GB2216732A - Ultrasonic linear motor employing piezoelectric elements - Google Patents

Abstract

An ultrasonic linear motor comprises a vibrator (4) driven by two piezoelectric elements (6, 7). The vibrator (4) has a pair of the parallel leg portions (1, 2) and a body portion (3) attached to one end of each of said leg portions (1, 2) to form a generally inverted 'U'-shaped configuration. The piezoelectric elements (6, 7) are attached, one to each leg portion to vibrate each leg portion (1, 2) and the body portion (3). The piezoelectric elements (6, 7) are energized by phase displaced sources so that when the lower ends of the leg portions vibrate in engagement with a common rail (8) there is a net translational motion of the vibrator (4) relative to the rail (8). <IMAGE>

Description

LINEAR MOTORS The present invention relates to ultrasonic linear motors. Such motors are used as the operation source in electronic equipment and in precision equipment.

Electronic equipment and precision equipment require actuators. Unsually, the installation space for such actuators is relativly narrow and the actuators have to be accurately positioned. However, where linear motion is required linear motors can be used instead of actuators, since a direction changing mechanism connected with the moving direction is not required.

It is an object of the present invention to provide an improved linear motor.

According to the present invention there is provided a vibrator having a pair of the parallel leg portions and the body portion attached to one end of each of said leg portions, to form a generally inverted 'U'-shaped configuration.

A piezoelectric element is attached to each said leg portion of the vibrator. Each piezoelectric element vibrates both said leg portions and said body portion. The angle between the piezoelectric element and the body portion is symmetrical.

Also the angle formed by each piezoelectric element along with the body portion or the leg portion, is not only set at the same angle but at that angle which ensures a proper distribution of the component forces throughout the vibrator. In addition the piezoelectric elements are driven at the resonant frequency of the vibrator, to obtain the vibrations, having the relative higher amplitude and effectively using less energy.

To achieve this effect, one piezoelectric element receives an AC voltage, which has the same vibration frequency as that of other piezoelectric elements but is phase displaced by 90". Therefore the lower extremity of each leg portion follows an oval or a circular path during rotation in the same sense. That is, the lower extremities of the two leg portions in turn engage the rail but are phase displaced by 90", S the net result is that the vibrator is moved in a predetermined direction along the rail.

Thus, the relative distance between the lower extremity of each leg portion and the rail is continuously changed, thereby the relative larger hitting force produces little damage even though the vibrating frequency is increased. Also, by equalizing the frequency of the power source with the resonance frequency of the vibrator, the energy convertion efficiency is superior.

Linear motors embodying the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a front elevation of a first embodiment of the linear motor; Figure 2 is a plan view of the motor of Figure 1; Figures 3a to 3d are schematic views of the motor of Figure 1 illustrating its motion; Figure 4 is a graph representing the results of the impedance analyzer for measuring the resonant frequency of the vibrator; Figure 5 is a front elevation of a second embodiment of the linear motor; Figure 6 is a plan view of the motor of Figure 5; Figure 7 to Figure 10 are fragmentary front elevations of four further embodiments of the linear motor; and Figure 11 is a cross-sectional view of a previously prepared linear motor in different operative modes.

Figure 11 shows a linear motor in the form of a winch worm. The winch worm includes shaft 21 and the traveller 22 in the form of a tube through which the shaft 21 extends. The traveller 22 comprises three piezoelectric tubular elements 23,24 and 25 adhesively bonded together. The central tubular element 23 is capable of expanding and contracting axially of the shaft in response to electrical excitation. The other tubular elements 24 and 25 are capable of expanding and contracting radially of the shaft in response to electrical excitation. When traveller 22 is to be moved towards the right hand side, the left hand tubular element 24 is caused to contract to grip the shaft 21 while the central tubular element 23 is caused to expand. This displaces the right hand end of the traveller 22 to the right.At this point the right hand tubular element 25 is caused to contract to grip the shaft 21 and the left hand tubular element 24 is caused to expand to release the shaft. If now the central tubular element is caused to contract this draws the left hand end of the traveller 22 to the right. By repeating the cycle the traveller 22 is moved progressively to the right.

The disadvantage of this arrangement is that the size of the gap between shaft 21 and the tubular elements 24,25 must be accurately managed and a high degree of precision is required. Accordingly it is very difficult to accurately process a very long shaft. Thus, either manufacturing cost is high or the moving distance is limited.

The motion of the traveller 22 is conducted stepwise. Due to it, when tube elements 24,25 grasp the shaft 21, the tubes 24,25 and shaft 21 engage each other and are vibrated. If they are moved at resonant frequency, the motor efficiency falls.

The linear motor shown in Figures 1 to 3 comprises a vibrator 4 of generally inverted 'U'shaped configuration. The vibrator 4 has a pair of parallel leg portions 1 and 2 interconnected by a transverse body portion 3.

The vibrator 4 is of elastic or reslient material, for example aluminium and is substantially square in cross-section. Each side of the crosssection may be 5mum, each leg may be 1Omm in length and the body portion 3 may be 26 mm in length.

The different materials used to construct the vibrator 4 may be as follows; the metal materials may be selected from the group consisting of zirconium, iron and stainless steel; the inorganic materials may be selected from the group consisting of alumina, glass silicate and carbon; and the organic materials may be selected from the group consisting of polyamide resin and nylon.

The portions of vibrator 4, where the leg portions 1 and 2 join the body portion g, have a surface 5 inclined at an angle of 450 to the axes of those portions. Two piezoelectric elements 6,7 are fixed by an adhesive agent one to each of said surfaces 5. Each piezoelectric element 6,7 is of piezoelectric ceramic and can expand and contract in a direction at right angles to the surface 5. Each piezoelectric element 6,7 may be 9 mm long and have a square cross-section of 5 mm.

The lower extremity of leg portions 1,2 of the vibrator make contact with the upper surface of rail 8. The upper surface of rail 8 has a longitudinal groove 8a which accommodates the lower extremities of the leg portions 1#,2 and constrains them for movement along the rail in the longitudinal direction.

The vibrator 4 is translated along the rail by ultrasonic vibration as will now be described.

The first piezoelectric element 6 (the left element in Figure 1) is energised with a sinusoidal voltage as shown by the equation; Va = E. sinwt the second piezoelectric element 7 is also energized by sinusoidal voltage as shown by the equation; Vb = E. sin (wt Each piezoelectric element 6,7 is vibrated in the direction of its longitudinal axis. This vibration is transferred through vibrator 4 to the extremity of each leg portion 1,2 to react with the rail 8. Thus each leg portion receives a horizontal vibration from one piezoelectric element and a vertical vibration from the other piezoelectric element, the horizontal and vertical vibrations produced by the same piezoelectric element being 180' out of phase.

The lower extremity of the first leg portion 1 is vibrated along coordinate X and Y axes as follows: X1 = A. sin (wt + r/2) Y1 = B. sin (wt +;r).

The lower extremity of the second leg portion 2 is vibrated along coordinate X and Y axes as follows: X2 = A. sinwt Y2 = B. sin (wt + for/2).

It will be seen that the lower extremities of the leg portions 1,2 perform an oval or a circular movement in a plane containing the axis of the rail 8. The vibrations of first leg portion 1 and second leg portion 2 have a phase difference of 90 between them.

The extremities of the leg portions 1 and 2 thus perform an oval or a circular movement 900 out of phase with each other.

These phase displaced movements produce a net translational movement along the rail 8 as will now be described with reference to Figures 3a and 3d.

Because the lower extremities of the legs make frictional engagement with the rail 8, the extremity which at any instant is the lower one will effect translational movement of the vibrator 4.

Thus as shown in Figure 3a the second leg portion 2 has preference and the vibrator moves a distance L1 to the right. In the case of Figure 3b, since both of the leg portions are raised the net amount of the movement of the vibrator 4 is small but in L2 to the right. In the case of Figure 3c, first leg portion 1 has preference and so the vibrator 4 is moved to the right. In the case of Figure 3d, both legs are moving the same direction; and so a relatively large displacement L4 of the vibrator is advanced. It is difficult to quantitively determine the magnitude of each movement, L1, L2, L3 and L4, but the amounts are roughly as represented by the lengths of the arrows L1, L2, L3 and L4 in the drawing. As a result, the sum of L1, L2, L3 and L4 is the net amount of movement performed in one cycle of the vibration.

The voltage of second leg portion 2 is as follows.

Vb = Ex ~ sin (wt + 1/2 t In the linear motor, since the leg portions 1,2 are always pressed in contact with the surface rail 8, the relative distance between the surfaces of leg portion 2 and rail 8 is little changed, even when the frequency of the vibration applied to vibrator 4 is increased. Thus, it is possible to select any frequency for the vibration.

To vibrate the general structure using as little energy as possible, it is most efficient to vibrate the structure at the resonant frequency.

Resonant frequencies have different values according to the materials, the shape and the size, of the structure.

In order to select the frequency for the power source supplying the piezoelectric elements 6,7 of the linear motor, the vertical resonant frequency of vibrator 4 in the linear motor had been investigated by an impedance analyzer. As a result, as shown in Figure 4 some peaks were detected. Among the peaks, the peak of 91.6 KH, was selected as the frequency for the power source. This frequency would be too high for the linear motor forming the conventional winch worm mechanism since the winch worm is normally operated at a frequency of 5 KH .

When the power source has a voltage of 5V and is operated at a peak frequency of 91.6 KXz, the vibrator 4 will move at a speed of 15cm/sec.

In the linear motor shown in Figures 5 and 6 parts similar to those in Figures 5 and 6 are similarily referenced. The vibrator 4 is urged towards the rail 8 by a mechanism which includes front and rear automobile wheels 11 supported on a frame 9 by axles 10. The wheels 11 engage the underside of the rail 8. A bridging member 12 is fixed to the upper surface of body portion 3 and coil springs 13 are mounted at opposite ends of bridging member 12 are coupled to the centre of the supporting frame 9. The lower extremities of the leg portions 1,2 are thus pressed into contact with the upper surface of rail 8 and the automobile wheels 11 are pressed in contact with the lower surface of rail 8 by means of the coil springs 13. The coil springs 13 are selected to have the proper elastic coefficient according to the load given.This embodiment, has the advantage that rail 8 may be used at a slant relative to the horizontal.

Figures 7 to 10 show different forms of the vibrator where the piezoelectric elements 6,7 are mounted in different positions.

In Figure 7 the piezoelectric elements 6,7 are attached to the inner of the leg portion in vibrator 4. In Figure 8 the piezoelectric elements 6,7 are attached to projections extending from the outer side of the leg portions. In Figure 9 the piezoelectric elements 6,7 are attached to projections extending from the inner side of the leg portions. In Figure 10 the one piezoelectric element 6 is attached to the inner side of the leg portion and the second piezoelectric element 7 is attached to the outer side of the leg portion. Each of these attaching methods have different structural characteristics and so can be selected according to the usage conditions required of the linear motor.

The relationship of the movement direction and the phase difference is different from the above description, but their fundamental principle is similar.

In this embodiment, the angle between the longitudinal axis of the body portion 3 and the axis of the piezoelectric element 6 is 45 but is not limited to this angle. Each piezoelectric element may have the same angle or the supplementary angle with respect to the body portion.

Also the phase difference of the power source voltages applied to both piezoelectric elements is preferably 90' but it can lie anywhere in the range of from 30 to 120'.

As described above, the vibrator is of generally inverted 'U'-shape in configuration and has a pair of parallel leg portions joined by a body portion. Piezoelectric elements are attached one to each leg portion of the vibrator to vibrate both leg portions and the body portions. In manufacturing it, it is not necessary to increase the processing precision and the manufacturing cost is not expensive. Also the movement distance is long.

Since the relative distance between the leg portion and rail will continuously change, the linear motor is not easily damaged even when the vibration frequency of the vibrator is increased, and also can have the good efficiency using the resonant frequency of the material.

Claims (8)

1. An ultrasonic linear motor comprising a vibrator of generally inverted 'U'-shape configuration and having a pair of parallel leg portions and the body portion extending between the leg portions a piezoelectric element attached to each leg portion to vibrate the body portion and each leg portion.

2. An ultrasonic linear motor according to Claim 1, including a power source coupled to said piezoelectric elements and operating at a frequency equal to the resonant frequency of the vibrator.

3. An ultrasonic linear motor according to Claim 1, or to Claim 2 wherein said piezoelectric elements are vibrated with a phase difference of 90".

4. A linear motor comprising a rail, a vibrating member having a pair of legs,spaced longitudinally of the rail and engaging said rail, and means for vibrating the legs to have a component of motion in the longitudinal direction of the rail, the components of motion of the two legs being phase displaced from one another whereby to enable the vibrating member to be translated along the length of the rail.

5. A linear motor according to Claim 4 wherein the vibratory means comprises two piezoelectric elements which act on both legs to cause that extremity of each leg which engages the rail to follow a closed loop path of motion in a plane containing the longitudinal axis of the rail.

6. A linear motor according to Claim 4 or to Claim 5 including means for urging the vibratory member against the rail.

7. A linear motor according to Claim 6 wherein the means comprises wheel means engaging the rail on the opposite side to said vibrating member, and resilient coupling means coupling the wheel means to the vibrating member.

8. A linear motor substantially as hereinbefore described with reference to any one of Claims 1 to 10.