Miniaturized remote monitoring intelligent power supply system based on ARM Cortex-M3

With the development of contemporary science and technology, the maintenance and management of power supplies for a huge number of various types of equipment requires a lot of manpower and material resources. The environment in which communication/power facilities are located is becoming more and more complex, with sparse population, inconvenient transportation, and high risk, which increases the difficulty and cost of maintenance. This puts higher requirements on the monitoring and management of power supply equipment. The power supply monitoring system needs to monitor each state quantity in the system and must also be able to control and manage each power supply branch. Maintenance managers can perform maintenance work such as data query and control remotely, and can use a friendly human-machine interface to easily obtain the required information.

The development of digital technology has shown unparalleled advantages over traditional technology. The signal sampling, processing, control, and communication of the entire power monitoring system can be realized through digital technology. Fully digital control technology can effectively reduce the size of the equipment and reduce the cost of the equipment, but at the same time greatly improve the reliability, intelligence, and user experience of the equipment. With the improvement of the intelligence of the module, the maintainability of the new power monitoring system has also been improved.

With the development of embedded technology, the use of embedded real-time operating systems is an inevitable choice for power monitoring systems. On the one hand, it is because embedded real-time operating systems have good portability and high reliability; on the other hand, with the continuous improvement of the performance of power monitoring systems, traditional microcontrollers alone can no longer meet new requirements. As the representative of today's embedded technology, ARM not only has all the advantages mentioned above, but also has a very low cost and a high cost-effectiveness. The system designed in this article uses the LM3S9B96 chip in the LuminaryCortex-M3 series ARM produced by TI.

1 Working Principle

Figure 1 takes the power supply monitoring of 8 power-consuming devices as an example and gives the principle block diagram of the monitoring system.

Figure 1 8-way power supply monitoring system block diagram

All 8 devices draw power from the main power supply, and each power supply branch works in exactly the same way. After the power monitoring system is started, the main chip is in a power-on reset state, and its 8 I / O pins of GPIOF are at a low level. At this time, the electric control switch remains in the off state, that is, the power supply branch is in a power-off state. When the main chip core and each peripheral are initialized successfully, the internal embedded program controls the output of the 8 I/O pins of GPIOF to become a high level, and accordingly, each power supply branch is powered on and starts to work normally.

The acquisition module includes a current sensor and a voltage divider circuit. The current sensor can measure the current value flowing through the power supply branch, and the voltage divider circuit adjusts the voltage value of the power supply branch to the range of the main chip ADC sampling. Both are analog values. After the detection value is sampled by AD, the current and voltage values ​​of each power supply branch can be calculated in the main chip and compared with the preset current and voltage thresholds. If it is within the threshold range, it means that the power supply branch is working normally, and if it is outside the threshold range, it means that the power supply branch has an abnormality such as overcurrent, overvoltage, or undervoltage. The main chip automatically cuts off the power to the branch by changing the output of the corresponding pin of GPIOF to a low level. After checking and troubleshooting, the host computer can send instructions to control the power supply branch to be powered on.

The host computer and the embedded slave computer communicate through Ethernet. The host computer can send instructions to the slave computer to control the on and off of the specified power supply branch, and can also set the current and voltage thresholds of each power supply branch. At regular intervals, the current, voltage value and various normal/abnormal states of each power supply branch are sent from the slave computer to the host computer. The working status of each power supply branch can be observed through the host computer display and control software. 2 Design and Implementation

2.1 Core Modules

The core module adopts the LM3S9B96 chip of the Luminary Cortex-M3 series ARM produced by TI   . The chip has an operating speed of 80MHz, and integrates a large-capacity 256KB single-cycle Flash ROM and 96KB single-cycle SRAM. It has a 16-channel 10-bit resolution AD sampling module, a 10/100M adaptive Ethernet module that supports the LwIP protocol, and rich I/O interfaces.

LM3S9B96 has 65 I/O interfaces. During the design, 8 I/O interfaces of GPIOF group are selected as control pins. Each power supply branch needs to collect two values, voltage and current. The 16-channel AD sampling module can meet the sampling requirements of 8 power supply branches. The integrated MAC+PHY peripherals can also realize Ethernet communication with the host computer. The large-capacity built-in storage space provides a suitable platform for complex programs. According to the above analysis, the LM3S9B96 chip is very suitable for this monitoring system and can greatly simplify the circuit design.

2.2 Control Module

The design of each power supply branch control module is shown in Figure 2. According to the current value required by each branch device, a suitable relay is selected as the electronic switch, and optocoupler isolation protection and power on/off indicator light are added between the control pin and the relay.

When the GPIOF control pin of the main chip is at a low level, the LED lights up, the input of the relay pin 3 and the output of the relay pin 5 are disconnected, and the power supply branch is powered off; when the GPIOF control pin of the main chip is at a high level, the optocoupler output is at a low level, the LED lights up, the input of the relay pin 3 and the output of the relay pin 5 are connected, and the power supply branch is powered on.

Figure 2 Design of power supply branch control module

2.3 Collection Module

The design of each power supply branch acquisition module is shown in Figure 3. The current sensor is connected in series in the power supply loop, and its internal Hall sensor will output the magnetic field generated by the branch current to the AD sampling module of the main chip in the form of voltage. The corresponding current value can be calculated according to the manual provided by the manufacturer. The voltage value acquisition circuit adopts the form of a resistor divider circuit, and the sampled voltage value is also output to the AD sampling module. The actual voltage value can be obtained by simple conversion. In practical applications, the appropriate current sensor and voltage divider circuit resistance can be flexibly selected according to the current and voltage values ​​of the electrical equipment. It should be noted that the current and voltage values ​​output to the AD sampling module must be within its 0-3V sampling range.

Figure 3 Design of power supply branch sampling module Figure 2.4 Communication module

The communication module is used to realize the communication between the upper computer and the lower computer. In this design, the Ethernet communication of the lower computer is realized by the built-in MAC+PHY of the main chip. The module supports 10/100M adaptive Ethernet.

Since the computing and storage resources inside the embedded processor are very limited compared to PC, the Internet protocol must be implemented and processed under resource constraints. Under such conditions, LM3S9B96 uses as little resources as possible to implement a lightweight TCP/IP protocol stack called LwIP. Like many other TCP/IP implementations, LwIP is based on layered protocols, and each protocol is implemented as a module. LwIP consists of a TCP/IP implementation module, an operating system simulation layer, a buffer memory management subsystem, a network interface function, and a set of Internet checksum calculation functions.

To facilitate secondary development, TI officially provides a wealth of underlying drivers and detailed API descriptions. This design writes the entire Ethernet communication program based on this. The realization of Ethernet communication function makes this power monitoring system not only intelligent, but also has the ability of remote monitoring, which greatly expands the application scope of the system.

2.5 Display and control module

The display and control module is a software function module developed by the host computer. In this design, the development of this module is based on VC++ 6.0. The display and control mainly realizes the communication control with the lower computer, the online setting of the current and voltage threshold values ​​of each power supply branch, and the visual display of the collected values.

The data format of the display and control module and the lower computer software must be determined during the design process. The commands sent by the upper computer include changing the threshold value, querying the threshold value, changing the on/off status, channel testing, etc. The parameters sent by the lower computer include changing the threshold value response, returning the current threshold value, returning the on/off status, returning the abnormal status, and channel testing, etc. These commands ensure that the entire monitoring system is in a closed-loop state, and the system status and detection values ​​are visible at any time, which improves the visualization and reliability of the entire system.

3 Conclusion

The power monitoring system described in this article has been actually applied in many projects, including some occasions with harsh environments. The whole system runs stably, and remote intelligent monitoring is realized with the help of Ethernet. In addition, there are also areas that can be improved and enhanced in this design. The first is to improve the accuracy of current and voltage sampling, so as to meet some fields with extremely high requirements for power supply accuracy; the second is to consider adding wireless communication functions, so as to reduce the complexity of system wiring and broaden the application occasions. With the continuous improvement of technology, this type of power monitoring system will surely be widely used in more fields.