Numerical control apparatus for a mechanical hydraulic servo valve
BACKGROUND OF THE INVENTION 
     This invention relates generally to a servo system. More particularly, this invention relates to a numerical control apparatus for a mechanical hydraulic servo valve. 
     A conventional numerical control apparatus for slide mechanism generally includes a ball screw driven by a servo motor or stepping motor. A ball nut carries a slide element thereon and is engaged with the ball screw. The rotation of the ball screw can be converted into the rectilinear movement of the ball nut because of the engagement between the ball screw and the ball nut. This kind of numerical control apparatus suffers from the following disadvantages: 
     (1) Because a ball screw of a very high precision is needed, it is difficult to make and maintain such a ball screw, resulting in high manufacturing costs. In addition, if the slide element is bulky, a large-size ball screw would be needed, also increasing its manufacturing costs. 
     (2) When a large-size screw is used, it must be driven by a high horsepower stepping motor or servo motor and the one motor can serve one slide element only. Thus, manufacturing cost is increased further. 
     (3) Due to uneven load, partial travel, long term serving and sudden impact by error, uncurable irregular worn-out backlash often occurs between the ball screw and the ball nut even when the ball screw and the ball nut are of a high precision. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a reliable numerical control apparatus for industry. 
     Another object of this invention is to provide an inexpensive numerical control apparatus for industry 
     To these ends the present invention provides, a servo system including a tubular drive shaft journalled in a housing. A screw is engaged threadably with a nut which is fixed in the drive shaft. A driven body is secured to an end of the screw in such a way that rectilinear movement of the the screw can cause rectilinear movement of the driven body but the screw cannot rotate. A motor rotates the drive shaft through a gearing so that the drive shaft moves in a first direction. A fork is then rotated at one end by an abutment element which is sleeved on the drive shaft. The other end of the fork activates a mechanical hydraulic servo, and hence a hydraulic driving unit, to move the driven body in a second direction which is opposite the first direction. The drive shaft is then moved back to the original position by the driven body. In this system, the assembly of ball screw and nut is used as a measuring and controlling unit. As a result, the backlash between the ball screw and the nut is minimized. Because the force applied to the ball screw and nut is even and small, the wear between the ball screw and nut is reduced. Furthermore, a low horsepower motor can be used to driven the ball screw. 
     In addition, the driven body can be easily driven by a hydraulic driving unit and a plurality of driven bodies can be driven by a single hydraulic driving unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of this invention will become apparatus in the following detailed description of a preferred embodiment of this invention with reference to the accompanying drawings in which: 
     FIG. 1 is a sectional view of a numerical control apparatus according to this invention; 
     FIG. 2 is a schematic view illustrating the fork of the numerical control apparatus according to this invention; 
     FIG. 3 is a block diagram of a servo system using the numerical control apparatus of this invention; and 
     FIG. 4 is a schematic view of the servo system using the numerical control apparatus of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, the numerical control apparatus 9 of this invention includes a housing 1, a tubular drive shaft 2, a screw 3, and a fork 4. 
     A mechanical hydraulic servo valve 5 and a motor 6 are attached to the housing 1. The motor 6 may be either a stepping motor or servo motor. The motor 6 has a motor shaft 61 is integral with an active gear 62. The drive shaft 2 is journalled in the housing 1 by roller bearings 11, 12 and thus the drive shaft 2 can move axially. A nut 21 is fixed in the drive shaft 2. A reactive gear 22 is integral with the drive shaft 2 and meshes with the active gear 62 so that the drive shaft 2 can be rotated by the motor 6. An annular abutment element 24 is sleeved rotatably on the drive shaft 2 by a gearing 23 and has an axially extending keyway 241 into which a key 15 is inserted. The key 15 is fixed in the housing 1 so as to prevent the rotation of the abutment element 24. 
     The screw 3 extends through the drive shaft 2 and is engaged threadably with the nut 21. Because a driven body 7, such as a slidable machine table, is secured to the left end of the screw 3, said screw 3 can effect an axial rectilinear movement of the driven body but cannot itself rotate. 
     The fork 4 is connected rotatably to a pivot pin 42 by a bearing 41 at an intermediate portion of the fork 4 thereof. The pivot pin 42 is fixed on a support frame 16 which is fixed in the housing 1. A wear pad 43 is secured to the upper end of the fork 4. The servo valve 5 includes a spool 51 disposed therein. The spool 51 is pushed by a spring (not shown) to abut against the wear pad 43. Referring to FIG. 2, the fork blades 44 of the fork 4 ride on the drive shaft 2 and are pulled by a tension spring 46 so as to bring the abutment element 24 into contact with two bearings 45 which are respectively mounted on the lower end portions of the fork blades 44. 
     When the motor 6 rotates the drive shaft 2 through the active gear 62 and the reactive gear 22, the drive shaft 2 moves a predetermined distance in a first direction due to the fact that the screw 3 cannot rotate and is engaged with the nut 21. Because the fork blades 44 of the fork 4 are pulled by the tension spring 46 so that they continually rest against the abutment element 24, the fork 4 rotates to cause the spool 51 of the servo valve 5 to move in a second direction which is opposite the first direction. Then, the servo valve 5 activates the piston rod 81 of a hydraulic cylinder 8 (see FIGS. 3 and 4) to move the driven body 7, the screw 3 and the drive shaft 2 the predetermined distance in the second direction. The fork 4 and the servo valve 5 thus return to their original positions, achieving an automatic feedback control. When the servo valve 5 returns to the original position, it no longer activates the hydraulic cylinder 8. 
     Because the key 15 engages with the keyway 241 of the abutment element 24 and the bearing 23 is disposed between the drive shaft 2 and the abutment element 24, when the shaft 2 rotates, the abutment element 24 cannot rotate allowing the abutment element 24 to contact firmly with the fork blades 44. Furthermore, because the bearings 45 are provided on the lower end portions of the fork blades 44, when the fork 4 rotates, a rolling friction is created between the abutment element 24 and said fork blades 44. 
     In a situation where the motor 6 is idle, when the driven body 7 is moved by an external force, the screw 3, drive shaft 2, fork 4, servo valve 5 and hydraulic cylinder 5 are in turn activated until the driven body 7 returns to its original position. A centering effect is therefore obtained in accordance with this invention. That is to say, the driven body 7 can be automatically and accurately positioned. 
     The hydraulic cylinder 8 may be replaced with a cylinder, motor, rotary actuator, turbine, or other hydraulic driving unit. The motor 6 may be replaced with any suitable manual rotary driving source. 
     While the invention has been disclosed with reference to a preferred embodiment, it is apparent that numerous modifications and variations can be made without departing from the scope and the spirit of this invention. It is therefore intended that this invention be limited only as indicated in the appended claims.