Design of fuel cell stack monolithic voltage detection system based on photoelectric isolation relay

The proton exchange membrane fuel cell engine is one of the important development directions of new automotive capabilities. It consists of five parts: fuel cell stack, hydrogen intake and exhaust system, air intake and exhaust system, thermal management system, and control system. Among them, the fuel cell stack is the core of the fuel cell engine. The

fuel cell stack is usually composed of dozens to hundreds of single cells. Due to the influence of operating parameters, the voltage of the fuel cell stack varies greatly. Usually, the normal single-chip working voltage of the battery stack is 0.7V, and it is about 0.9V when no load. Abnormal voltage indicates that the system has failed and measures must be taken immediately, otherwise the fuel cell stack will be damaged.

In order to protect the fuel cell stack, it is necessary to develop a battery stack single-chip voltage detection system to measure the battery stack single-chip voltage in real time and cooperate with the fuel cell engine main controller to handle voltage abnormalities.

1 Design of fuel cell stack single-chip voltage detection system The

vehicle-mounted fuel cell stack single-chip voltage detection system includes two parts: voltage detection card and communication network; fuel cell stacks in non-mobile applications, such as laboratory research and fixed power station applications, should also add computer monitoring functions. This design is applied to laboratory research, and the system structure is shown in Figure 1.

Figure 1 Block diagram of the fuel cell stack single-chip voltage detection system

A fuel cell stack with hundreds of single cells may use multiple voltage detection cards. Considering the communication between the voltage detection card and the main controller, and taking into account the problem of future application in vehicles, the communication of the fuel cell stack single-chip voltage detection system uses the CAN network. The

computer monitoring part uses a third-party CAN card to simulate CAN communication, receive the single-chip voltage data sent by the voltage detection card, and display and save these data at the same time.

The voltage detection card is the core of the system, which realizes the continuous acquisition of the single-chip voltage of the fuel cell stack and judges the voltage value at the same time. If the voltage value is abnormal, the corresponding abnormal condition indicator code and single-chip serial number are sent to the main controller of the fuel cell stack engine through the CAN network, and the main controller takes corresponding actions according to the received information. The difficulty of measuring the single-chip voltage of the fuel cell stack lies in the high accuracy requirement (about ±10mV), the large number of voltage paths and the high potential accumulation. Considering that the leakage current of the photoelectric isolation relay is extremely small, the loss of 1V voltage measurement can be ignored, which can meet the accuracy requirements; at the same time, by controlling the input control terminal of the photoelectric isolation relay, the voltage of any one of the fuel cell stacks can be effectively selected to solve the problem of multiple voltage paths and potential accumulation; in addition, the photoelectric isolation relay has the advantages of no contact, high stability and long life. Therefore, the photoelectric isolation relay method is used for design. In addition, the designed system must also meet the requirements of inspection speed and earthquake resistance.

2 Design of single-chip voltage detection card for fuel cell stack

2.1 Overall design of voltage detection card

The single-chip voltage detection card for fuel cell stack is mainly divided into two parts: signal acquisition module and digital core module. The signal acquisition module realizes the acquisition of the voltage of a specified single cell from multiple single cells in the battery stack and sends it to the digital core module. The digital core module mainly realizes analog/digital conversion, control signal acquisition, and communication with the main controller and microcomputer. The specific structure of the voltage detection card is shown in Figure 2.

Figure 2 Schematic diagram of the voltage detection card structure

The input control terminals of adjacent photoelectric isolation relays are connected to the output terminals of a pair of decoders respectively; the signal input terminals of the photoelectric isolation relays are connected to the voltage signal terminals of the two ends of the single-chip battery of the fuel cell stack in turn; the signal output terminals of the odd-numbered photoelectric isolation relays are connected together, recorded as the COMA terminal. Similarly, the signal output terminals of the even-numbered photoelectric isolation relays are also connected together, recorded as the COMB terminal. The COMA and COMB terminals are connected to the analog signal input terminals of the A/D conversion chip. [page]

It is only necessary to ensure that only one pair of adjacent photoelectric isolation relays are closed at any time to measure the corresponding single-chip voltage value. This can be achieved by controlling the decoder with a single-chip microcomputer. For example, to measure the voltage of the first single-chip, the single-chip microcomputer first shields other pairs of decoders, turns on the first pair of decoders, and then controls the pair of decoders to close the first and second photoelectric isolation relays, so that the voltage signal of the first chip can be led to the COMA and COMB terminals. It is worth noting that when measuring the voltage of odd-numbered and even-numbered single-chips according to this method, the voltage at the COMA terminal is positive or negative relative to the COMB terminal. This problem can be solved by adding an absolute value circuit or an external bipolar A/D conversion chip. This design chose an external A/D converter.

2.2 Speed ​​and accuracy analysis of voltage detection card

Based on the requirements of signal real-time performance, the voltage monitoring system is required to complete signal acquisition and data transmission within 1s. From the acquisition algorithm, it can be seen that if the sum of chip selection, A/D conversion and microcontroller operation time is in the order of μs, then the time to complete the acquisition of 120 voltage signals is in the order of ms. At the same time, the communication rate of the CAN bus can reach 1Mbps. It can transmit 8000 frames/second, and the transmission of 120 voltage signals only requires 40 frames (assuming that one frame can transmit three signal data), which shows that the communication rate is sufficient.

The factors affecting the measurement accuracy are mainly the accuracy of the A/D converter, noise error, etc. When the input of A/D conversion is bipolar and the full-scale voltage is 12V, if it is accurate to 0.01V, the required conversion accuracy is 0.01/12/2=1/2400 (divided by 2 because of bipolar input), so an A/D conversion chip with an accuracy of at least 11 bits is required. Noise error can be reduced by taking the average value of multiple measurements.

2.3 Main hardware selection

The selection of microcontroller mainly considers its computing power and its integrated functional modules. In order to match the running speed of the microcontroller with the A/D acquisition rate and communication rate, the time for the microcontroller to run a single instruction should be around 1μ. The integrated functional modules should mainly include CAN communication module, SPI communication module, and 16-channel digital output module (taking the measurement of 124-channel voltage as an example).

Considering further development in the future, the C8051F040 microcontroller was finally selected. The microcontroller has the following features:

·CAN bus 2.0B
·Pipeline instruction structure
·Clock frequency is 25MHz, speed can reach 25MIPS
·4352 bytes of internal data RAM
·64K bytes of FLASH memory
·64 I/O lines, all lines are 5V resistant
·Hardware SMBus TM (I2C compatible), SPITM and two UART serial ports can be used simultaneously.

The selection of A/D conversion chip should mainly consider the accuracy and its interface with the microcontroller. The A/D conversion chip with an accuracy of at least 12 bits, SPI communication and bipolar should be selected.

The A/D conversion chip currently selected is Max1132. Max1132 has the following features:

·Bipolar 200ksps and unipolar 100ksps sampling rate
·16-bit conversion accuracy
·Input voltage range is -12V~12V
·SPI bus interface

Its accuracy and conversion rate can fully meet the requirements of voltage detection.

The decoder selects the 4-wire/16-wire 74h154 decoder. Since the use of decoders is already very common, we will not go into details here.

The selection of photoelectric isolation relays mainly considers leakage current, withstand voltage and volume. Finally, the photoelectric isolation relay model AQW210S was selected.

AQW210S has the following characteristics:

·Dual-unit photorelay
·Only tens of ohms of resistance when turned on
·Withstand voltage of 350V, leakage current does not exceed 100pA
·Ultra-small SOP package

3 Experimental analysis

This system is currently being used on a small proton exchange membrane fuel cell stack in the State Key Laboratory of Automotive Safety and Energy Conservation of Tsinghua University, and the speed and accuracy of the system have been tested. The system can scan the voltage of 124 single cells 9 times in 1 second, with a measurement accuracy of less than 0.01V, which can meet the needs of detecting and protecting the fuel cell stack.

Experimental research found that the opening and closing time of the photoelectric isolation relay is the main factor affecting the number of scans, which takes about 0.5ms each time. It takes about 60ms to measure the voltage on 120 photoelectric isolation relays, accounting for half of the total inspection time.

The system can measure the output voltage of a single battery at any time, solving the problems of high precision, multiple voltage paths and high potential accumulation; and the system's acquisition accuracy and speed can meet actual requirements through experiments. (end)