Control System and Control Method

Abstract

To improve the accuracy of displacement of a base having a piezoelectric device disposed thereon in response to a driving voltage.SOLUTION: A control system comprises: a piezoelectric device having an amount of strain that changes in accordance with a magnitude of a driving voltage being applied; a base on which the piezoelectric device is arranged; a strain sensor that detects a displacement of the base caused by straining of the piezoelectric device and outputs the displacement as a displacement signal; and a driving device that applies a driving voltage to the piezoelectric device. The driving device adjusts the magnitude of the driving voltage so as to reduce a difference between the value of the displacement signal and the value of a setting signal.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This embodiment relates to a control system and a control method for controlling the displacement of a base on which a piezoelectric element is disposed.

Background Art

[0002] By applying a driving voltage to a piezoelectric element, strain is generated in the piezoelectric element due to the inverse piezoelectric effect. A device that deforms a base on which a piezoelectric element is disposed by utilizing the stress caused by the strain of the piezoelectric element is used. For example, a vibration system such as a piezoelectric speaker that vibrates a laminated structure of a piezoelectric element and an electrode has been realized. As the material of the piezoelectric element, a ferroelectric material such as lead zirconate titanate (Pb(Zr,Ti)O 3 :PZT) is often used.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] It is known that the relationship between the driving voltage applied to the ferroelectric material and the strain of the piezoelectric element is different between when the driving voltage is rising and when it is falling. For this reason, for example, even if a predetermined driving voltage at which the base reaches a desired displacement is applied to the piezoelectric element when the driving voltage is falling, the displacement of the base cannot be made the desired magnitude even if the predetermined driving voltage is applied to the piezoelectric element when the driving voltage is rising.

[0005] An object of this embodiment is to improve the accuracy of the displacement of the base on which the piezoelectric element is disposed with respect to the driving voltage.

Means for Solving the Problems

[0006] According to one aspect of the present embodiment, a control system is provided that includes a piezoelectric element, a base on which the piezoelectric element is disposed, a strain sensor that detects displacement of the base caused by distortion of the piezoelectric element, and a driving device that applies a driving voltage to the piezoelectric element. The amount of strain of the piezoelectric element changes according to the magnitude of the driving voltage applied to the piezoelectric element. The strain sensor outputs the displacement of the base as a displacement signal. The driving device adjusts the magnitude of the driving voltage so as to reduce the difference between the value of the displacement signal and the value of the setting signal.

Effects of the Invention

[0007] According to the present embodiment, it is possible to improve the accuracy of the displacement of the base on which the piezoelectric element is disposed with respect to the driving voltage.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Modes for Carrying Out the Invention

[0009] Next, embodiments will be described with reference to the drawings. In the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals. However, it should be noted that the drawings are schematic, and the relationship between the thickness and the planar dimensions, the ratio of the thicknesses of the respective parts, etc. are different from the actual ones. Also, it goes without saying that there are parts where the dimensional relationships and ratios are different between the drawings.

[0010] In addition, the embodiments shown below exemplify devices and methods for embodying the technical idea, and do not specify the shape, structure, arrangement, etc. of the components as the following. Various changes can be made to these embodiments within the scope of the claims.

[0011] (First Embodiment) As shown in FIG. 1, a control system 1 according to the first embodiment includes a piezoelectric element 100, a base 20 on which the piezoelectric element 100 is disposed, a strain sensor 30 disposed on the base 20, and a driving device 40 that applies a driving voltage Vd to the piezoelectric element 100. The control system 1 controls the mechanical displacement of the base 20 in accordance with a setting signal S0 from a signal source 2.

[0012] In the control system 1, when a driving voltage Vd is applied from the driving device 40 to the piezoelectric element 100, a mechanical strain is generated in the piezoelectric element 100 due to the inverse piezoelectric effect. When the piezoelectric element 100 is strained, the base 20 on which the piezoelectric element 100 is disposed is mechanically deformed. Hereinafter, the amount of deformation of the base 20 at this time is also referred to as "displacement". The amount of strain of the piezoelectric element 100 changes according to the magnitude of the applied driving voltage Vd. Therefore, the displacement of the base 20 changes according to the magnitude of the driving voltage Vd.

[0013] The strain sensor 30 detects the displacement of the base 20. Then, the strain sensor 30 converts the detected displacement into an electrical displacement signal Sd and outputs it.

[0014] The drive device 40 applies the adjusted drive voltage Vd to the piezoelectric element 100 while adjusting the magnitude of the drive voltage Vd so that the value of the displacement signal Sd approaches the value of the set signal S0 from the signal source 2. That is, in the control system 1, the displacement of the base 20 is fed back to set the drive voltage Vd.

[0015] The set signal S0 is an electrical signal set so that a predetermined displacement occurs in the base 20 when the set signal S0 is applied to the piezoelectric element 100. The set signal S0 is, for example, a signal of a sound source that vibrates the piezoelectric element 100 as a piezoelectric speaker. In that case, the signal source 2 is a sound source.

[0016] In the control system 1, the drive voltage Vd is adjusted so as to reduce the difference between the value of the displacement signal Sd and the value of the set signal S0. Therefore, even when a predetermined displacement does not occur in the base 20 when the set signal S0 is directly applied to the piezoelectric element 100, according to the control system 1, a predetermined displacement occurs in the base 20 by bringing the displacement signal Sd closer to the set signal S0.

[0017] FIG. 2 shows an example of the configuration of the piezoelectric element 100. The piezoelectric element 100 shown in FIG. 2 has a piezoelectric film 10, a first electrode film 11 and a second electrode film 12 that sandwich the piezoelectric film 10. The piezoelectric film 10 has piezoelectricity such that it is distorted according to an increase or decrease in the intensity of the electric field applied to the piezoelectric film 10. Therefore, distortion occurs in the piezoelectric film 10 according to the electric field generated in the piezoelectric film 10 by applying a voltage between the first electrode film 11 and the second electrode film 12. The piezoelectric film 10 is a ferroelectric film such as a PZT film, for example.

[0018] The material of the first electrode film 11 in contact with the base 20 is, for example, platinum (Pt). The second electrode film 12 has, for example, a two-layer structure in which an iridium oxide (IrO 2 ) layer and an iridium (Ir) layer are laminated in this order from the piezoelectric film 10 side.

[0019] As shown in FIG. 2, the direction in which the first electrode film 11, the piezoelectric film 10, and the second electrode film 12 are laminated is defined as the Z direction. A plane perpendicular to the Z direction is defined as the XY plane.

[0020] When a driving voltage Vd is applied to the piezoelectric element 100, an electric field in the Z direction is generated in the piezoelectric element 100. Due to the electric field generated in the piezoelectric film 10, strain is generated in the piezoelectric film 10. The greater the electric field generated in the piezoelectric film 10, the greater the amount of strain in the piezoelectric element 100.

[0021] As shown in FIG. 3, for example, the base 20 includes a beam portion 21 on which the piezoelectric element 100 is disposed and a support portion 22 to which the end of the beam portion 21 is fixed. Displacement occurs in the beam portion 21 in response to the strain of the piezoelectric element 100. The material of the base 20 is, for example, silicon. In the base 20 shown in FIG. 3 having a cantilever beam structure, the strain sensor 30 is disposed in a region near the joint between the beam portion 21 and the support portion 22. The strain sensor 30 detects the displacement generated in the beam portion 21 due to the strain of the piezoelectric element 100.

[0022] The strain sensor 30 converts the detected displacement of the base 20 into an electrical displacement signal Sd and outputs it. The strain sensor 30 outputs the displacement signal Sd by a conversion function f that satisfies the relationship Sd = f(d) with respect to the displacement d of the base 20. A configuration including a piezoresistor whose electrical resistance changes due to mechanical deformation may be used for the strain sensor 30.

[0023] The piezoresistor may be attached or laminated on the surface of the base 20. Alternatively, when the material of the base 20 is a silicon substrate, the piezoresistor may be formed in the base 20 by impurity diffusion or ion implantation. When the piezoresistor formed by impurity diffusion or the like into the base 20 is used for the strain sensor 30, the resistance value of the piezoresistor may exhibit characteristics that are easily affected by temperature. In that case, a circuit or operation for temperature correction of the resistance value of the piezoresistor is provided in the driving device 40.

[0024] The driving device 40 shown in FIG. 1 includes a conversion circuit 41, an error amplifier 42, and a driver circuit 43.

[0025] The conversion circuit 41 converts the displacement signal Sd into a comparison signal S1 that compares with the setting signal S0. For example, when the strain sensor 30 has a structure with piezoresistors, the conversion circuit 41 outputs a comparison signal S1 obtained by converting the resistance value of the piezoresistors into a voltage. If the displacement signal Sd and the setting signal S0 can be directly compared, the conversion circuit 41 may be omitted.

[0026] The error amplifier 42 outputs the difference between the comparison signal S1 and the setting signal S0. For the error amplifier 42, for example, a differential amplifier may be used. Although FIG. 1 shows the case where the driving device 40 has the conversion circuit 41, an arithmetic processor that converts the displacement signal Sd into the comparison signal S1 may be incorporated in the error amplifier 42.

[0027] The driver circuit 43 adjusts the driving voltage Vd so that the output of the error amplifier 42 input to the driver circuit 43 becomes smaller. That is, the driving device 40 adjusts the magnitude of the driving voltage Vd so that the displacement signal Sd approaches the setting signal S0. The fact that the difference between the displacement signal Sd and the setting signal S0 is small means that the difference between the predetermined displacement that the setting signal S0 is intended to cause in the base 20 and the actual displacement occurring in the base 20 is small. Thus, according to the control system 1, by reducing the difference between the value of the displacement signal Sd and the value of the setting signal S0, the accuracy of the displacement of the base 20 can be improved.

[0028] Here, the reduction in the accuracy of the displacement of the base 20 with respect to the driving voltage Vd, which is caused by the relationship between the driving voltage Vd and the strain of the piezoelectric element 100 being different when the driving voltage Vd rises and falls, will be described.

[0029] The relationship between the electric field E applied to the ferroelectric material and the induced strain S generated in the ferroelectric material is shown in FIG. 4. In the ferroelectric material, induced strain begins to occur at the electric field E0 from the unpolarized state. Then, as the electric field E increases, the induced strain S increases. After that, as the electric field E decreases, the induced strain S decreases, but even when the electric field E becomes zero, residual strain occurs in the ferroelectric material, and the induced strain S generated in the ferroelectric material does not become zero.

[0030] When a ferroelectric material is used for an actuator application that converts an electrical signal into physical motion, the region A enclosed by the dashed line in FIG. 4 is generally used. In region A, the change characteristics of the induced strain S with respect to the electric field E are different when the electric field E is increasing and decreasing.

[0031] Referring to FIG. 5, in the region A shown in FIG. 4, consider applying a driving voltage Vd to the ferroelectric material to obtain the displacement d generated in the base on which the ferroelectric material is disposed.

[0032] When the displacement d is obtained linearly with respect to the driving voltage Vd, the driving voltage Vd and the displacement d are represented by the equation Vd = G×d using an amplifier with a gain of G times. In this case, the characteristic T0 shown by the dashed line in FIG. 5 is obtained. That is, the displacement d1 is obtained by the driving voltage v1, and the displacement d2 is obtained by the driving voltage v2.

[0033] However, the displacement d of the ferroelectric material with respect to the driving voltage Vd has a hysteresis characteristic such as the characteristics T1 and T2 shown by the solid lines in FIG. 5. That is, when the driving voltage Vd is increasing from the voltage v0 to the voltage v3, as shown in the characteristic T1, the driving voltage v1u is required to obtain the displacement d1, and the driving voltage v2u is required to obtain the displacement d2. On the other hand, when the driving voltage Vd is decreasing from the voltage v3 to the voltage v0, as shown in the characteristic T2, the driving voltage v2d is required to obtain the displacement d2, and the driving voltage v1d is required to obtain the displacement d1.

[0034] As described above, when the driving voltage Vd is increasing from the voltage v0 to the voltage v3, for example, the relational expression Vd = Fu(d) holds using the non-linear function Fu. On the other hand, when the driving voltage Vd is decreasing from the voltage v3 to the voltage v0, the relational expression Vd = Fd(d) holds using the non-linear function Fd different from the non-linear function Fu. Hereinafter, the fact that the displacement d with respect to the driving voltage Vd is different when the driving voltage Vd is increasing and decreasing is also referred to as "displacement hysteresis".

[0035] In a ferroelectric material in which displacement hysteresis occurs, for example, when moving from the position of displacement d1 to the position of displacement d3, the curve of the voltage applied to the ferroelectric material differs depending on whether the drive voltage Vd is increasing or decreasing. For this reason, the accuracy of the amount of strain of the base on which the ferroelectric material is disposed with respect to the drive voltage Vd decreases. For example, in a piezoelectric speaker using a ferroelectric material in which displacement hysteresis occurs, distortion of the output sound of the piezoelectric speaker occurs.

[0036] On the other hand, the control system 1 compares the displacement signal Sd corresponding to the displacement of the base 20 with the set signal S0 from the signal source 2, and adjusts the drive voltage Vd so as to reduce the difference between the value of the displacement signal Sd and the value of the set signal S0. By applying the adjusted drive voltage Vd to the piezoelectric element 100, the displacement generated in the base 20 follows the predetermined displacement of the base 20 that is to be set by the set signal S0. For this reason, according to the control system 1, even when the dependency of the amount of strain of the piezoelectric element 100 with respect to the drive voltage Vd is different between when the drive voltage Vd is increasing and when it is decreasing, the actual displacement of the base 20 can be made closer to the value that is to be generated in the base 20 by the set signal S0.

[0037] As described above, according to the control method using the control system 1, the accuracy of the displacement of the base 20 with respect to the drive voltage Vd is improved. For example, the sound quality of headphones or earphones including the piezoelectric element 100 as a component of a piezoelectric speaker can be improved.

[0038] <Modification Example> The control system 1 according to the modification example of the first embodiment shown in FIG. 6 has a double-beam structure in which the base 20 has a first support portion 22A and a second support portion 22B to which the ends of the beam portion 21 are fixed. The strain sensor 30 is disposed near the center of the beam portion 21. In the base 20 having the double-beam structure, the amount of strain of the beam portion 21 is maximum near the center. For this reason, it is preferable to dispose the strain sensor 30 near the center of the beam portion 21.

[0039] (Second Embodiment) As shown in FIG. 7, in the control system 1 according to the second embodiment of the present invention, the first strain sensor 30A and the second strain sensor 30B are arranged on the base 20 with the piezoelectric element 100 interposed therebetween. The first strain sensor 30A and the second strain sensor 30B are also collectively referred to as the "strain sensor 30". The control system 1 shown in FIG. 7 is different from the control system 1 shown in FIG. 1 in that the strain sensors 30 are arranged on both sides of the piezoelectric element 100. Regarding other configurations, the second embodiment is the same as the first embodiment.

[0040] In the control system 1 shown in FIG. 7, the first strain sensor 30A is arranged near one of the two opposing side surfaces of the piezoelectric element 100, and the second strain sensor 30B is arranged near the other side surface. The first strain sensor 30A converts the detected displacement of the base 20 into a first displacement signal Sd1 and outputs it. The second strain sensor 30B converts the detected displacement of the base 20 into a second displacement signal Sd2 and outputs it.

[0041] In the control system 1 shown in FIG. 7, for example, the average value of the first displacement signal Sd1 and the second displacement signal Sd2 may be used as the displacement signal of the base 20. Alternatively, the first displacement signal Sd1 and the second displacement signal Sd2 may be added to obtain the displacement signal of the base 20. Also, after weighting the first displacement signal Sd1 and the second displacement signal Sd2, the average value of the first displacement signal Sd1 and the second displacement signal Sd2 may be calculated or added to calculate the displacement signal of the base 20. For example, in order to consider the displacement in the region where the displacement of the base 20 is large more, weighting may be applied to the larger one of the first displacement signal Sd1 and the second displacement signal Sd2.

[0042] The arithmetic processing such as calculating, adding, and weighting the average values of the first displacement signal Sd1 and the second displacement signal Sd2 may be executed by, for example, the conversion circuit 41. Alternatively, an arithmetic processor that executes these arithmetic processes may be built into the error amplifier 42.

[0043] According to the control system 1 according to the second embodiment, by arranging the strain sensors 30 on both sides of the piezoelectric element 100, even when the base 20 is deformed so as to be twisted, for example, the displacement of the base 20 can be accurately detected.

[0044] Further, when a driving voltage Vd having a frequency close to or equal to the natural frequency of the piezoelectric element 100 is applied to the piezoelectric element 100, the base 20 may vibrate more than the desired amplitude. However, according to the configuration of the control system 1 according to the second embodiment, the large vibration of the base 20 as described above is suppressed. Therefore, a desired amplitude can be obtained for the base 20.

[0045] In FIG. 7, an example in which two strain sensors 30 are symmetrically arranged on both sides of the piezoelectric element 100 is shown, but three or more strain sensors 30 may be arranged around the piezoelectric element 100. Otherwise, the control system 1 according to the second embodiment is substantially the same as the first embodiment, and redundant descriptions are omitted.

[0046] (Other Embodiments) As described above, although described by this embodiment, the discussions and drawings forming a part of this disclosure should not be understood as limiting this embodiment. Various alternative embodiments, examples, and operation techniques will be apparent to those skilled in the art from this disclosure.

[0047] For example, the piezoelectric element 100 may have a configuration including a bulk ferroelectric material other than the piezoelectric film 10 formed in a film shape.

[0048] Also, in the above, the case where the control system 1 suppresses a decrease in the accuracy of the displacement of the base 20 due to displacement hysteresis has been described. Further, according to the control system 1, a decrease in the accuracy of the displacement of the base 20 due to other factors can be suppressed. For example, even with respect to a change in characteristics caused by a change in mechanical characteristics due to mechanical deterioration of the base 20, the piezoelectric film 10, etc., the control system 1 can suppress a decrease in the accuracy of the displacement of the base 20. Mechanical deterioration is, for example, the growth of minute defects or cracks.

[0049] Also, when a large acceleration is applied to the base 20 due to an external impact or the like, the beam portion 21 of the base 20 and the piezoelectric element 100 are greatly shaken. At this time, the difference between the set signal S0 and the displacement signal Sd may be larger than the assumed value. Therefore, when the difference between the set signal S0 and the displacement signal Sd is larger than a predetermined difference set in advance, the result may be recorded in the drive device 40. From this record, for example, when repairing a failure of the base 20, it is possible to take measures in the case of "having a history of excessive impact". Alternatively, the life of the piezoelectric element 100 may be estimated using the fact that the difference between the set signal S0 and the displacement signal Sd is larger than the assumed value as information.

[0050] FIG. 8 shows an example of the configuration of a control system 1 that records the result of comparing the set signal S0 and the displacement signal Sd (hereinafter also referred to as "comparison result"). The comparison result is, for example, the difference between the set signal S0 and the displacement signal Sd, or the difference between the difference between the set signal S0 and the displacement signal Sd and the assumed value. The drive device 40 of the control system 1 shown in FIG. 8 includes a recording device 44 that records the comparison result between the set signal S0 and the displacement signal Sd, and a reference device 45 for referring to the comparison result recorded in the recording device 44 from the outside. The recording device 44 records the comparison result. The reference device 45 enables the comparison result recorded in the recording device 44 to be referred to by referring to the reference device 45. The recording device 44 may be, for example, a hard disk device or a semiconductor memory. The reference device 45 may be, for example, a display device such as a display or a printer that displays the comparison result. The recording device 44 may record the result of comparing the comparison signal S1 obtained by converting the displacement signal Sd with the set signal S0 as a result equivalent to the comparison result between the set signal S0 and the displacement signal Sd. Alternatively, when the recording device 44 directly compares the displacement signal Sd and the set signal S0, the recording device 44 may record the comparison result between the displacement signal Sd and the set signal S0.

[0051] According to the control system 1 shown in FIG. 8, for example, when the difference between the set signal S0 and the displacement signal Sd is larger than an assumed value, the result is recorded in the recording device 44. The user of the control system 1 can refer to the comparison result recorded in the recording device 44 by the reference device 45 and take corresponding measures in the case of "having a history of excessive impact". Note that the recording device 44 and the reference device 45 may be prepared outside the driving device 40.

[0052] Thus, this embodiment includes various embodiments not described herein.

Explanation of Reference Numerals

[0053] 1…Control system 10…Piezoelectric film 11…First electrode film 12…Second electrode film 20…Base 30…Strain sensor 40…Driving device 41…Conversion circuit 42…Error amplifier 43…Driver circuit 44…Recording device 45…Reference device 100…Piezoelectric element

Claims

1. A control system for controlling the mechanical displacement of a base according to a setting signal from a signal source, comprising: a piezoelectric element whose amount of strain changes according to the magnitude of an applied driving voltage; the base on which the piezoelectric element is disposed; a strain sensor that detects the displacement of the base caused by the piezoelectric element being strained and outputs the displacement as a displacement signal; a driving device that applies the driving voltage to the piezoelectric element while adjusting the magnitude of the driving voltage so as to reduce the difference between the value of the displacement signal and the value of the setting signal; wherein the driving device includes an error amplifier that outputs the difference between the displacement signal and the setting signal; and a driver circuit that receives the output of the error amplifier and applies the driving voltage to the piezoelectric element; the driver circuit adjusts the driving voltage such that as the value of the displacement signal approaches the value of the setting signal, the output of the error amplifier decreases. A control system characterized by the above.

2. The strain sensor has a piezoresistor whose electrical resistance changes due to mechanical deformation, and the driving device converts the resistance value of the piezoresistor into a voltage to adjust the magnitude of the driving voltage. The control system according to claim 1, characterized by the above.

3. The control system according to claim 1 or 2, characterized in that at least two of the strain sensors are disposed on the base with the piezoelectric element interposed therebetween.

4. A control system for controlling the mechanical displacement of a base according to a setting signal from a signal source, comprising: a piezoelectric element whose amount of strain changes according to the magnitude of an applied driving voltage; the base on which the piezoelectric element is disposed; a strain sensor that detects the displacement of the base caused by the piezoelectric element being strained and outputs the displacement as a displacement signal; a driving device that applies the driving voltage to the piezoelectric element while adjusting the magnitude of the driving voltage so as to reduce the difference between the value of the displacement signal and the value of the setting signal; wherein the base includes a beam portion on which the piezoelectric element is disposed; and a support portion to which an end portion of the beam portion is fixed. The control system is characterized in that the strain sensor is disposed in a region close to the joint between the beam portion and the support portion to detect the displacement generated in the beam portion.

5.

5. The piezoelectric element has a piezoelectric film; and a first electrode film and a second electrode film sandwiching the piezoelectric film. The control system according to any one of claims 1 to 4, characterized by the above.

6. The control system according to any one of claims 1 to 5, characterized in that the dependency of the amount of strain of the piezoelectric element on the drive voltage is different between when the drive voltage rises and when it falls.

7. A recording device that records the comparison result between the setting signal and the displacement signal, and a reference device for externally referring to the comparison result recorded in the recording device The control system according to any one of claims 1 to 6, further comprising the above.

8. A control method for controlling the mechanical displacement of a base according to a setting signal from a signal source, wherein the base has a double-cantilever structure having a beam portion, and a first support portion and a second support portion to which the ends of the beam portion are fixed, In the base in which a piezoelectric element whose strain amount changes according to the magnitude of the applied drive voltage is disposed on the beam portion, the displacement of the base caused by the piezoelectric element being strained is detected by a strain sensor disposed in a region close to the vicinity of the center of the beam portion where the strain amount is maximized, The detected displacement is output as a displacement signal, The drive voltage is applied to the piezoelectric element while adjusting the magnitude of the drive voltage so as to reduce the difference between the value of the displacement signal and the value of the setting signal. A control method characterized by the above.

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