Design of vector control system for AC asynchronous motor

1. Introduction

In the field of industrial flaw detection, various problems in the welding process will cause defects such as pores and cracks in the weld, affecting the quality of the product, so the detection of defects in the welding image is very important. Influenced by the technical quality and experience of the film reviewers of traditional X-ray weld image detection, the detection of weld defects has gradually transitioned from manual film review to computer intelligent recognition. The use of X-ray real-time detection system for online detection and analysis can effectively overcome the misjudgment caused by manual film review, so that the online detection of weld film defects can be objective, standardized and standardized. It is a high-tech project that is born from the effective combination of many disciplines such as computers, automatic control, mechanical transmission, and non-destructive testing. Its successful development will make the non-destructive testing technology of oil and gas pipeline welds in my country have a "qualitative" leap, and bring a huge impact on the traditional process of weld detection. The portable weld film digital detector has a greater development prospect because its portability can bring great convenience to customers.

The design and development of a portable weld film digital detector requires the following work to be completed. The system can be divided into the following four parts according to its functions:

(1) Mechanical part: The mechanical part includes the film transmission mechanism and the automatic control part. The film transmission mechanism drives the weld film to move in the horizontal direction. This part is composed of wear-free guide rails, rubber transmission wheels, synchronous transmission belts and stepper motors.

(2) System lighting: A cold light source with an energy-saving electronic ballast is used to illuminate the target field of view.

(3) Image acquisition part: This part uses a high-speed industrial camera to acquire images, which can ensure image quality and scanning speed compatibility.

(4) Pattern recognition part: The computer preprocesses, segments and recognizes patterns of the images captured by the camera, extracts weld defects and classifies them.

This paper mainly designs the hardware structure of the portable digital weld film detector. For the design of the mechanical part, the film transmission mechanism must be designed first to make it portable. Then the single-chip microcomputer is used to control the stepper motor to realize the movement of the film transmission mechanism. Design the circuit control schematic diagram and write the single-chip microcomputer control program.

2. Overall design

According to the requirements of nondestructive testing of weld films of oil and gas pipelines, the system structure design is shown in Figure 1.

The portable digital weld film detector is mainly composed of two parts: a laptop computer and an acquisition system. The main components of the acquisition system are: line scan camera, film transmission mechanism, lighting system, stepper motor control circuit board, stepper motor driver and stepper motor, etc.

This system first completes the digital input of the X-ray film, and then processes and analyzes it. Its working principle is as follows: Under the coordinated work of the cold light source lighting system, the heat dissipation system and the power supply system, the host computer and the slave computer establish various information data communications, the computer sends film moving commands and moving direction commands to the serial interface, the single-chip microcomputer receives and processes related commands, and sends CP signals and DIR signals to the stepper motor driver in the mechanical servo transmission system. The driver subdivides the CP signal and identifies the DIR signal to drive the three-phase hybrid stepper motor to work. The stepper motor drives the film transmission mechanism through the synchronous belt and drives the film to run smoothly in a certain direction; when the film passes directly under the industrial camera, the host computer software accurately controls the camera electronic shutter action, combined with the lighting system, the camera accurately collects the film image frame to the host computer in a line scanning manner; the host computer then divides the collected image data for synchronous processing, sends part of it to the computer for auxiliary processing, and sends the other part to the graphic image processing board for processing; dumps the processed large batch of image data into the mass storage system to complete the entire workflow.

The volume of the acquisition system shell is 200mm×200mm×400mm, and the weight does not exceed 10kg. The main layout is shown in Figure 2:

3. Film transmission mechanism design

Since the width of industrial radiographic film is not fixed, in order to improve work efficiency and work quality and ensure the accuracy of the film detection, it is very necessary to design a continuously adjustable weld film detection system. In this paper, we can achieve accurate positioning of industrial radiographic films of different widths (70mm-120mm) by adjusting the width of the side plates at both ends of the film transmission mechanism to ensure the centering of the camera. The design of the film transmission mechanism is shown in Figure 3:

4. Stepper control

The stepper motor is one of the main control elements of industrial process control. It is an electromagnetic mechanical device that converts electrical pulse signals into corresponding angular displacement or linear displacement, and has the ability to start and stop quickly. When the load does not exceed the dynamic torque value provided by the stepper motor, it can start and stop in an instant.

The central command processing and control unit of the stepper motor control system uses the cost-effective ATMEL_AT89C51 single-chip microcomputer, which realizes the centralized control of the whole machine hardware system through its internal software program and combined with peripheral circuit components. Its basic functions are: interpreting the relevant control commands of the software system to the hardware actuator, outputting the CP pulse signal and DIR direction signal required by the stepper motor driver, etc.

In the single-chip microcomputer control system of the stepper motor, the following two basic control tasks must be implemented:

(1) Control the direction of the stepper motor: This is achieved by changing the phase sequence of power supply.

(2) Control the speed of the stepper motor: This is achieved by adjusting the pulse frequency.

This system uses an 8-bit microcontroller ATMEL_AT89C51 to implement the control function. The circuit diagram of the circuit board is shown in Figure 4:

This control circuit is mainly composed of control unit chip ATMEL_AT89C51, serial port interface chip MAX232, voltage regulator circuit and ULN2003A motor pulse distribution chip. The upper computer continuously sends 4 1-byte data to the lower computer, namely speed increase, speed decrease, direction control (0X01 for forward rotation, 0X00 for reverse rotation) and offline state control (0X01 for normal operation, 0X00 for offline operation), to achieve the speed regulation and forward and reverse rotation functions of the control motor.

When the DIR direction signal and CP pulse signal are sent to the corresponding port of the driver, the driver responds to the command and converts the pulse signal sent by the control system into the angular displacement required by the stepper motor. The frequency of the CP pulse is proportional to the speed of the stepper motor, and the number of CP pulses determines the angle of rotation of the stepper motor. In this way, the control system controls the pulse signal CP to achieve the purpose of speed regulation and positioning of the stepper motor, and the film transmission mechanism can drive the film to move smoothly at a very stable speed.

This system uses the MS series stepper motors jointly produced by SHAPHON Corporation of the United States and Beijing Start Microstep Control Technology Co., Ltd.

As shown in Figure 4, when the positive end of the signal is at a high level, the diode is turned on, and after it is turned on and emits light, the photosensitive tube is turned on and drives the stepper motor to operate. On the contrary, the diode is off, the photosensitive tube stops working, and the stepper motor stops running.

5. MCU programming

Use Keil to program the microcontroller to achieve the above control functions. Before programming, set the address, data and control signals first. The address of the programming unit is added to P2.0-P2.3 of P1 port and P2 port (the 11-bit address range is 0000H-0FFFH), and the data is input from P0 port. The levels of pins P2.6, P2.7 and P3.6, P3.7 need to be set, PSEN is high, RET remains high, and the programming voltage is added as required. The programming pulse is input to the ALE pin. When programming, a 4-20MHz clock oscillator can be used. The AT89C51 programming method is as follows:

(1) Add the address signal of the unit to be programmed on the address line.

(2) Add the data byte to be written to the data line.

(3) Activate the corresponding control signal.

(4) In high voltage programming mode, add +12V programming voltage to the Vpp terminal.

(5) Each time a byte is written to the Flash storage array or each time a program encryption bit is written, an ALE programming pulse is added. Change the address of the programming unit and the data to be written, and repeat steps 1-5 until all files are programmed. Each byte write cycle is self-timed, usually about 1.5ms.

6. Conclusion

Through the design and research of the film transmission mechanism and stepping control system of the weld film detector, the problem of structural miniaturization has been well solved, laying a good foundation for the better application of the detector in engineering.