Development of low-voltage protection measurement and control device based on MCF51EM256

Abstract: On the basis of analyzing the needs of low-voltage power distribution monitoring, a solution based on MCF51EM256 single-chip SOC is proposed. This solution uses its on-chip high-precision 16-bit ADC and high-speed 32-bit COLDFIRE-V1 core to better protect Measurement, control, management and communication functions, so as to cost-effectively meet the complete needs of low-voltage measurement and control devices, with high reference value.

Keywords: Low voltage measurement and control microcomputer protection MCF51EM256 MQX operating system
Abstract On the basis of analyzing the demand of low voltage distribution monitoring, this paper put forward a solution based on single chip SOC MCF51EM256, it uses the 16-bit high precision ADC and 32-bit high speed COLDFIRE-V1 core of the SOC to realize protection, measurement, control and communication, thus the whole needs of the low voltage measuring and control device are met economically and efficiently, it has very high reference value.
Keywords low voltage measuring and control, microcomputer, protection, MCF51EM256, MQX operation system

0 Preface
In AC 50Hz, 380V (660V) low-voltage system, there are many distribution circuits, the space of the distribution cabinet is narrow, the motor or other feeder equipment on the low-voltage terminal needs an intelligent integrated measurement and control device, which works with contactors and circuit breakers to achieve intelligence It can improve the intelligent management level of equipment and facilitate users' use. The low-voltage measurement and control device developed in this paper can completely realize the above-mentioned integrated functions of control, measurement, protection, communication and management.
 
Control device functions outlined low-pressure low-pressure flow measurement and control device can directly deposit the sample phase AC current, three-phase AC voltage, temperature of the heat resistance, the leakage current signal, a high-speed MCU processor calculates the load current, ground current, voltage, imbalance, Parameters such as power and electric energy, combined with built-in protection algorithm processing, can realize protection input, withdrawal, alarm or trip according to needs, and achieve selective protection functions; through the input and output functions of the switch, it can be completed with contactors and circuit breakers The relevant control logic realizes a certain process interlocking or control protection function; if used in the MCC circuit, it can complete the direct start of the motor, reversible start, Y- â–³ start control; has a powerful network communication function, using the standard RS485, MODBUS_RTU protocol (extensible PROFIBUS-DP, industrial Ethernet, DEVICENET of CAN network) protocol can easily realize the networking function, which is convenient for user maintenance and management [1]. The structure of a typical low-voltage measurement and control device is shown in Figure 1. It collects voltage, current, temperature and other signals from the main loop, which is processed by the internal MCU at high speed, combined with the digital input signal DI for digital output control processing, with human-computer interaction Display function, rich network communication interface can facilitate user management.
Figure 1 Schematic diagram of low-voltage measurement and control device
2 System hardware design
2.1 MCU selection
MCU is the core of the low-voltage measurement and control device. It implements data acquisition, data processing, output control, network communication, and human-computer interaction functions. Here uses the MCF51EM256 processor recently released by Freescale, which uses a high-speed 32-bit ColdFire V1 core (with hardware multiply-accumulate MAC unit), on-chip integrated with programmable delay module PDB and four 16-bit SAR ADC, PDB can be directly Control the sampling of the trigger ADC, complete the high-accuracy AC sampling and measurement of voltage, current, leakage current, temperature. Combined with the high-speed 32-bit COLDFIRE core, various measurement and control protection arithmetic algorithms can be realized. The hardware independent RTC and two secure FLASH memories can complete the event record management function, and at the same time, through remote firmware update, reduce the maintenance cost of the device. Up to 3 SPI, 3 SCI and 1 I2C interface, can realize a variety of network communication functions, internally integrated 2 TPM multi-function timer modules and 1 16-bit, 2 8-bit timing modules and rich GPIO The port can easily complete the input collection and output control of timed interrupts and various switch digital quantities. With the help of CODEWARRIOR-IDE development platform and free MQX operating system, software development can be carried out efficiently and quickly [3].
Figure 2 MCF51EM256 functional block diagram
 
Figure 3 CODEWARRIOR development environment with built-in MQX operating system
2.2 Signal processing
The low-voltage protection measurement and control device mainly collects 8-phase analog signals of three-phase AC current, three-phase AC voltage, leakage current and temperature. Signal sampling is carried out through resistance voltage divider, current sensor, etc., after the internal reference of the MCU is raised, the operational amplifier is amplified and filtered, and then directly connected to the four 8-channel 16-bit ADC pins inside the MCU. The AC sampling conversion of the ADC provides interrupt service for the MCU to collect the ADC conversion results. This data collection method that is not controlled by the MCU program improves the operation efficiency of the MCU. The input signal of the 9-channel switch dry contact switch used to monitor the switching signals of circuit breakers and contactors is connected to the GPIO pin of the MCU after the optocoupler isolation. The MCU obtains the on-site switch information by interrupting the regular query. MCU through high-speed signal acquisition and intelligent analysis and logic processing through the on-chip GPIO output 5 relays to protect and control the output operation, signal processing circuit shown in Figure 4.
Figure 4 Signal processing circuit
2.3 Communication circuit
The low-voltage protection measurement and control device adopts a bus-type distributed network communication structure. At present, the mainstream is RS485-ModBus bus, RS485-ProfiBus bus, CAN-DeviceNet bus, of which the most commonly used is RS485 ModBus bus. Through the SCI unit in the MCF51EM256 chip, using the high-speed optocoupler 6N137 isolation and TI's anti-ESD SN75LBC184 485 chip hardware design, the software writes the MODBUS-RTU protocol module to support 01, 02, 03, 04, 06, 10 and other command operations . At the same time, using the other two SCI parts on the chip, the standard ProfiBus-DP, CAN-DeviceNet, and Ethernet modules can be extended to complete the communication of other bus networks. The RS485-ModBus communication circuit is shown in Figure 5.
Figure 5 RS485 communication circuit
2.4 MCU peripheral and human-computer interaction
Through the MCU's on-chip LCD module, it can support 8 × 36-point segment code LCD display, which can facilitate users' simple interactive debugging; connect with an external display module through the I2C / SCI interface to complete human-computer interactive operations. The peripheral circuit diagram of MCF51EM256 is shown in Figure 6.
Figure 6 MCF51EM256 peripheral circuit
3 Software design
Using FREESCALE's CODEWARRIOR development environment and the free and open source MQX (Message Queue eXecutive: message queue execution) operating system, combined with the low-voltage protection measurement and control equipment protection algorithm, the task is divided and the task module is written reasonably. Pre-processing, protection judgment, driving output, human-computer interaction and communication divide tasks, and program related task modules to achieve good transplantation maintenance and high reliability requirements in the software.

3.1 MQX operating system transplantation
In order to adapt to processors and development boards of different architectures, MQX must retain a part of the code related to specific hardware devices as an abstract interface, and the hardware driver is specifically implemented by the hardware designer. This is MQX transplantation. In MQX BSP, PSP Modified and tailored in the folder, MQX3.6 and later versions support MCF51EM256 BSP, PSP driver code, developers can adapt to the needs of different projects with only a small amount of migration, and configure user_config.h , You can choose to turn on / off the peripheral to cut.

3.2 Protection principle and algorithm realization
The algorithm of the low-voltage protection measurement and control device includes the auxiliary algorithm in AC sampling (calculation of the fundamental component), the calculation of the sequence component (positive sequence, zero sequence, negative sequence), and various calculations after the operation parameter measurement and the calculation of the above characteristic quantities Fault judgment, such as short circuit, overload, phase loss, low voltage, over voltage judgment and fault output according to the setting requirements. At the same time, the corresponding DI and DO control operations are performed according to the logic control requirements of the system configuration.

3.3 Software block diagram
According to the real-time and accuracy requirements of the low-voltage protection measurement and control device software, the task is decomposed into data collection and data preprocessing, feature quantity calculation and protection judgment, driving input and output, communication, human-computer interaction, and semaphores are used for communication between tasks . The block diagram of the program is as follows. It can be seen that the programming method of the operating system is adopted. Since each module and task are running independently and in real time, it is very suitable for the transplantation of modules under the low-voltage protection measurement and control device and the requirement of real-time task running.
Figure 7 Software block diagram
4 Conclusion
This article uses Freescale's latest MCF51EM256, using its on-chip high-precision 16-bit ADC and high-speed 32-bit processor, with the help of CODEWARRIOR and MQX operating system software environment, through modular software architecture and rich low-voltage protection and industrial field control It requires experience to write software to realize intelligent protection, measurement, control, network communication and management functions. It has good advancedness and economy, and has certain reference value.
The article comes from: "Intelligent Building Electrical Technology" 2012 Issue 3.
references
[1] General Administration of Quality Supervision, Inspection and Quarantine of the People ’s Republic of China, China National Standardization Administration. GB14048.4-2010 low-voltage switchgear and control equipment [S]. Beijing: China Standard Press, 2011.
[2] MCF51EM256RM MCF51EM256 Series ColdFire® Integrated Microcontroller Reference Manual MCF51EM256 Datasheet [Z]. Freescale: 2010,4.
[3] MQXRM Freescale MQX â„¢ RTOS Reference Manual [Z]. Freescale: 2010, 8.

About the Author:

Yu Jing, female, undergraduate, engineer of Wuhan Ankerui Electric Co., Ltd., the main research direction is intelligent power monitoring and power management system

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