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液压系统(文献翻译) 11

management standards are comparatively backward, coupled with a strong sense of quality, resulting in low levels of product performance, quality, Reliability poor services in a timely manner, lack of user satisfaction and trust of the brand-name products;

(3) industry specialization of production low, scattered strength, low repeat serious, between regions and enterprises of convergence products, blindly compete with each other, driving down prices, the decline of enterprise returns, lack of funds, liquidity difficulties, product development and technological transformation is inadequate and seriously restricted the industry to improve the overall level of competition and the increase of strength;

(4) The internationalization of the domestic market and the increasing degree of foreign companies have entered the Chinese market and participate in competition with the domestic private and cooperative enterprises, individuals, foreign-funded enterprises, such as the rise of state-owned enterprises due to the growing impact. 1.32 the development trend

As the socialist market economy continues to deepen, hydraulic, pneumatic and sealing products in the market supply and demand and there is a greater change, long ago to a \characteristics of the buyer's market place . Overall capacity, is already in oversupply situation, in particular the general low level of hydraulic, pneumatic and seals, the general oversupply; and the host of urgent high-tech high-parameter, high value-added high-end products, and they do not satisfy the market needs, only dependent on imports. China joins the WTO, its impact may be even greater. Therefore, the \the output value of industry, must not depend on volume growth and the industry should address the structural problems of their own, increase the intensity of the adjustment of the industrial structure and product mix, is, we should rely on the improvement of quality, and promote technical upgrading of products to meet market demand and stimulating, seek greater development.

2 The application of hydraulic power sliding stage

2.1 Power sliding Taiwan introduced

This paper deals with the investigation for slide unit's impact and motion stabil?ity in modular machine tool fay means of the method of power bond graph and state space analysis. The dynamic mathematical model of self-adjusting back pressure speed control system used to drive slide unit is established. Main reasons and affect?ing factors for slide unit impact and motion unstability are analysed through compu?ter digital simulation, It is concluded from those that, if the structural dimensions of hydraulic cylinder and back pressure valve are designed rationally, the slide unit's dynamics will

markedly be improved.

NOMENCLATURE

Sf Sei R Ii h Ci,C2

flow source

sliding friction force in slide unit

equivalent viscous friction coefficient in slide unit mass of slide unit and cylinder mass of SABP valve spool

hydraulic capacitances of rod chamber and non-rod chamber in cylinder re-

液压系统(文献翻译) 12

C3 R]rR2 R9 Se2 I4J5 C^Cg

V-j V^ P,r:, P-i F V

spec-tively

spring compliance of SABP valve

hydraulic resistances of damping holes

hydraulic resistance of orifice of SABP valve presetting force of spring in SABP valve equivalent liquid inertia in pipe lines

equivalent hydraulic capacitances in pipe lines equivalent hydraulic resistances in pipe lines

oil-containing volumes in non-rod chamber and rod chamber respectively oil pressures in non-rod chamber and rod chamber respectively load acted on slide unit slide unit velocity

* Department of Mechanical Engineering, Dalian Ur.iversity of Technology, Dalian. China.

2.2 Introduction

During operation of modular machine tool, the changes of slid unit's speed and load acted on it in both magnitude and direction will affect working performar.ee to a different extent Particularly the impact caused by sudden vanishing of load and the motion unstability due to periodical change of load in operation will affect the surface quality of the workpiece machined, and the tool would be broken off under serious conditions, By using the method of power bond graph and state space analy-sis, the dynamic mathematical model of the system used to drive slide unit is estab-lished, that is called as self-adjusting back pressure speed control system and abbre-viated to SABP system. In order to improve slide unit's dynamics, it is necessary to find out the main reasons and affecting factors, that must be based on computer digital simulation and study on the results.

2.3 Dynamic Mathematica Modeld

The schematic diagram of SABP system is shown in Fig.l, the system is used to perform the cycle of feeding, stopping and returning. Four way control valve works in the right position during slide unit's feeding. The supply pressure of the pump is approximately constant under the action of pressure relief valve, the oil through the control valve and pressure compensated flow control valve enters the non-rod chamber to put slide unit forward. At the same time, the oil from the rod chamber is discharged through SABP valve and directional control valve to tank. In this process, the state of two check valves and pressure relief valve is not changed, To establish the mathematical model as reasonably and simply as possible, consid-eration must be focused on main affecting factors for a complex non-linear system such as the SABP system. It is illustrated by theoretical analysis and test result ' ,

液压系统(文献翻译) 13

that the transient time of the system is much longer than that of the flow control valve, and the flowrate overshoot of the valve in transients affects very small to slide unit speed because of the ;large effective sectional area of non-rod chamber in cylinder. For investigating the system's dynamics widely and deeply, the initial mod-eltn is further simplified in this paper, and so the study can be efficiently made with microcomputer. It is assumed that the flowrate through the flow control valve is constant in the whole transient process, and is denoted to a flow source.Fig.2 shows the structure diagram of the dynamic model of the system, it is composed of cylin-der, slide unit, SABP valve and pipe line; etc.

By using the method of power bond graph and state space analysis in this paper, the dynamic mathematical model of the system is to be established- The power bond graph is a power flow diagram, which expresses abstractly the actions among sub-systems as three effects, i.e. resistance effect, capacitive effect and inertia effect, according to the way of energy transform, on the basis of practical structure and by means of method of lumped parameters. The model is characterized by a clear conception in physics, and non-linear system can be accurately analysed in combination with method of state space analysis, thus it is a effective method used in the dynamic investigation of complex non-linear system in the'timedomain.

From main performances of components in SAEP system, the power bond graph of the system has been formed by means of the rule of model establishing ' and is shown in Fig.3. Half arrow in each bond indicates a direction of power How, two variahles of power are effort variable and flow variable. O-junction illustrates algebraic summation of flow variables at common effort, i.e. parallel connection, 1-junction does algebraic summation of effort variables at common flow, i.e. series connection. The symbol TF represents power transformer between two types of energy, and transforming modulus between efforts or flows is noted below the sym-bol TF. Short transverse bar across one end of each bond shows causality between two variables. A full arrow expresses a control action. Among three actions, there is an integration or differential form in capacitive effect and inertia effect between two variables. So state equation may be derived from Fig.3, there are nine state vari-ables in this complex nonlinear equation. Studying on the slide unit's dynamics is started with impact and motion stability. The equation is simulated by using the method of 4th order Runge-Kutta integration procedure on IBM-PC computer. Fig.4 and Fig.5 illustrate the results respectively.

Slide unit's impact phenominon results from load's vanishing in the transients, ■ for example, the situation of drilling through workpiece, Fig.4 expounds the varia-tions of the load and speed of slide unit, the pressures of chambers in cylinder. When slide unit motions evenly under the action of load, the oil pressure in non-rod chamber is very high, and there is a lot of hydraulic energy accumulated in side. The pressure decreases at once with load's discharging rapidly. During the process of oil pressure converting from high to low, the system absorbs some of the energy, so slide unit impacts forward with high speed. And then the oil in rod chamber is

液压系统(文献翻译) 14

compressed to increase back pressure, some of the energy is consumed, which plays a part of restraining the impact of the slide unit. It must be noted that inlet pressure of SABP valve telys on the interaction of pressures of two chambers, and increases rapidly at the instant of load's vanishing, and then stabilizes at some value greater than initial one. This pressure is also greater than one of traditional speed control system, therefore the energy can be absorbed much more in the rod chamber. In result, the impact of slide unit in SABP system is 20% lawer than in traditional's. It is thus clear that slide unit with SABP system for driving has a good performance in restraining the impact and SABP valve plays an important part in that,

2.4 Motion stability

When load acting on slide unit varies periodically, such as the situation of mill-ing, slide unit's speed will bring about some pulse. In order to meet the requirements of manufacturing quality, the magnitude of the speed pulse must be reduced as small as possible. The variation of the load is assumed to be of sine wave, in order to sim-plify discussion of the problem, The result of digital simulation is shown in Fig.5 It can be seen that, the response of the system is the sameas traditionl's and the differ-ences between them are very small. The reason for this is that the variation of the load is not targe, there the pressures in chambers vary very little that is, the effect of the SABP valve is not obvious.

2.5 Improvement

It is shown by studying, that dynamics of slide unit which used SABP speed control system as driving system is better than that of traditional system. To reduce the slide unit's impact, the back pressure of rod chamber has to be increased rapidly in the transients of load's vanishing; on the other hand, to enhance the slide unit's motion stability, it is necessary to raise the system rigidity. However, main recom-mendation lies in decreasing the volume of oil. It is known from system structure that, there is a lot of oil-containing volume between the rod chamber and drain pipe as shown in Fig.6a. Because the volume exists, not only the effect of SABP valve is delayed and reduced, hut also the rigidity of the system is decreased. Therefore, it is hindered to further improve the impact and motion stability. To make the slide unit dynamics better, the structural dimensions of cylinder's chamber and the SABP valve must be designed suitably. Based on simulations under the various structural dimensions and comparison among the results, the following two measures can be taken for improvements:

The ratio between volumeV4 and V3 is changed from 5.5 to 1 approximately, as shown in Fig,6b;

15

the bottom diameter of spool of the SABP valve is increased from 10 mm to 13 mm and the length of side of triangular damping slot is decreased from 1 mm to 0.7mm the slide unites impact quantity can be reduced by 30%, and the time of dyna-mic response is shortened. In addition, slide unit's motion stability may be improved obviously. It is thus evident that improvements are very effective.

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