Design of an intelligent power supply in a vehicle-mounted unit

Abstract: High performance and low power consumption are the inevitable trends in the development of electronic systems. The power consumption of embedded systems has attracted more and more attention, especially for battery-powered portable embedded systems. In most embedded system designs, performance improvement is always accompanied by an increase in system power consumption. Solving the contradiction between the two, exploring low-power power management technology, and providing practical solutions will be opportunities and challenges faced by related industries around the world. Based on the electronic non-stop toll collection system, this paper proposes an effective intelligent power management solution for the on-board unit in the system.

The research field of Intelligent Transport System (ITS) is the development direction of future transportation systems. It is an integrated transportation management system that effectively integrates advanced information technology, data communication transmission technology, electronic sensor technology, control technology and computer technology and applies them to the entire ground transportation management system. It is a comprehensive transportation management system that can play a role in a wide range and in all aspects, and is real-time, accurate and efficient.

1 Introduction to the ETC system

Electronic Toll Collection (ETC) is designed to consist of several major parts, including roadside reading and writing equipment (Road Side Unit, referred to as RSU), on-board unit (On Board Unit, referred to as OBU), IC card, computer security control technology, network and accounting.

Its composition is shown in Figure 1.1:

Figure 1.1 ETC system composition diagram

The OBU contains the vehicle identification information, such as the license plate number and car ID number. It is generally installed on the windshield in front of the vehicle, the RSU is installed next to the toll booth, and the loop sensor is installed under the lane ground. The central management system has a large database that stores a large amount of registered vehicle and user information. When a vehicle passes through the toll booth, the loop sensor senses the vehicle, the roadside unit sends an inquiry signal, and the on-board unit responds. Two-way communication and data exchange are carried out. The central management system obtains vehicle identification, vehicle model and other information and compares and judges with the corresponding information in the database, and controls the management system to produce different actions according to different situations. Through mutual communication and information exchange between the roadside unit and the on-board unit, the vehicle can be automatically identified, and the toll is automatically deducted from the user's special account, thereby realizing automatic charging.

Because OBU plays a very important role in the entire ETC system, and the performance of the power module is directly related to whether OBU can work normally. There are generally two existing OBU power supply methods, but each has its own shortcomings. Based on this, a new low-power power supply design method should be generated.

2 Design of low power intelligent power supply for electronic toll collection system

For example, the power consumption of a system like OBU, which is based on an embedded processor, is mainly composed of the processor power consumption and the peripheral circuit power consumption. Considering the actual operation situation, the time when a car passes through a toll station during the entire process of driving on the road is very short. The OBU only needs to work when the car passes through the toll station, and the working time is less than one second. It does not need to work for most of the other time. The OBU intelligent power management adopts a battery-powered method to control the OBU to work in two modes, sleep mode and activation mode; when the car enters the toll station, the OBU is activated and starts working in the activation mode; when the transaction is completed and the car leaves the toll station, the OBU immediately stops working and enters the sleep mode. According to this feature, a hierarchical management strategy is adopted for the power supply, that is, it is divided into standby power supply and working power supply, as shown in Figure 2.1.

Figure 2.1 OBU power management hierarchical module structure diagram

At the same time, in order to achieve further time-sharing power supply control, the working power supply module is divided into independent power supply circuits according to functional modules. The power supply of each module is independently controllable and coordinated by the main controller.

The activated MCU uses interrupt programming to judge each I/O and then control it in time-sharing mode, outputting DC/DC enable high-level signals for each power-consuming module, and providing power for RF reception, RF transmission, baseband circuit, IC card reading and writing, account management and human-machine interface in time-sharing mode; at any time, the system only supplies power to a maximum of 2 modules in addition to the main controller, which greatly reduces the power consumption of the system.

3 Intelligent power circuit design and device selection

3.1 Amplification circuit design

The signal detected by the detector is very weak, which may be several milliamperes or even microamperes. Such a small signal generally cannot meet the signal amplitude requirements of various circuits in the subsequent stage, so it must be amplified. The quiescent current of the operational amplifier is proportional to the bandwidth. The larger the bandwidth, the larger the quiescent current. This can be clearly seen in the selection tables of major semiconductor manufacturers. Through comparison, in order to design an amplifier with large gain, low noise and a certain bandwidth, this design chooses TLV2382 as the wake-up signal amplifier.

Figure 3.1 Operational amplifier single power supply circuit

3.2 Low-power MCU hardware and software design

In order to achieve low power consumption, the MCU should choose a model with low energy consumption. An ultra-low power MCU should be considered from the following aspects: system average current, clock system, interrupt, on-chip peripherals, BOR protection, pin leakage current, and processing efficiency. Based on this feature, this design selected a 16-bit ultra-low power mixed signal processor (Mixed Signal Processor) MSP430 series microcontroller that Texas Instruments (TI) began to market in 1996.

3.3 System Clock Design

For a system with low voltage sleep wakeup, the choice of crystal oscillator is very important. This is because low supply voltage reduces the excitation power provided to the crystal, especially when waking up from sleep, causing the crystal oscillator to start very slowly or not at all, and too long startup time will significantly increase the power consumption of the system. The system designed by MSP430F2001 can solve this problem well.

The low-speed clock uses 12kHz VLOCLK, and the high-speed clock is generated by the internal integrated DCO oscillator. The appropriate output frequency can be selected by adjusting the control parameters.

3.4 Power supply design

This system uses 2 dry batteries as the test power supply, and generates a +3.3V voltage through the power chip. Since the input voltage range of TPS79633 is 2.7~5.5V, the enable signal is high level and low level, so the I/O pin of the microcontroller can be directly used as the enable signal.

Whether the MCU works or not is completely determined by the output POW of the main switch. The microcontroller only switches between working and power-off. Other I/O interfaces serve as outputs to control other modules for time-sharing control.

3.5 Software Design

To reduce power consumption, the combination of software and hardware is necessary to achieve the ideal effect. The following aspects should be paid attention to in software design:

The system uses interrupt programming to judge each I/O, time-sharing control, and time-sharing power supply to each module, thereby reducing power consumption. Secondly, it outputs a feedback signal to control the main switch to achieve the power-off function.

The power consumption of the system will increase as the frequency of the system increases. Using a 12kHz main clock after the system starts working can reduce the power consumption during operation.

4 Conclusion

The low-power design of embedded systems is a design principle that must be considered in embedded system design. A successful low-power design should be a combination of hardware design and software design. Starting from hardware design, we should fully realize the characteristics of a low-power application, choose a suitable MCU, and design a system solution by understanding its characteristics; in software design, we should take into account the particularity of low-power programming and try to use the low-power mode of the microcontroller.