1. Field of the Invention
The present invention relates to a drive device using an electromechanical transducer suitable for driving general fine machine devices such as drive units of an XY drive table, a photographing lens of a camera, a projecting lens of an overhead projector, a lens for a binocular and the like.
2. Description of the Prior Art
Although a drive device using an electric motor has conventionally been used for driving or the like of an XY drive table or a photographing lens of a camera, inconvenience of enlargement of device, generation of magnetic field, generation of noise or the like has been pointed out. Hence, as means for solving such various problems, the applicant has proposed an actuator using an electromechanical transducer, that is, an actuator in which a moving member is frictionally coupled to a drive member that is fixedly coupled to an electromechanical transducer and displacements in elongation and contraction directions having different velocities are caused by applying drive pulses in a sawtooth-like waveform to the electromechanical transducer by which the moving member frictionally coupled to the drive member is moved in a predetermined direction.
FIG. 14 through FIG. 16 show an example of an actuator using the above-described electromechanical transducer. FIG. 14 is a perspective view showing the actuator by disassembling the actuator into constituent members, FIG. 15 is a perspective view showing an assembled state of the actuator and FIG. 16 is a sectional view showing the constitution of contact portions of a drive shaft, a slider block and a pad.
In FIG. 14 through FIG. 16, an actuator 100 is constituted by a frame 101, support blocks 103 and 104, a drive shaft 106, a piezoelectric element 105, a slider block 102 and the like. The drive shaft 106 is supported by a support block 103a and the support block 104 movably in the axial direction. One end of the piezoelectric element 105 is fixedly adhered to the support block 103 and the other end is fixedly adhered to one end of the drive shaft 106. The drive shaft 106 is supported displaceably in the axial direction (arrow mark "a" direction and direction opposed thereto) when a displacement in a thickness direction of the piezoelectric element 105 is caused.
The drive shaft 106 penetrates the slider block 102 in the horizontal direction, an opening 102a is formed at the upper portion of the slider block 102 which the drive shaft 106 penetrates and the upper half of the drive shaft 106 is exposed. A pad 108 which is brought into contact with the upper half of the drive shaft 106 is inserted and fitted to the opening 102a, a projection 108a is provided at the upper portion of the pad 108 and the projection 108a of the pad 108 is pushed down by a leaf spring 109 by which a downward urging force F for bringing the pad 108 into contact with the drive shaft 106 is applied on the pad 108. Incidentally, numeral 110 designates screws for fixing the leaf spring 109 to the slider block 102. The constitution of contact portions of the drive shaft 106, the slider block 102 and the pad 108 is apparent by referring to FIG. 16.
By the above-described constitution, the slider block 102 including the pad 108 and the drive shaft 106 are brought into press contact with each other and frictionally coupled by the urging force F of the leaf spring 109.
Next, an explanation will be given of the operation. First, when drive pulses in a sawtooth-like waveform each having a slowly rise portion and a rapidly fall portion as shown by FIG. 17(a) are applied to the piezoelectric element 105, the piezoelectric element 105 is slowly displaced to elongate in the thickness direction at the slowly rise portions of the drive pulses and the drive shaft 106 coupled to the piezoelectric element 105 is also slowly displaced in a positive direction (arrow mark "a" direction). At this moment, the slider block 102 frictionally-coupled to the drive shaft 106 is moved in the positive direction along with the drive shaft 106 by a frictional coupling force.
At the rapidly fall portion of each of the drive pulses, the piezoelectric element 105 is rapidly displaced to contract in the thickness direction and the drive shaft 106 coupled to the piezoelectric element 105 is also rapidly displaced in a negative direction (direction opposed to arrow mark "a"). At this moment, the slider block 102 frictionally-coupled to the drive shaft 106 substantially remains at the position and is not moved by overcoming the frictional coupling force by inertia. By continuously applying the drive pulses to the piezoelectric element 105, the slider block 102 can continuously be moved in the positive direction.
Incidentally, the expression "substantially" used here includes a motion moving in the arrow mark "a" direction as a whole by a difference in drive time periods in either of the positive direction and the direction opposed thereto where the slider block 102 follows the drive shaft 106 while causing slippage between frictional coupling faces of the slider block 102 and the drive shaft 106.
In moving the slider block 102 in the opposed direction (direction opposed to arrow mark "a"), the waveform of the sawtooth drive pulses applied on the piezoelectric element 105 is changed and drive pulses each comprising a rapidly rise portion and a slowly fall portion as shown by FIG. 17(b) are applied by which the movement can be achieved.
According to the actuator explained above, the pad 108 that is brought into contact with the upper half of the drive shaft 106 is inserted and fitted to the opening 102a at the upper portion of the slider block 102 and although a fitting clearance between the opening 102a and the pad 108 is set to be as small as possible, a clearance necessary for assembly operation remains.
Meanwhile, if there is a clearance between the slider block 102 and the pad 108, when the drive shaft 106 is moved in the axial direction, the pad 108 is moved by an amount of the clearance along with the drive shaft 106 and drive energy received from the drive shaft 106 is not sufficiently transmitted to the slider block 102 whereby loss of drive energy is caused.
Further, in order to make the fitting clearance between the opening 102a and the pad 108 as small as possible, the opening 102a and the pad 108 need to be fabricated with high finishing accuracy which results in an increase in cost. Further, there is a fabricating means of press-fitting the pad 108 into the opening 102a or the like to nullify the fitting clearance. However, when the fitting clearance is nullified by such a fabricating means, there causes inconvenience in which a pertinent frictional force cannot be provided between the pad 108 and the drive shaft 106 by pushing down the pad 108 by the leaf spring 109.
Further, according to the actuator explained above, the frictional force caused between the slider block and the pad, and the drive shaft, is produced by the downward urging force F of the leaf spring in which the leaf spring having a pertinent modulus of elasticity capable of producing an optimum frictional force in accordance with anticipated load is selected and fine adjustment of the frictional force is carried out by the screws 110 for fixing the leaf spring 109. The constitution is simple and functions sufficiently when variation in load is inconsiderable.
However, the frictional force is varied in accordance with load and therefore, the actuator having the above-described constitution cannot efficiently correspond to usage having a large amount of variation of load and the drive efficiency is lowered.
According to the actuator explained above, the slider block (driven member) is frictionally coupled to the drive shaft when the slider block is not driven. As is apparent from the operational mode of the actuator, the slider block follows the drive shaft while causing slippage between the frictional coupling faces of the slider block and the drive shaft. Therefore, the frictional coupling force is not strong and the slider block is easily moved when external force is applied to the slider block. Accordingly, when unintentional external force is applied to the slider block, the slider block may be unpreparedly moved, the actuator may be destructed or parts driven by the actuator may be destructed.