Research On The Electro-hydraulic Depth Adjustment System Based On PID Control Algorithm

Keeping the stability of tillage depth is one of the important measures to improve the quality of cultivating in the process of cultivation.At present,the tillage depth control way is mostly force adjustment or position adjustment.The effect of tillage depth control is not so good,and the tillage depth can only be received after the farming by measuring the gap from surface to the bottom of ditch.This measurement is inaccurate and can not be real-time feedback to the driver,so that the driver can not adjust the tillage according to the tillage effect.With farming machinery developing towards intelligent and lightweight,as well as the gradually maturing of electronic control technology and hydraulic technology,it has become an inevitable trend to achieve the automatic adjustment of tillage depth.In order to improve the machine's stability of tillage depth in the process of cultivation,this paper designed an automatic control system of tillage depth.The main research contents are as follows:This paper designed an electro-hydraulic tillage depth control system,based on the rear rotary component of a cultivator.This system uses electronic control and hydraulic technology to achieve automatic control of depth,while adding PID control algorithm to optimize the control effect.Different from traditional tillage depth control system,the system set the goal depth by a potentiometer and two dip sensors were used to detect the depth of tillage.The control chip collect the preset signal and the real-time detection signal.Then,the system produces an error signal by comparing the two signal and transform it to a control signal.The signal control the action direction and opening size of the electromagnetic proportional valve.The system achieves the control of the hydraulic system and the depth of rotary tillage.The electronic system adopt AT89S52 as the processing chip.This paper designed the corresponding module circuit and the interface circuit of the tilt sensor.It realized the signal acquisition and PWM output.The software system was designed based on C programming language.It included the main program,sampling program,IIC communication program and PWM output program.The function of software system was to calculate the real-time tillage depth according to the signal of tilt angle sensor and compare the depth to the setting value.Then it output PWM wave after the conversion operations.The wave was transformed into analog signal by RC filter circuit.Then the signal controls the action of electro-hydraulic proportional valve action.On the part of the hydraulic system,this paper designed the hydraulic control circuit and the hydraulic drive rotary tillage circuit according to the requirements of regulating tillage depth.The power of the rotary tiller was calculated.According to it,the working pressure was determined as 16 MPa,the oil pump model was determined as PFE-31036.The hydraulic motor model was determined as 1JMD-40.On this basis,the design and selection of hydraulic components and hydraulic pipes were completed.The dynamic response of the control system has an important influence on the instant of the depth adjustment.In order to study the dynamic response characteristics of the system,the dynamic simulation of the tillage control system is carried out by using Simhydraulic of Simulink.In the simulation software,the closed-loop electro-hydraulic control system model is built,which parameters are set according to the design value.Then the simulation of system under different operating conditions is carried out.The operating conditions include increasing tillage depth from 0 to 100 mm and decreasing tillage depth from 175 mm to 100 mm.The step signal is used to indicate the setting depth.The tilt sensor is replaced by sensor module in Simhydraulic.The reaction force from cantilever to hydraulic rod is replaced by damping module.The friction between piston and cylinder block,piston rod and cylinder head is replaced by friction module.Also,the liquid resistance and mechanical inertia are indicated by some suitable module.The simulation results show that there is a 13% depth overshoot in the tillage control system,and it takes 5.0 seconds to reach steady state.In order to eliminate the overshoot phenomenon of tillage depth adjustment and shorten the adjustment time required for reaching the steady state,the integral separation PID control algorithm was used to process the error signal and the integral threshold was set to 0.08.When the error is large,the system accelerate the speed by canceling the integral cycles.When the tillage depth is close to the target value,the integral effect is added to improve the control precision.Adopting the trial and error method with experience,the PID parameters are tuned.When Kp=0.08,Ki=1.2,Kd=0.001,the dynamic response of the system is very well.Compared with the simulation result of without PID control algorithm,the depth overshoot is almost disappeared.Also,the response time is reduced to about 1.4s so that the dynamic response characteristics of the system reach a good level.On this basis,the stability test of tillage depth is carried out.The cultivator works in preset depth conditions,the variation of tillage depth stability coefficient is 6.05% and 3.54% when the depth is 10 cm and 16 cm.reached the prescribed agronomic requirements of rotary tillage.It meets the agronomic requirements.