Battery online monitoring hardware platform based on ARM9 and LEM sensor

As a backup power source, batteries often play an extremely important role in power supply systems. Once a battery pack has problems when AC power is lost or other accidents occur, the power supply system will be paralyzed, causing equipment shutdown and other major operating accidents. In recent years, with the widespread use of valve-regulated sealed lead-acid batteries (hereinafter referred to as valve-regulated batteries), coupled with poor use environment and conditions, accidents caused by premature battery failure have occurred from time to time. Due to the special valve-regulated sealing structure of valve-regulated batteries, we cannot accurately grasp the health status of the battery. Its advantage of "maintenance-free" has become a disadvantage and difficulty in battery operation management, and it is extremely easy to mislead users. While improving battery performance and reducing maintenance workload, how to quickly and effectively detect early-stage failed batteries and predict the trend of battery performance changes has become a new topic in battery operation management. At present, in addition to conventional maintenance and detection methods such as verification discharge and voltage inspection, some new detection methods have emerged with the development of technology, and the new detection technology of battery online monitoring has gradually been used [1].

Beijing LEM Electronics is a wholly-owned subsidiary of the Swiss LEM Group (LEM). This Sentinel module, developed in Switzerland and produced in Beijing, is a battery sensor specially designed for online battery monitoring. The sensor is only the size of a matchbox. Each module monitors a battery. The module adopts a four-wire design, which is connected to the positive and negative poles of the battery to achieve power supply and measurement. Then, up to 254 modules are connected through the RJ11 interface (telephone line interface), and the entire battery pack is monitored through open serial port protocol communication. The sensor can directly measure the impedance, voltage and surface temperature of a single battery (the module is attached to the surface of the battery). It is easy to connect and install. At the same time, the module has a five-year warranty period, which completely spans the battery failure cycle, so it is very convenient to build an online battery monitoring system.

2 Composition of the battery online monitoring hardware platform

On the one hand, the battery online monitoring system needs to complete the human-computer interaction of battery parameters in industrial sites such as substations and data centers, so that users can observe the overall voltage, current, impedance, voltage and temperature of each single battery on site; on the other hand, it needs to provide a network interface so that users can timely understand the situation on site at remote locations such as the central control room; finally, a GSM interface needs to be added so that once a fault occurs, the on-duty personnel can be notified by text message or phone call. Therefore, it is necessary to design a field monitoring host to complete the above functions and match it with the Sentinel module to realize the construction of the entire battery online monitoring system, as shown in the following figure:

It is worth mentioning that:

1) As can be seen from the above figure, due to the unique design of the sentinel module, the battery impedance can be tested directly, so the system does not need to install a separate discharge module.

2) Since the sentinel module needs to be identified by the address, which is 8 bits, the above connection can connect up to 254 batteries.

3) Theoretically, the Sbus communication line indicated by the dotted line in the figure does not need to be connected. However, considering the complexity of the industrial site, if the communication line between two modules is damaged, some modules will not be in the line. Therefore, for the sake of redundant design, try to arrange this cable if the site allows.

4) The current transmitter in the figure is generally used by the host to measure the charging and discharging current and sense the status of the battery. It is very necessary. Considering the convenience of on-site construction, try to use an external current transmitter instead of placing the transmitter in the host. Otherwise, the battery charging and discharging cables (often very thick cables) need to go around the host, which is uneconomical and unsafe.

5) When using a GSM antenna, you need to measure the local signal strength on site. Some equipment rooms have severe signal shielding. In this case, you can consider moving the antenna outdoors to ensure smooth signal transmission.

From the above analysis, the requirements for on-site monitoring include at least human-computer interaction function, network function, GSM transmission function, Sbus communication function and A/D conversion interface.

3 Battery online monitoring host based on ARM9

The host is essentially an embedded system with a human-computer interaction interface. In order to complete the construction of the embedded platform and facilitate the communication of the network protocol, the ARM9+ operating system is adopted. Considering the industrial-grade working temperature and the convenience of the LCD interface, ATMEL's AT91SAM9261 is selected as the main control CPU of the system.

In order to facilitate debugging, the core board + expansion board method is adopted. A minimum ARM9 system is arranged on the core board, all interfaces are brought out, and the functional part is arranged on the expansion board.

3.1 Core board design

The design block diagram of the core board is as follows:

illustrate:

1) Since the Dataflash startup mode of AT91SAM9261 can only work in the temperature range of above 0℃ and below 70℃, it will not be able to start once the temperature is below 0℃. In order to solve this problem, ARM can only use external startup, that is, NOR FLASH startup. Therefore, it is necessary to select the startup mode as external startup (BMS=0) to meet the temperature requirements of the industrial site.

2) The Norflash memory chip is AMD's AM29LV160DB, which has a capacity of 4M*16bit and is used to store BOOT programs, small operating systems and small applications. The design uses word alignment, that is, the A0 address line of the chip corresponds to the A1 address line of the ARM chip. In addition, since the SAM-BA burning program officially provided by ATMEL only supports Dataflash and Nandflash, it is necessary to modify the SAM-BA script file to realize the burning of Norflash

3) The Nandflash memory chip selects Samsung's K9F1208U0B, and its storage capacity is: 64M*16bit. When using wince or linux operating system, this chip is used to store the operating system and application programs; if a small operating system such as ucos is used, the chip can be omitted and not soldered, and the system and BOOT program can be stored in Norflash.

4) The Sdram chip uses MT48LC16M16A2TG-75IT:D, each with a capacity of 16M*16bit. Two SDRAMs are used in this system to form a 32 data bus. Since the Sdram chip is the memory of the entire embedded platform, it needs to frequently interact with the CPU for data. In order to achieve better signal integrity, a 22-ohm balancing resistor is connected in series on the address and control bus close to ARM to absorb signal reflection. When a small operating system is used, the operating system can run in the SRAM inside the ARM, and the Sdram can be omitted without welding.

5) The expansion interface expands all available interfaces of the ARM chip for connection with the expansion board.

6) Due to the dense signals and the need to bring out all interfaces and ensure good electromagnetic compatibility, the PCB adopts a six-layer PCB design method, using the signal layer - ground layer - signal layer - power layer - ground layer - signal layer.

In order to ensure the effect of high-frequency operation, the design considers designing the buses of the two SDRAMs to be of equal length, and adopts technical means such as double-sided layout and serpentine routing.

3.2 Expansion board design

The design block diagram of the expansion board is as follows:

illustrate:

1) The SPI flash chip is used to store the data collected by the battery sensor. Here, the write protection pin of the chip is managed using an ARM I/O port to prevent the data in the chip from being modified when the power is on or off.

2) The GSM module uses Siemens' TC35i module, which communicates with the expansion interface (connected to the ARM new chip) through the serial port. In addition, an ARM I/O port is used to control the IGT pin to activate the module. In order to ensure normal communication between the module and the SIM card, the wiring distance between them should be as short as possible.

3) The network card interface chip uses DM9000, through which the data packet is transmitted to the Ethernet and then to the host computer software. At the same time, the network protocol can be used to achieve remote firmware upgrades to ensure that the host runs the latest application software.

4) Sbus is LEM's own protocol, which can be converted into a serial port protocol. Its conversion circuit is open. This circuit is designed on the expansion board to realize the communication between the main control board and the sentinel module.

5) Since AT91SAM9261 provides a liquid crystal data interface, it can be directly connected to the LCD.

6) The touch screen interface chip uses the dedicated chip ADS7843.

4. Machine joint debugging

We experimented with the system in a substation, using two battery groups, each with 54 2v 300Ah batteries, as shown below:

Write a test program to run in the system, collect data from each battery module every half an hour, and then print the information through the debug serial port. The following is the result of a certain collection:

# 1 battery : 2.28v 24.29 404.9 uohm # 2 battery
: 2.24v 24.08 362.1 uohm # 3 battery : 2.22v 24.29 426.1 uohm # 4 battery : 2.29v 24.29 350.1 uohm
# 5 battery : 2.25v 24.29 3 81.8 uohm # 6 battery : 2.28 v 24.29 392.6 uohm
# 7 battery : 2.28v 24.29 359.0 uohm # 8 battery : 2.31v 24.29 373.2 uohm

The parameters of each battery are voltage, temperature and impedance. In future practical applications, the health status of each battery can be known through comprehensive analysis of these parameters; at the same time, this experiment also verifies that the platform can be applied to online battery monitoring.

In addition, the display function and network communication function of the platform have been tested and they are completely normal. The focus of subsequent work is to further develop the upper management software and build a complete battery online monitoring system using this hardware platform.

5 Outlook

The battery online monitoring hardware platform based on ARM9 and LEM sensors can not only be used for the research of battery failure models, monitoring algorithms and even battery activation technology, but also with the matching monitoring software, it can be applied to various practical occasions that need to monitor batteries, such as DC power supply systems in the power, communication, petroleum, chemical, railway, coal and other industries, as well as online monitoring of batteries in UPS systems, thus truly providing an insurance for the weak link of batteries and safeguarding the safe use of electricity in my country.