Design of digital automatic leveling control system based on CAN bus

    Design of digital automatic leveling control system based on CAN bus

    With the development of microelectronics technology and the reduction of costs, digital control systems have been more and more widely used in modern engineering machinery, and control systems based on CAN bus are gradually becoming popular in modern engineering machinery. Digital automatic leveling control systems can improve the performance of asphalt concrete pavers, improve maneuverability, etc., because of their high reliability and flexibility. Therefore, research on them has important practical significance. There are many solutions to realize digital control systems. The control system developed by the author is based on CAN bus and is designed with C8051F040 single-chip microcomputer as the core.

    2 Characteristics of CAN bus

    CAN (Controller Aera Network) bus is a serial data communication protocol developed by the German company BOSCH in the early 1980s to solve the data exchange between numerous control and test instruments in modern automobiles.

    Compared with other buses, CAN bus has the following characteristics:

    1) CAN bus is a multi-master bus structure, that is, each node machine can become a host, and the node machines can communicate with each other.

    2) The CAN bus communication interface integrates the physical layer and data link layer functions of the CAN protocol, and can complete the framing processing of communication data, including bit filling, data block encoding, cyclic redundancy check, priority determination, etc.

    3) The communication medium can be twisted pair, coaxial cable or optical fiber, and the communication rate can reach up to 1 Mb/s.

    4) One of the biggest features of the CAN protocol is that it abolishes the traditional station address coding and instead encodes the communication data block. The identification code of the data block can be composed of 11 or 29 binary digits. Therefore, 11 or 29 different data blocks can be defined. This data block encoding method can also enable different nodes to receive the same data at the same time.

    5) The maximum length of the data segment is 8 bits, which can meet the general requirements of control commands, working status and test data in the general industrial field. At the same time, 8 bits will not occupy the bus time for too long, thus ensuring the real-time nature of communication.

    6) The CAN protocol uses CRC check and can provide corresponding error handling functions to ensure the reliability of data communication.

    CAN bus has the characteristics of high communication rate, simple structure and strong reliability. Therefore, choosing CAN bus as the controller communication method can better meet the system requirements.

    3 Design of automatic leveling control system for paver

    The automatic leveling control system automatically adjusts the solenoid valve to achieve automatic leveling according to the distance between the measuring surface and the reference surface (line). The control system consists of a controller, a transverse slope sensor, a longitudinal slope sensor, a display unit and a cable. The system control structure is shown in Figure 1. The longitudinal slope sensor can be equipped with different types of sensors such as contact, ultrasonic, ultrasonic balance beam, laser, etc. The longitudinal controller, transverse controller, display and sensor adopt modular design to meet their different control requirements. The communication between modules adopts CAN bus. Modular design is conducive to centralized system operation and system expansion.

    The controller in the system controls the action of the solenoid valve according to the corresponding sensor signal; the display is an optional unit installed in the operation room, which displays the parameters and information of each controller and can also control the corresponding controller through buttons.

    3.1 Controller

    The controller is the core part of the automatic leveling system. The data processing part uses the C8051F040 single-chip microcomputer. The C8051F040 is a fully integrated mixed-signal system-on-chip SoC (System on Chip) produced by Cygnal, USA, with a CIP-51 core that is fully compatible with the 8051 instruction set. It integrates all analog and digital peripherals and other functional components required to constitute a single-chip data sampling or control system. The C8051F040 includes 64 kB Flash, 4352 B RAM, CAN controller 2.0, 2 serial interfaces, 5 16-bit timers, 12-bit A/D converter, 8-bit A/D converter, 12-bit D/A converter and 6 PCA modules. It also has a JTAG interface inside, which makes debugging very convenient. The communication part mainly uses the CAN controller integrated inside the C8051F040, and other mainstream communication interfaces are reserved. The design based on the CAN bus can arbitrarily expand any new sensor to achieve a variety of leveling control functions. [page]

    3.1.1 Display function

    LCD display: The digital LCD display screen uses the DMF-50840 dot-matrix graphic LCD display module of Hong Kong Jingdian Company, and its features are as follows: resolution 320x240; brightness 350 lumens; partition display system status. Including left and right longitudinal slope display area and transverse slope display area; LCD interface uses CPU bus, CPLD, and video memory for expansion; uses combined display, strong readability. Display controller upward and downward output; display slope value and direction, superelevation value and direction, sensitivity, controller parameters and fault information, etc.

    LED display: only used to indicate the current valve driving direction. The direction it indicates is consistent with the arrow direction of the LCD. When the operator is far away from the controller, the role of the LED will be very obvious.

    Function indicator light: used for automatic function indication, direction indication, and reference rope function indication.

    3.1.2 Menu Function

    Function selection and parameter setting, including: cross slope sensor identification; display of the slope measured horizontally; sensor selection; selection of the corresponding sensor type; external control settings; whether to allow control commands from other devices (such as display keyboards, etc.) to be effective. This function is used to determine whether this command is allowed to participate in the operation of the controller.

    Sensitivity setting: If different types of sensors are configured, the sensitivity of the system needs to be reset. The adjustment range is 1 (low sensitivity) to 10 (high sensitivity). This set of data is actually set for the two parameters of "dead zone" and "proportional zone" that are affected by many factors. The longitudinal sensor and slope sensor need to be set separately. If the sensor needs to be replaced, the sensitivity value will be automatically saved. If the system works very unstable in the automatic state, it is necessary to reduce the sensitivity.

    Control range setting: If the external control deviation exceeds the preset control range, the system will consider it as a fault signal and the controller will stop the relevant drive. Only valid for longitudinal sensors.

    Length unit setting: Preset the length unit of the displayed value. The length unit can be selected between "cm" and "inch". Only valid for longitudinal sensors.

    Position coefficient: Position coefficient = position change of actuator / position change of sensor, actual descent height = sensor position change × position coefficient. Only valid for longitudinal sensors.

    Self-check: Check the controller status.
 
    3.1.3 Operation mode selection

    It includes automatic mode (the set value can be changed, the controller works automatically, and the actuator will be driven), semi-automatic mode (the set value can be changed, and the controller has no output), and manual mode (the controller stops working automatically and the set value does not change).

    The controller also has the following functions:

    Zero point setting: set standard value; automatically identify the type of sensor; fault diagnosis function; detect errors caused by sensors, solenoid valves, wiring or operating procedures, and display information or prompt with sound or light signals; when the system is powered on, the controller self-checks; has a solenoid valve circuit drive; correction function; when the displayed value is not equal to the actual value, parameter correction is required; power supply polarity protection, voltage transient protection.

    3.2 Slope sensor

    The SEIKA solid-state angle sensor NB43210 is used, which has analog signal output, inclination sensitivity of 0.2°, and is fully sealed, and is used to detect the slope value of the actuator. The CPU is C8051F040, and its built-in CAN bus interface is used, and external photoelectric isolation is performed.

    3.3 Slope sensor

    Different types of sensors such as contact type, ultrasonic type, and ultrasonic balance beam type can be selected.

    Contact type: The rotary potentiometer uses a conductive plastic potentiometer WDD35D series, with a resistance of 1 kΩ to 5 kΩ. Independent linearity is 0.2%. The shaft can rotate in two directions from the zero position, and its mechanical parts are used to sense the reference surface for distance detection. Processed by C8051F040. Sent through the CPU's built-in CAN bus interface, and externally photoelectrically isolated. The reference rope and reference surface must be fixed and reliable.

    Non-contact type: Ultrasonic sensors often use multi-probe ultrasonic technology. As shown in Figure 2. The sensor can accurately detect subtle changes in the surface or line through multi-point detection, which are processed by the microprocessor and sent to the controller through the CAN interface. If the reference ground is measured, the principle of an ultrasonic probe can be expressed as follows:

    L=1／2vt

    Where t is the propagation time, and v is the known propagation speed of the ultrasonic wave in the medium. The ultrasonic propagation speed is affected by environmental factors such as temperature and air medium. This can be corrected by calibrating the ultrasonic probe to detect the ultrasonic propagation time t1 between L1, that is, from L1=1/2vt1, V=2L1/t1, so L=t/t1L1. Ultrasonic sensors are non-contact measurements and are easy to use.

    4 Conclusion

    The digital control system has a friendly human-machine interface and is easy to operate. The specific deviation value and direction can be clearly understood through the display. Different control methods can be realized by modifying the software. The system can be intelligent. It has the ability to analyze faults and send out alarm signals to eliminate accidents in the bud and reduce economic losses. The digital controller with a microprocessor as the core can easily realize communication between controllers. The use of CAN bus communication makes the structure of the entire system more compact, the system more reliable, more practical, and easy to expand the system and functions.