digital Fourier algorithm to eliminate the noise component in the vortex signal and improve the anti-interference ability of the vortex flowmeter;

⑸ By adding the USB data transmission function, online download of flow data and online modification of measurement parameters can be realized, making the use of the flow meter more convenient and flexible.

2.2 Realizability

⑴The high-speed computing performance of the PIC32 series microcontrollers makes complex digital signal processing and calculation possible;

⑵The rich peripherals of the PIC32 series microcontrollers simplify the design of the external circuit of the vortex flowmeter, making the flowmeter smaller, lower in cost and more reliable;

⑶The OTG module of the PIC32 series microcontroller reduces the amount of analysis of the USB data transmission protocol and shortens the development cycle of USB data transmission.

• Design

3.1 Hardware Design

The whole system consists of two parts: the host and the slave. The host is a field instrument with functions such as flow measurement, display and data recording. The slave is a handheld management instrument. Users can manage the field instrument through a USB cable. The management content includes downloading measurement data and modifying measurement parameters. The specific hardware design is shown in the figure below.

3.2 Software Design

The software program consists of three parts. The first part is the startup initialization program, which completes the initial state setting of the vortex flowmeter. The second part is the measurement main program, which realizes the collection, processing, calculation and display of flow signals. The third part is the USB data transmission interrupt program, which is triggered by a button to realize data exchange between the host and the slave.

3.3 MCU Selection

According to the design requirements, the selected microcontroller should have the following basic configuration:

⑴80MHz main frequency;

⑵32-bit bus design;

(3) 32kB RAM space;

⑷128kB Flash storage space

⑸2 interrupt signal channels

⑹4 A/D conversion channels;

⑺4-way I/O conversion interface;

⑻USB OTG hardware analysis module;

⑼1 D/A conversion channel.

Therefore, we choose PIC32MX440F128H and above microcontrollers as the processing chip of the vortex flowmeter.