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Piezo-Electric Motor

This invention relates to a controllable piezo- electric motor designed to move objects by utilizing friction forces developed between piezo actuators and the object being moved. More specifically, it relates to a piezo-electric step motor designed to move an object in a step-by-step manner. By different arrangements of the piezo-electric actuators, the movement of the object can be linear, planar, rotation about a fixed axis, or rotation about a fixed point, etc.
Such piezo-electric motors are especially useful in applications requiring manipulation of small objects in environments where mechanical access is not possible or not desired. Depending on their construction, these devices can also be used in harsh environmental
conditions such as ultra-high vacuums, variable
(especially low) temperatures, and strong magnetic fields.
A particularly useful application is in microscopes, such as scanning tunnelling microscopes or atomic force microscopes, where the piezo-electric motors may be used to move the sample stage or the microscope probe tip.
An example of a piezo-electric linear actuator is described in US Patent Nos. 3,902,084 and 3,902,085.
These documents describe a device known as the Inchworm™ which comprises an assembly of piezo-electric tubular elements wherein a coaxial rod, which is to be moved, is alternately gripped and released by end tubes together with appropriate stretching and shrinking of a centre tube, thus causing the rod to be translated along the axis of the device. This stepwise motion is achieved by the indirect action of the extending and contracting centre piezo-electric tube on the outer piezo-electric tubes which grip the rod.
Another device, which can be used to provide smooth and continuous translation of a load, is described in US Patent No. 4,928,030. This motor uses a number of piezo-electric actuators which can be either individually or in combination brought into contact with the object to be moved, and then relatively slowly sheared by the
application of a voltage to the piezo-electric actuators, thus moving the load by means of the frictional force developed between the end of the actuator and the load, and finally retraced back to their original position by rapidly decreasing the voltage to zero such that the surface of the actuator slides over the load without moving it. This sliding is achieved as a result of the inertia of the load. The friction force between the load and the actuators is a result of a normal force applied to their contacting surfaces, and this can be achieved either by static loading or by using correctly aligned piezo-electric transducers to lift the actuator into contact with the load. Use of controlled sequences of actuator movements results in smooth, continuous
translation of the load, for example by moving one pair of actuators whilst another pair are retracing and so on sequentially. The driving signal for this device
consists of a gradually increasing voltage during the moving portion of the operation, and a rapid decrease to zero during the sliding or retrace portion.
The device of US Patent 4,928,030 moves the load by means of sequential operation of each piezo-electric actuator, and because of the variable friction forces present between the actuators and the load, precise step-by-step control of the motion of the load is not
possible. Furthermore, in order to attempt to reduce these friction losses, and to reduce the mass which must be moved by the actuator when retracing so that
acceleration is maximised, thus minimising friction during the retrace motion, the actuator is" pyramid-shaped and therefore the contact surface between the load and actuator is made as small as possible. This can result in difficulties due to localised imperfections in the contact surfaces, thus reducing the minimum controllable single step size, adding a requirement for the smoothness of the contact surfaces which increases manufacturing costs, and increasing the risk of localised deformation and wear of the components. A further problem is that if the frictional forces are kept to a minimum, the forces supporting the load and holding it in position are also minimised, thus decreasing the mechanical stability of the overall system. Similarly, since it is preferred for the actuators to only be in contact with the load when being operated to move it, in the off state the load must be held by some other means, or a residual voltage must be applied to the 'lifting' piezo-electric transducers so that the actuators are brought into contact with the load and thus hold it in place. This consumes excess power and can be noisy, etc. If some of the actuators are held in contact with the load, even when not being operated to move it, to help support and stabilise it, the frictional forces that are therefore present result in additional loads on the driving actuator and, due to their varying values, make precise control of the motion of the load impossible. Furthermore, since environmental influences such as temperature changes can vary these friction forces, causing more problems, the use of minimum area contact surfaces means that very local environmental changes can considerably alter the friction forces present.
It is an object of the present invention to provide a piezo-electric motor which is capable of precise manipulation of a small object in a step-by-step manner, by using the friction forces developed between the piezo-electric actuators and the object being moved.
A further object of the invention is to provide a piezo-electric motor having a simple, compact structure which is very stable and rigid at all times, even when fully deactivated, and requires no mechanical components or voltage supply for holding or clamping any of the components .

Another object of the invention is to provide a motor which is substantially unaffected by environmental changes and can operate in harsh environments such as ultra-high vacuum, low temperatures and magnetic fields. It is also an object of the invention to provide a motor which is capable of smooth, continuous positioning of the load within a single step range.
Yet another object of the invention is to provide a piezo-electric motor for which the waveform of the driving signal is not critical.
According to the invention there is provided a method of translating a load using a piezo-electric motor wherein a plurality of piezo-electric actuators are disposed about and held in constant contact with the load by normal forces so that frictional forces hold the load stationary, the method comprising supplying driving voltages to the actuators to:-

(a) simultaneously actuate and move all of the
actuators to translate the load without any
sliding between the actuators and the load;
(b) actuate all of the actuators in ones or groups at different times to slide them relative to
the load while the remaining actuator or
actuators hold the load stationary.

In a preferred embodiment, step (b) comprises actuating each one or group of the actuators sequentially to slide them across the load until they have all been moved to new positions, and step (a) is performed before or after this step.
In a further preferred embodiment, control of a constant driving signal supplied to every "actuator can be used to accurately position the load within a single step range. Furthermore, by controlling the driving signal for step (a) , the force or torque applied to the load can be adjusted.

In another preferred embodiment, the driving signals in step (b) to each actuator are separated in time and rapidly rising. The step (a) driving signal is preferred to be the smooth combination of two parabolic curves in order to maintain constant acceleration and deceleration.
According to another aspect of the present
invention, there is provided an apparatus for translating a load comprising: -

(a) a body having a V-shaped groove having a
plurality of piezo-electric actuators disposed on its sloping surfaces, onto which a
triangular cross-sectioned load can be placed; (b) a plate having a plurality of piezo-electric
actuators disposed on one surface thereof, the plate being placed on top of the load so the
actuators rest thereon;
(c) means for applying a force to the plate to
push it,towards the groove, thus applying in a normal pressure on the contact surfaces
between the actuators and load; and
(d) control means for supplying driving signals to each of the actuators.

In a preferred embodiment, spring means extending across the groove and coupled to the plate by a ball placed therebetween are used to generate the normal pressures acting on the contact surfaces between the load and the actuators.
The motor of the present invention can accurately position small objects with well-controlled steps in submicron ranges. The use of friction forces alone removes the dependence on inertia for successful
operation of the motor and negates gravitational effects, so the present device can move objects with very small masses (in principle even zero mass) and is not sensitive to mounting orientation. Similarly, the invention provides a motor which is insensitive to vibration.
An advantage of the invention is that the motor has a very low minimum step size which is unaffected by orientation or load (within a certain range) . The invention also allows the maximum output force or torque to be used to move the load, with no frictional
resistance, and this force or torque to be readily adjustable.
Further, the motor can hold the load by frictional forces and does not need to be provided with driving signals, or to retain a charge, to hold the load still. Thus, unlike prior design, the motor of the invention can hold a load still (when charges would leak away in prior designs) with no power applied.
A number of embodiments of the invention will now be described by way of example only and with reference to the accompan ing drawings, in which: - Figure 1A depicts the working principle of the present invention;
Figure IB is a graph of the driving voltages applied to each actuator in Fig. 1A;
Figure 2A shows an embodiment of the present invention;
Figure 2B shows a section along the line II-II in Figure 2A;
Figure 3A is a diagram of another embodiment according to the present invention;
Figure 3B is a diagram of a piezo-electric leg according to one embodiment of the present invention.
Figure 4A is a view of yet another embodiment of the present invention;
Figure 4B is a view of yet another embodiment of the present invention;
Figure 5 shows still another embodiment of the present invention;
Figure 6 is a graph of a preferred driving signal according to the present invention;

Figures 7A and 7B are a circuit diagram showing an embodiment of the control circuit used in the present invention; and
Figure 8 shows explanatory waveforms.
Figure 1A illustrates the working principle of the present invention. In this schematic drawing, a bar 1 made of a hardened material with flat and smooth surfaces is held by a plurality (four are shown) of piezo-electric legs 2a, 2b, 2c, 2d. The piezo legs are arranged such that the normal pressures to keep each leg against the bar surface are self-adjusted to be equal and constantly hold the bar in position. The legs are rigidly fixed relative to a support means 3.
Each leg is comprised of a piezo-actuator, which can be a section tube, shear plates, extension bar, or other configuration, as are known in the art, and a friction shoe mounted thereon which provides frictional contact with the surface of the bar. Electrical contacts are connected to each individual piezo-actuator, such that actuating voltages can be applied thereto, causing the actuator to shear in its polarising direction
according to the piezo-electric effect. When an electric potential is applied to a correctly orientated piezo-actuator, the friction shoe will be moved in a direction tangential to the contact surface between the load and the leg, which in this embodiment is in the direction of the arrow in Figure 1A.
A single step of movement of the load is
accomplished by moving all the legs in the same direction simultaneously such that they translate the load in that direction because of the friction forces therebetween, and returning the legs to their start positions in such a way as to leave the load stationary. The immobilising of the load can be achieved by friction forces present between the load and actuators which remain stationary whilst others move.
In this embodiment, this is carried out by applying an electrical potential V0 to each leg or combination of legs in sequence, and then ramping down the potential to zero for all of the legs together. Since all of the legs 2a, 2b, 2c, 2d are pressed on to the bar 1 surfaces individually, the friction forces between each leg and the bar surface are approximately the same provided that the size, shape and arrangement of the legs is correct.
As depicted in Figures 1A and IB, a voltage of V0 is applied to leg 2a first, thus causing the friction shoe on leg 2a to slide on the bar surface when the shearing force generated by the actuator is larger than the static friction force between the friction shoe and the bar surface. The bar 1 should not, in principle, move since it is held by the frictional forces developed from the contact between the remaining legs and the bar surface (in this embodiment approximately three times as large as the frictional force between friction shoe 2a and the bar surface) .
After leg 2a stops sliding it is held in its new position by holding the voltage across the actuator at V0 and then the same value of potential is applied to the second leg 2b. Thus, similarly, the friction shoe on leg 2b will slide along the bar surface and move to its new position, without moving the bar itself. This process is then repeated sequentially for the remaining legs.
Once all of the legs have moved to their new positions, and are held there by the applied potential V0, the potential on all the legs 2a, 2b, 2c, 2d is ramped down from V0 to zero simultaneously, preferably at the same rate. Since all the friction shoes are now moving in the same direction and at substantially the same rate, there is no frictional resistance to the bar and thus it is carried by all of the legs to move to its new
position, as shown in Figure 1A. Thus when the potential is ramped down to zero, a single step motion of the motor is completed.
Simply by repeating this process, the bar can be moved stepwise in a given direction. By reversing the polarity of the potential or reversing the time axis of - S - the driving signal, shown in Figure IB, the direction of movement can be reversed.
In principle the sliding distances of each friction shoe will be the same and controlled by factors such as the static frictional force, the sliding frictional force, the sensitivity of the piezo-actuators and the electric potential V0 applied. Thus the step size can be easily controlled simply by changing the amplitude of the potential V0 applied.
In addition to this single step motion, continuous micro-positioning of the load within the range of a single step size can be obtained by applying the same residual electrical potential simultaneously to all of the legs. Adjustment of this residual electric potential can therefore be used for high resolution movement and positioning of the load..
The driving signals supplied to each leg relative to time are depicted in Figure IB. This shows the sliding period b-e and the carry period e-f of a single step. These signals can be varied so that 'carrying' occurs before 'sliding' or so that some actuators are 'shearing' while others are 'unshearing' during the carrying stage, etc.
Since the invention does not rely on inertia for the legs to slide over the load surface, the quality and timing of the driving signal is not especially critical. The rising periods for each leg do not have to be
especially fast to achieve sliding but if they are, the total time for each step is reduced, the sliding friction is reduced and the effective stiffness of the piezo legs is increased, therefore fast rising signals are
preferred. The rising periods for each leg are
preferably separated in time and not overlapping. It may be necessary to employ a delay period between consecutive signals to ensure that any adverse effects due to piezo creeping are avoided. Of course, it is possible to slide more than one leg along the load at the same time, provided that the load remains stationary during this operation. In the "carry" period, during which all the moved legs return to their original (i.e. deactivated) positions simultaneously, the best results are achieved if the signals to each leg are identical, otherwise differential movement of the legs may result, thus leading to losses. Preferably, the simultaneous ramping down or "carry" period signal is the smooth combination of two parabolic curves of voltage against time in order to maintain constant acceleration and deceleration of the load, thus enabling the acceleration force to be
maintained below the frictional force needed to prevent sliding. Such a waveform is depicted on the graph in Figure 6. The start of the 'carry' period is shown as reference 61.
When in use, any electrical charges on the piezo legs due to applied potential or bending, shearing, and expansion of the legs should be reserved during the holding period. One way to accomplish this is to simply disconnect the piezo legs during that time, since the charge will remain if there is no leakage. This will also help in increasing the effective stiffness of the piezo legs.
An example of a suitable electronic control circuit is depicted in Figures 7A and 7B and discussed further below.
The simultaneous movement of the piezo legs to translate the load, means that the load is moved with no frictional resistance. This removes the adverse effects of frictional losses when moving the load, and any problems associated with variations in the frictional forces present on the load when it is moved due to such factors as environmental effects. This enables the positioning of the load to be smoothly and precisely controlled, even within a single step range and removes the limitation of a minimum single step distance.
Furthermore, since there are no frictional losses
associated with the motion of the load, the maximum obtainable output force or torque can be fully transferred and, in addition, can be precisely adjusted by means of varying the driving signal V0.
The normal force to hold the piezo legs in contact with the load can be applied by any number of means known in the art. This normal force should be sufficient such that the resulting friction force is large enough to ensure that the load is held stationary when the friction shoes are moved independently over its surface, and is preferably much larger than the gravitational force on the moving object so that any effects due to orientation of the motor can be minimised. Furthermore, if the normal force is sufficient then a very rigid structure is obtained, with no loose components even when not in operation. A compact structure is therefore obtained and no mechanical components are necessary for holding or clamping the load and components together, even when no voltage is applied to any of the actuators.
By careful arrangement of each of the individual piezo legs around the load, equal forces on each leg can be obtained and any environmental changes will result in self-alignment and adjustment of the system. This frictional holding also gives protection against
overloading and catching of the components.
As the present invention is based on achieving frictional forces between the contact surfaces of the load and piezo legs, it is desirable for this surface area to be as large as possible. This is because the frictional force is proportional to the normal pressure and the area of contact. The use of a large surface area of contact has the advantage of averaging the effects of any defects on the contact surfaces and thus lowers the requirements of smoothness for the contact surfaces and the risk of localised deformation and wear of the
individual components. This also has the advantage of reducing any limitation on the minimum controllable single step size. The use of larger piezo legs also increases the rigidity and stability of the whole
structure .

Using the principles described above, various different modes of motion can be achieved by correct arrangement of the piezo legs. Four different preferred embodiments are described below. These embodiments are motors for: linear motion; planar motion; rotation about a single axis; and rotation of a sphere.
An embodiment of a linear motor is depicted in Figure 2. The support body 10 of the motor is made of a hard material and has a V-section groove cut in it. In this embodiment the groove 14 is triangular with 60° angles. A plurality of piezo legs 11 having shear plates 18 are arranged about the load. A total of six is preferred. Four of these six legs are arranged to be fixed in the groove, and the other two are mounted on another piece of body 12, made of the same material, which is mounted above the load 13 which has been placed in the groove 14. The top piece of body 12 is single point spring loaded in order to apply a normal force F on the load to hold the structure together.
Each leg 11 preferably has a friction shoe 15 on the top thereof to contact the surface of the load object 13. In this embodiment the moving object 13 is a
triangular cross-sectioned bar. The surfaces of the bar 13 which contact the friction shoes 15 need to be flat and polished to a sufficient smoothness. The contact area between the shoe 15 and the bar surface is as large as possible in order to maximise the frictional forces therebetween.
The normal force which gives rigidity to the motor should be large and in this embodiment is achieved by means of the pressure supplied by the spring plate 16 on to the top piece of the body 12 through a hard ball 17. This arrangement results in self-adjustment of the top piece of the body to press the friction shoes 15 evenly on to the top surface of the bar 13. In a preferred embodiment, the spring plate 16 has a bellows to absorb any differences in thermal expansion of different
components, therefore enabling the motor to be used in variable temperatures without adverse changes in its properties. Of course, the normal force F to hold the structure in contact can be applied by any number of known methods.
By using a similar driving signal sequence to that described earlier, the motor can be activated to move the bar 13 along the axis of the groove 14. In this
embodiment, the groove is depicted with a triangular cross-section, but this can, of course, be varied
according to the shape of the load, as can the
arrangements of the legs.
The use of a triangular cross-section shape does however have advantages. Firstly, the normal force F can simply be arranged to press on the top piece of the body 12 and the force to all six legs will automatically be equal providing all six are the same shape and arranged correctly. Secondly, the load bar is self-positioning within the groove, and finally, the direction of movement is well defined as the axis of the groove.
The construction of the piezo legs 11 themselves can be by a number of ways, as are known in the art. For example, they can be section tubes in bending modes, shear plates or extension bars. Shear plates are
preferred since they can have a large surface area and small heights thus resulting in high rigidity. Friction tubes, extension bars or shear plate stacks can give larger step sizes for lower driving voltages but with a corresponding loss of rigidity. The friction shoes 15 are also preferably thin, for similar reasons.
A second embodiment of the present invention is illustrated in Figure 3A. This embodiment allows the load to be moved in the x-y plane. A load comprising a flat plate 20 is held between two support plates 21 on which are disposed a plurality of piezo legs 22. Again, the supports 21 are arranged so as to apply a normal force F on to the load 20. The piezo legs 22 can
comprise either sectioned piezo tubes or shear piezo stacks, etc.

The preferred construction of piezo legs using shear plates is illustrated in Figure 3B. Two shear plates 23 arranged so as to have orthogonal polarising (i.e. shear) directions 25 are stacked together with a friction shoe 24 to make up each leg 22. By applying voltages to the different shear plates or to different sections of the piezo tubes, if used, planar motion of the flat plate can be achieved. Orthogonality in the x-y plane can be assured by use of mechanical means to confine the motion.
An embodiment permitting rotation about a single axis is illustrated in Figure 4A. A disc-shaped load 40 having a fixed axis 41, about which it is constrained to rotate, is held between two supports 42 on which a plurality of piezo legs 43 are mounted. The legs are arranged such that their polarising (i.e. movement) directions 45 are tangential to the contact plane and, in addition, tangential to the circle of rotation. Thus independent sliding of the piezo legs 43 over the load followed by simultaneous motion can be used to rotate the disc about its axis.
Figure 4B illustrates a variant of the previous embodiment to effect rotation about an axis. A plurality of piezo legs 46 are mounted so as to bear against the circumference of a disc 47 and arranged so as to rotate the disc about a fixed axis 48. The piezo legs 46 are spaced by an angle θ and are brought into contact with the disc 47 by spring plates 49 which also hold the disc in place. Only one spring plate 44 is shown for clarity. Figure 5 illustrates an adaptation of the above embodiment to effect rotation about a single point i.e. to extend the previous embodiment to three dimensions. The spherical motor illustrated shows a ball 50 held by four piezo legs 51. The legs are again sectioned piezo tubes or orthogonal shear plate stacks, and are held in contact with the ball by means of normal force F
resulting from means not shown in the Figure. Correct arrangement of the movement directions of the legs can be used to give rotation about any axis passing through the centre of the ball.
Figs. 7A and 7B show a circuit suitable for
producing driving voltages for the transducers. Anti-phase pulses synchronised with the mains and produced by comparators 70 and differentiators 71 are provided on lines A and B. Fig. 8 shows explanatory waveforms.
Output pulses may be provided continuously on line X or by switching switch 72 the output may be triggered following a trigger input at terminal 73. Output X is supplied to a chain of onostable multivibrators 74 which produces successive 100ns pulses to respective
transistors 75. The transistors drive respective pulse transformers 76 (Fig. 7B) which fire triacs 77 to drive the respective piezoelectric actuators. The trigger circuit includes a power-on reset circuit 78. The trigger signal may be produced using Schmitt trigger circuit 80 as shown in Fig. 8.
As can be seen, the present invention provides a piezo-electric friction step motor which allows precise step-wise motion of a load in a number of directions, with a well-defined and controllable step size, and smooth, continuous positioning of the load within a single step range. The maximum output force can be transferred to the load with no losses, and its magnitude is readily adjustable. The motor is a compact structure, which is rigidly held together when a load is in place, thus conferring high stability and removing the need for mechanical components or residual voltage to support the load or hold it still in air. This means that
construction of the motor is very simple and requires a minimum number of components. The invention also provides a motor for which the driving signal is not critical, thus further simplifying the construction "of the motor as simple electronics can be used. Finally, the motor can be operated in adverse environmental conditions, and is substantially unaffected by environmental changes.