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2022-08-15
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Application of touch screen combined with PLC in variable frequency power supply

Abstract: This paper introduces the functional characteristics of touch screen and the application of connecting with PLC in variable frequency power supply system. As the control unit of the system, PLC has some shortcomings in human-computer interaction of parameters. As the human-computer interface, the touch screen makes the system interface friendly, simple and intuitive, easy to operate, and achieves obvious results in practical application

key words: touch screen PLC variable frequency power supply system man-machine interface

I. Introduction

programmable controller (PLC) as a central processing unit is widely used in various industrial control occasions because of its high computing speed, rich instructions, strong function to withstand the explosion of TV screen, high reliability, and strong anti-interference ability. However, because it does not have human-computer interaction function, when there are many process parameters and human-computer interaction is required, using touch panel monitor with touch operation function is a good choice. In this set of variable frequency power supply system, through the combination of touch screen and PLC, the target voltage value and target frequency value can be directly set in the touch screen, and the actual current, voltage and frequency of the power supply system can be monitored in real time, as well as the alarm function. It has simple operation, powerful function, friendly and intuitive interface, and has achieved good results in production and application

II. System hardware composition and structural principle

this system is used for the test platform of variable frequency air conditioning compressor. The test platform requires a variable range of power output voltage of 80 ~ 480v and a variable range of frequency of 20 ~ 200Hz, so as to comprehensively test the working performance of the air conditioning compressor. This set of variable frequency power supply system is composed of frequency converter, excitation unit, asynchronous drive motor and synchronous generator unit, PLC and man-machine interface. The descriptions of each part are as follows:

1) the asc600 frequency converter of ABB company is selected as the frequency converter. This series has the characteristics of high reliability, accurate control, excellent self-protection system and so on. The function of the frequency converter is to control the speed of the asynchronous drive motor in real time through PID adjustment of certain parameters according to the motor speed given by PLC and the actual speed value fed back by the coding disk, so as to make the motor run at the given speed, so that the output voltage of the synchronous generator coaxially connected with the motor is the given frequency

2) the excitation unit adopts the DC power supply DCS500 of ABB company. The device adopts silicon controlled rectifier and has good current regulation performance and protection function. After selecting external control, the current can be directly controlled according to the given value of PLC to adjust the excitation current of synchronous motor and then control the terminal voltage of synchronous generator

3) the asynchronous motor is coaxially connected with the synchronous generator, and the frequency of the generator output current can be accurately controlled by controlling the motor speed through the frequency converter, while the synchronous generator output voltage amplitude E and magnetic flux Φ Therefore, adjusting the excitation current provided by the excitation unit can control the amplitude of the generator output voltage. Finally, the voltage and frequency can be adjusted respectively, and the waveform is sine wave

4) as a control unit, PLC is the control core of the whole system. Use Omron CPM2A. First, output the speed signal to the frequency converter to control the motor speed; Second, according to a certain control strategy, the control signal of the excitation unit is given to realize the adjustable output voltage of the synchronous generator. Thus, the output frequency and amplitude of the whole variable frequency power supply can be continuously adjusted. At the same time, it communicates with the touch screen in real time, provides data for the display of the touch screen, and processes the information input by the touch screen

5) the touch screen adopts Omron nt31c. The touch screen realizes the real-time display of current, voltage and frequency, the setting of voltage and frequency target values, and the generation of alarm information

the hardware structure of the system is shown in Figure 1:

Figure 1 hardware structure of variable frequency power supply system

III. touch screen features and functions

touch screen monitor (PT for short) is a new programmable terminal that appeared in the 1990s and a new generation of high-tech human-machine interface products. It is suitable for application in harsh industrial environment and can replace ordinary or industrial computers as human-machine interface. It has the characteristics of good interaction, high reliability, simple programming, simple connection with PLC and so on. The main functions of PT are [1]:

1) Pt is mainly used to display real-time information about the operating status of equipment or system in real time

2) the touch button on the touch screen can generate corresponding switch signals, or input values and characters to PLC for data exchange, so as to generate the operation of corresponding action control system or equipment

3) multiple pictures can be overlapped or switched for display. Due to their high fineness, strings, alarm information, historical records, trend charts, etc

IV. The role of PLC in excitation regulation

different from the synchronous motor excitation regulator of the power system, this equipment serves as a variable-frequency power supply to supply power to independent loads, rather than infinite power. Therefore, in excitation regulation, the possibility of many faults is reduced, but at the same time, there are wide voltage and frequency regulation range, high regulation accuracy and certain dynamic response requirements. Since the frequency regulation range is required to be 20-200hz, the voltage regulation range is 80-480v, and the load is switched frequently, these have increased the requirements for the excitation regulator. For this system, the dynamic process of synchronous generator mainly includes voltage change, frequency change, sudden load addition, sudden load rejection, pulse interference and so on. How to ensure that various appropriate control strategies can be applied flexibly and accurately in various operating states, so that the synchronization function can work normally, and at the same time, having a good response speed is the key to the realization of this system. The following is a qualitative analysis of the two most important dynamic processes, voltage regulation and frequency regulation

voltage regulation: under the condition that the frequency remains unchanged, when the given voltage changes by V, the regulator changes the magnetic flux by adjusting the excitation current output Φ m. Make the excitation electromotive force E0 rebalance the armature reaction electromotive force, and compensate the voltage change caused by the change of stator current at the same time

frequency adjustment: when frequency adjustment is required, PLC controls the frequency converter to adjust the speed of the prime mover so that the prime mover operates at a given speed. At the same time, due to the change of frequency, the excitation electromotive force and the internal parameters XD and XQ of the motor change significantly under unsaturated conditions, and the set frequency increases ω, Then E0 will increase, the motor parameters XD and XQ will also increase, and the terminal voltage will change accordingly. In order to keep the terminal voltage unchanged, it is necessary to adjust the excitation current to change the magnetic flux Φ M to reach a new equilibrium point

in view of the regulation requirements of this system, the regulator adopts a closed-loop control system based on the negative feedback principle of the regulated voltage deviation. According to the characteristics of frequency and voltage regulation and the process of high frequency, low frequency, high voltage and low voltage, subsection control and variable parameter regulation are adopted to achieve the optimal performance of the system

program flow chart is shown in Figure 2:

Figure 2 system program flow chart

v. touch screen image design

the application of touch screen in this set of variable frequency power supply system is introduced below. The touch screen image is designed by the support software, and then downloaded from the support tool (personal computer) to PT for use. The connecting cable between the support tool and Pt is shown in Figure 3. The touch screen and PLC are connected by RS232 communication cable in the form of host link. The cable connection mode between Pt and PLC is shown in Figure 3. PLC reads and writes Pt status control area and notification area to achieve information interaction between them. PLC reads the data in the PT status notification area to get the current picture number, and forcibly switches the picture by writing the data in the PT status control area

Figure 3 connecting cable between touch screen and support tools and PLC

5.1 design of welcome page

on the support tool, create a welcome page, the whole page is a transparent touch button, and its function is to switch the screen. When you click any part of the page, you will enter the parameter display page. When there is no action outside, the data in the state control area will be rewritten and the parameter display page will be loaded after 30 seconds of programming by PLC. Text can be placed on the touch button on the welcome page to explain and mark the system

5.2 design of parameter display page

in this system, the quantity that users need to observe is the output three-phase current, voltage and frequency. Therefore, three groups of digital display areas are set to display the voltage, current and frequency values respectively, occupying the corresponding storage space in the digital memory table. At the same time, the address of the digital memory table corresponds to the location of the PLC digital storage area. After the system is started, the voltage, current and frequency signals are sent to PLC for processing by sensors, coding disks, etc. because the address of PLC digital storage area is associated with Pt memory table, Pt can display these three groups of data in real time. The precision and number of bits of data are set in the attributes of PT's digital memory table according to the actual situation

5.3 design of parameter setting page

the quantity that users need to set is the target value of voltage and frequency, which is achieved by using the dial switch type digital setting input area. The user can directly increase or decrease the dial switch to get the desired target value. After setting on the dial switch, click the "OK" button, and the value will be sent to the PLC target value saving unit. Click the "Cancel" button, and the original value will remain unchanged. The number of digits, precision and the address of the value in the PLC memory unit are set in the properties of the dial switch. See Figure 4

Figure 4 parameter setting page

in the adjustment process, the frequency and voltage must conform to a certain relationship, so the frequency and voltage cannot be increased indefinitely. At the same time, both of them should have the function of interlocking electronic universal testing machine to test the static bending strength and elastic modulus according to the figure (the figure is omitted). The indenter must coincide with the center line of the length of the test piece. The specific relationship between the two is determined according to the relationship between the frequency and voltage of asynchronous motor and the relationship between the excitation and voltage of synchronous machine. See Figure 5. The system works in the working area shown in the shadow. Interlock is realized in PLC programming

Figure 5 system working area

5.4 design of alarm information page

set the alarm page according to the actual possible three faults of motor over temperature (detected and reported by the built-in temperature sensor of the motor), over-current and system emergency stop. The alarm page is composed of some standard lights in the PT component library. When the PLC detects the alarm information, the data in the PT control area changes accordingly, and it is forced to switch to the alarm page. Each standard lamp corresponds to a bit in the PLC indicating whether there is an alarm. In this way, the fault can be specifically determined according to the on and off status of each lamp, so as to take corresponding measures

5.5 help page design

considering the needs of the industrial site, a large number of help pages are added in the page design, including Pt maintenance, system use, parameter setting, etc

VI. summary

in this set of power supply system, the amount to be updated is the set value of voltage and frequency. The touch screen is used as a human-computer interaction tool, which is simple, intuitive and easy to operate. However, there are still some limitations in actual use, such as the distance between PLC and touch screen cannot be too large, and the touch screen itself is relatively expensive. As a central processing unit, PLC and PLC are combined in the variable-frequency power supply system to realize the variable-frequency voltage regulation of the system. The output waveform is an ideal sine wave, with adjustable

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