Patent Description:
In some cases, the image on the display is created in part by piezoelectric MEMS mirrors (also termed a "piezo MEMS mirror" or simply "MEMS mirror") which provide horizontal and vertical scanning. Piezo MEMS mirrors require alternating current (AC) driving voltage and current. The driving voltages are very high often in the order of <NUM> to <NUM> volts. Compare this to a typical portable device battery (often a Li-ion battery) which provides a direct current (DC) voltage typically less than <NUM>. The MEMS mirror typically has a mechanical resonant frequency on the order of tens of kilohertz (kHz). In the addition to the real power component that mirrors need to overcome friction, up to <NUM> VA imaginary power component could be needed to agitate the Piezo actuators. The AC driving voltage includes a frequency component that is at this mechanical resonant frequency so as to properly operate the piezo MEMS mirror. A drive systems operates to use the smaller DC voltage provided by the battery to provide an appropriate higher voltage AC driving voltage at the proper current.

<CIT> discloses an image display apparatus including a mirror caused by first and second actuators to oscillate about intersecting axes. The first actuator is driven based on a drive signal having a frequency component around a resonant frequency relevant to oscillatory motion of the mirror. The drive signal is generated by regulating a reference drive signal based on a correction signal for use in correcting distortion of a displayed image, the distortion occurring when the first actuator is driven based on the reference drive signal for use as the drive signal to cause oscillatory motion of the mirror.

<CIT> discloses an optical deflection device including a mirror having a reflection face for deflecting light that enters the reflection face; and a support member to support the mirror including a torsion bar having one end being continuously connected to the mirror, a beam being continuously connected to another end of the torsion bar; and a plurality of piezoelectric elements disposed on the beam including a first piezoelectric element and a second piezoelectric element.

The invention is defined by the independent claim, and preferred embodiments are set out in the dependent claims.

At least some embodiments described herein relate to a piezo MEMS mirror system.

Such a mirror system may be used, for instance, to perform scanning when producing an image on a display of a portable device. Portable devices are battery powered where the battery provides DC power at a relative low DC voltage. However, the piezo MEMS mirror is driven by AC power at a much higher peak AC voltage.

The piezo MEMS mirror system includes a drive system that is connected at its input to DC power provided by the battery. The drive system uses that DC power to create AC power at high peak voltage at the output of the drive system, so as to drive the piezo MEMS mirror. The drive system generates properly conditioned AC power based on such low voltage batteries. Such properly condition power appropriately drives the piezo MEMS mirror to allow for high quality image generation. Furthermore, in some embodiments, the drive system is small allowing the portable device to remain lightweight, and wastes less power, thereby lengthening battery life.

The drive system includes a DC-AC converter that operates to convert the DC power provided by the battery to AC power. In one embodiment, the DC-AC converter generates the AC power having a peak voltage that is at an intermediate level - being between the DC voltage of the battery, and the peak AC voltage generated by the drive system. For instance, the DC-AC converter could comprise a DC boost circuit that boosts the DC voltage up to approximately that intermediate voltage, followed by an AC voltage converter configured to convert the DC power provided by the DC boost circuit to the intermediate voltage AC power. According to the invention, the AC voltage converter generates differential AC power. This could be accomplished by using an H-bridge.

The drive system also includes an output filter that is coupled to receive the AC power provided by the DC-AC converter. The output filter using an inductance system coupled in series between the input of the output filter and the output of the output filter and drive system. The output filter uses the inductance system in conjunction with a capacitance of the piezo MEMS mirror to amplify the AC voltage of the AC power at a mechanical resonant frequency of the piezo MEMS mirror. That conditioned AC power (having high power at the resonant frequency of the piezo MEMS mirror) is provided at the output of the drive system for driving the piezo MEMS mirror.

The output filter also operates differentially, such that an inductor is coupled in series between each of the two inputs of the output filter, and each of the two outputs of the output filter. To allow for higher inductances, those two inductors could be inductively coupled. This allows the drive system to be relatively small notwithstanding having such inductors. The presence of such inductors in conjunction with the capacitance of the piezo MEMS mirror also efficiently conditions such AC power so as to reduce lost power.

A tuning capacitor may also be coupled between each of the output nodes of the output filter and a fixed voltage (such as ground). Such may be used to tune the drive circuit to generate a power at a frequency of the mechanical resonant frequency of the actually fabricated piezo MEMS mirror. This is helpful as semiconductor processing technology at small scales can introduce some small variation in the actually fabricated MEMS structure, thereby causing some variation in the actual mechanical resonant frequency of the piezo MEMS mirror.

At least some embodiments described herein relate to a piezo MEMS mirror system. Such a mirror system may be used, for instance, to perform scanning when producing an image on a display of a portable device. Portable devices are battery powered where the battery provides DC power at a relative low DC voltage. However, the piezo MEMS mirror is driven by AC power at a much higher peak AC voltage.

The output filter operates differentially, such that there is an inductor coupled in series between each of the two inputs of the output filter, and each of the two outputs of the output filter. To allow for higher inductances, those two inductors could be inductively coupled. This allows the drive system to be relatively small notwithstanding having such inductors. The presence of such inductors in conjunction with the capacitance of the piezo MEMS mirror also efficiently conditions such AC power so as to reduce lost power.

<FIG> illustrates a piezo MEMS mirror <NUM>. The principles described herein are not limited to the structure, form, or size of the piezo MEMS mirror. Accordingly, the piezo MEMS mirror <NUM> is shown by way of example only. In this particular example, the piezo MEMS mirror <NUM> includes a reflective mirror <NUM> at its center, which is supported by two orthogonally pivoting axes. The vertical scan and the horizontal scan are controlled via respective piezoelectric actuators. By applying oscillating electrical voltages to the respective piezoelectric actuators, the mirror is caused to oscillate, thereby causing appropriate scanning to occur on a display. This oscillation can be most efficiently obtained and maintained if occurring at a mechanical resonant frequency of the piezo MEMS mirror.

The voltages required to actuate the piezo mirror are quite high and often orders of magnitude higher than the maximum DC voltage that can be provided by a battery of a portable device. In accordance with the principles described herein, a high voltage piezoelectric actuation voltage is attained at the mechanical resonant frequency of the piezo MEMS mirror so as to efficient drive mechanical oscillation of the piezo MEMS mirror.

<FIG> illustrates a piezo MEMS mirror system <NUM> in accordance with the principles described herein. The piezo MEMS mirror system <NUM> includes a battery <NUM> and a piezo MEMS mirror <NUM>. A drive system <NUM> operates to receive at its input <NUM> the DC power of the battery <NUM>, and generate at output terminal(s) <NUM>, a high-voltage AC signal for driving the piezo MEMs mirror <NUM>. It is an extremely difficult task to obtain such high gain driving, with a high voltage and high quality output signal at finely controlled frequency, while maintaining power efficiencies, and a small size footprint. But the principles described herein achieves these benefits.

A single output node 212A might be used if the AC output used to drive the piezo MEMS mirror <NUM> is non-differential mode (i.e., single-ended and not according to the invention). Alternatively, two output nodes 212A and at least one other (as represented by the ellipses 212B) might be used if the AC output used to drive the piezo MEMS mirror <NUM> is differential mode. As an example, the piezo MEMS mirror <NUM> may be the piezo MEMS mirror <NUM> of <FIG>.

The mirror system <NUM> is represented quite abstractly in <FIG>. <FIG> illustrates a more concrete example of a piezo MEMS mirror system <NUM> which operates to drive the piezo MEMS mirror <NUM> in non-differential mode (single-ended and not according to the invention) using a single output terminal <NUM>. <FIG> illustrates a more concrete example of a piezo MEMS mirror system <NUM> which operates to drive the piezo MEMS mirror in differential mode using two output terminals 412A and 412B. <FIG> illustrates an example circuit diagram of a MEMS mirror system <NUM> that represents an example of the MEMS mirror system <NUM> of <FIG>.

Referring to <FIG>, a DC-AC converter <NUM> operates to convert the DC power provided by the battery <NUM> to AC power. The DC-AC converter <NUM> receives the DC power provided by the battery <NUM> on the input terminal <NUM>, and provides AC power at intermediate node(s) <NUM>. In the illustrated embodiment, the intermediate node(s) <NUM> that carry the AC power includes at least one intermediate node 213A. A single intermediate node might be used if the AC output of the DC-AC converter <NUM> was single ended (not according to the invention), which could be the case, for instance, if the output of the drive system <NUM> were also single-ended. However, the ellipses 213B represent that the AC power may be provided by the DC-AC converter <NUM> using multiple intermediate nodes. According to the invention, two intermediate nodes are used with the AC output of the DC-AC converter <NUM> and the drive system <NUM> being in differential mode.

The output filter <NUM> receives the AC voltage from the intermediate node(s) <NUM>. An inductance system <NUM> is coupled in series between the intermediate node(s) <NUM> and the output terminal(s) <NUM>. The output filter <NUM> uses the inductance system <NUM> in conjunction with a capacitance Cmirror of the piezo MEMS mirror <NUM> to <NUM>) amplify an AC voltage of the AC power provided by the DC-AC converter at a mechanical resonant frequency of the piezo MEMS mirror <NUM>, and <NUM>) provide the AC power at the amplified voltage at the output terminal(s) <NUM> of the drive system <NUM> for driving the piezo MEMS mirror <NUM>. In one embodiment, the peak voltage of the amplified voltage (at a mechanical resonant frequency of the piezo MEMS mirror) provided at the output of the drive system <NUM> is at least ten times the DC voltage provided by the battery <NUM>.

In one embodiment, the DC-AC converter <NUM> operates to convert the DC power provided by the battery <NUM> to an intermediate voltage AC power. That intermediate voltage AC power has a peak voltage that is between <NUM>) at the low end, a voltage of the DC power provided by the battery <NUM> and <NUM>) at a high end, a peak voltage of the amplified AC voltage provided by the output filter <NUM>. In one embodiment, a peak voltage of the amplified voltage provided at the output of the drive system <NUM> is at least three times a peak voltage of the intermediate voltage AC power at a mechanical resonant frequency of the piezo MEMS mirror <NUM>.

The output filter <NUM> may also have tuning capacitor(s) <NUM> coupled between an output of the output filter <NUM> and a fixed voltage terminal <NUM>, which may be ground. As will be described hereinafter, the tuning capacitor(s) <NUM> allow for there to be some deviation in the actual capacitance Cmirror of the piezo MEMS mirror <NUM> that often occurs during fabrication of the piezo MEMS mirror <NUM>. The tuning is helpful to be able to obtained amplified AC power at the mechanical resonant frequency of the piezo MEMS mirror <NUM>. This is because slight fabrication deviations in the piezo MEMS mirror <NUM> can cause the actual mechanical resonant frequency of the mirror to vary from the designed mechanical resonant frequency. Such fabrication deviations are unavoidable when fabricating small devices using semiconductor processing technology. The tuning capacitor (s) <NUM> includes at least one tuning capacitor 232A, which is the case with each example of <FIG>. However, the ellipses 232B represent that there may be multiple tuning capacitors 232B, which is the case of <FIG> and <FIG> when the output filter <NUM> operates in differential mode.

<FIG> illustrates a piezo MEMS mirror system <NUM> operating so as to be single-ended, having but a single output terminal <NUM>. The piezo MEMS mirror system <NUM> of <FIG> is an example of the piezo MEMS mirror system <NUM> of <FIG>. In that case, the output terminal <NUM> of <FIG> is an example of the output terminal 212A of <FIG>. Furthermore, the drive system <NUM> of <FIG> is an example of the drive system <NUM> of <FIG>. The DC-AC converter <NUM> of <FIG> is an example of the DC-AC converter <NUM> of <FIG>. The intermediate node <NUM> of <FIG> is an example of the intermediate node 213A of <FIG>. Also, the output filter <NUM> of <FIG> is an example of the output filter <NUM> of <FIG>. The lower terminal of the capacitance Cmirror of the piezo MEMS mirror <NUM> is shown as grounded.

The DC-AC converter <NUM> operates to convert the DC power provided by the battery <NUM> to singled-ended AC power applied to the intermediate node <NUM>. The output filter <NUM> receives the AC voltage from the intermediate node <NUM>. An inductor <NUM> of <FIG> acts as an example of the inductance system <NUM> of <FIG>. The inductor <NUM> is coupled in series between the intermediate node <NUM> and the output terminal <NUM>. A resistor <NUM> (possibly modelling parasitic resistance) is additionally represented in series between the intermediate node <NUM> and the output terminal <NUM>. The output filter <NUM> also has a tuning capacitor <NUM> coupled between an output terminal <NUM> of the output filter <NUM> and the fixed voltage terminal <NUM>, which may be ground.

Again, the output filter <NUM> uses the inductor <NUM> in conjunction with a capacitance Cmirror of the piezo MEMS mirror <NUM> to <NUM>) amplify an AC voltage of the AC power provided by the DC-AC converter at a mechanical resonant frequency of the piezo MEMS mirror <NUM>, and <NUM>) provide the AC power at the amplified voltage at the output terminal(s) <NUM> of the drive system for driving the piezo MEMS mirror <NUM>. More about how this may be done is described with respect to the differential mode embodiment of <FIG>.

<FIG> illustrates a piezo MEMS mirror system <NUM> operating so as to be in differential mode, having two output terminals 412A and 412B. The piezo MEMS mirror system <NUM> of <FIG> is an example of the piezo MEMS mirror system <NUM> of <FIG>. In that case, the output terminal 412A and 412B of <FIG> are example of the output terminal 212A (and another output terminal as represented by the ellipses 212B) of <FIG>. Furthermore, the drive system <NUM> of <FIG> is an example of the drive system <NUM> of <FIG>. The DC-AC converter <NUM> of <FIG> is an example of the DC-AC converter <NUM> of <FIG>. The intermediate nodes 413A and 413B of <FIG> are examples of the intermediate node 213A (and another intermediate node as represented by the ellipses 213B) of <FIG>. Also, the output filter <NUM> of <FIG> is an example of the output filter <NUM> of <FIG>. The output voltage across terminals 412A and 412B is applied to the capacitance Cmirror of the piezo MEMS mirror <NUM>.

The DC-AC converter <NUM> operates to convert the DC power provided by the battery <NUM> to differential mode AC power applied across the intermediate nodes 413A and 413B. The output filter <NUM> receives the differential mode AC voltage from the intermediate nodes 413A and 413B. Two inductors 431A and 431B of <FIG> act as an example of the inductance system <NUM> of <FIG>. The inductor 431A is coupled in series between the intermediate node 413A and the output terminal 412A. The inductor 431B is coupled in series between the intermediate node 413B and the output terminal 412B. A resistor 434A (perhaps a parasitic resistor) is additionally represented in series between the intermediate node 413A and the output terminal 412A. Likewise, a resistor 434B (perhaps a parasitic resistor) is additionally represented in series between the intermediate node 413B and the output terminal 412B. The output filter <NUM> also has a tuning capacitor 432A coupled between the output terminal 412A and the fixed voltage terminal <NUM>. The output filter <NUM> has a tuning capacitor 432B coupled between the output terminal 412A and the fixed voltage terminal <NUM>.

Again, the output filter <NUM> uses the inductors 431A and 431B in conjunction with a capacitance Cmirror of the piezo MEMS mirror <NUM> to <NUM>) amplify a differential AC voltage of the AC power provided by the DC-AC converter <NUM> at a mechanical resonant frequency of the piezo MEMS mirror <NUM>, and <NUM>) provide the differential AC power at the amplified voltage at the output terminal(s) 412A and 412B of the drive system <NUM> for driving the piezo MEMS mirror <NUM>.

In one embodiment, the output filter <NUM> is balanced in that the inductances of the inductors 431A and 431B are approximately the same. Likewise, the capacitors 432A and 432B may have approximately the same capacitances. The resistors 434A and 434B may likewise have approximately the same resistances, although the resistors 434A and 434B may be parasitic in nature thereby being used to model inherent resistances in the wiring of the inductors and so forth.

In the illustrated embodiment, the inductors 431A and 431B are inductively coupled. This allows for the total size of the inductors to be smaller while providing an inductance system of a suitable level of total inductance. For maximum benefit, the inductors 431A and 431B may be inductively coupled such that a mutual inductance of the inductors 431A and 431B is approximately equal to a self-inductance of each of the inductors 431A and 431B. The ability to inductively couple inductors in the differential mode drive system <NUM> makes the embodiment of <FIG> particularly advantageous.

As previously mentioned, the output filters <NUM>, <NUM> and <NUM> are used to amplify the AC power at a frequency that is close to the mechanical resonant frequency of the mechanical mirror. This is of great benefit because the piezo MEMS mirror requires a high voltage AC signal to cause the piezo MEMS mirror <NUM> to move. Since the AC power is particularly amplified at the mechanical resonant frequency of the piezo MEMS mirror <NUM>, the mechanical resonance of the piezo MEMS mirror <NUM> may be used to cause the piezo MEMS mirror <NUM> to properly oscillate. Furthermore, this may be done without wasting too much power amplifying the AC power at frequencies other than that mechanical resonant frequency. Thus, mirror oscillation is properly achieved for scanning, while reducing the amount of power required to perform the oscillation.

Operation of the output filter <NUM> will now be described. In describing the operation, the inductor 431A and 431B are assumed to be the same inductances, which is represented herein as Lf. The tuning capacitors 432A and 432B are assumed to have the same capacitances and is represented herein as Cf. The resistors 434A and 434B are assumed to have the same resistance, which is represented herein as Rf. As a reminder, the capacitance of the piezo mirror itself is represented herein as Cmirror.

To properly design the AC driving system, we can model the mirror itself as another series-resonant RLC equivalent circuit with very high Q. Since the effect of the mirror load to the output filter <NUM> is minimal at any frequency not at mirror resonance, we can characterize the filter <NUM> mainly by Rf, Lf, and Cpiezo/Cf. The AC output voltage across output terminals 412A and 412B resonates at a frequency f<NUM> and with total capacitance (Cf+<NUM>*Cpiezo), which is to be at the mechanical resonant frequency of the piezo MEMS mirror <NUM>. The AC output frequency fo is represented as follows: <MAT>.

When running at the series resonant frequency, the resistor Rf is the major factor that limits the current. Here, Idrive and Vdrive represent the AC current and voltage, respectively, across intermediate nodes 413A and 413B. The current Idrive is represented by the following formula.

Another constrain is the Q factor, which represents the quality of the produced signal across the output terminals 412A and <NUM>, and is represented by the following: The actual Q expression is more complicated, due to the mirror loading effect. However, the mirror effect will only show up around a narrow frequency range near the mechanical resonance.

The voltage gain of the output filter <NUM> (represented by the expression <MAT> of the mirror voltage (represented by Vmirror) over frequency f is represented by the following: <MAT>.

As apparent from the above equation, given the parasitic resistances Rf and the mirror capacitance Cpiezo and with the output of the output filter <NUM> resonating at capacitance (Cf+<NUM>*Cpiezo) (after appropriate adjustment with the tuning capacitor Cf), it is possible to select the size of the inductance Lf to obtain an appropriate frequency response in which gain is maximized at the mechanical resonant frequency of the piezo MEMS mirror <NUM>. To realize the inductanceLf, and minimize the physical size of the inductors 431A and 431B, the inductors 431A and 431B may be coupled as illustrated. When the two windings of the couple inductors are connected in the way shown in <FIG>, the mutual inductance would add up to the self-inductance.

<FIG> illustrates a specific circuit diagram of a piezo MEMS mirror system <NUM>, and represent an example of the piezo MEMS mirror system <NUM> of <FIG>. Here, the details of an example DC-AC converter are represented by the combination of a boost converter <NUM> and an H-bridge converter <NUM>. Boost converters are well known in the art, and thus the details of the boost converter <NUM> are not described in detail here. The boost converter <NUM> boosts the DC voltage of the battery from the area of <NUM> to <NUM> volts to the area of <NUM> to <NUM> volts. The DC output of the boost converter <NUM> is represented as Vboost.

The H-Bridge converter <NUM> also has an operation that is well known in the art, and is designed to provide an output AC signal (of an appropriate square wave), that oscillates between positive and negative Vboost. It does so at the mechanical resonant frequency of the piezo MEMS mirror <NUM>. The Q controlled filter <NUM> of <FIG> is the same design as that described and illustrated with respect to the output filter <NUM> of <FIG>.

Considering the Boost voltage gain <MAT>, we can design and optimize between the size of the inductor (changing inductance Lf) and total power consumption. The most important design criteria is to minimize the total capacitance, so it is recommended that only the necessary tuning capacitance of Cf is added on top of Cmirror, in such way that it will help absolve the parameter variance of the practical mirrors. With the minimum necessary capacitance determined, the value of Lf and Rf can be found, which will give us needed resonant frequency, voltage gain and power losses.

One acceptable value of the tuning capacitance Cf is <NUM> nanofarad (<NUM> nF) given a mirror capacitance of <NUM> nF. However, the appropriate value will dependent on the precision and consistency of the fabrication technology employed to make the piezo MEMS mirror, and the desired yield and reliability for the piezo MEMS mirror or corresponding piezo MEMS mirror system. One acceptable selection of an inductor value Lf is <NUM> millihenries (<NUM> mH) given a mechanical resonant frequency of the mirror at <NUM> kilohertz (<NUM> kHz), and with resistance Rf of <NUM>Ω.

With these values, and with the design illustrated in <FIG>, the waveforms of <FIG> may be obtained. In <FIG>, the horizontal axis represented time. The vertical axis represents the time-wise voltage of the signal. Four voltage signals are stacked (each centered vertically at zero volts) so that one can see the time-wise relationship between the four signals.

The uppermost signal represents the signal input to the Q controlled filter <NUM>. It is a substantially square wave, with a positive and negative value at approximately the DC voltage provided by Vboost. In this case, Vboost is <NUM> volts, which is about <NUM> or <NUM> times the DC voltage output of the battery. So the AC voltage that is input to the Q controlled filter <NUM> is already amplified to an intermediate value between the voltage of the battery, and the resonant voltage provided to the piezo MEMS mirror. The second signal from the top represents the voltage across the resistors having the resistance Rf. The third signal from the top (second signal from the bottom) represents the voltage across the inductor having inductance Lf.

The bottom signal shows the voltage across the output terminals as applied to the piezo MEMS mirror. Note that the signal is now oscillating with an almost exact sinusoidal wave at the mechanical resonant frequency of the mirror (in this case <NUM> kHz, which converts to a wavelength period of <NUM> microseconds). Furthermore, the amplitude of the signal varies between plus and minus <NUM> volts for a total voltage swing of <NUM> volts. <FIG> illustrates the frequency response of the voltage applied to the mirror. Note the sharp peak at the mechanical resonant frequency of the mirror (in this case <NUM> kHz), which steadily decline higher than that value, and negligible value below that value.

Thus, truly an appropriate very high voltage for driving a piezo MEMS mirror has been obtained. Furthermore, by amplifying primarily at the mechanical resonant frequency of the mirror, energy is preserved by avoiding heavy amplification at other frequencies that might not benefit, or may even interfere with, the resonating of the piezo MEMS mirror. Also, because of the coupled inductors, the inductors may be made much smaller. With power preservation, lightness of weight, and the ability to generate high voltage AC signals, the principles described herein are well suited to drive piezo MEMS mirror systems that are incorporated into mobile devices.

Claim 1:
A piezo MEMS mirror system (<NUM>) comprising:
a drive system (<NUM>) that is configured to be connected at its input to DC power provided by a battery (<NUM>), and configured to be connected at its output to a piezo MEMS mirror (<NUM>), the drive system comprising:
a DC-AC converter (<NUM>) that operates to convert the DC power provided by the battery (<NUM>) to AC power; and
an output filter (<NUM>) that is coupled to receive, at least one input of the output filter, AC power provided by the DC-AC converter,
the output filter comprising an inductance system (<NUM>) coupled in series between the at least one input of the output filter and at least one output of the output filter, and
the output filter further comprising at least two input nodes and at least two output nodes, such that the at least one input of the output filter comprises the two input nodes, and such that the at least one output of the output filter comprises the two output nodes, the AC power provided by the DC-AC converter being in differential form across the two input nodes of the output filter, and AC power provided by the output filter being in differential form across the two output nodes of the output filter,
wherein the output filter is configured to use the inductance system (<NUM>) in conjunction with a capacitance of the piezo MEMS mirror to amplify an AC voltage of the AC power provided by the DC-AC converter at a mechanical resonant frequency of the piezo MEMS mirror, and provide the AC power at the amplified voltage at the output of the drive system for driving the piezo MEMS mirror (<NUM>), the inductance system of the output filter comprising:
a first inductor (431A) coupled in series between a first input node (413A) of the two inputs nodes of the output filter and a first output node (412A) of the two output nodes of the output filter, and
a second inductor (431B) coupled in series between a second input node (413B) of the two inputs nodes of the output filter and a second output node (412B) of the two output nodes of the output filter.