CN112817144B

CN112817144B - MEMS scanning mirror and laser projector

Info

Publication number: CN112817144B
Application number: CN202011635318.9A
Authority: CN
Inventors: 林育菁; 畠山庸平
Original assignee: Goertek Inc
Current assignee: Goertek Inc
Priority date: 2020-12-31
Filing date: 2020-12-31
Publication date: 2023-04-14
Anticipated expiration: 2040-12-31
Also published as: CN112817144A

Abstract

The invention discloses an MEMS scanning mirror and a laser projector. A specific embodiment of the MEMS scanning mirror includes a mirror, an angle sensor for sensing a deflection angle of the mirror about a first rotation axis and outputting a signal to the control unit, a control unit including a driver and a charge readout circuit for outputting a constant voltage signal to the driver according to an output from the angle sensor, and an actuator serving as a cantilever for rotating the mirror about the first rotation axis according to a control of the control unit. The embodiment can reduce the noise power of the output signal of the angle sensor and improve the signal-to-noise ratio.

Description

MEMS scanning mirror and laser projector

Technical Field

The invention relates to the technical field of laser projection. And more particularly, to a MEMS scanning mirror and a laser projector.

Background

The laser projector adopting the MEMS (Micro Electro Mechanical System) scanning mirror has the advantages of low cost, miniaturization and the like, and has wide market prospect.

The traditional MEMS scanning mirror usually adopts a torsion bar actuating mode, the reflecting mirror is driven by two, three or more torsion bars for supporting the reflecting mirror to incline and twist so as to execute optical scanning, a large scanning angle is realized by adopting resonance driving, and the resonance frequency of the inclined movement of the reflecting mirror needs to be matched with the driving frequency through structural design.

In order to maintain a resonance state when driving a MEMS scanning mirror or monitoring an angle of a mirror, a current method is to provide an angle sensor for sensing a deflection angle of the mirror, and a driver controls a driving voltage or a driving frequency applied to an actuator according to a signal output from the angle sensor to drive the mirror to rotate. For a traditional torsion bar type MEMS scanning mirror, the angle sensor is an angle sensor which is arranged at the edge position of a torsion bar and adopts a piezoelectric effect or a piezoresistive effect.

For applications with relatively low driving frequencies, it is necessary to use a lower resonance frequency, for which reason the prior art also proposes a solution using an external piezoelectric actuator forming a curved cantilever (i.e. a folded spring structure, a plate hinge) as a design suitable for low frequency driving to lower the resonance frequency. For example, as shown in fig. 1, the two-dimensional MEMS scanning mirror includes: a reflector 10; a movable support 11 (inner movable frame), the movable support 11 surrounding the mirror 10 to support the mirror 10 by a pair of

torsion bars

12a and 12 b; first and second internal

piezoelectric actuators

13a and 13b (may be collectively referred to as internal piezoelectric actuators), the first and second internal

piezoelectric actuators

13a and 13b being fixed between the movable support 11 and the

torsion bars

12a and 12b, respectively, and serving as cantilevers for rotating the mirror 10 about the X-axis by the

torsion bars

12a and 12b, respectively; a fixed support 14 (external fixed frame), the fixed support 14 surrounding the movable support 11; and a first external piezoelectric actuator 15a and a second external piezoelectric actuator 15b (which may be collectively referred to as external piezoelectric actuators), the first external piezoelectric actuator 15a and the second external piezoelectric actuator 15b being respectively fixed between the fixed support 14 and the movable support 11 and respectively serving as a curved cantilever for rotating the mirror 10 about the Y axis (the Y axis is perpendicular to the X axis) via the movable support 11, thereby implementing two-dimensional scanning, wherein the driving signal for driving the first internal piezoelectric actuator 13a and the second internal piezoelectric actuator 13b is typically a sine wave or rectangular wave signal having a frequency of 20kHz or more, and the driving signal for driving the first external piezoelectric actuator 15a and the second external piezoelectric actuator 15b is typically a sawtooth wave signal having a frequency of about 60Hz (i.e., the X axis is a fast axis and the Y axis is a slow axis), thereby implementing fast lateral scanning and slow longitudinal scanning of the two-dimensional MEMS scanning mirror.

In the two-dimensional MEMS scanning mirror structure shown in fig. 1 using the external piezoelectric actuator forming the bent cantilever, when the deflection angle sensing of the rotation of the mirror 10 about the X axis and/or the Y axis is performed using, for example, an angle sensor of the piezoelectric effect, the signal-to-noise ratio (SNR) of the output signal of the angle sensor is low, which causes a reduction in detection accuracy, thereby affecting the accuracy of actuator control. One of the main reasons for the low signal-to-noise ratio of the angle sensor output signal for various configurations of MEMS scanning mirrors is that: the angle sensor of the piezoelectric effect is a variable capacitance element, and thermal noise is determined by a resistance value of a readout circuit connected thereto, whereas a conventional voltage reading method generally employs a high resistance element, and thus noise in a low frequency range increases.

Accordingly, there is a need to provide a new MEMS scanning mirror and laser projector.

Disclosure of Invention

An object of the present invention is to provide a MEMS scanning mirror and a laser projector to solve at least one of the problems of the prior art.

In order to achieve the purpose, the invention adopts the following technical scheme:

a first aspect of the present invention provides a MEMS scanning mirror including a mirror, an angle sensor for sensing a deflection angle of the mirror about a first rotation axis and outputting a signal to the control unit, a control unit for rotating the mirror about the first rotation axis according to control of the control unit, and an actuator serving as a cantilever, the control unit including a driver and a charge readout circuit for outputting a constant voltage signal to the driver according to an output from the angle sensor.

Optionally, the charge sensing circuit comprises an inverting amplifier and an impedance in parallel with the inverting amplifier.

Optionally, the driver is for controlling a driving voltage or a driving frequency applied to the actuator according to a constant voltage signal output from the charge readout circuit.

Optionally, the actuator is a piezoelectric actuator.

Optionally, the actuator is an external actuator, the MEMS scanning mirror further comprising a movable support for supporting the mirror.

Optionally, the external actuator forms a curved cantilever.

Optionally, the movable support comprises torsion bars for supporting the mirror, the MEMS scanning mirror further comprising an internal actuator acting as a cantilever for rotating the mirror about a second axis of rotation via the torsion bars.

A second aspect of the invention provides a laser projector comprising a MEMS scanning mirror as provided in the first aspect of the invention.

The invention has the following beneficial effects:

the technical scheme of the invention can reduce the noise power of the output signal of the angle sensor, improve the signal-to-noise ratio, improve the control precision of the actuator and be beneficial to realizing stable driving.

Drawings

The following describes embodiments of the present invention in further detail with reference to the accompanying drawings;

FIG. 1 shows a schematic diagram of a prior art two-dimensional MEMS scanning mirror.

FIG. 2 shows a schematic diagram of a control unit in a MEMS scanning mirror provided by an embodiment of the invention.

Fig. 3 shows a schematic diagram of a thermal noise profile.

Detailed Description

In order to more clearly illustrate the invention, the invention is further described below with reference to preferred embodiments and the accompanying drawings. Similar components in the figures are denoted by the same reference numerals. It is to be understood by persons skilled in the art that the following detailed description is illustrative and not restrictive, and is not to be taken as limiting the scope of the invention.

An embodiment of the present invention provides a MEMS scanning mirror including a mirror, an angle sensor for sensing a deflection angle of the mirror about a first rotation axis and outputting a signal to the control unit, a control unit for rotating the mirror about the first rotation axis according to control of the control unit, and an actuator serving as a cantilever, the control unit including a driver and a charge readout circuit for outputting a constant voltage signal to the driver according to an output from the angle sensor.

In the MEMS scanning mirror provided by the embodiment, the charge readout circuit can be connected with the low resistance, so that the noise power of the output signal of the angle sensor can be reduced, the signal-to-noise ratio can be improved, the control precision of the actuator can be improved, and the stable driving can be realized.

In some alternative implementations of this embodiment, the charge sensing circuit includes an inverting amplifier and an impedance in parallel with the inverting amplifier.

In some optional implementations of this embodiment, the driver is to control a driving voltage or a driving frequency applied to the actuator according to a constant voltage signal output from the charge readout circuit.

In some alternative implementations of this embodiment, the actuator is a piezoelectric actuator.

In some alternative implementations of this embodiment, the actuator is an external actuator, and the MEMS scanning mirror further includes a movable support for supporting the mirror.

In some alternative implementations of this embodiment, the external actuator forms a curved cantilever.

In some alternative implementations of this embodiment, the movable support includes torsion bars for supporting the mirror, and the MEMS scanning mirror further includes an internal actuator serving as a cantilever for rotating the mirror about the second axis of rotation via the torsion bars.

In the above implementation, in the case that the actuator is an external actuator, the actuator rotating the mirror around the first rotation axis according to the control of the control unit should be understood as the actuator rotating the movable support to bring the mirror to rotate around the first rotation axis according to the control of the control unit. In this case, the actuator rotates the mirror about the first rotation axis under the control of the control unit, which is understood to mean that the actuator rotates the mirror about the first rotation axis through the torsion bar under the control of the control unit. Of course, a mode of accessing the outputs of two angle sensors for sensing the deflection angles of the mirror about the X axis and the Y axis respectively to the corresponding charge readout circuits may be adopted, in short, the control unit including the driver and the charge readout circuit in the MEMS scanning mirror provided in this embodiment may be applicable to angle sensors of various types and various mounting positions, and is not limited to other structures of the MEMS scanning mirror (for example, the type of the actuator, the type of the cantilever formed by the actuator, and the like).

In a specific example, in combination with the foregoing implementation manner, the two-dimensional MEMS scanning mirror is further described with the MEMS scanning mirror provided in this embodiment: on the basis of the conventional two-dimensional MEMS scanning mirror as shown in fig. 1, as shown in fig. 2, the two-dimensional MEMS scanning mirror of this example includes a mirror device 100 and a control unit 200, wherein the mirror device 100 includes a movable inner frame 110 and a fixed outer frame 120 (for convenience of illustration, fig. 2 does not show a structure in which the actual fixed outer frame 120 surrounds the movable inner frame 110), the movable inner frame 110 includes movable supports (not shown in the figure), a torsion bar 111, a mirror 112, and an internal piezoelectric actuator (not shown in the figure), the fixed outer frame 120 includes fixed supports (not shown in the figure), an external piezoelectric actuator 121, and an angle sensor 122 based on the piezoelectric effect, the control unit 200 includes a driver 210 and a charge readout circuit 220, and the charge readout circuit 220 includes an inverse amplifier 221 and an impedance 222 connected in parallel with the inverse amplifier 221. Wherein the charge readout circuit 220 may be connected with a low resistance. For charge readout, an inverting amplifier is connected to apply a constant electric field to the piezoelectric element. Thereby, the electric charge is converted into a voltage. Basically, in a piezoelectric effect-based angle sensor having a constant capacitance, if noise power is integrated over the entire frequency band, the amounts of noise power are equal in the voltage reading circuit and the charge reading circuit. However, since the thermal noise density and bandwidth are different, as shown in fig. 3, the thermal noise outside the system bandwidth can be cut off by a filter at a subsequent stage in the charge reading circuit.

In this example, the mirror 112 is made of a single crystal silicon supporting layer serving as a vibration plate, a metal layer serving as a reflector, and a hard mask layer. The movable support is composed of a monocrystalline silicon active layer and a silicon dioxide layer. The internal piezoelectric actuator, the angle sensor 122 based on the piezoelectric effect, and the external piezoelectric actuator 121 are respectively constituted by a vibration plate, a lower electrode, a piezoelectric body (PZT), and an upper electrode. The fixed support is composed of a single crystal silicon layer, an intermediate silicon layer, a single crystal silicon active layer, a silicon dioxide layer, and a hard mask layer.

Another embodiment of the present invention provides a laser projector, which includes the MEMS scanning mirror provided in the above embodiments, and further includes a laser light source, optical devices such as collimation shaping, a laser driving circuit, a MEMS scanning mirror driving circuit, and the like.

In the description of the present invention, it should be noted that the terms "upper", "lower", and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, which are merely for convenience in describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed in a specific orientation, and operate, and thus, should not be construed as limiting the present invention. Unless expressly stated or limited otherwise, the terms "mounted," "connected," and "connected" are intended to be inclusive and mean, for example, that they may be fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.

It should also be noted that, in the description of the present invention, relational terms such as first and second, and the like are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising a … …" does not exclude the presence of another identical element in a process, method, article, or apparatus that comprises the element.

It should be understood that the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention, and it will be obvious to those skilled in the art that other variations and modifications can be made on the basis of the above description, and all embodiments cannot be exhaustive, and all obvious variations and modifications belonging to the technical scheme of the present invention are within the protection scope of the present invention.

Claims

1. A MEMS scanning mirror comprising a mirror, an angle sensor for sensing a deflection angle of the mirror about a first rotation axis and outputting a signal to the control unit, a control unit, and an actuator serving as a cantilever for rotating the mirror about the first rotation axis according to control of the control unit, characterized in that the control unit comprises a driver and a charge readout circuit for outputting a constant voltage signal to the driver according to an output from the angle sensor;

the charge readout circuit includes an inverting amplifier and an impedance in parallel with the inverting amplifier; wherein the charge readout circuit may be connected to a low resistance.

2. The MEMS scanning mirror of claim 1, wherein the driver is configured to control a drive voltage or a drive frequency applied to the actuator based on a constant voltage signal output from the charge readout circuit.

3. The MEMS scanning mirror of claim 1, wherein the actuator is a piezoelectric actuator.

4. A MEMS scanning mirror as claimed in claim 1, wherein the actuator is an external actuator, the MEMS scanning mirror further comprising a movable support for supporting the mirror.

5. A MEMS scanning mirror as claimed in claim 4, wherein the external actuator forms a curved cantilever.

6. A MEMS scanning mirror as claimed in claim 4, wherein the movable support comprises torsion bars for supporting the mirror, the MEMS scanning mirror further comprising an internal actuator acting as a cantilever for rotating the mirror about a second axis of rotation via the torsion bars.

7. A laser projector comprising a MEMS scanning mirror as claimed in any one of claims 1 to 6.