Design of CAN Intelligent Sensor Based on PIC16F877A

With the progress and development of science and technology, the requirements for vehicle driving performance, safety and comfort have been greatly improved, which has led to a gradual increase in the number of electronic control units on vehicles. However, the connection of electronic control units on vehicles (such as various switches, actuators, sensors, etc.) is still achieved with traditional wiring harnesses, which makes the wiring harnesses in the car too many and complicated, thus causing serious electromagnetic interference and reducing the reliability of the system. In high-end cars, electronic components and their systems account for more than 20% of the price of the whole vehicle, and there is a trend of increasing day by day. In this case, the electronic control circuits in the car will become more complicated. How to network the devices in the car and reduce the number of wiring harnesses has become a key research direction for improving the in-car system.

2 Principle and structure of the electronic throttle valve control system

Electronic throttle control technology first appeared in the early 1980s and was initially only used in high-end cars. With the increasing development of electronic technology, the increasingly prominent energy and environmental issues, and the increasing requirements for automobile performance, the electronic throttle has become the most important control device on the fully electronically controlled engine and has begun to be widely used in various vehicles. Its advantages are that the throttle can be quickly and accurately controlled at the optimal opening according to the driver's wishes, emissions, fuel consumption and safety requirements, and a variety of control functions can be set to improve driving safety and comfort. At present, companies such as BMW, BOSCH, and Toyota are conducting research on this technology, and manufacturers such as BMW, GM, Toyota, and AUDI have successfully applied it on some of their models.

As shown in Figure 2, the system consists of an accelerator pedal position sensor and an electronic throttle body. The throttle body includes three parts: an actuator, a throttle valve, and a throttle position sensor, which are packaged as one. The actuator consists of a DC motor and related transmission components. The accelerator pedal is a high-precision linear potentiometer, which serves as a sensor device for the driver's desired throttle opening. Its output is an analog voltage signal proportional to the pedal stroke; the throttle body consists of two forward and reverse position sensors as the throttle opening feedback signal in the control. It obtains the corresponding voltage feedback value at the current opening through a pair of high-precision potentiometers inside the throttle body. The feedback value changes linearly with the throttle opening angle.

3 Intelligent sensor CAN bus interface design

The design of the intelligent sensor contact is based on Microchip's PIC16F877A microcontroller, independent CAN bus controller MCP2510 and CAN transceiver PCA82C250.

PIC16F877A is a high-performance 8-bit microprocessor with RISC instruction system, Harvard bus structure, low power consumption and high speed. It integrates ADC, serial peripheral interface (SPI) and Flash program memory, and has multiple functions such as PWM output. PIC16F877A can achieve seamless connection with CAN controller MCP2510 through SPI interface.

The hardware schematic diagram of the CAN intelligent sensor node based on PIC16F877A is shown in Figure 3.

The communication module of the intelligent sensor CAN node consists of an independent CAN controller MCP2510 and a CAN transceiver PCA82C250. MCP2510 can complete all the functions of the physical layer and data link layer of the CAN bus, support high-speed SPI interface (the maximum data transmission rate can reach 5MB/s), and support CAN2.0A/CAN2.0B protocol. The CAN transceiver PCA82C250 is the interface between the CAN controller and the physical bus, providing differential transmission capability for the physical bus and differential reception capability for the CAN controller. At the same time, it can increase the communication distance and improve the anti-interference ability of the embedded CAN intelligent node.

PIC16F877A is connected to CAN controller MCP2510 via SPI, with its serial data input (SDI) pin connected to SO pin of MCP2510, its serial data output (SDO) pin connected to SI pin of MCP2510, and its serial clock (SCK) pin connected to SCK pin of MCP2510. The reset signal and chip select signal of MCP2510 are provided by the microcontroller.

By setting the SPI interface status register and control register of PIC16F877A, the SPI interface works in active mode. The timing of the communication between PIC16F877A and MCP2510 is very important. When sending data, send the write instruction first, then send the register address, and finally send the data. When MCP2510 receives data from the bus, an interrupt will be generated, and the microcontroller responds to the interrupt. When reading data, send the read instruction first, then send the register address, and the data will be automatically written into the buffer of the microcontroller SPI interface.

Since the microcontroller itself has a 10-bit A/D converter, the analog signals output by the pedal position sensor and the throttle position sensor are directly connected to the microcontroller for digital-to-analog conversion, without the need to add a new A/D conversion device. In Figure 3, the sensor is input via RA0/AN0. In order to filter out high-frequency noise, an RC filter circuit is connected to the analog-to-digital input port. At the same time, in the control of the DC motor of the electronic throttle device actuator, the PIC16F877A has a PWM port, which can drive the DC motor by connecting the drive circuit. The driver of this device uses L298.

The whole CAN bus control network consists of pedal intelligent position sensor nodes, throttle position sensor and actuator nodes and controller nodes. Among them, the pedal intelligent position sensor nodes, throttle position sensor and actuator nodes are completed by the single-chip CAN bus mechanism. Its main function is to transmit the pedal position and feedback signal throttle position signal to the controller, and at the same time, receive the drive command signal sent by the controller to the actuator. The controller uses a microcomputer to realize CAN bus communication and corresponding control algorithms through Advantech PCL-841 card to complete the control of the wire-controlled electronic throttle.

4 System control principle and experimental results

The system control flow is shown in Figure 4.

The control system is a closed-loop control process. The pedal position sensor is used as the input of the system and is sent to the controller through the CAN bus after A/D conversion. Similarly, the throttle position sensor is used as the feedback signal and is sent to the controller through the CAN bus after A/D conversion. The two signals are compared in the controller, and the controller uses the corresponding control algorithm (such as PID, etc.) to make a decision. The decision result is sent to the throttle body position sensor and the actuator node through the CAN bus. The microprocessor of the node generates a corresponding PWM signal to drive the operation of the actuator through the drive device.

In order to verify the performance of the control system, the adaptive PID control algorithm was used to conduct experimental platform and real vehicle experiments. The experimental results are shown in Figure 5. Among them, PPS represents the pedal position, TPS1 represents the throttle position experimental result under the experimental platform, and TPS2 represents the throttle position experimental result under the real vehicle. From the control results, it can meet the real-time and accuracy requirements of electronic throttle control. At the same time, after testing in the real vehicle environment, the system has a certain anti-noise ability.

5 Conclusion
CAN bus, as a reliable automotive computer network bus, has been applied in many advanced cars. Applying CAN bus to intelligent sensors enables the signals obtained by sensors to be transmitted through the bus in real time, reliably, at high speed and accurately, so that each automotive computer control unit can share all information and resources through CAN bus, thus simplifying wiring, reducing the number of sensors, avoiding duplication of control functions, improving system reliability, reducing costs, and better matching and coordinating various control systems. At the same time, since the entire intelligent sensor network adopts fully digital communication, the bus also has good anti-interference ability, which is the development trend of intelligent sensors and intelligent control networks in the future.