Design and Implementation of Intelligent Measurement and Control System for Low Voltage Circuit Breaker
0 Preface
Low-voltage circuit breaker products are mainly used in low-voltage power distribution networks to distribute electrical energy and protect circuits, power supplies, and electrical equipment from overload, undervoltage, and short circuit hazards, improving the reliability of power supply. The low-voltage circuit breaker intelligent measurement and control system integrates computer technology, intelligent control technology, and detection technology into an integrated control system. It implements the current overload measurement and control function of the low-voltage circuit breaker bimetal (thermal release), which solves the manual adjustment in the traditional process. The problem of large randomness and difficult to control quality greatly improves product detection accuracy and production efficiency to meet the high demand in the low-voltage electrical appliance market. This paper introduces the detection technology of low-voltage circuit breakers, as well as the basic structure, working principle and design and implementation of the intelligent measurement and control system. Through repeated debugging, the intelligent measurement and control system of the low-voltage circuit breaker runs stably and efficiently, and it has been applied on-site at a circuit breaker manufacturer in Shanghai.
1 Low-voltage circuit breaker detection and control process requirements
Low-voltage circuit breakers have functions such as overload protection and short circuit protection. At present, the applicable scope of this system is the protection form that adopts bimetallic protection (thermal release) in the overload protection of circuit breakers, and pushes the tripping mechanism to cut off the power supply when the bimetallic heat deforms. Working principle of bimetallic sheet during overload protection: When the circuit current of the circuit breaker exceeds the rated value, the bimetallic sheet deforms due to heat and shifts to the side of the trip unit until the tripping screw on the bimetallic sheet reaches the tripping At this time, the trip unit trips and cuts off the circuit current. When the circuit breaker does not operate within the specified tripping time, adjust the position of the tripping screw and nut on the bimetal to make the circuit breaker meet the requirements of the tripping time of the circuit breaker [2] [3].
In view of the current long time for the detection and calibration of low-voltage circuit breakers, the analysis of the deformation mechanism of the bimetallic sheet is carried out, and the existing detection process is transformed. In the case of meeting national standards, increase the detection current, greatly shorten the measurement and control time, and improve production efficiency. This system adopts the single-phase 3 times current method, that is, the three contact groups are loaded with three times the prescribed current in sequence, and the time t1, t2 and t3 of each group of contacts from loading to trip are recorded. When the TI (i = 1,2,3) of a group of contacts exceeds the allowable error range, it is necessary to adjust the screw nut on the bimetal. The specific detection methods of circuit breaker products are:
(1) T1 is counted until the trip mechanism operates.
(2) If t1 is greater than the specified Tmin (the shortest time for the circuit breaker qualification standard), and the trip mechanism has not yet operated, the screw and nut adjustment mechanism is activated until the bimetallic sheet pushes the trip mechanism.
(3) If t1 is greater than the specified Tmax (the longest time for the circuit breaker qualification standard), but has not yet acted, it will be treated as a fault.
(4) Repeat the above test steps for phase B and C test.
(5) After the three-phase test is qualified, record the test data and the system prompts that the test is complete.
2. General overview of low-voltage circuit breaker intelligent measurement and control system
The intelligent system adopts a three-layer distributed control structure (as shown in Figure 2), which is divided into a three-layer architecture of field device layer, field control layer and monitoring management layer.
(1) Field equipment layer
The field control equipment needs to complete a series of interlocking measurement and control operations such as pressing, closing, closing, energizing, screw and nut adjustment and loosening of the measured object (plastic case circuit breaker). The designed screw and nut adjustment mechanical compound mechanism, combined with the laser tracking CNC tracking method, simultaneously adjusts the screw and nut on the bimetallic sheet to realize the concentric double flexible shaft adjustment function of the system without additional force.
(2) Field control layer
The field control layer is composed of two units: one is the strong current unit, which provides the current required for measurement and control; the second is the weak current unit, which is composed of the Siemens S7-200 series PLC as the core of the lower computer controller and its related control program. Control System. On-site control uses a control scheme that combines automatic control and random control. Automatic control realizes the small closed-loop control tasks of each adjustment mechanism. Random control is based on the randomness of the bimetallic sheet heating of the circuit breaker. Based on the experience of on-site experts, an expert system is established, and the knowledge base is established using the "IF-THEN" grammatical structure to achieve low voltage. Fast reasoning function of circuit breaker measurement and control system.
(3) Monitoring management
The supervisory management layer (host computer) is based on the Advantech industrial computer and performs data communication with the PLC controller through the OPC general interface (see Figure 2). The man-machine interface system adopts Siemens industrial configuration software WinCC programming environment, screen configuration standardization, and program design modularization method to complete the data collection, program processing, and operation instruction output of the entire intelligent control system, and realize the chain control of the field control layer And equipment adjustment.
3. Software design of low voltage circuit breaker intelligent measurement and control system
The low-voltage circuit breaker intelligent measurement and control system software is mainly composed of the upper computer monitoring software and the lower computer detection process control software, which respectively realize the system control function and system monitoring function. The lower computer of the system is based on Siemens PLC-200, using Siemens programming software STEP7-Micro / Win programming environment, ladder diagram programming language, and modular programming mode to build the control program of the intelligent measurement and control system. The upper computer of the system is based on Advantech industrial control computer, adopts Siemens industrial configuration software WinCC programming environment, C script programming language, OPC general interface, and configures the man-machine interface monitoring program of the intelligent measurement and control system.
3.1 Design of the control software for the detection process of the lower computer based on PLC
The PLC control program of this system adopts ladder diagram modular programming to complete the technological requirements of field devices for circuit breaker monitoring. The ladder diagram program module mainly includes: power-on self-test, parameter initialization, circuit breaker positioning compression, circuit breaker closure, screw nut adjustment mechanism stepping, laser positioning, energization detection, screw nut adjustment, screw nut adjustment mechanism X axis shift Phase, circuit breaker measurement and control product counting and system failure alarm processing.
In the monitoring process, the screw and nut adjustment mechanism needs to adjust the screw nut on the bimetallic piece of the circuit breaker in real time to ensure that the circuit breaker will jump up within the qualified standard time. Therefore, the positioning problem of the screw and nut adjustment mechanism is one of the key technologies and difficult points of the entire intelligent measurement and control system. This intelligent measurement and control system has designed a control unit composed of a stepper motor, magnetic grid displacement sensor, Siemens PLC and positioning ladder program to achieve closed-loop control of the stepper motor and improve the positioning of the stepper motor in the screw nut adjustment mechanism problem.
The UML flow chart of the ladder diagram program for the positioning of the screw and nut adjustment mechanism (see Figure 3), specific implementation:
(1) Initialize the parameters of the stepper motor and its high-speed pulse counter. These program blocks are integrated in STEP7-Micro / Win, which is very convenient for the pulse cycle, pulse number and working mode of the stepper motor and the work of the high-speed counter Initialize the mode and initial count value [4], some codes are as follows:
X axis high-speed counter parameter initialization:
(2) The system establishes an XY axis two-dimensional positioning platform for laser scanning positioning. According to the set parameters and scanning method, the X-axis and Y-axis stepper motors perform positioning scanning. The magnetic grid sensor transmits the detected signal to the high-speed counter in real time. At the same time, the program monitors the scanning status of the stepper motor to make the stepper motor completely Under closed-loop control.
3.2 Design of monitoring software based on WinCC
The man-machine interface system of the upper computer of this system uses Siemens advanced industrial configuration software WinCC to realize real-time monitoring of the on-site production process and offline management of production data. The function implements user hierarchical authority management and is divided into three major functional modules such as login, detection, and system management (see Figure 4). Among them, the login module implements user account verification management; the detection module implements on-site working condition monitoring (see Figure 5); the system management module implements advanced production expansion functions such as offline production data management and system intelligent diagnosis.
4 Conclusion
In the low-voltage circuit breaker intelligent measurement and control system, the upper and lower computers cooperate to realize an intelligent control scheme that combines automatic control and random control. The control program of the lower computer adopts a modular design to complete the technical requirements of the field equipment for detection; the design of the human-machine interface configuration screen of the upper computer is standardized, and the functional planning is modularized, especially the offline management of production data, which expands the production monitoring function of the human-machine interface. At present, the system is running stably and well, and has entered the stage of measurement and control experiments, and the test results have been approved by actual production.
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