Zheng-xiong CHEN, Xiao-gang ZHANG, Jian-peng SHI,Xiao-yan XIONG, Long QUAN

(Key Lab of Advanced Transducers and Intelligent Control System of Ministry of Education and Shanxi Province，Taiyuan University of Technology，Taiyuan 030024，China)

Abstract: In order to adapt to the intelligent operation of mobile machinery and the development of high quality operation, a new position control strategy of excavator boom is proposed. According to the target position and operation velocity, the expected operating trajectory is designed. With the proposed strategy, the boom can move to the target position along the design trajectory and ensure the position accuracy. Then a test prototype controlled by the control strategy is established, and relevant tests are carried out. The research work has universal significance to improve the automation operations efficiency and accuracy of excavator boom.

Key words: Position control, Excavator boom, Flow matching

1 Introduction

Proportional valves are often used in traditional construction machinery to control the velocity of actuators. There are some shortcomings such as the simultaneous throttling and throttle loss of the import and export[1]. The solution for this problem is to control the pressure and flow of the two chamber of the actuator. Yao[2] used five throttle valves to compose independent metering valve control (IMV)system, and adopted an adaptive robustness control strategy to improve the stability of the system. Nielsen [3] put forward the meter-in flow control and meter-out pressure control compound strategy, and studied the characteristics of boom cylinder of loader crane controlled by this strategy. J.C Lee [4] designed an optimized energy distribution algorithm to improve the energy-saving characteristics of the IMV system consisting of five poppet valves. The experiment results show that the system can save energy more than 10% compared to conventional system. Quan [5-6] proposed a pump and valve combined principle based on IMV system, which further increase the system control degree of freedom. The principle was used to control the boom cylinder, arm cylinder, and swing motor to improve the operating characteristics and energy efficiency of the system. According to different working conditions of the hydraulic cylinder of mobilemachinery, Xu[7-8] designed different working modes based on pump and valve combined control principle, and used it to control the boom and arm of hydraulic excavator. Test results show that the operating characteristics and energy efficiency are improved. Xia[9] designed an integrated drive and energy recuperation system based on a three-chamber hydraulic cylinder. The test results show that with the proposed system, the energy consumption of the boom is reduced by 50.1% and the peak power of the power source is reduced by 64.9%. Ge[10] designed an electric hydraulic excavator configuration combining with an independent metering in and metering out system to improve the overall energy efficiency. The results show that, compared with pure displacement variable concept, the electric power consumption during the idle period can be reduced from 2.05 kW to 0.7 kW, the energy saving ratio under partial load condition can be up to 33%, while it is 28.5% under digging condition. The above research works about independent metering system are mainly focused on the characteristics of actuator velocity and pressure, as well as the energy efficiency of the system. However, with the trend of intelligent mobile machine and the improvement of task quality requirements, each actuator should not only meet the velocity and output force characteristics but also the higher position accuracy during the operation. Therefore, the motivation of this paper is to propose a new position control strategy of excavator boom. With the proposed strategy, the boom can move to the target position along the design trajectory and ensure the position accuracy.

2 System principle

Fig.1 shows the system principle of independent metering system used in excavator in this paper. The inlet and outlet of boom cylinder are separately controlled by two proportional direction valves. The pump is an electronic proportional variable piston pump and the pressure and flow rate can be adjusted continuously. A displacement sensor is assembled on the rod of cylinders. Pressure sensors are separately arranged at the two chambers of cylinder and the outlet of the pump. The whole system is controlled by the hardware in the loop system DS1103 produced by dSPACE company.

Fig.1 system principle of independent metering system

3 Control strategy

3.1 The overall control strategy

The overall position control strategy of excavator boom is as follows. According to target position signal, the controller can identify the working modes. Then corresponding signalsu1,u2anducare output to control the valve of piston chamber, the valve of rod chamber and the swing angle of variable displacement pump respectively. The final objective is to control the actuator move to target position along the designed trajectory and ensure position accuracy at the same time. In order to distinguish the two valves in the following section, the one of piston chamber is defined as valve 1 and the other of rod chamber is defined as valve 2.

During operation process, the acceleration-velocity-displacement curves of desired trajectory can be designed based on the target displacement, the maximum velocity and the maximum acceleration as shown in Fig.2. By operating as the designed trajectory, both the displacement and the velocity of actuators will not change suddenly, avoiding rigid impact and flexible impact. In addition, similar to the curves shown in Fig.2 just for extension, the corresponding curves for retraction can be generated easily.

Fig.2 Expected acceleration-velocity-displacement curves

The control modes can be identified based on the difference of target positionxdand the actual valuexrelof cylinder. The recognized criterion for three control modes is given by equation (1).

(1)

Where,xthrepresent the switching threshold for different operating modes.

3.2 Extension and retraction mode

When the boom cylinder in the extension and retraction mode, more attention should be given to running velocity and energy consumption characteristic. The combination of velocity feed forward and position feedback (VFPB) is adopted. Firstly, the desired trajectory including velocity and displacement curves can be designed as shown in Fig.3. Then corresponding feed forward signal are calculated by the desired velocity curve. Due to the influence of various factors, such as leakage, oil compression, and response time, etc, it is difficult to achieve desired trajectory just rely on the feed forward. Thus, based on the expected displacement curve, position feedback is added as compensation and correction to feed forward. The combination of velocity feed forward and position feedback is capable of achieve the desired trajectory as much as possible.

The control principle of boom extension mode is as follows. The valve 1 open to maximum to reduce the throttle loss and the valve 2 also fully open to decrease the back pressure. Moreover, the extension velocity is controlled by changing the swing angle of variable displacement pump.

The input signals of valve 1, valve 2 and swing angle of pump are given by equation (2).

{u1,u2,uc}={u1max,u2max,uup}

(2)

Where,u1maxandu2maxrepresent the maximum opening of two valves,uuprepresents the pump swing angle which is obtained by velocity feed forwardUvand position feedbackUcas given by equation (3).

uup=Uv+Uc

(3)

The relationship of the expected velocity and feed forward signal can be express as given by equation (4).

(4)

Where,vris the expected extension velocity,A1is the piston area,nis the rotational speed of the motor,Vpmaxis the maximum displacement of pump. The expression of position feedback can be obtained as given by equation (5).

(5)

In addition, if the difference between actual displacement and target displacement is taken as the input signal, the effect of position feedback keep too big to play the role of micro regulation. Therefore, the difference of actual displacement and the integration of desired velocity curve is selected, the function of which keep small during the process. In this way, the feed forward plays a major role while the position feedback plays a regulating role to achieve the expected trajectory.

The control principle of boom retraction mode is shown in Fig.3. The inlet and outlet are coupled together and both of them are connected to oil tank through a back pressure valve. Due to the function of back pressure valve, the oil flowing out from the piston chamber is divided into two parts. One of them flows into the rod chamber to achieve flow regeneration, while the other flows back to tank.

Fig.3 Control principle of boom retraction mode

The corresponding input signals of valve 1, valve 2 and swing angle of pump are given by equation (6).

{u1,u2,uc}={udown,u2max,up}

(6)

Where,u2maxrepresents the maximum opening of valve 2,upmeans that the pump is under pressure control and just need to maintain a small pressure,udownrepresents the partial opening of valve 1 to control flow rate. The signal ofudownis determined by the combination of feed forwardUvand feedbackUcas given by equation (7).

udown=Uv+Uc

(7)

The pressure difference between the two ends of valve 1 is given by equation (8).

Δp=p1-pT

(8)

Thus, the velocity feed forward signalUvof retraction is given by equation (9).

(9)

Where, ΔpNis the rated pressure difference andQNis the rated flow of valve. In the above equation,p1is related to the load force andvris related to expected velocity, so different calculating equations can satisfy various velocity and load force conditions. The expression of position feedbackUcis the same as the equation (5) with different parameters.

3.3 Positioning mode

In order to improve the position accuracy, the controller switches to positioning control (PC) mode when the difference between actual displacement and target displacement is less than the given thresholdxth. The control principle of positioning mode is shown in Fig.6.

Fig.4 Control principle of positioning mode

At this time, the corresponding input signals of valve 1, valve 2 and swing angle of pump are given by equation (10).

{u1,u2,uc}={u1x,u2x,up}

(10)

Where,u1xandu2xseparately represent the signals of valve 1 and valve 2 under positioning mode. Andupindicates that the variable displacement pump is under pressure control, the pressure of which keep constant and is able to overcome the gravity of boom at least.

During the process of positioning control, the accuracy should be mainly considered. The proportional valve has the characteristics of high pressure gain, fast positioning and high position accuracy, etc. Fig.5 shows the test relationship between the pressure and the opening of proportional valve. In order to make the pressure gain of IMS similar to that of proportional valve, the valve 1 and valve 2 are linkage controlled at the same time. Thus, the function of two valves is similar to one proportional valve.

If the control signals of two valves are equal, the role of the system is equivalent to the symmetric valve control asymmetric cylinder, causing the problem of pressure mutation, pressure overrun and cavitation. In order to match the flow rate of inlet and outlet with the area of two chambers, different signals are transmitted to two valves respectively as given by equation (11). The ratio R of two signals is determined by the ratio of rod area and piston area.

u2x=Ru1x

(11)

One of the signals is produced by a PI-controller as given by equation (12).

(12)

Thus, the effect of two valves is equivalent to an asymmetric valve because the signals of two valve spools are always proportional. In this way, the system is equal to the asymmetric valve control asymmetric cylinder, avoiding the mismatch of flow and pressure. In addition, the conventional asymmetric valve with different area gain is only applicable to the asymmetric cylinder with specific area ratio because the area gain of the valve is unchangeable. However, the function of asymmetric valve here is realized by adjusting the spool displacements, which can be changed arbitrarily to be applicable to asymmetric cylinder with any area ratio.

4 Simulation and test research

In order to verify the feasibility of the proposed strategy and study the operation characteristics, a test rig adopting the independent metering system is established on a 6t excavator and relevant tests are carried out.

4.1 Simulation system

The simulation system of hydraulic excavator with IMS is shown in Fig.1. The boom cylinder and the boom cylinder are separately controlled by four proportional directional valves.

4.2 Simulation results

There are two kinds of results are given for comparison. The first kind is only velocity feed forward during the whole process. The second is velocity feed forward and position feedback.

Fig.5 shows the simulation results of excavator boom controlled by velocity feed forward only. Fig.6 shows the control signal of the pump swing angle and the two valves.

It is obvious that there is a big difference between actual and expected displacement during the operation. The reason is that open loop control cannot compensate the influence of response time of pump and system leakage.

Fig.7 shows the result of boom controlled by velocity feed forward and position feedback. Fig.8 shows he control signal of the pump swing angle and valves. The actual displacement is close to the designed trajectory and the final position accuracy is less than 1mm. The operation characteristic is improved by the combined function of velocity feed forward and position feedback.

Fig.5 Simulation result with velocity feed forward

Fig.6 Control signals of pump and valves

Fig.7 Simulation result with feed forward and position

Fig.8 Control signals of pump and valves

Fig.9 shows the actual and desired displacement when boom moves to different target positions with the same velocity requirement. It can be concluded that the boom can move to target position along the desired trajectory for different target position by using the proposed strategy.

Fig.9 Simulation result for different target positions

4.3 Test results

Fig.10 shows the test result when boom moves to the same position with different velocity, which are 70 mm/s, 100 mm/s and 120 mm/s respectively. The results indicates that the strategy is available for different velocity requirement.

Fig.10 Test result for different operate velocities

5 Conclusion

In order to reduce throttling loss, the volume control method should be used for lifting, and flow regeneration method should be adopted for falling. With the control strategy, the boom cylinder can move along the expected trajectory smoothly and reach to target position accurately, under the premise of low energy consumption. The research work in the paper can make some contribution on smooth, high-precision and low energy consumption automation operation for excavator boom.

Acknowledgements

This work was supported by the National Natural Science Foundation of China(No.U1510206) and National Natural Science Foundation of China(No.51575374).