Actuator and actuator driving method

Abstract

To provide an actuator and an actuator driving method which can be widely applied to various fields.SOLUTION: A platform with magnetic properties is moved in one direction with a driving force caused by extension of a piezoelectric element while reliably holding the platform with an electromagnetic force of a first electric magnet. Thereafter, the electromagnetic force of the first electric magnet is cancelled, and at the same time, an electromagnetic force of a second electric magnet fixed to a base member is generated so that while fixing the platform, the piezoelectric element is contracted and returned to an initial position. By repeating this basic operation, the platform can be linearly moved relative to the base member using an amount of extension of the piezoelectric as a unit of movement.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an actuator and an actuator driving method, and particularly to the development of a precision linear actuator having high resolution, high stability, low friction and wear, and a large driving force / holding force.

Background Art

[0002] Conventionally, techniques related to actuators that generate linear motion using an inchworm mechanism are known (see, for example, Patent Document 1). Also, techniques for generating linear motion by a walking mechanism using a piezoelectric actuator by walking are known (see, for example, Patent Document 2).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the case of the above prior art, since the movement of the piezoelectric element is transmitted to the actuator by friction, slippage occurs and the resolution decreases. In addition, wear of the friction surface progresses, and it is difficult to operate stably for a long period of time.

[0005] An object of the present invention is to provide an actuator that can be widely applied to various fields and a method for driving the actuator.

Means for Solving the Problems

[0006] To solve the above problems, the invention according to claim 1 is an actuator, comprising: A magnetic platform configured to move freely in linear or rotational motion on a base member, A piezoelectric element drive unit comprising a first electromagnetic attachment / detachment mechanism capable of holding / releasing the platform, A second electromagnetic attachment / detachment mechanism is configured to allow the platform to be held / released from the base member, A linear encoder that functions as a position detection sensor for the platform, A capacitance sensor that functions as a displacement sensor for detecting the amount of displacement of the piezoelectric element drive unit, Control for driving the piezoelectric element drive unit and controlling the timing of holding / releasing the first electromagnetic attachment / detachment mechanism and the second electromagnetic attachment / detachment mechanism. means and, the law of nature, The control means is capable of performing a first process that controls the driving of the piezoelectric element drive unit and the timing of holding / releasing the first electromagnetic attachment / detachment mechanism and the second electromagnetic attachment / detachment mechanism in conjunction with the position signal from the linear encoder, and a second process that controls the amount of drive of the piezoelectric element drive unit in conjunction with the displacement signal from the capacitance sensor. The control described in the first process is executed until the platform approaches the target position, and the control described in the second process is executed once the platform approaches the target position. It is characterized by the following:

[0007] The invention described in claim 2 is the actuator described in claim 1, The piezoelectric element drive unit is characterized by being configured to reciprocate linearly in accordance with the expansion and contraction of the piezoelectric element.

[0010] Claim 3 The invention described is an actuator driving method, A magnetic platform configured to move freely in linear or rotational motion on a base member, A piezoelectric element drive unit comprising a first electromagnetic attachment / detachment mechanism capable of holding / releasing the platform, A second electromagnetic attachment / detachment mechanism is configured to allow the platform to be held / released from the base member, A linear encoder that functions as a position detection sensor for the platform, A capacitance sensor that functions as a displacement sensor for detecting the amount of displacement of the piezoelectric element drive unit, Control for driving the piezoelectric element drive unit and controlling the timing of holding / releasing the first electromagnetic attachment / detachment mechanism and the second electromagnetic attachment / detachment mechanism. means and, the law of nature, The control means is capable of performing a first process that controls the driving of the piezoelectric element drive unit and the timing of holding / releasing the first electromagnetic attachment / detachment mechanism and the second electromagnetic attachment / detachment mechanism in conjunction with the position signal from the linear encoder, and a second process that controls the amount of drive of the piezoelectric element drive unit in conjunction with the displacement signal from the capacitance sensor. The control described in the first process is executed until the platform approaches the target position, and the control described in the second process is executed once the platform approaches the target position. In actuators, A first step of linearly moving the piezoelectric element driving unit in a certain direction while holding the platform by the first electromagnetic attachment / detachment mechanism; A second step of linearly moving the piezoelectric element driving unit in the opposite direction by releasing the first electromagnetic attachment / detachment mechanism while holding the platform by the second electromagnetic attachment / detachment mechanism following the first step; The method is composed of these steps. By repeating the first step and the second step, the platform is linearly moved or rotated.

[0011] Claim 4 The invention according to claim 3 In the actuator driving method according to claim The position of the platform is detected by a position detection sensor, and the displacement amount or displacement speed of the piezoelectric element driving unit in the first step is controlled in conjunction with the position signal.

[0013] In the present invention, while securely holding a magnetic platform by the electromagnetic force of a first electromagnet, it is moved in one direction by the driving force due to the elongation of a piezoelectric element, and then, while releasing the electromagnetic force of the first electromagnet, the electromagnetic force of a second electromagnet fixed to a base member is generated to fix the platform while contracting the piezoelectric element to return it to the initial position as the basic operation. By repeating this basic operation, the platform can be linearly moved with respect to the base member with the elongation amount of the piezoelectric element as the movement unit. Since the driving force of the piezoelectric element is reliably transmitted to the platform by the electromagnetic holding force, it is possible to control the movement amount according to the precise elongation amount of the piezoelectric element. Also, since wear between transmission surfaces is significantly reduced, long-term stable operation is possible.

Effect of the Invention

[0014] According to the present invention, it is possible to provide an actuator that has precise positioning, large thrust and stroke, and is widely applicable to technical fields that require long-term operation stability.

Brief Description of the Drawings

[0015] [Figure 1] It is a detailed view of an X stage and an electromagnet according to a first embodiment of the present invention. [Figure 2] The actuator according to the first embodiment of the present invention is shown, which is a plan view (a), a front view (b), and a view (c) seen from the right side of the paper. [Figure 3] It is a process diagram of the left linear motion according to the first embodiment of the present invention. [Figure 4] It is a process diagram of the right linear motion according to the first embodiment of the present invention. [Figure 5] It is a time chart regarding the applied voltages of the piezoelectric element and the electromagnet according to FIG. 3. [Figure 6] It is a control flowchart according to the present invention. [Figure 7] The actuator according to the second embodiment of the present invention is shown, which is a plan view (a) and a view (b) seen from the lower side of the paper. [Figure 8] The bidirectional X stage according to the third embodiment of the present invention is shown, which is a front view (a) and an A-A cross-sectional view (b) of FIG. 8(a). [Figure 9] The actuator according to the third embodiment of the present invention is shown, which is a plan view (a) and a view (b) seen from the right side of the paper.

Modes for Carrying Out the Invention

[0016] FIGS. 1 and 2 show the first embodiment. FIG. 1 shows an X stage (piezoelectric actuator) 1 provided with a feedback capacitance sensor 2. An end piece 4 is adhered to one end of the piezoelectric element 3, and both ends of the piezoelectric element 3 are incorporated in contact with each other within a frame 6 that can be elastically deformed at the bending portion 5. The pressing force of the bolts 7 screwed into the frame 6 preloads the piezoelectric element 3 via the end piece 4. When a voltage is applied to the piezoelectric element 3 and it expands, a strong pressing force is generated on the frame 6 according to the amount of expansion, causing the upper end surface 6a of the frame to be displaced in the direction of expansion of the piezoelectric element 3, i.e., to the left of the plane of the paper. Furthermore, when the voltage of the piezoelectric element 3 decreases or is released and the piezoelectric element contracts, the upper end surface 6a of the frame returns to the rightward direction in the plane of the paper accordingly. The displacement of the upper end surface 6a of the frame is measured using the capacitance sensor 2, and by controlling it to achieve the desired displacement, displacement control with nanometer resolution is possible. Furthermore, an electromagnet 8 is fixed to the upper end surface 6a of the frame, and when the X-stage is displaced, the electromagnet 8 is also displaced in the direction of displacement. The electromagnetic force of the electromagnet 8 acts on the upper end surface 6a of the frame and the opposite surface, and the electromagnetic force can be applied via a screw 9 that is magnetic and can be adjusted in position.

[0017] Figure 2 is an overall view of the first embodiment. Two X-stages 1a and 1b are fixed to the base 10. Electromagnets 8a and 8b are fixed to X-stages 1a and 1b, respectively. The platform 11 is attached to the guide rail 12 so as to be able to move linearly in the left-right direction of the paper, and the guide rail 12 is fixed to the base 10. The platform 11 is configured to be held and released by the electromagnetic force of electromagnets 8a and 8b via screws 9a and 9b, respectively. 2a and 2b are capacitive sensors that measure the displacement of the X-stage. 13 is a linear encoder used as a position detection sensor for platform 11, which measures the real-time position of platform 11 and transmits it to the controller. The detailed position control flow using these linear encoders and capacitive sensors will be described later.

[0018] Next, the process of moving the platform 11 in a linear motion to the left of the paper in Figure 2 using the actuator configured above will be explained step by step with reference to Figure 3. In step (1), a voltage is applied to the electromagnet 8b to hold the magnetic platform 11 by electromagnetic force. In step (2), a voltage is applied to the electromagnet 8a to hold the platform 11, while the voltage to the electromagnet 8b is released to release the hold. In step (3), a desired voltage is applied to the piezoelectric element 3a, and by extending it by an amount of displacement corresponding to the voltage, the X-stage 1a is displaced in the direction of extension of the piezoelectric element 3a (to the left on the page). At this time, since the electromagnet 8a is holding the platform 11, the platform 11 can be displaced to the left by the amount of displacement of the X-stage 1a. In step (4), the electromagnet 8b is activated to maintain the displacement position of the platform 11. In this state, the voltage between the electromagnet 8a and the piezoelectric element 3a is released, and the X-stage 1a is returned to its original position. By repeating this process, the platform 11 can be moved in a linear motion in the direction of extension of the piezoelectric element 3a, i.e., to the left.

[0019] Figure 5 shows a time chart relating to the switching of voltages applied to electromagnets 8a and 8b and piezoelectric element 3a in steps (1) to (4) described above. Td1 is the cycle time of process (1) to process (4), Td2 is the operating time of the electromagnet 8a, Td3 is the time difference from when the electromagnet 8a is activated until the piezoelectric element 3a is activated, and Td4 is the operating time of the piezoelectric element 3a. By simultaneously switching the operating timing of electromagnets 8a and 8b, the platform 11 is held in place by either electromagnet. Furthermore, by setting the time Td3 to the time it takes for a certain electromagnetic force to be secured by the operation of electromagnet 8a, it is expected that the displacement of the X-stage 1a will be transmitted more reliably without the electromagnet 8a and platform 11 slipping. The voltages applied to electromagnets 8a and 8b are set appropriately to produce the required electromagnetic force. Furthermore, the actuator's motion speed and operating accuracy can be changed by adjusting the voltage and frequency applied to the piezoelectric element.

[0020] The process of moving platform 11 in a linear motion to the right of the paper in Figure 2 will be explained step by step using Figure 4. In step (1), a voltage is applied to the electromagnet 8a to hold the magnetic platform 11 by electromagnetic force. In step (2), a voltage is applied to the electromagnet 8b to hold the platform 11, while the voltage to the electromagnet 8a is released to release the hold. In step (3), a desired voltage is applied to the piezoelectric element 3b, and by extending it by an amount of displacement corresponding to the voltage, the X-stage 1b is displaced in the direction of the piezoelectric element 3b's extension (to the right on the page). At this time, since the electromagnet 8b is holding the platform 11, the platform 11 can be displaced to the right by the amount of displacement of the X-stage 1b. In step (4), the electromagnet 8a is activated to maintain the displacement position of the platform 11. In this state, the voltage between the electromagnet 8b and the piezoelectric element 3b is released, and the X-stage 1b is returned to its original position. By repeating this process, the platform 11 can be moved in a straight line in the direction of extension of the piezoelectric element 3b, i.e., to the right.

[0021] The time chart for switching the voltage applied to electromagnets 8a and 8b and piezoelectric element 3b in steps (1) to (4) as explained in Figure 4 can be set according to Figure 5, so a detailed explanation is omitted.

[0022] Based on the actuator operation described above, a method for high-resolution position control will be explained using the control flowchart in Figure 6.

[0023] The system controller 14 receives the current position signal of the platform 11 from the linear encoder 13 via the linear encoder amplifier 15, calculates the difference from the target position, and outputs drive signals for the X stages 1a and 1b to the servo controller 16. At the same time, it outputs attachment / detachment operation signals for the electromagnets 8a and 8b to the electromagnet driver 17 in synchronization with the drive signals for the X stages 1a and 1b. The drive signal from the servo controller 16 is input to the driver 18, which controls the required voltage and timing for the piezoelectric element 3a or 3b of the X-stage 1a or 1b. Furthermore, the electromagnet driver 17 receives the aforementioned attachment / detachment operation signal and controls the voltage and timing for attachment / detachment operations on the electromagnets 8a and 8b. If the platform is far from the target position, increasing the amount of movement of the piezoelectric element or increasing the extension / retraction speed can shorten the time it takes for the platform to approach the target position. Also, once the platform is close to the target position, decreasing the amount of movement of the piezoelectric element and slowing down the extension / retraction speed can improve the positioning accuracy of the platform relative to the target position. This position control system of platform 11 allows for high-precision, closed-loop control of the actuator to the target position based on the position signal from the linear encoder 13.

[0024] This section describes the control flow for further improving position control accuracy. After the platform 11 approaches the target position based on the position signal from the linear encoder 13, position control switches to the X-stage displacement signal from the capacitive sensor 2a or 2b. Specifically, the displacement signal of the X-stage 1a or 1b detected by the capacitance sensor 2a or 2b is input to the servo controller 16, and based on this position signal, a drive signal to the target position is output to the driver 18, and closed-loop control is performed in which the subsequent X-stage displacement is detected again by the capacitance sensor. For example, if the difference from the target position is about 20 μm, the high detection accuracy of the capacitance sensor allows the platform to be positioned at the target position with nanometer resolution. Furthermore, the control conditions for the system controller 14 can be pre-configured from a PC.

[0025] Figure 7 shows a second embodiment, which is a rotating stage capable of vertical movement. This actuator is capable of being driven in the vertical direction and has the advantage of being able to maintain its vertical stopping position even when the holding force by the electromagnet is released.

[0026] The lower end of the male screw shaft 21 is fixed on the base 20, and a female thread, which is located at the center of the magnetic rotating body 22, is screwed onto the male screw shaft 21, allowing it to move up and down. The X-stages 23a and 23b have a configuration similar to that shown in Figure 1, with the built-in piezoelectric elements 24a and 24b positioned opposite each other with their extension directions parallel and opposite to each other around the rotating body 22, and are fixed onto the base 20 via the support 25. Electromagnets 26a and 26b are fixed to the upper end faces 23a-1 and 23b-1 of the X-stages 23a and 23b, respectively, and magnetic screws 27a and 27b are attached to allow for fine adjustment of their tip positions. The electromagnet 26a is positioned such that when a voltage is applied to it, an electromagnetic force is generated, which holds the rotating body 22 via the screw 27a, and when the voltage is released, the holding is released. Similarly, the electromagnet 26b is positioned such that when a voltage is applied to it and an electromagnetic force is generated, it holds the rotating body 22 via the screw 27b, and when the voltage is released, the holding is released. Furthermore, the X-stage is equipped with capacitive sensors 28a and 28b, which measure the displacement of the X-stage and transmit the data to the controller.

[0027] First, a voltage is applied to the electromagnet 26b to hold the rotating body 22 in place by electromagnetic force. Subsequently, a voltage is applied to electromagnet 26a to hold the rotating body, while the voltage to electromagnet 26b is released to release the holding. Subsequently, a desired voltage is applied to the piezoelectric element 24a, causing it to extend downwards in Figure 7(a), thereby displacing the X-stage 23a. At this time, since the rotating body 22 is held in place by the electromagnetic force of the electromagnet 26a, the rotating body 22 can be rotated by a clockwise rotation angle corresponding to the displacement of the X-stage 23a. Next, a voltage is applied to the electromagnet 26b to hold the rotating body 22. In this state, the voltage between the electromagnet 26a and the piezoelectric element 24a is released, and the X-stage 23a is returned to its original position. By repeating this process, the rotating body 22 can be rotated clockwise in the direction of extension of the piezoelectric element 24a.

[0028] In the second embodiment, the process diagram for driving the platform and the time chart for the applied voltages to the piezoelectric element and electromagnet described in the first embodiment can be implemented in the same manner. Similarly, high-resolution control using position detection sensors can also be implemented. Furthermore, even if the voltage to electromagnets 26a and 26b is released, the weight of the rotating body 22 is converted into a horizontal component force on the male thread portion of the male screw shaft and a static frictional force on the thread portion, so the vertical position of the rotating body 22 can be maintained.

[0029] When rotating the rotating body 22 in a counterclockwise direction, the X-stage 23b is activated to generate a displacement direction opposite to the displacement direction of the X-stage 23a. By applying a desired voltage to the piezoelectric element 24b and extending it upward in Figure 7(a) by an amount of displacement corresponding to the voltage, the X-stage 23b is displaced in the direction of extension of the piezoelectric element 24b. In this way, the controller can switch the voltage between the piezoelectric elements of the two X-stages, thereby switching the rotational motion direction of the rotating body 22.

[0030] Figures 8 and 9 show a third embodiment. This actuator is capable of linear motion using a single X-stage that can be driven in both directions, and since the actuator can be constructed with fewer components than in the first and second embodiments, it has the advantage of providing a more compact actuator.

[0031] Figure 8 shows the bidirectional X-stage 30. The movable portion 31a of the bidirectional X-stage 30 is supported by the frame 31 in a parallel spring structure by elastically deformable portions 31b, which have a thin shape at their four corners. Furthermore, piezoelectric elements 32a and 32b are positioned opposite each other at both the left and right ends of the movable part 31a. The preload screws 33a and 33b generate a pressing force to bring the piezoelectric elements 32a and 32b into close contact with the moving part 31a, respectively. When an upward voltage is applied to the piezoelectric element 32b to extend it, and a downward voltage is applied to the piezoelectric element 32a in synchronization with this, causing the piezoelectric element 32a to contract to an amount slightly greater than the amount of extension of the piezoelectric element 32b, the moving part 31a is elastically displaced to the left of the plane of the paper due to the extension of the piezoelectric element 32b. On the other hand, when a downward voltage is applied to the piezoelectric element 32b in sync with the upward voltage applied to the piezoelectric element 32a, causing the piezoelectric element 32b to contract to a degree slightly greater than the expansion of the piezoelectric element 32a, the moving portion 31a is elastically displaced to the right in the plane of the paper. The voltage controller for the piezoelectric element can be implemented in accordance with the method described in the first embodiment.

[0032] Figure 9 is an overall view of the third embodiment. A magnetic platform 34 is attached to a guide rail 35 so as to be able to move in a straight line, and the guide rail 35 is fixed to a base 36. The aforementioned bidirectional X-stage 30 is fixed to the base 36. Furthermore, an electromagnet 37 is fixed to the moving portion 31a of the bidirectional X-stage 30. The electromagnet 37 is positioned so that when a voltage is applied to it and an electromagnetic force is generated, it holds the platform 34, and when the voltage is released, the holding is released. Additionally, an electromagnet 38 is fixed to the base 36 via a support 39. The electromagnet 38 is positioned so that when a voltage is applied to it and an electromagnetic force is generated, it holds the platform 34, and when the voltage is released, the holding is released.

[0033] First, the procedure for moving the actuator of the third embodiment to the left of the paper will be described. First, the platform 34 is held in place by electromagnetic force by applying a voltage to the electromagnet 38. Subsequently, voltage is applied to the electromagnet 37 to hold the platform 34, while simultaneously releasing the voltage to the electromagnet 38 to release the hold. Next, an upward voltage is applied to the piezoelectric element 32b to extend it, and simultaneously, a downward voltage is applied to the piezoelectric element 32a to contract it so that the amount of contraction is slightly greater than the amount of extension of the piezoelectric element 32b. As a result, the moving part 31a is elastically displaced to the left of the plane of the paper due to the extension of the piezoelectric element 32b. At this time, since the electromagnet 37 is holding the platform 34, it can displace the platform 34 by the amount of displacement of the bidirectional X-stage 30. Next, the electromagnet 38 is activated to maintain the displacement position of the platform 34. In this state, the voltage between the electromagnet 37 and the piezoelectric element 32a is released, and the bidirectional X-stage 30 is returned to its original position. By repeating this process, the platform 34 can be moved in a linear motion in the direction of extension of the piezoelectric element 32b, i.e., to the left of the plane of the paper.

[0034] When moving platform 34 in a linear motion to the right in the plane of the paper, applying a rising voltage to piezoelectric element 32a and simultaneously applying a falling voltage to piezoelectric element 32b, thereby contracting piezoelectric element 32b to a contraction amount slightly greater than the extension amount of piezoelectric element 32a, causes the moving part 31a to elastically displace to the right in the plane of the paper. By controlling the voltages of piezoelectric elements 32a and 32b using the controller based on the above procedure, the direction of movement of platform 34 can be switched between forward and backward.

[0035] In the third embodiment, the process diagram for driving the platform and the time chart for the applied voltages to the piezoelectric element and electromagnet described in the first embodiment can be implemented in the same manner. Similarly, high-resolution control using position detection sensors can also be implemented.

[0036] As described above, the present invention makes it possible to provide an actuator that has precise positioning, large thrust and stroke, and is widely applicable to technical fields that require long-term operational stability.

[0037] The application of the present invention is not limited to the embodiments described above, and can be modified as appropriate without departing from the spirit of the invention. [Explanation of symbols]

[0038] 1, 1a, 1b X-stage 2 Capacitive Sensor 3. Piezoelectric element 8, 8a, 8b electromagnet 10 base 11 Platforms

Claims

1. A magnetic platform configured to move freely in linear or rotational motion on a base member, A piezoelectric element drive unit comprising a first electromagnetic attachment / detachment mechanism capable of holding / releasing the platform, A second electromagnetic attachment / detachment mechanism is configured to allow the platform to be held / released from the base member, A linear encoder that functions as a position detection sensor for the platform, A capacitance sensor that functions as a displacement sensor for detecting the amount of displacement of the piezoelectric element drive unit, It consists of a control means for controlling the driving of the piezoelectric element drive unit and the timing of holding / releasing the first electromagnetic attachment / detachment mechanism and the second electromagnetic attachment / detachment mechanism, The control means is capable of performing a first process that controls the driving of the piezoelectric element drive unit and the timing of holding / releasing the first electromagnetic attachment / detachment mechanism and the second electromagnetic attachment / detachment mechanism in conjunction with the position signal from the linear encoder, and a second process that controls the amount of drive of the piezoelectric element drive unit in conjunction with the displacement signal from the capacitance sensor. An actuator characterized in that it performs control according to the first process until the platform approaches the target position, and then performs control according to the second process once the platform approaches the target position.

2. The actuator according to claim 1, characterized in that the piezoelectric element drive unit is configured to reciprocate linearly in accordance with the expansion and contraction of the piezoelectric element.

3. A magnetic platform configured to move freely in linear or rotational motion on a base member, A piezoelectric element drive unit comprising a first electromagnetic attachment / detachment mechanism capable of holding / releasing the platform, A second electromagnetic attachment / detachment mechanism is configured to allow the platform to be held / released from the base member, A linear encoder that functions as a position detection sensor for the platform, A capacitance sensor that functions as a displacement sensor for detecting the amount of displacement of the piezoelectric element drive unit, It consists of a control means for controlling the driving of the piezoelectric element drive unit and the timing of holding / releasing the first electromagnetic attachment / detachment mechanism and the second electromagnetic attachment / detachment mechanism, The control means is capable of performing a first process that controls the driving of the piezoelectric element drive unit and the timing of holding / releasing the first electromagnetic attachment / detachment mechanism and the second electromagnetic attachment / detachment mechanism in conjunction with the position signal from the linear encoder, and a second process that controls the amount of drive of the piezoelectric element drive unit in conjunction with the displacement signal from the capacitance sensor. A method for driving an actuator, wherein control is performed according to the first process until the platform approaches the target position, and control is performed according to the second process once the platform approaches the target position, A first step is to move the piezoelectric element drive unit linearly in a certain direction while the platform is held by the first electromagnetic attachment / detachment mechanism, The process comprises the first step followed by a second step in which the first electromagnetic attachment / detachment mechanism is released while the platform is held by the second electromagnetic attachment / detachment mechanism, thereby causing the piezoelectric element drive unit to move linearly in the opposite direction. An actuator driving method characterized by causing the platform to move in a linear or rotational manner by repeating the first and second steps described above.

4. The actuator driving method according to claim 3, characterized in that the position of the platform is detected by the linear encoder, and the amount of displacement or displacement speed of the piezoelectric element driving unit in the first step is controlled in conjunction with the position signal.

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