Switching Delay Analysis And Compensation Methods For The Pilot Operated Proportional Directional Valves

Electro-hydraulic proportional directional control valves(EH-PDCV for short)are flow control devices which have been commonly used in industrial hydraulic systems such as the injection molding machine and die-forging press.However,the dynamic performance is limited since there is a large switching delay when changing the direction of the flow path,especially in the piloted operated EH-PDCV.The main reasons include the large structural dead-zones,imperfect manufacturing and the conservative control strategy used in the EH-PDCV.Eliminating the dead-zone directly by a zero overlap design in structure just like the servo valves is a traditional way,but it results in high manufacturing costs,large energy consumption and low anti-contamination ability.Therefor it is important to reduce the switching delay with structural dead-zones in the EH-PDCV.This thesis focuses on analysis and compensation for the switching delay in a typical piloted operated EH-PDCV.In this thesis,three new methods were proposed to improve the dynamic performance,which includes online dead-zone estimation and compensation method,a dual-solenoid drive method with a non-linear current controller and a new design for the pilot control valve.The first method was proposed in the fact that the coupled dead-zones in the EH-PDCV can be varied extensively and the uncertain dead-zones can lead to a conservative feedback control.An online dead-zones estimation method was developed to detect the dead-zones in a specific EH-PDCV precisely.Then a proper feedback gain can be chosen based on the dead-zones.The experimental results showed that this method is effective for improving the tracking performance with lower tracking error.This method can be used to adjust the control parameters in traditional valve controller for better performance.The second method was innovatively designed to solve the problem that the traditional analogy current controller was not fast enough due to the limitation of PI control method.A non-linear controller was designed based on the Embedded Model Control(EMC)and new drive scheme with two solenoids act synchronously was developed.The experimental results indicated that the-3db bandwidth of the current controller can be increased by 36.4%with the proposed method.And the switching delay of the EH-PDCV can be reduced by 20%.What's more,the dual-solenoid drive scheme was also proven to be effective with switching delay reduced by 11%further.This method can be used to design a better digital current controller for the EH-PDCV.The third method was a new design proposal with two independent spools in the pilot valve,which can be used to replace the traditional one with all the control orifices distributed in a spool.The moving part in the new pilot valve was significantly reduced and the resistant forces exerted on the spool was also decreased.What's more,the pressure-flow gains for the pilot control valve can be adjusted independently since the control orifices in the spool was decoupled.Based on this novel structure,the dead-zones can be overcome by setting initial solenoid currents to skip the overlaps.A pilot operated EH-PDCV prototype with this new pilot control valve was designed and manufactured.The static as well as dynamic performance of the new design and the traditional valve were tested on the ISO standard test rig.The comparative results indicated that the dynamic performance can be improved significantly,the switching delay can be reduced from 25ms to 8ms.The-3db bandwidth of±90%input can be increased from 7Hz to 13Hz.The-900°bandwidth of±90%input was also improved from 9Hz to 18Hz respectively.It can be concluded that this new design remained to be a proportional valve with uncertain dead-zones,but the dynamic performance can be improved as good as that of some servo valves.The thesis is outlined as follows:In chapter 1,the principles and the characteristics of typical piloted operated EH-PDCVs were reviewed.The research on improving the dynamic performance at home and abroad was investigated into two categories:improved piloted control methods and dead-zones compensation methods.And the main research subjects and contents in the thesis were discussed.In chapter 2,a dynamic simulation model for the piloted operated EH-PDCV was built.Three important non-linear characteristics found in the drive circuit,the solenoid and the flow-pressure relationship in the orifices were investigated in detail.The effectiveness of this model was validated by experiments tests both for the parts of the valve and the whole valve.Then,based on this model,the dynamic behavior of a step input was simulated and the main factors causing the large switching delay were obtained.In chapter 3,an online dead-zones estimation and compensation method were developed.The dead-zones including center dead-zones and switching dead-zones were analyzed and indicated to be sensitive to the clearance and overlaps between the spool and valve body.Then an online estimation method was introduced and the preconditions were also discussed.Based on the precise dead-zone values,an optimized dead-zone compensation method was developed with a gain estimation function embedded to calculate the feedback gains.Finally,comparative experimental results were given to verify the effectiveness.In chapter 4,a dual-solenoid drive method with a non-linear current controller was introduced.In order to improve the drive speed of the solenoid,a non-linear controller based on the Embedded Model Control(EMC)was designed to reduce the harmful effect of the piecewise In-out characteristics in the inverse discharging drive circuit.Then a dual-solenoid drive method was introduced.Both of the EMC current controller and the new drive method were validated by comparative tests with a traditional design.In chapter 5,a new design for the pilot control valve with two dependent control spools was discussed.Firstly,the principles and features of the new design were presented and the advantages related to dynamic performance,manufacturing costs and fail-safe function were given.The damping coefficients and the open-loop dynamic behavior were analyzed both for the new design and the traditional valve.Then a new drive mode was introduced to overcome the dead-zone.Finally,the static as well as dynamic performance of the new design and the traditional valve were tested on the ISO standard test rig.The comparative results were given and analyzed.In chapter 6,the research contents and the conclusions were summarized and some future work was suggested.