Research On Electro-Hydraulic Control System Of Cold Milling Machine

PREFACEThe cold milling machine is one of the main models of maintaining machinery, used for the highway, town road, airport and bituminous concrete .Because of starting to produce lately, the large milling machines mainly depend on import at present, and the domestic research of the machine is of lack too. The main research contents of the paper include: analyzing the function request of the milling machine's electro-hydraulic control system, comparing and studying the ideal control project, studying the control characteristic of the electric- proportional pump and valve and studying the core part of the control system-load control of speed elaborately.CONSTITUTES OF ELECTRO-HYDRAULIC CONTROL SYSTEM The electro-hydraulic control system can be divided into five parts: the control of drive system, of the conveyor, of the milling depth, of the steering system and of the subsidiary organization. Control of the driving speed mainly concentrated at regulation of the displacement of the drive pump and the drive motor. Displacement of the drive pump is regulated by proportional current. There are two kinds of motor: two-position motor and pressure-control-proportional motor. And the later is better because it can raise the construction efficiency at light load. Including the former, control of the brake, of the anti-skid and the efficiency of the hydraulic is important also。The core of the conveyor control also lies in the rotating speed control which should adapt the load. On one hand the conveyor's speed must big enough to send the waste material to the truck, on the other hand we can make use of the system pressure Pc to be the feeding parameter to adjust the displacement of the conveyor pump. Thus we can make full use of the ability of the conveyor and also it contributes to reduce the damage of the conveyor belt. The request of the steering system is high. But we can manipulate the front and rear steering separately through close-loop position control. The depth control usually adopts the standard controller, so the key of the depth control lies in the selection of the proportional direction valve and the fit control parameter. Control of the subsidiary organization mainly include cooling fan and sprinkle water pump which droved by a hydraulic motor.ANALYSIS ON THE CONTROL PARAMETER OF THE PROPORTION- AL PUMPThe double direction proportional pump is the key parts of the control of the drive speed and the conveyor. The displacement of the pump is regulated by PWM current which generated by the controller. It is vital to find out how the PWM signal parameters including the frequency and magnitude influent the displacement of the pump. By which, we can make sure the dead-band current and the saturated current. The PWM output of the controller posses the electric current feedback, which usually looked as the direct current, and take no account of the influence of the frequency of PWM within the scope of certain frequency. However, experiments indicate that it's very different about the effect of the PWM input according to the double direction pump and the single direction pump. The PWM signal frequency shows evidently influence on the dead-band current I0 and saturated current Im of the double direction pump. The value of the I0 and Im of the low frequencies input is much bigger than that of the DC. In addition there is increasing trend of these values with the increasing PWM frequency. Which make clear that it maybe a fault if we give the same value of the I0 and Im to different pump and we may not achieve the anticipant result.LOAD CONTROL OF THE DRIVE SPEEDThe Efficiency of the planer is measured by the physical volume of the destroyed road layer. Which means that we must guarantee the drive speed is faster as it could at all possible work condition. However, because of the existence of the milling drum, the machine is subjected to the resistance during milling work. Study expresses that the resistance is proportional to drive speed. And also there are other uncertain reasons which can lead to the change of the speed.There are two ways to modulate the speed- changing the displacement of the motor or the pump automatically according to the work load. The proportional motor could be regulated automatically according to the pressure and the proportional pump can be modulated automatically according to the engine speed. This kind of control method can achieve ideal drive speed on one hand; the engine can work at ideal speed area on the other hand. Which means the work efficiency is the highest. This kind of control way changed the traditional theory which equates the speed control and the power distributed control, pointing out that the final purpose of the speed control is increasing the product efficiency. DESIGN FRAME OF PLC CONTROL SYSTEMControl system of the milling machine is very complicated. The drive system and conveyor system are close-loop control with load adaptation; steering system is close control of steering angle and the control of auxiliary mechanism is complex too. The whole machine control needs to handle a great deal of data and carries out the complicated logic operation, the PID control, and fault diagnosis function, which must adopt the high performance PLC to carry out these tasks. The synopsis in text gives a PLC control system frame design of soft and hardware.