Based on OSEK / VDX the electric power system design
With the electronics and motor control technology, electric power steering (ElectronicPowerSteering, EPS) system has made great progress. At present, the automotive electric power steering has replaced the traditional hydraulic power steering (Hydraulic Power Steering, HPS). After 20 years of development, EPS improved technology has made considerable achievements in the light car, van vehicle is widely used .
Traditional software design is not easy to maintain, poor portability, and real-time can not be guaranteed. With the continuous development of the automobile industry, electronics technology in vehicles has been an increasing number of applications. In order to meet the increasingly complex automotive electronics to control software development needs, the realization of application software portability and the control of different vendors can be compatibility between modules, the German automotive industry in 1993 jointly launched the "automotive electronic systems and open interfaces software specification ", that is, OSEK (open systems and the corresponding inteRFaces for automotive electronics) specification, aimed at the distribution of vehicle control unit to provide an open structure of the industry standard. Automotive industry in France in 1994 a motor vehicle for use in Distributed Operation System VDX (Vehicle Distributed Executive) and OSEK specification merged to form OSEK / VDX specification . At present, the norm has become the ISO international standards (ISO17356). Based on this, this paper presents real-time operating system based on embedded software design of the EPS method.
1 EPS system structure and working principle
1.1 EPS structure
According to the different location of the electrical installation, EPS can be divided into the steering shaft-type power, small power and rack gear-type power. Figure 1 is a typical electric power steering system of the diagram. EPS control system the main components :
Figure 1 Schematic diagram of electric power steering system
① torque sensor signal, the role of measurement in the steering wheel on the driver's size and direction of torque.
② speed signal sensors, measuring speed of vehicles in different speed and torque under the steering wheel will help to provide different, to ensure that under the EPS system in the low-speed flexibility, with better under high-speed "road sense."
③ motor power is EPS system, one of the most critical components of its ability to help low-speed large motor torque, moment of inertia of small, high reliability, vibration small, noise, and small size, light weight.
④ electromagnetic clutch, mainly from the role of security, when the failure of EPS, the electromagnetic clutch will cut off the motor and steering column in time connections to the traditional mechanical automotive steering device, so as to ensure traffic safety.
⑤ deceleration agencies, EPS system is an integral part of, used to generate speed by twisting the role.
⑥ electronic control unit ECU, the main processing unit, including information and its peripheral circuits. It is the core of EPS control system. Control unit in accordance with the steering wheel torque sensor and speed sensor signal, after A / D conversion and the logic of analysis and calculation, to give directions to control the electrical work.
1.2 EPS system working principle
Although EPS are three kinds of types, but the principle is the same: through the torque sensor and the speed sensors, for signal acquisition, after A / D conversion, the signal sent to the electronic control unit (ECU), the single-chip microcomputer in accordance with acquisition The speed signal and torque signal, and the corresponding control strategy based on the DC servo motor control for real-time.
Driving under the car towards the requirements of different situations, EPS control by different means of control, usually have three kinds of basic control mode.
(1) Power control
Range of vehicles in low-speed driving, the steering wheel to turn and leave the middle when the electronic control unit of the conventional motor control; determined by calculating the motor current power to help get the right torque, so that manipulation of light-sensitive steering.
(2) back to control
Back control can improve vehicle performance back. Range of vehicles in low-speed driving after shift back to the middle of the steering wheel near the motor electronic control unit so that current rapid decrease in order to turn round quickly to now; at a high speed, the use of damping control, so that both ends of the electrical short circuit, and back to produce Back to the contrary is torque damper is to improve the steering wheel back to the overshoot.
(3) damping control
Damping control can be attenuated when a high-speed car steering wheel jitter phenomenon, due to the elimination of steering wheel and the road surface caused by the importation of shimmy. The principle is very simple, that is, when the car in a high-speed, short-circuit the motor, the terminal voltage to zero, the motor will not provide assistance, but because of the role of induced electromotive force, the motor will result in the opposite direction with the rotation of the torque. This process is tantamount to increasing the damping of the steering system, the driver can get a sense of the appropriate way, will not we have a floating feeling.
2 EPS control system ECU design
EPS system's main function is to collect the torque sensor signal, the speed sensor signals and motor current feedback signal, the controller of the control strategy and control algorithm, through pulse width modulation control for servo motor-assisted power steering provides the driver. In addition, taking into account the specificity of the object of their application, the security requirements of the absolute status, the system also need to provide a number of emergency treatment.
EPS system hardware design mainly includes the following major modules: the core control system design, control unit interface circuit, motor drive and protection circuits, electromagnetic clutch control circuit, sensor signal processing circuit and power supply system circuit design in Figure 2 shown. Introduce here the main controller and the motor drive circuit.
Figure 2 EPS hardware circuit diagram
EPS microcontroller system is used by Microchip's PIC18F458 chip. The chip has the following properties:
① 16-bit wide instructions, 8-bit wide data path, 2 MB of program memory, 4 KB of data memory, up to the implementation of the speed of 10 MIPS.
② 40 MHz clock input, 4 ~ 10 MHz crystal oscillator with PLL active PLL / clock input.
③ with priority interrupt and 8 × 8 single-cycle hardware multiplier.
④ capture / compare / PWM (CCP) modules:
Capture input - 16, maximum resolution of 6.25 ns;
Comparison of unit - 16, maximum resolution of 100 ns;
Pulse width modulation (PWM) output - a resolution of 1 ~ 10;
The maximum PWM frequency - a frequency of 8, when 156 kHz, 10 bit frequency is 39 kHz.
⑤ Enhanced CCP module CCP addition to the above characteristics, but also with 1,2,4 Road PWM output, PWM polarity selectable, programmable dead time of PWM.
⑥ 10, and 8-channel A / D conversion.
⑦ CAN bus module.
2.2 Drive Circuit Design
Motor control circuit in the electric power steering system design is a more crucial part. With the computer into the control fields, as well as a new type of power electronics components emerging power, DC motor control structures and have undergone great changes, the use of full-controlled power switching components PWM (Pulse Width Modulation , PWM) control methods have become the absolute mainstream. In this system, the motor is used to control pulse width modulation PWM control mode. Full-bridge bipolar drive circuit shown in figure 3.
ECCP pin PIC18F458 mcu to connect two driver IC IR2110 (each IR2110 control two MOSFET), to control the four MOSFET turn-on and cut-off in order to achieve the control of electrical power. EPS system needs to achieve the three kinds of control: Conventional control, back to the control and damping control.
Figure 3 power motor drive circuit
3 EPS Software Design
With the embedded applications and to further complicate real-time, the improvement of the reliability requirements, in order to legitimate a variety of task scheduling and the use of system resources, embedded real-time operating system based on embedded software design gradually became the design and development of embedded systems mainstream. At present there are hundreds of embedded real-time operating system types, their characteristics. Open-source embedded real-time operating system in terms of cost and technology has unique advantages, and occupy an increasingly important position. In this paper, choice of open-source embedded real-time operating system PICOS18 as a software development platform EPS. PICOS18 in accordance with the OSEK / VDX standards to achieve real-time operating system. PICOS18 is a multi-tasking can be deprived of their real-time micro-kernel, a very small, accounting for program space (ROM) is less than 1KB, accounting data space (RAM) only 7B, the system capacity and operational requirements of the code of ROM and RAM are also very few ; provides task management, timer management, event management, interrupt management functions; priority-based scheduling tasks with priority 16, the system occupied by one, the user can create 15 tasks, each task can have up to 8 events .
3.1 application Software Development
Embedded real-time operating system to function-oriented application development into a task-oriented application development, so the process of software development is in accordance with the function of application system is subdivided into a number of tasks, and then the realization of each task and the task to determine the appropriate priority; for demanding real-time operation, the need to prepare the relevant interrupt service routine.
According to the working principle of EPS can be divided into eight tasks.
(1) Task1 - speed signal acquisition
Expansion of tasks, used to calculate the speed. Task1 power running in wait state, waiting for the speed calculation of the incident EventSpeed. The use of timer / counter module when the TMR0 overflow counter (for the number of speed after the signal pulse) have interrupted, interrupt service routine to enter the speed, recording the total pulse cycle time, and then set the incident EventSpeed, activation of Task1. Task1 in this state of readiness for scheduling mechanisms in operating systems (complete preemptive) the management, ready to wait until the priority queue is higher than the Task1 to run upon the completion of tasks, Task1 run, according to the recorded number of pulse duration and pulse to calculate the speed and filtering. Implementation after activation Task2, clear the event EventSpeed, Task1 is in wait state.
(2) Task2 - torque signal acquisition
The basic tasks for the acquisition of torque signal. Task1 activated by the task, the implementation of the same frequency and Task1. Because of the speed signal and torque signal EPS system is the most important two parameters, so these two parameters must be updated in a timely manner to ensure that the choice of power modes and help to determine the size of the control to be timely and accurate.
(3) Task3 - current feedback signal acquisition
The basic tasks for collecting electric current feedback. Task5 activated by the task, the system only activated when power to control this task. Parameters and objectives of the current margin, through the PID regulator controlled to rapidly provide the corresponding motor torque to achieve the purpose of assistance.
(4) Task4 - Fault Diagnosis
Extension of the mandate for the monitoring and fault diagnosis. Power running, waiting for news MsgSpeedErr, to determine the normal speed; wait MsgVoltErr, to determine the normal voltage; wait MsgTorqueErr, to determine the normal torque. The event of failure, the task will be immediately disconnected from the relay, so that in a mechanical steering system steering state, to avoid the accident.
(5) Task5 - Power Mode Selection
Basic tasks, as well as assistance for the choice of power control mode to determine the target current. Task2 activated by this task, through the speed and torque of the size, help to determine the mode under the control of the power curve through the help target current. The implementation of this task and the number of Task1 and Task2 the same manner and help to ensure that the size of real-time and accurate help.
(6) Task6 - Power control
Basic tasks, power control, activated by Task3. Task5 get through the current goal, as well as motor feedback Task3 current regulator for the use of closed-loop PID control, and finally through the PWM pulse width modulation to control electrical power.
(7) Task7 - back to control
Basic tasks, back to being controlled by the activation Task5. When the car at high speed, so that both ends of the electrical short-circuit, resulting in back is the damping, reducing the overshoot is back; when the car in a low speed so that rapid motor circuit at both ends, reducing the electrical resistance, so that is quickly turning.
(8) Task8 - Damping Control
Basic tasks, damping control, activated by Task5. Damping control for a variety of high-speed state at the time (back is turned and a straight line running). Back to when the damping control system can reduce the overshoot; turn, the increased resistance, so that drivers get a better sense of the way; straight-line motion, the face of the steering wheel to reduce the impact of the road.
3.2 task priority
Preemptive Scheduling PICOS18 used, that is, all the tasks are to take precedence, and each has a mandate only to determine the priority, the more important task of higher priority. Since the power control (Task6) tasks to be run at the right moment, so the highest priority Task6, back to the control (Task7), damping control (Task8) second, followed by the task of fault diagnosis (Task4), the remaining tasks according to their priority determine the implementation of the order of activation. Task4 at the beginning of run-time monitoring in a wait state if the signal is no longer unusual to implement. Task1, Task2 and Task5 in order to finish a cycle, the speed to continue to respond to interrupt and re-implementation. This scheduling can be collected not only the latest signal of the speed and torque signals, so that EPS systems provide accurate and real-time help, but also increase the CPU utilization, make full use of hardware resources.
3.3 configuration tasks (OIL)
PICOS18 through taskdesc.c the various parameters of the definition of tasks and is OSEK / VDX specification of OIL (OSEK / VDX the realization of the language, similar to the definition of a c structure) prepared . PICOS18 as a result of the task did not provide GUI for configuration, it can only be prepared sentence by sentence. The definition of the parameters of the task is structured as follows:
This paper analyzes the structure of the EPS system, working principle and the three kinds of control. PIC18F458 microcontroller through the ECCP module control motor, the realization of the EPS system in all cases the way of help. Embedded real-time operating system, not only improve the utilization of the CPU to ensure that the EPS system for real-time requirements, but also improve the stability of the system operation, reliability and portability.
OSEK / VDX is an international automotive electronics development standards, the use of OSEK / VDX real-time system specification developed software modules to improve the efficiency of transplantation, to achieve the reuse of software modules and between the different electronic control unit communication. The use of OSEK / VDX for the development of automotive electronic control unit has become a trend.
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