| Bulldozers are widely used in construction, mining industry, agriculture and forestry, national defense and other areas. However the main method to adjust the bulldozer working device is by manual operation, which normally leads to low work efficiency, poor job quality, the operating lag etc. Besides that, it also requires a very skilled driver who needs carrying out frequent operation so it is rather labor intensive, which causes security accidents easily. So a electro-hydraulic proportional control system of bulldozers working device automatic fluctuation that is used to flat areas was in need to be designed.The research object of this paper is ZD120-3 crawler bulldozer which is used widely among this kind of engineering machinery. The hydraulic, sensor, signal processing and micro-control technology were integrated to be used during the design of the system. And the position adjustment way of the tractor was be used as control principle to build the electro-hydraulic proportional control system scheme of bulldozers working device automatic fluctuation. The main contents of this paper are follows.(1) The overall design scheme of the control system of working device automatic fluctuation of the bulldozer was raised, which included hydraulic system scheme and electrical control modules.(2) A new hydraulic system scheme was improved and designed based on the original hydraulic system, which was consisted by two proportional pilot valves, a main reversing valve, a manual/automatic switching valve and cylinders. And the proportional pressuring-reducing valves were used as pilot valves, the hydraulic slide valve was used as main one and PWM signal was selected to control the movement of the pilot valves.(3) The mathematical model of the hydraulic system was established and the characteristic equation and force balance relations of the pilot valves, main valve and cylinder were analyzed. And then the transfer function between the cylinder displacement and the PWM signal dutyfactor in the whole hydraulic system was obtained and simulated by SIMULINK in MATLAB7.1 in order to get the dynamic performance of the system step response, which showed that the overshoot was 30% and the system reached stable state within 0.3 second. It meant the hydraulic system scheme was reasonable. Also the simulation block diagram between the pressure of the main valve control cavity and the outlet-flow was got by the state equation method that was based on the mathematical model established before. By simulating the subsystem in SIMLINK of MATLAB7.1, the direct proportion relationship between the pressure and the outlet-flow was acquired, which would provide a theoretical basis for the following design of electrical control.(4) The design of hardware circuit was finished, including A/D converter circuit, liquid crystal display circuit, electromagnetic valve driving circuit, power switching circuit and peripheral circuit design of MCU. Each module mentioned above was integrated and PCB board of the hardware circuit was made.(5) It finished the design of software through MCU C Language, mainly including main program, A/D converter subprogram, liquid crystal display subprogram, Judgment algorithm subprogram, position adjustment interruption subprogram. All of them were debugged successfully in the development environment of Keil uvision2.(6) Each module of the system was debugged and experimented and then the integration experiment was also carried out. It showed that the A/D converter could work accurately and it could display on the LCD 1602, and the PWM signal controlled by electronic system can export precisely by observing from OSC RIGOL DS 5102CA,which only existed 2% absolute error at end-point compared with theoretical value. In the reversing experiment of main valve, when the set value and real value different, the two pilot valves could get power separately and when the D-value between the two being the allowed band, the two pilot valves did not move. Lastly, when the duty ratio of PWM signal was different, the voltage value that was controlled to output by PWM signal was also different. And to be more precise, the duty ratio was proportional to voltage value, so the output voltage value could be controlled by different duty ratio PWM signal.Through the experiment, it showed that the electro-hydraulic proportional control system of bulldozers working device could realize the function and meet the requirements of the system. |
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