A supercapacitor controller for electric vehicles
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1 Introduction
Regenerative braking plays a prominent role in the energy recovery of electric vehicles. In the energy management system of electric vehicles, it is required to utilize as much energy as possible from regenerative braking feedback. Usually, the energy fed back by regenerative braking is absorbed by charging the battery [1]. The disadvantage is that it is difficult to achieve high-power charging of the battery in a short time, the number of charge and discharge cycles is limited, and the cost is high. Supercapacitors are energy storage elements between batteries and electrostatic capacitors. They have a much higher energy density than electrostatic capacitors and a much higher power density than batteries. They are suitable for use as a short-term power output source [2]. They have advantages such as high specific power (power output per unit mass or volume), high specific energy (electricity output per unit mass or volume), and high energy storage. Therefore, they can greatly improve the driving range of electric vehicles and effectively improve the movement characteristics of electric vehicles when starting, accelerating, and climbing. In addition, the use of supercapacitors in electric vehicles can smooth the charge and discharge current of the power battery, and the service life of the power battery can be greatly extended, even by 1.5 times. Based on this, this paper proposes a method of using supercapacitors as energy storage devices for electric vehicles in order to obtain a high level of energy recovery, and briefly introduces the design of the supercapacitor controller.
2 System Overview
2.1 Regenerative braking experimental design
Regenerative braking is to store the kinetic energy of the vehicle in the energy storage device of the electric vehicle by the generator of the motor for recycling. In the experiment, a domestically produced permanent magnet DC motor (18kW/288V) and two parallel supercapacitors (350V/0.7F/400A, 400V/0.58F/400A) were used, and a small power chopper (DC/DC converter) was designed to control the charging and discharging of the supercapacitor. The schematic diagram of the regenerative braking control system test bench is shown in Figure 1. In the figure, the AC power supply is rectified into DC power through AC/DC, which is the power source in the experiment. In fact, it is equivalent to the main power supply of the electric vehicle - the power battery, which is responsible for driving the motor and charging the supercapacitor when necessary.
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Figure 1 Schematic diagram of the regenerative braking control system test bench
The drive motor of an electric vehicle should have a wide speed regulation range and high speed, sufficiently large starting torque, high efficiency, strong dynamic braking and energy feedback performance. Permanent magnet DC motors have such characteristics when used as drive motors, and their drive control systems are relatively simple. A chopper (DC/DC converter) is a periodically on-off switch control device between a DC power supply and a load. Its function is to change the voltage supplied to the motor or supercapacitor. In fact, it works as a voltage regulation system. This type of regulator is named because it cuts off the input voltage and turns it into a pulse output that is intermittent in time. Power semiconductor devices such as thyristors, GTOs, GTRs or IGBTs can be used as electronic switches. In the above experimental scheme, supercapacitors and DC motors are used as DC power supplies or loads respectively. When the electric vehicle is starting, accelerating and running at a constant speed, the supercapacitor discharges and supplies electric energy to the motor. The motor is in an electric state, realizing the conversion of electric energy to mechanical energy and driving the vehicle forward. When an electric vehicle decelerates, the DC motor is required to be in a generating braking state, that is, in a regenerative braking state, to charge the supercapacitor, an energy storage device serving as a power source, to achieve the conversion of mechanical energy into electrical energy and to realize regenerative energy recovery.
2.2 DC-DC main circuit design
In order to achieve the above control requirements, a bidirectional step-up and step-down DC-DC converter is designed, and its main circuit topology is shown in Figure 2.
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Figure 2 Topology of the main circuit of the DC-DC converter
The control scheme is as follows:
(1) Electric boost
Smdw is fully turned on, Smup is chopped; Sbdw and Sbup are turned off. At this time, the supercapacitor works in the discharge mode, and its voltage is greater than the rated voltage required by the load (motor). The DC-DC converter can only perform electric boost conversion to provide electrical energy to the load.
(2) Electric step-down
Smup is completely cut off, Smdw is chopped; Sbdw and Sbup are turned off. At this time, the supercapacitor works in the discharge mode, and its voltage is less than the rated voltage required by the load (motor). The DC-DC converter can only perform electric step-down conversion to provide electrical energy to the load.
(3) Electric direct transmission
Smup is completely cut off, Smdw is completely turned on; Sbdw and Sbup are turned off. At this time, the supercapacitor works in the discharge mode, and its voltage is approximately equal to the rated voltage required by the load (motor). The DC-DC converter does not perform step-up or step-down conversion, and the voltage input by the supercapacitor is directly output to the load (motor).
(4) Brake boost
Sbdw is fully turned on, Sbup is chopped; Smdw and Smup are turned off. At this time, the supercapacitor works in charging mode, and its required rated voltage is greater than the voltage supplied by the load (motor). The DC-DC converter can only perform braking boost conversion to provide electrical energy to the supercapacitor.
(5) Braking pressure reduction
Sbup is completely cut off, Sbdw is chopped; Smdw and Smup are turned off. At this time, the supercapacitor works in charging mode, and its required rated voltage is less than the voltage supplied by the load (motor). The DC-DC converter can only perform braking and step-down conversion to provide electrical energy to the supercapacitor.
(6) Braking direct transmission
Sbup is completely cut off, Sbdw is completely turned on; Smdw and Smup are turned off. The supercapacitor is now operating in discharge mode, and its required rated voltage is approximately equal to the voltage supplied by the load (motor). The DC-DC converter does not perform step-up or step-down conversion, and the input voltage supplied by the load (motor) is directly output to the supercapacitor.
3 Design of DC-DC Controller
3.1 DC-DC controller based on CAN bus and characteristics of CAN bus
The electric vehicle energy management system needs to detect and exchange a large amount of data. It is difficult to solve the problem by hard-wiring signal lines, and the cost is high. Using CAN bus to realize internal data communication is an effective method. The designed electric vehicle supercapacitor controller is a fully distributed control system based on CAN bus.
The CAN (Controller Area Network) bus was designed by Bosch of Germany for automobile monitoring and control systems. It has excellent performance and high reliability. It can work in multi-master mode, enabling the modules of the system to achieve multi-master communication. In multi-master mode, any node on the network can actively send information to other nodes at any time, regardless of master or slave, and the communication mode is flexible. The node information on the CAN network is divided into different priorities to meet different real-time requirements. The biggest feature of the CAN protocol is that it breaks the traditional node address encoding method and encodes the communication data block. This method allows different nodes to receive the same data at the same time. 211 or 229 different data types can be defined. The network capacity is huge, and at the same time, bus "conflicts" can be avoided [3].
3.2 DC-DC controller hardware system design
According to the control scheme of bidirectional step-up and step-down DC-DC converter, the DC-DC controller based on CAN bus developed in this paper is essentially a CAN node. Its hardware principle is shown in Figure 3. The main functional modules are as follows.
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Figure 3 DC-DC controller hardware schematic diagram
(1) Measurement and control module
The CPU uses 80C196KC single-chip microcomputer. The voltage and current signals are adjusted to signals suitable for collection by the single-chip microcomputer A/D converter through sensors and signal conditioning circuits. It is mainly used to measure and monitor faults of the acceleration and deceleration state of electric vehicles, the voltage and current of the main circuit, and the voltage and current of the supercapacitor.
(2) Storage information module
Extended EPROM 32K×8-bit UV-erasable electrically programmable read-only memory 27256.
(3) Signal output module
This system requires the output of 4-channel PWM waveforms. The programmable logic device GAL16V8 is directly connected to the PWM port of 80C196KC to achieve 4-channel PWM output, time-sharing control of the switch duty ratio of the 4 IGBT tubes in the main circuit, and voltage regulation.
(4) Communication interface module
The communication interface of the controller extension is the CAN bus interface. The CAN bus interface extension uses the CAN communication controller SJA1000 + high-speed optical coupler 6N137 + CAN bus transceiver 82C250 circuit, and can be connected to the RS232C serial port of the host computer through the MAX232 circuit to achieve two-way communication between the host computer and the controller. The circuit principle is shown in Figure 4. In addition, due to the above-mentioned CAN bus characteristics, CAN nodes can be easily added or reduced in the CAN network. Any node can actively send information to other nodes at any time, regardless of master and slave, to achieve multi-master communication, so that multiple control units in the energy management system can be turned into network nodes to form a network-integrated fully distributed control system.
Figure 4 CAN bus interface circuit schematic
3.3 Software Design[4]
The function of the system software is to judge the running state of the electric vehicle. If the accelerator pedal of the car is pressed, the supercapacitor works in the discharge mode and adjusts the electric voltage increase and decrease subroutines; if the brake pedal of the car is pressed, the supercapacitor works in the charging mode and adjusts the voltage increase and decrease subroutines.
In order to facilitate the writing and debugging of software, the change and analysis of control algorithms, the software adopts a modular structure. The system software consists of a main program, a subroutine, and an interrupt service program. During initialization, the initial value and global variables should be set, the interrupt vectors used by each interrupt service program should be initialized, the software structure should be set, and the priority order should be reset. The A/D sampling interrupt service program uses the CAM lock bit of the high-speed output HSO of the 80C196KC to start the ACH0 channel at a fixed time. The high-speed input HIS of the 80C196KC is used in interrupt service program 4 to record the time when an external event occurs, which is used to judge the accelerator pedal and brake pedal signals. The functional description can be completed with only a few judgment statements, making the program writing very concise.
4 Conclusion
In the experimental study of regenerative braking of electric vehicles, the use of supercapacitor controllers makes full use of the energy storage capacity of supercapacitors, which is of great significance for achieving high-level energy recovery. The software and hardware of this system adopts modular design, which has good versatility and strong flexibility. It can not only realize the function of DC-DC conversion, but also can be used as a development platform for easy expansion and can be applied to the design of various controllers. It has a relatively broad application prospect and is an open distributed control system.
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