Computer control of dynamic weighing process

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Computer control of dynamic weighing process (Part 1)

Abstract: in view of the shortcomings in the automatic control of flour and other processing, a set of electromechanical integrated dynamic weighing process computer control system is designed. 5. The laboratory space 1 must be larger than the size of the universal experimental machine itself, equipped with measurement and control interfaces and application software, to realize the dynamic detection and quantitative control of the mechanical blanking process, The measurement accuracy and reliability of dynamic weighing process in flour processing are solved

key words: automatic weighing; Computer control; Measurement accuracy; Dynamic weighing; Reliability

1 overview

over the years, most of China's flour packaging production lines have adopted mechanical weighing. When the feeding amount reaches the fixed quantity, the mechanical scale scale displacement is used to realize the fixed quantity control by means of lever amplification mechanism or photoelectric effect control actuator. This method has the advantages of low weighing accuracy, complex operation, high labor intensity, and must carry out zero calibration and finished product re inspection at any time. Once there is an out of tolerance, it can only be adjusted by increasing or reducing the additional weight on the scale, which is extremely inconvenient. In order to change this situation, in recent years, people have applied mechanical technology, microelectronics technology, automation technology and computer technology to design and construct some electromechanical quantitative weighing equipment; Force time, elongation time and strain time have fundamentally changed the weighing structure of the original mechanical system, replaced the mechanical lever with electronic sensing elements, made the measurement electronic, and improved the conditions of the measurement and packaging production line to a certain extent, but the measurement accuracy and reliability are still not perfect. With the development and in-depth application of computer technology, further improve the automation technology level of flour processing process, improve the accuracy and reliability of measurement process, and develop more advanced dynamic measurement equipment with high-precision digital and intelligent functions. It is an important research and development direction for the production automation technology of Chongqing Zhongxian Shenan chemical construction general factory, which participated in the drafting of this standard for flour processing. Therefore, we have researched and designed a set of computer control system for dynamic weighing process of flour processing, which solves the problems of measuring accuracy and reliability in flour processing, and improves the functions of the equipment to make it more universal and practical

2 system structure

this flour automatic weighing control system is an electromechanical computer control system. The machine adopts a single-chip computer with measurement control interface and control software to realize real-time dynamic detection and quantitative control of the mechanical blanking process. The control system has a high performance price ratio, and the weighing error of the system is ± 4 ‰. It is very suitable for quantitative measurement control of powder and granular objects. The system is divided into two parts: mechanical feeding and signal detection and control

2.1 mechanical feeding structure

the mechanical feeding part is mainly composed of screw feeder, surge hopper, weighing hopper, speed regulating motor, electromagnet, etc. The performance of mechanical structure directly affects the technical indicators of weighing and measuring instruments, and is the most basic condition for the success of mechatronics products. Therefore, in the structural design, we absorbed the advantages of weighing equipment from relevant manufacturers, created our own characteristics, and designed the feeder, buffer hopper, weighing hopper and bucket door mechanism

the feeder is composed of two screw propellers with different diameters. In the high-speed loading stage, in order to improve the loading speed, the twisted pair wheels run at high speed at the same time; In the low-speed fine adjustment stage, the large winch is disengaged by the clutch, and the small winch is loaded separately. The whole loading process is equipped with the stepless speed regulation characteristic of the slip motor, so that the accurate loading can be realized. It is found that the uniformity and stability of cutter feeding have a great influence on the accuracy of symmetrical quantity. Therefore, we have adopted a special structure with multiple blades and a short section at the front end of the propeller impeller, which greatly improves the uniformity and stability of the material flow, especially the low-speed material flow, thus effectively improving the weighing accuracy. The surge hopper is composed of a storage bin and a double door. It is connected with the outlet of the screw feeder. When the material reaches the rated value, close the buffer material door to intercept the material retained in the air, so as to reduce the impact of falling materials on the measurement accuracy

the weighing hopper is composed of a storage bin, a sensor hanging mechanism and a double leaf material door. The hanging structure is adopted, and the weighing bucket is hoisted in a symmetrical structure by three tension sensors through a twisted pair chain structure. The weight of the object is transformed into superimposed MV signals by three tension sensors and sent to the computer for processing. This structure eliminates the problem of signal distortion due to the deviation of the force point caused by the uneven distribution of materials in the weighing hopper

the hopper doors are all equipped with pull rod interlocking mechanism, whose action is controlled by electromagnet. The door opening and closing response speed is fast and the consistency is good. In the sensor hanging mechanism, a damping device is added to reduce the impact of mechanical vibration on the sensor signal

2.2 signal detection and control

the system is an intelligent instrument with MCS-51 single chip microcomputer 8031 as the core component (Fig. 1). It consists of single chip computer, tension sensor, signal transmitter, a/D, D/a converter and AC contactless disconnector. Because this machine adopts single-chip computer and large-scale integrated circuit technology and modular circuit structure design, it has the advantages of simple structure, strong function, stable performance, high control precision and convenient operation and maintenance

the analog input channel consists of three S-type shear beam tension sensors (accuracy 0.05%) that transmit the signal (0-5mv) superimposed by the material weight to a two-stage differential input signal amplifier (accuracy and more perfect different curve 0.01%) composed of a high-precision, low-temperature drift integrated operational amplifier (OP07) for amplification. It turns into a 0-5V DC signal and sends it to a 12 bit a/D converter to convert it into a 0-4095 digital for computer processing

the analog output channel adopts digital trigger, photoelectric isolation and speed feedback closed-loop control to control the excitation voltage of the slip motor for stepless speed regulation. The switch output channel adopts photoelectric isolation to drive contactless AC switch to control the action of electromagnet. The hall switch is used as the detection element in the switch input channel to feed back the opening and closing status of the hopper door to the computer, so as to ensure the accuracy of the weighing sequence and improve the reliability of the system

the operation panel adopts intuitive LED digital display and operation status analog screen display. The communication network interface (RS-485) is set. It supports full duplex asynchronous communication protocol, laying the foundation for modern management

2.3 control process

the flour output from the milling machine is directly sent to the packaging workshop through the feeder (or through the buffer bin). In the packaging workshop, a number of dynamic quantitative scales and corresponding packaging machinery are installed according to the production scale. The incoming material from the feeder is divided into a plurality of feeding ports to feed the flour into the buffer material trough of each dynamic quantitative scale. The computer controls the speed change of the screw feeder of the quantitative scale to send the flour into the weighing hopper, and real-time detects the weight (flow) of the flour in the weighing hopper reflected by the electronic sensing element. The speed of the feeder is controlled according to the best scheme. When the quantitative value is reached, the feeder is stopped and the buffer gate is closed to intercept the flour in the air, so as to realize the quantitative measurement control. After the quantitative measurement, the computer will judge the unloading switch status. If unloading is required, open the weighing bucket door to pour the flour into the packaging machine, and the packaging machine will complete the bagging and sealing

3 control software

the control software is written in Intel PL/M-51 high-level language, with a modular structure design. It is divided into main program module, test module, timing sampling module, adaptive module and port refresh module. The functions of some main modules are described below

the interrupt timing time of this system is 2ms. During the interrupt, a/D sampling, adaptive control and I/O port refresh processing are carried out. (see Figure 2 for the program block diagram)

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