Experimental studies of the hydraulic engines synchronization process in a multi-engine unit

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Experimental studies of the hydraulic engines synchronization process in a multi-engine unit

Gavrylenko A.N.

M. Engdirector ”EM.A. ” Ltd

Kulinich S.P., Ph.D.

Ass. Prof. Sumy State University

Abstract

stand hydraulic drive

In this paper, the experimental stand of a multi-engine hydraulic drive providing the synchronous movement of engines is described. An automated control system to move the elements of a multi-engine hydraulic drive has been developed. The program code of the control program is based on the model to calculate the hydraulic drive elements' motion errors (positioning) in the conveyor. The experimental studies results of the synchronization process regarding the motion of multi-engine hydraulic drive elements have been shown. A method to estimate the experimental data uncertainty and to check the adequacy of the mathematical model has been presented. The research results will form the basis for the engineering calculation methodology of the multiengine hydraulic drive.

Key words: multi-engine hydraulic drive, synchronization, automated control system, experiment, the modeladequacy

Introduction

The mechanisms of a wide range of machines in mechanical engineering, building sector, chemical industry are moved with the help of several working bodies driven by multi-engine drives [1,2]. Synchronization of several working bodies' work to provide an accurate motion of one working element (the use of multi-engine hydraulic units) is an urgent problem in many industries [3-7]: equipment for pressing and forging, lifting and transporting devices, devices to create vibration, excavating machines and etc.

Strict coordination in motion time, speed or acceleration of the working body (synchronization of several working bodies' operation to ensure an accurate motion of one working element) is the main task that must be solved at the design stage of the device depending on the links motion conditions [8,9].

A promising direction of synchronization is to use the hydraulic engines, which ensure the coordination of the output features of the executive mechanisms with a variable load at their input.

Background

In [10] there are study results regarding the operation of the drive with two hydraulic engines, working on a common load with synchronization according to the pressure value in the cavities of the engines. The mutual influence of hydraulic engines on the consistency regarding the motion of the engine's output links is observed.

In [8] synchronous hydraulic drives to lift heavy building structures have been reviewed, and the criteria for their selection have been proposed, based on the load (carrying) capacity and motion value, not taking into account the speed synchronization errors.

The [6] observes a vibration mechanism with four unbalanced masses, driven by four synchronized hydraulic engines. Synchronization on the rotation frequency of hydraulic engines shafts is carried out through the implementation of four identical pumps, individually feeding each engine. Authors believe that such a scheme provides synchronism regarding the rotation of the hydraulic engines' shafts and observe only the vibration mechanism dynamics, not taking into account the synchronization errors of the rotation frequency of the hydraulic engines' shafts.

The proportionality of the working bodies' motion speeds is the proportionality of the working fluid flow rate in hydraulic engines, for mechanisms driven by hydraulic motors. Ways to synchronize the motion of hydraulic engines are analyzed in more detail in [3, 4, 9, 10-16].

In [5], the criteria for synchronization of two hydraulic engines in an eccentric rotary vibration machine are observed. The authors believe that the flow rate of the working fluid in hydraulic engines is the same and take into account only the effect of fluid leakage in the gaps between the pistons and the holes in the cylinder block of an axial-piston hydraulic engine.

The theoretical and experimental studies results of the hydraulic engines' motion synchronization in the hydraulic drive using a throttle flow divider are demonstrated in [15]. The authors observe the influence of working fluid leaks on the accuracy of the hydraulic engines' motion speed synchronization.

In the considered sources, attention is usually paid to the operation of hydraulic engines in the steady regime, when the loads on their output links are unchanged, the motion occurs at a constant speed. Therefore, the dependence of the synchronization error of the movement speed on the load asymmetry is defined and methods for minimizing this error are proposed [11-14].

During the operation of a hydraulic drive with synchronized engines, there are problems in nonstationary modes. These modes of the work are conditioned upon the following:

- switching on (or switching off) the hydraulic engines of the group drive. As a result, the pressure at the inlet to the flow divider is changed, since the overflow valve reacts to a change in flow rate with a delay;

- a sudden change of the load on one of the hydraulic engines.

In [17], the synchronization control of two hydraulic cylinders motion used to oscillate the heavy form of a continuous casting machine by a fuzzy logic controller is proposed. A drive, in which each hydraulic cylinder is controlled by a servo valve independently in an accurate closed control system, is presented.

Unlike the methods, described in the above literature, we propose to use a flow divider, which adds additional feedback on pressure drop in inter-throttle divider chambers, implemented through the regulated spool-type throttles.

Also, transient modes of the hydraulic engines' operation are rarely mentioned, only in works [17-23] there is general information, but there is no detailed analysis of such modes.

Aim of the article

In parallel with checking the adequacy regarding the created mathematical model of the multi-engine hydraulic units' work to synchronize the working body motion, the problem to ensure the energy efficiency of this process should be considered. Some approaches to evaluate (including the exergy approach) hydraulic drives' work is described in [18-21]. As can be seen from the above list of publications (which, of course, is not complete), this topic is given special attention considering changes in the project designers' “worldview”, rejection of energy saving in favour of energy efficiency, including through the compressed environments energy use [10].

The above theses form aims of this article:

to construct the experimental stand of the multiengine unit taking into account the optimization of energy costs to implement the synchronous motion process of the working bodies in the machine;

to create the automated process control system for the synchronous motion of working bodies;

to check the adequacy of the developed mathematical model [24, 25].

Description of research object

In order to test technological modes of the hydraulic drive operation for the synchronous motions, an experimental stand has been created in the laboratory of the Applied Aerohydromechanics Department. The computational model of the experimental stand is demonstrated in fig. 1.

Units of the stand:

two hydraulic cylinders (diameter of the piston is 25 mm, the diameter of the stock is 16 mm, run is 125 mm);

distributor with an electric control;

pipe station (supply is 6 l/min, the pressure is 25 MPa);

flow divider;

linear movement detectors;

manometer to measure the pressure at the outlet of the pump station.

The hydraulic cylinders are located vertically, they transfer a load of 500 kg (5000 N). On half a way, one of the hydraulic cylinders is additionally loaded with 100 kg (1000 N).

Fig. 1. The computational model of the hydraulic drive scheme for synchronous motions of the working bodies in machines: 1 - valve; 2, 3 - unregulated throttles; 4, 5 - hydraulic cylinders: 6 - distributor; 7 - pump station

During the experiment, the velocity of the rods movement is measured at the place of the additional load of the hydraulic cylinder, the length of the path, where the measurement 5 mm + is carried out (2-3 mm before the start of the additional load). The pressure in the divider's inter-throttle chambers is also measured (it is evaluated simultaneously with the velocity measurement), the pressure is 8-16 MPa, the maximum error of the sensor is 1%.

In general, the functional block diagram of measurements and graphs based on the results of these evaluations is presented in fig.2.

Fig. 2. The functional block diagram regarding the evaluation of the hydraulic drive rods motion velocity

A proportional integral regulator with a negative feedback was used to control the velocity of rods' movement. The velocity of the rod's movement without an additional load is taken as the reference signal. The block diagram of the regulator is shown in fig. 3.

Fig. 3. The block-diagram of the regulator of the rod's velocity

Measurements were carried out by the linear The control cabinet of the hydraulic drive is shown in movement detectors 8740 of the Burster company, fig. 4.

Fig. 4. The control cabinet of the hydraulic drive for the synchronous motion of working bodies in machines

The determination of the measurement error and the calculation results of the main features in the experimental sample of a multi-engine gyro drive system is based on generally accepted techniques and recommendations [26] regarding the engineering experiment and the data processing. Identification of measurements error and the calculation results of the main properties of the experimental sample of a multiengine hydraulic drive system is based on generally accepted methods and recommendations [26] regarding the engineering experiment and the data processing.

In order to define the optimal number of investigations and to achieve the highest accuracy and reliability degree of the obtained results, as well as the processing of these results, the mathematical statistics methods have been used [27, 28].

During the experiment, two types of errors may occur, they are random and systematic [28].

A random error reduces the accuracy of the experiment results. An analysis of this type of error is possible using the average square deviation c calculated by the following formula [28]:

A random error reduces the accuracy of the experiment results. An analysis of this type of error is possible using the average square deviation у calculated by the following formula [28]:

?? = vУ (???????)2 ???? =1 ???1 , (1)

where ?? is the arithmetic mean; ??is a single parameter value;?? is a number of measurements.

The largest possible error of a single measurement is determined by three sigmas rule [28, 29]

Д = 3??. (2)

The two-sided confidence interval of the arithmetic mean ??was estimated by the dependence [28] if its probability to reach the confidence interval was 95%.

?? = ??????v??, (3)

where ????- Student's criterion [28, 29].

The standard error of indirect measurements [29]

???? = vУ (??????? Д ? ???? )2????=1 . (4)

where ?? = ??(??1, ??2, . . . , ???? ).

The accuracy of the obtained regression equations is defined by the least square method [28].

A systematic error shifts equally all the variables that are controlled during the experiment. In order to exclude this type of error, all the devices used during the study were tared using exemplary devices in the experiment. The communication of measurement devices with devices on the control board is provided with a maximum error of signals processing within 1.5%.

Results and discussion

The experimental studies results of the hydraulic drive working bodies' motion velocity and pressure in the hydraulic cylinders are shown in figs 5-7.

Fig. 5. Theoretical (1,2) and experimental average (3,4) data to estimate the pressure in the hydraulic cylinders (MPa)

Fig. 6. Theoretical (1,2) and experimental average (3,4) data to evaluate the velocity of the hydraulic drive working bodies' motion (mm/s)

Fig. 7. Experimental average data (3 - the fist cylinder, 4 - the second cylinder) to evaluate the pressure in the hydraulic cylinders (MPa)

According to the results of theoretical and experimental research, the possibility to improve the process of hydraulic engines synchronization in a hydraulic drive thanks to the regulated throttles use in the flow divider of the working fluid has been confirmed.

Comparison of the results, obtained for a flow divider with a double-slotted throttle distributor, with results for a flow divider with unregulated throttles, shows:

Implementation of the double-slotted throttle distributor, regulated by the pressure fall in the interthrottle chamber, allowed reducing the error in the velocity synchronization from 0.43 to 0.27, i.e. 1.6 times, and the relative pressure fall in the inter-throttle chamber from 1 to 0.53, i.e. 1.9 times.

In the transient process there are higher-order harmonics for the velocity and pressure, caused by the motion of the double-slotted distributor's throttle.

The higher-order harmonic in fluctuations of pressure and velocity does not significantly affect the operation of hydraulic engines, since the amplitude of oscillations is insignificant.

The reduction of the synchronization error velocity is caused by the simultaneous change of the throttle area, which stabilizes the differential pressure and the adjustable throttle area.

The experimental indicators deviate from the calculated indicators in the confidence interval, so we can conclude that the proposed synchronization method of the hydraulic engines is effective.

The automated system enables to synchronize the motion of the hydraulic drive elements in two modes:

Advisor mode. In this mode, the control program recommends the optimal value of the process parameters (an increase in the flow rate of the working

environment for the “lagging” link and a decrease in it for the “advanced” link) to the operator.

Interactive mode. The operator, if necessary, can adjust the formulation and conditions of the task (for example, the minimum and maximum value of the displacement error, the positioning parameters of the working body), solved by the system itself when developing recommendations for managing the transportation process and dosing materials.

Conclusion

The conducted researches have established that the transitional modes of synchronized hydraulic engines' operation have the motion velocity error. The error is caused by the inertia of the flow divider throttle. It is revealed in the late compensation of the changing load effect on the pressure fall in the unregulated throttles. Experimental studies confirmed the theoretical conclusions regarding the reduction of the velocity synchronization error in the implementation of a flow divider with a double-slotted throttle distributor. The research results confirmed the adequacy of the proposed mathematical model [24, 25]. Based on the results of the experiment, an improved automated technical control system for synchronous movement of hydraulic cylinders has been developed.

Acknowledgements

The automated system has been successfully introduced into the production line of porous ammonium nitrate to dose the dispersed phase at the granulation and final drying stage of the obtained granules in the framework of the study "Small-scale energy-saving modules with the use of multifunctional devices with intensive hydrodynamics for the production, modification and encapsulation of granules" (with the support of the Ministry of Education and Science of Ukraine, project number 0119U100834).

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