Push-pull high force piezoelectric linear motor

[0002] 1. Field of the Invention

[0003] The present invention relates to linear electric motors and more particularly to a high force, step and repeat piezoelectric linear motor.

[0004] 2. Description of the Related Art

[0005] Many types of electrical motors have been developed that provide rotational force such as three-phase motors, induction motors, split-phase motors, etc. However, there is a need for motors that provide linear displacement for use in applications such as aircraft flaps, electrical powered sunroofs or electrically powered car seats. Some of the more heavy duty applications require a linear displacement force in excess of 220 Newtons.

[0006] Currently, ball and screw type motors are frequently used for producing linear displacement and comprise a threaded shaft and a metal ball with a threaded hole that mates with the threaded shaft. As the shaft turns, the metal ball rides up or down the shaft to linearly displace the metal ball and any attached mechanism. One problem with this motor is the high tolerance that is required between the ball and the shaft, particularly when used in high accuracy applications. Through use, the threads on the shaft and the ball tend to wear, resulting in a backlash when the motor is stopped or encounters a load, thus reducing positioning accuracy.

[0007] The ball and screw motor is powered by an electric motor which can overheat and be damaged or destroyed if the motor stalls under an excessive load. Also, electric motors generally work most efficiently at one speed; when the motor is slowed under load its efficiency drops.

[0008] Furthermore, the force produced by the shaft and ball displacement is directly related to the power of the electrical motor. For increased power, the size of the electrical motor must be increased. In applications requiring a great deal of linear displacement force, the size of the motor can become prohibitive. Ball and screw motors also tend to be relatively expensive.

[0009] Piezoelectric materials have been used for many different types of motors, primarily in motors that produce a rotation as opposed to displacement. As an example, see U.S. Pat. No. 5,780,956 to Oliver, Neurgaonkar, et.al. Certain piezoelectric materials are useful because of their ability to directly convert electrical energy into motion (mechanical energy). When a voltage is applied to the piezoelectric material, the material will experience a strain that causes it to expand. When the voltage is removed, the strain is removed and the material contracts.

[0010] Piezoelectric materials are generally formed from ceramics. One particularly valuable type of piezoelectric device has a plurality of laminated piezoelectric layers that can expand and contract quite rapidly, and combines the expansion of all the layers. The purpose of such layering is to keep the necessary drive voltage to a practical level, while obtaining significant expansion. The expansion can vary, but is generally on the order of 0.002 times the length of the layered piezoelectric material.

[0011] Linear piezoelectric motors have been developed using an “inchworm” piezoelectric mechanism to linearly translate a shaft. An example of this type of motor is the Burleigh PZ-577 Inchworm™ Translator System which comprises three piezoelectric cylinders coupled together on a shaft. One of the end cylinders is fixed to a support structure and the other cylinders are allowed to move linearly in relation to the fixed cylinder. The cylinders rely on an inchworm type motion to move the shaft. The first and third cylinders fit around the shaft with near zero clearance, while the middle cylinder has a clearance fit over the shaft. If the first cylinder were fixed, a voltage is applied to the first cylinder and it grips the shaft. A voltage is then applied to the middle cylinder causing it to expand longitudinally down the shaft, pushing the third cylinder ahead of it. A voltage is then applied to the third cylinder, causing it to grip the shaft. The voltage is next removed from the first cylinder causing it to release the shaft, and also from the middle cylinder, causing it to contract and pull the shaft with it. This inchworm cycle results in moving the shaft in the direction of the first cylinder and is repeated to move the shaft linearly.

[0012] The primary problem with inchworm type motors is that they typically provide linear push or pull forces in the range of 10 to 15 Newtons and cannot be used for heavy duty applications requiring a greater linear force. Such motors also require precise machining and are not easily adjusted for optimum performance

[0013] The present invention provides a improved linear pushpull motor that relies on the expansion characteristics of piezoelectric materials to produce linear displacement. The new motor employs a step and repeat action, using two clamps fixed on a motor base and a split motor shaft that travels within the clamps such that the clamps hold the shaft when closed.

[0014] The split motor shaft preferably comprises two shaft segments and a displacement actuator, with the two shaft segments coaxially aligned and attached to opposite ends of the displacement actuator. Each shaft segment travels within a respective clamp. The displacement actuator has a piezoelectric body that expands when a voltage is applied, with the body oriented such that the expansion is coincident with the axis of the two shaft segments. Expansion of the piezoelectric body results in an increase in the length of the split motor shaft.

[0015] Linear movement of the split motor shaft is produced by coordinating the opening and closing of the clamps with the expansion of the piezoelectric body. For example, the split motor shaft can be moved linearly by opening one of the clamps, activating the piezoelectric body to expand the split motor shaft in the direction of the open clamp, closing the open clamp, opening the other clamp, and deactivating the piezoelectric body. When the body is deactivated it contracts, pulling the shaft segment in the open clamp toward the clamped shaft. This cycle results in the split motor shaft moving in the direction of the originally opened clamp. It can be repeated to move the shaft in the same direction or reversed to reverse the shaft direction.

[0016] The new motor is less complicated than other linear motors while providing consistent force output over a wide range of speeds with precision and reliability. The new motor is relatively small, but can provide sufficient linear force to pull or push in excess of 220 Newtons. It does not have gears and does not experience gear related power loss, wear and backlash, nor is it overheated or damaged when slowed or stalled. Furthermore, a uniform force output is produced at any motor speed.

[0017] These and other further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which:

[0018] FIG. 1 is an elevation view of the new push-pull piezoelectric motor;

[0019] FIG. 2 is a series of simplified elevation views of the new motor showing different states of the motor during operation;

[0020] FIG. 3 is a plan view of a displacement actuator for the motor;

[0021] FIG. 4 is a sectional view of one embodiment of a clamp used in the motor;

[0022] FIG. 5 is a sectional view of an alternate clamp taken along section line 5—5 of FIG. 1;

[0023] FIG. 6 is a plan view of the clamp in FIG. 5; and

[0024] FIG. 7 is a block diagram and a timing diagram for an electrical control circuit for the new motor.

[0025] FIG. 1. shows one embodiment of the new high force push-pull piezoelectric motor 10 which consists of a split motor shaft 12 and two clamps 13aand 13bthat are fixed on a motor base 14. The split motor shaft has two shaft segments 15aand 15band an intervening displacement actuator 16, each shaft segment traveling within a respective clamp. The clamps 13aand 13bopen and close upon the respective shaft segment 15aand 15bto either allow the shaft segment to slide within the clamp, or to hold the shaft segment in place. The displacement actuator 16 is preferably implemented as a piezoelectric body 17 that expands when a voltage is applied and contracts when the voltage is removed. The expansion of the piezoelectric body 17 pushes the two shaft segments 15aand 15bfurther apart, lengthening the overall split motor shaft 12. In operation, the two clamps 13aand 13bopen and close upon the shaft segments 15aand 15bin cooperation with the expansion and contraction of the piezoelectric body 17 to cause linear motion of the split motor shaft 12.

[0026] FIG. 2 shows a series of simplified diagrams of the new motor 10 illustrating the different states of the motor during one of its cycles. Linear motion of the split motor shaft 12 is accomplished by a step-and-repeat sequence. When the motor is off, clamps 13aand 13bare closed on the respective shaft segments 15aand 15b, holding the split motor shaft 12 in place. In the first state 20, clamp 13bis opened. In the second state 22, the piezoelectric body 17 is energized and expands moving shaft segment 15bto the right. In the third state 24, with the piezoelectric body 17 still energized, clamp 13bis closed and clamp 13ais opened. In the fourth state 26, the voltage is removed from the piezoelectric body 17, causing the piezoelectric body 17 to contract and pull the shaft segment 15ato the right. In the fifth state 28, clamp 13ais closed, completing the cycle.

[0027] Repeating the cycle causes linear motion of the split motor shaft 12 at a speed determined by the cycling frequency and the length of expansion of the piezoelectric body 17. The total travel of the split motor shaft is limited to the travel distance of the piezoelectric body between the two fixed clamps 13aand 13b. The direction of motion is controlled by the timing sequence of the two clamps 13aand 13b. By reversing the timing sequence of FIG. 2, the direction of travel of the split motor shaft 12 can be reversed; this reversal can be accomplished within one cycle. The new motor 10 can either push a load at the end of one of the shaft segments 15aor 15bor pull a load attached to one of the shaft segments.

[0028] FIG. 3 shows an embodiment of displacement actuator 16 in which a piezoelectric body 17 is held between two thrust plates 34aand 34bby a pair of compression screws 36 an 37. The displacement actuator 16 relies on the expansion properties of the piezoelectric body 17 to separate and restore the positions of the shaft segments 15aand 15brelative to each other. Many types of piezoelectric bodies can be used but it is preferably formed from layers of piezoelectric ceramic that are laminated together. The layer thickness typically ranges from 0.1 to 0.4 millimeters, with corresponding operating voltages of 200 to 800 volts. The layers are laminated with metal foil electrodes using epoxy bonding. When a voltage is applied, the overall expansion of the piezoelectric body 17 is the accumulation of the expansion of its individual layers. Many types of piezoelectric ceramics can be used but the preferred embodiment uses layers of high strain Nb-doped PLZT piezoelectric ceramic as disclosed and described in U.S. Pat. No. 5,595,677 to Neurgaonkar, Oliver and Nelson and assigned to the same assignee as the present application.

[0029] The piezoelectric body 17 is oriented such that its expansion axis is coincident with the axis of the two shaft segments 15aand 15b. The two thrust plates 34aand 34bare attached to respective shaft segment 15aand 15bby set screws, but could also be threaded or welded to the shaft segments. The piezoelectric body 17 is held between the thrust plates 34aand 34b. The body 17 is not fixed to the thrust plates but instead is held under compression between the thrust plates 34aand 34bby compression springs 38 and 39 and compression screws 36 and 37. Different compression springs can be used but in the preferred motor the compression springs are commercially available Belleville springs.

[0030] The compression screws 36 and 37 pass through openings in thrust plate 34band are threaded into thrust plate 34awith the springs 38 and 39 lodged between the screw heads and thrust plate 34b. The compression screws 36 and 37 are tightened until the piezoelectric body 17 is held between the thrust plates 34aand 34b, while leaving compression play in the compression springs 38 and 39. When the piezoelectric body 17 is energized it expands against the compression springs, lengthening the split motor shaft 12. The split motor shaft 12 functions as a single shaft whose overall length is varied by the action of the piezoelectric body 17.

[0031] By holding the piezoelectric body 17 under compression, the new motor 10 provides the unique ability to operate in either push or pull mode with equal force and reliability. In the push or pull mode of operation a load is attached to one of the shaft segments, in this example segment 15b. To push the load to the right, the clamp on shaft segment 15ais closed and piezoelectric body 17 is expanded. The expansion of the body 17 compresses springs 38 and 39 and increases the length of the split motor shaft 12, pushing the shaft segment 15band its attached load to the right a distance equal to the expansion of the body 17. To pull the load attached to shaft segment 15bto the left, the piezoelectric body 17 is contracted from and expanded state and the clamp on shaft segment 15ais held closed while the clamp on segment 15bis open. This action pulls shaft segment 15band its attached load to the left toward thrust plate 34a. The above cycles can be repeated to further push or pull the load.

[0032] Layered and laminated piezoelectric bodies tend to delaminate if they are placed under excessive tension. If the layered piezoelectric body were simply attached directly to the shaft segments without springs 38 and 39, the tension imposed by the load in the pull mode would be experienced by the layered body, resulting in possible delamination of the layers. By keeping the piezoelectric body 17 under compression (and not attaching it to the thrust plates 34aand 34b) and allowing the compression springs to pull the load, equal reliability is provided in both modes by preventing pull mode delamination.

[0033] Furthermore, since the piezoelectric body 17 is held securely in position by the thrust plates 34aand 34b, the compression screws 36 and 37 and the compression springs 38 and 39, it does not require a permanent bond to the thrust plates and shaft segments. The installation, removal and/or replacement of the piezoelectric body 17 is easily accomplished by loosening the compression screws 36 an 37 to remove the piezoelectric body 17, and tightening the screws on the piezoelectric body 17 when it is replaced.

[0034] Many different clamps 13aand 13bcan be used in the new motor. The push/pull force of the motor is limited by the ability of the clamps 13aand 13bto hold the shaft segments 15aand 15b. The maximum expansion force of the piezoelectric body 17 normally exceeds the holding force of the clamps. For example, a piezoelectric body can commonly produce an expansion force in excess of 1000 Newtons while the clamps 13aand 13bmay be able to hold the shafts up to loads on the order of only 250 Newtons. Thus the motor's maximum load capability is limited to the clamps' holding capacity. If the shaft segments slip within the closed clamps, the motor will stall.

[0035] Another important feature of the clamps 13aand 13bis the frequency at which the clamp can reliably open and close. If the clamp is made of a material that is susceptible to flexing, it may experience bouncing at high frequencies which reduces the clamp closing force and holding force. To avoid bouncing, the clamp material should be stiff. However, for a high operation frequency, the mass of the clamp material should be low. Accordingly, the clamp material should have low mass and high stiffness, thereby minimizing clamp flexing while allowing for high frequency operation.

[0036] FIG. 4 shows one embodiment of the clamps 13aand 13b. Each clamp has a layered piezoeletric body 41 similar to the body 17 used in the displacement actuator 16. The piezoelectric 41 is housed within the motor base 14 and extends out a sufficient distance to engage a clamp arm 42. One end of the clamp arm 42 serves as a hinge 42a, while the other end 42bclamps on one of the shaft segments 15aor 15b, the upper portion of which are cut to mate with the lower clamp edge. When a voltage is applied to the piezoelectric body 41 it expands and provides a vertical lifting force to open the clamp arm 42. Clamp arm 42 can be made of various materials, but is preferred made of titanium. Titanium has a low mass and high stiffness, which allows for a high clamping force and low clamp arm flexing at high frequency. In addition, the clamp arm 42 can have a hole 43 or other cutout that further reduces the its mass, provided the hole does not increase clamp arm flexing

[0037] The clamp arm 42 is mounted on the motor base 14 by a clamp screw 44 which passes through the clamp arm 42 and is treaded into the motor base 14. A compression spring 45 is held between the screw head 44 and the upper surface of clamp arm 42. Different springs can be used such as Belleville springs. The compression spring 45 maintains a downward force on the clamp arm 42 holding it against the upper end of the piezoelectric when the clamp is open and against the shaft segment 15a/15bwhen the clamp is closed; the play in the spring 45 allowing upward movement of the clamp arm 42. The clamp screw 44 is also used to adjust the clamping force of the clamp arm 42; the tighter the screw the greater the clamping force. However, if the screw 44 is too tight, the compression spring 45 can become fully compressed and/or the upward force of the piezoelectric 41 may not be sufficient to overcome the downward force of the screw 44. In either case, the piezoelectric 41 will be unable to open the clamp arm 42.

[0038] The clamp 13a/13bhas a hinge which is made of a steel dowel pin 46 lodged within a notch 47aat the hinge end 42aof the clamp and an aligned notch 47bon the motor base. The clamp arm 42 rotates up about the hinge to open when the piezoelectric body 41 is expanded. An adjustment screw 47 is threaded though the clamp arm 42 above the piezoelectric body 41 and engages the body 41. The lower end of the screw 47 serves as the clamp arm's point of contact with the piezoelectric 41. By adjusting the screw 47, the closing point of the clamp arm 42 can be changed. For example, as the adjustment screw 47 is rotated to move it further toward the piezoelectric body 41, the gap between the motor base 14 and the shaft end 42bis increased. By adjusting the screw, the clamping force can be adjusted

[0039] The motor base 14 has a slot 48 within which one of the shaft segments 15aand 15brests, the shaft segment sliding within the slot during operation. When used with the clamp shown in FIG. 4, the shaft segments 15aand 15bhave a flat surface 49 that serves two purposes. First, the flat surface allows for greater contact area with the clamp arm 42, resulting in reduced deformation of the shaft when the clamp is closed. Second, the flat surface prevents rotation of the shaft segment within the clamp during operation. As shown in FIG. 1, if cylindrical shaft segments are used, anti-rotation pins 18 extending from the thrust plates are provided to mate with a longitudinal slot 19 in the motor base 14 to prevent rotation.

[0040] During operation, a voltage is applied to the piezoelectric body 41, causing it to expand and rotate the clamp arm 42 about hinge 46, against the force of the compression spring 45. The hinged clamp 42 provides mechanical amplification of the piezoelectric 41 expansion at the shaft end 42bof the arm. As the clamp arm 42 rotates up, its shaft end 42bopens away from the motor base 14 and the clamping force on the shaft segment 15aor 15bis removed. The shaft segment 15aor 15bis allowed to slide in the motor base slot 48.

[0041] FIG. 5 shows a second embodiment of the clamp 13a/13bwhich also uses a layered piezoelectric body 51 similar to the piezoelectric bodies 17 and 41. The piezoelectric 51 is housed within the motor base 14 and is connected to a yoke 1252 which has a slot 53 for a shaft segment 15aor 15b. A floating clamp 54 provides a downward force on the shaft segment 15aor 15band has a recess 55 that provides the contact with the shaft segment 15aor 15b. The slot 53 is larger than the diameter of the shaft segment 15aor 15b, and the segment slides in the slot 53 when downward pressure is removed.

[0042] Two spring adjustment screws 56aand 56battach the yoke 52 and floating clamp 54 to the motor base 14. The screws pass through the yoke 52 and floating clamp 54, with the ends of the screws threaded for insertion into corresponding threaded holes in the motor base 14. The remainder of the screw shafts are smooth to allow the yoke 52 and floating clamp 54 to move up and down the screws with expansion and contraction of the actuator 51. Compression springs 57aand 57bare held between the heads of screws 56aand 56brespectively, and the floating clamp 54. The compression springs 57aand 57bmaintain a downward force on the floating clamp and yoke, while still allowing them to move upward. The downward force of the compression springs 57aand 57bforce the floating clamp down on the shaft segment 15aor 15b, to hold it in place between the floating clamp 54 and bushings (shown in FIG. 6).

[0043] A clamp adjustment screw 58 is threaded through the motor base 14 opposite the yoke 52. The adjustment screw 51 engages the piezoelectric body 51 and is used to adjust the starting and finishing points for the expansion of the body 51. As the screw 58 in rotated to move against the piezoelectric 51, its expansion range will be moved toward the shaft segment 15aor 15b. By adjusting the screw 58, the clamping force and range can be adjusted.

[0044] In operation, a voltage is applied to the piezoelectric body 51, lifting the yoke 52 and the floating clamp 54 against the compression springs 57aand 57b. The downward pressure of the floating clamp 54 is removed from the shaft segment 15aor 15b, allowing it to slide within the slot 53.

[0045] As shown in FIG. 6, the clamp of FIG. 5 includes bushing 59aand 59bon the sides of the floating clamp 54. The bushings provide a guide for the shaft segments 15aand 15bwhen the clamp is open and the shaft segment is sliding. In the preferred embodiment, each bushing 59aand 59bis a metal block fixed to the motor base adjacent the yoke and floating clamp, with the bushings having respective shaft openings 60 and 61. In operation, a shaft segment 15aor 15bpasses through the bushings 59aand 59ballowing the yoke and floating clamp 54 to act upon it. When the piezoelectric is contracted, the downward force of the floating clamp 54 holds the shaft segment between the floating clamp 54 and the bushings 59aand 59b. When the clamp is opened, pressure from the floating clamp is removed from the shaft segment 15aor 15b, which is allowed to slide within the bushing holes.

[0046] The control voltages to energize the piezoelectric bodies 17,41 and 51 can be produced by many different circuits. FIG. 7. is a block diagram one such circuit and also shows the waveform that is produced to excite each of the piezoelectric bodies. Each of the waveforms is generated from a clock signal that is delayed for a predetermined amount of time by one or more commercially available dual 555 timers such as the LM 556 timer by National Semiconductor. The purpose of these timing delays is to account for the finite time required to energize and de-energize the piezoelectric bodies.

[0047] For clamp 13athe output of a clock 71 is connected to the input of a delay circuit 72, the output of which transitions from LO to HI after delay period τ_Cfollowing the positive transition of a clock signal. The transition from HI to LO is not delayed. For clamp 13b, the output of clock 71 is inverted and connected to delay circuit 73, the output of which transitions from LO to HI when a negative clock transition is received after one-half clock cycle, following the same delay period τ_C; the output of buffer circuit 77 goes LO at the next positive transition of the clock pulse. Only one of the waveforms 79 and 80 for clamps 13aand 13bwill be HI at one time. The respective waveforms 79 and 80 are output from respective buffers 76 and 77 to power amplifiers which are connected their respective piezoelectric bodies in clamps 13aand 13b.

[0048] The control signal for the split shaft piezoelectric 17 is generated with two delay circuits 74 and 75. The first delay circuit 74 provides a delay τ_D(greater than τ_C)in the LO to HI transition response to the clock signal, while the second delay circuit 75 is connected to buffer 78 for delay circuit 74 to provide a delay of τ_Din the transition from HI back to LO when the positive clock pulse has passed. This results in a waveform 81 that transitions from LO to HI after clamp 13aopens, and then transitions from HI to LO after clamp 13acloses and actuator 13bopens. The waveform is connected to a power amplifier and then to the split shaft piezoelectric 16.

[0049] When the motor is off, both clamps 13aand 13bare preferably closed, thereby holding the split motor shaft 12 and preventing movement. By closing both clamps the blocking or holding force of the motor when off is at least twice that of its single-clamp stall force during operation. Manual manipulation of the motor position can by accomplished by electrically activating and opening both clamps 13aand 13b. The split motor shaft can then be freely moved by hand to the desired position. Alternatively clamps 13aand 13bcan be normally open when the motor off, thereby permitting manual movement of the motor without an electrical input.

[0050] The expansion force of each of the piezoelectric bodies 17, 41 and 51 is related to its cross section, the larger the cross section the greater the expansion force. However, as the diameter increases the ability of the piezoelectric to dissipate heat decreases. As the frequency of expansion and contraction increases, larger diameter piezoelectrics can overheat. Accordingly, an optimal size should be chosen for the particular motor to provide the desired displacement force and motor speed. In one embodiment the piezoelectric body has a 1 cm (0.4 inch) diameter and is 2 cm (0.8 inch) long, with the frequency of the motor ranging from 0 to 1000 hertz (Hz). At 500 Hz the displacement speed is 2 cm per second.

[0051] The length of expansion or strain of each piezoelectric body when a voltage is applied is the aggregate expansion of its constituent layers. Typically, the expansion is about 0.002 times the length of the piezoelectric body at maximum voltage.

[0052] Below the stall force, the motor exhibits a linear speed vs. load characteristic (the load line) which is governed by the force capability and the mechanical stiffness of its piezoelectric drive. If the motor begins to experience slippage due to excess loading, its speed drops until the longitudinal displacement stops. Practical operating load capacities in excess of 220 Newtons have been achieved, with a potential load capacity in excess of 440 Newtons. The upper limit is determined by the attainable clamping forces and flexing in the clamps and the motor base.

[0053] Although the present invention has been described in considerable detail with reference to certain specific implementations, other versions are possible. For instance, the motor can have more than two clamps. The split motor shaft can have more than two shaft segments with more than one displacement actuator, and the displacement actuator can have more than one piezoelectric body. The displacement actuator can also have additional compression screws and springs, which can be mounted in different ways. Therefore, the spirit and scope of the appended claims should not be limited to their specific versions.