With the development of society, people have higher and higher requirements for the comfort and safety of automobiles, and manual transmission vehicles increase the difficulty of driving because of heavy gear selection and clutch operation.For novice drivers, there will be problems such as easy flameout, high fuel consumption, and serious clutch wear when starting on a hill.[ 1]. Although automatic transmission vehicles are simple to drive, they are expensive and difficult to develop. The Electronic control automatic clutch ACS (Automatic Clutch System) designed in this paper is to install the electronic control system on the basis of the manual gearbox, cancel the clutch pedal, and realize the automatic clutch. The advantages of ACS are very obvious: compared with manual transmission, its driving control is simpler, with the characteristics of fast acceleration and comfortable driving; compared with automatic transmission vehicles, ACS has the advantages of low cost, convenient maintenance, economy and fuel saving.
1 System function
ACS uses modern electronic control technology to control the dry friction clutch, simulates the manipulation action and feeling of an excellent driver, and achieves the best clutch coupling law. function. The ACS controller designed in this paper mainly realizes the following major functions.
(1) Shift clutch: After the controller receives the shift signal, the clutch is quickly and automatically disengaged, and the clutch is automatically combined after the shift is in place. The combination rule is determined by the electronic control unit according to the driving conditions of the vehicle.
(2) Starting on a hill: The driver steps on the brake pedal, starts the engine, puts the shift handle in first gear or reverse gear, releases the handbrake, and the car can automatically drive at a slow speed without stepping on the accelerator pedal after the brake is released, and start Stable, small impact, no flameout.
(3) Flameout protection: When the vehicle is running, the clutch will automatically disengage when the vehicle speed and engine speed are lower than the set value, and the clutch will be automatically engaged when the vehicle speed and engine speed are higher than the set value.
(4) CAN communication: The ACS controller realizes data communication with the engine controller through the CAN bus interface, providing data support for the coordinated control of the clutch and the engine.
2 The hardware design of the system
2.1 Controller composition
The schematic diagram of the automatic clutch controller is shown in Figure 1. The microprocessor of this system selects Infineon’s high-performance 8-bit microprocessor XC878CM, and the operating frequency can reach up to 27 MHz. Its on-chip hardware resources are very rich, and the on-chip integrated MultiCAN controller, capture/compare unit 6 ( CCU6), high-performance ADC module, etc. The excellent performance of XC878CM fully meets the design needs of this system. The hardware part of this system mainly includes power module, data acquisition module, CAN communication module, executive motor drive module and so on.
Figure 1 Schematic diagram of automatic clutch controller
(1) Power module The low-voltage control system of the vehicle is powered by a 12 V battery, and the 8-bit MCU is powered by 5 V. Therefore, the system needs to use a power chip for voltage conversion and isolation. This system uses Infineon power chip TLE4290, which can provide a stable 5V voltage, the error is within 2%, and the input voltage can be up to 42V. After testing, it works reliably and meets the system requirements.
(2) CAN communication module CAN communication module uses XC878CM on-chip MultiCAN controller and Infineon high-speed CAN transceiver IFX1050G as the hardware composition of CAN communication. The CAN module is responsible for the data exchange and sharing between the clutch controller and the engine controller, and provides data communication support for the coordinated control of the engine and the clutch.
(3) Execution motor drive module The execution motor used in this system is a DC motor with a rated voltage of 12V. The microcontroller uses one IO port to control the rotation direction of the motor, and one PWM output controls the speed of the motor.
The PWM wave is generated by the CCU6 module included in the microcontroller configured in the compare mode. The single-chip microcomputer realizes the control of the executive motor through the Infineon motor driver chip BTS7810K.
(4) Data acquisition module There are mainly three types of data collected by this system: switch quantity, analog quantity and frequency quantity. The switching value mainly refers to the ignition signal and the driver’s gear shift signal, etc., which are collected through the I/O port of the single-chip microcomputer.
The XC878CM one-chip computer integrates a high-performance 10-bit analog-to-digital converter with 8 analog input options, which can be easily used for analog quantity acquisition. The CCU6 module included in the XC878CM can be configured to work in the capture mode to collect the frequency signal sent by the vehicle speed sensor. Due to the large interference of the automobile environment, the signal acquisition circuit needs to add filtering, voltage conditioning and other circuits. In addition, for the frequency quantity acquisition, since the received pulse signal, it is also necessary to use a Schmitt trigger to shape the pulse signal.
2.2 Motor drive circuit design
The clutch actuator is driven by a 12 V DC motor, and the single-chip microcomputer uses the pulse width modulation PWM technology to control the motor speed. PWM speed regulation method has become a widely used speed regulation method due to its advantages of simple control, good dynamic response effect, and wide speed regulation range.
The control of the rotation direction of the DC motor needs to be realized by building an H-bridge circuit, because the H-bridge circuit and gate drive circuit built by themselves are often difficult to guarantee in terms of reliability. Therefore, this paper chooses the integrated motor driver chip BTS7810K to drive the clutch execution motor. The chip BTS7810K is a full-bridge motor driver chip, which integrates an H-bridge motor driver circuit and a gate driver circuit. Its operating frequency is as high as 1 kHz or more, which can conveniently and reliably control the DC motor. The input and output characteristics of the BTS7810K in normal working mode are shown in Table 1.
Table 1 BTS7810K input and output characteristics
The motor drive circuit is shown in Figure 2. The single-chip microcomputer uses an I/O port output to control the motor rotation, and a PWM output to control the motor speed. The two control signals are connected to the input terminals IH1 and IH2 of the driver chip through an interface circuit composed of an AND gate and two NOT gates. This is done to ensure that the two input terminals are not at a high level at the same time, to prevent the occurrence of the bridge arm shoot-through problem, and to improve the safety and reliability of the system.
Figure 2 Motor drive circuit
2.3 CAN node interface design
CAN bus is a serial communication protocol network developed by German Bosch company in the early 1990s to solve the information exchange between many control and test instruments in modern automobiles.[ 3]. It has the characteristics of high transmission rate, strong reliability and good real-time performance, which just meets the communication needs of ACS and engine coordinated control. Comprehensive control of the engine and clutch, making full use of the characteristics of the engine electronic control system to control the engine speed in a timely and accurate manner, so as to coordinate with the clutch, will help the clutch to achieve better control effect, thereby improving the shifting quality.
The hardware circuit of CAN node mainly includes: Microcontroller with CAN controller and CAN transceiver for data transmission and reception. The microprocessor XC878CM selected in this paper has an on-chip CAN controller, which is mainly responsible for CAN initialization and data processing. The MultiCAN module integrates all functions of a CAN bus controller except the transceiver. In addition, MultiCAN also has advanced acceptance filtering function, advanced data management, advanced interrupt management and other excellent features. There are many types of CAN transceivers, and the high-speed transceiver IFX1050G of Infineon Corporation is selected in this design. The interface circuit diagram of the CAN node is shown in Figure 3.
Figure 3 Interface circuit diagram of CAN node
3 Software Design
The control software of the electronic control unit ECU is mainly composed of the clutch control program and the CAN bus communication program.
3.1 Design of clutch control software
The control program of the clutch includes three parts: the clutch disengagement control program, the start-up combination control program, and the shift combination control program. Among them, the separation control procedure is relatively simple. After the ECU gets the separation command, the clutch is released at full speed, and it can be stopped exactly at the complete separation point. The difficulty of clutch control lies in the start-up combined control. The start-up combination process of the clutch should not only ensure the smoothness and comfort of the vehicle start, but also ensure the rapidity of the start, reduce the generation of friction and friction, and prolong the service life of the clutch. Therefore, in order to obtain a better control effect, in addition to controlling the engagement amount of the clutch, it is also necessary to control the engagement speed of the clutch, and through the coordinated control with the engine, the control effect is improved. Figure 4 is the flow chart of the start-up combined control software. The coupling control of the clutch in the shifting process is similar to the starting control in terms of control strategy, and will not be repeated here.
Figure 4 Flow chart of start-up combined control software
3.2 CAN Communication Protocol Design
CAN communication protocol includes physical layer, data link layer and application layer. The physical layer and the data link layer are implemented by hardware. When using CAN communication, the developer needs to define the application layer protocol. The main task of constructing the application layer protocol is to assign ID, define the message period, and determine the mapping relationship between signals and messages. The main factors to be considered in the design are the real-time requirements of data transmission, the relative importance of data, and the time requirements of data-related application control algorithms for data. There are some existing standards in the world, such as CANopen, SAE J1939 and so on.
In some cases where simple communication protocols can be used to meet requirements, the use of complex protocols will result in waste of resources, and users will also feel a lot of inconvenience when applying, which limits flexibility. There are only two nodes in the CAN bus network designed in this paper, the clutch controller and the engine controller. For the experimental platform with only two nodes, this paper defines a simple and reliable CAN protocol from the perspectives of the amount of code implemented by the protocol, the amount of information in the target system, and the cost of software development. The specific communication protocol definition is shown in Table 2. The identifier is used to indicate the priority of the information. The smaller the identifier, the higher the priority.
Table 2 CAN bus communication protocol
4 CAN communication test experiment
The experiments in this paper are carried out on a self-built clutch simulation experimental platform. The experimental platform is composed of clutch control board, accelerator pedal, brake pedal, related sensors, clutch actuator and engine simulation control board. Communication between the clutch control board and the engine simulation control board is via CAN bus. Figure 5 is the gear change information transmitted through the CAN bus during the experiment, and Figure 6 is the accelerator pedal opening signal transmitted through the CAN bus.
Figure 5 Gear information
Figure 6 Accelerator pedal opening signal
In this paper, a controller scheme of electronically controlled automatic clutch is proposed, and the software and hardware of the system are developed, the basic functions of the automatic clutch are initially realized, and the CAN bus interface is designed. The feasibility and reliability of the controller scheme and CAN communication module are verified on the experimental platform, which lays the foundation for the real vehicle test.
