Patent Description:
In such display devices, it is desirable for the MEMS driver to be driven at a frequency close to its resonant frequency. However, due to changes in environmental conditions, wear and tear on display device components, and/or other changes to the display device, the resonant frequency of the MEMS driver may change. Thus, when changes to the resonant frequency occur, the efficiency of the MEMS driver may be reduced due to the MEMS driver being driven at a frequency other than its resonant frequency. Imprecise control of the mirrors due to changes in resonant frequency may result in distortion of the displayed image.

In some existing systems, a number of control loops are required to coordinate in order to detect and accommodate changes in the resonant frequency. For instance, in order for a fast scan (FS) mirror to operate at its resonance, multiple controllers of the FS mirror must coordinate with each other and with controllers of a slow scan (SS) mirror. This coordination requires each controller to have the necessary wiring and functionality to communicate with one another. In addition, it takes a number of computational resources for each controller to determine a change of a resonant frequency of a mirror and communicate the detected resonant frequency to the SS mirror controller. In turn, the SS mirror controller has to have the capabilities to process the signal indicating the resonant frequency and coordinate that input with a frame rate of the input media defining the displayed images. This type of architecture can be expensive from a computational resource standpoint. The high complexity of existing systems leads to high power consumption and low control efficiency and power efficiency. In addition, the complex coordination required by existing architectures do not always lead to optimal stability with respect to image quality. Each controller may not have the ability to react quickly enough when there are large swings in a resonant frequency of a mirror. Given that the mirror property variations are unpredictable, and given the fact that the resonant frequency could vary beyond the controllable range of the system, the complex coordination of the various control loops of existing devices can be ineffective as it is challenging to predict all possible use scenarios. Given these issues, there is an ongoing need to improve the robustness and efficiency of laser beam scanning devices.

<CIT> discloses a resonance-type optical scanner which includes a reflection mirror which reflects incident light, a first beam portion which is connected to one side of the reflection mirror, a second beam portion which is connected to the other side of the reflection mirror, and first piezoelectric element portions for elastically deforming the first beam portion.

<CIT> discloses a MEMS scanning mirror with a tunable natural frequency.

The present invention provides a display device according to claim <NUM>.

More generally, the techniques disclosed herein provide a laser beam display device that can dynamically control the resonant frequency of a mirror. The techniques disclosed herein address the above-described issues by controlling the resonant frequency of a mirror instead of requiring components of a display device to react to changes in resonant frequency. In one embodiment, a controller can drive a mirror with an input signal. The controller can also receive a signal or data indicating a target resonant frequency. The controller can bias the input signal to control the resonant frequency of the mirror. In some embodiments, the controller can also receive a feedback signal from the mirror indicating a current resonant frequency. The controller can then bias the input signal to increase or decrease the current resonant frequency. By dynamically controlling the current resonant frequency of a mirror, a device can minimize any difference between the current resonant frequency and the target resonant frequency.

As described in more detail below, a laser beam display device having improved robustness and image quality is provided. By stabilizing and controlling a resonant frequency of a mirror, a more simplistic, cost-effective architecture can be achieved. The architecture described herein does not require coordination between the various control loops to manage variances of a resonant frequency of a mirror. In addition, certain components, such as a slow scan mirror controller can operate autonomously without the need to coordinate with other components that track and measure a resonant frequency. Such features can also provide lower-cost designs.

<FIG> illustrates an example of a display device <NUM> for controlling the resonant frequency of a mirror. In this example, the display device <NUM> comprises a slow scan control <NUM> for controlling a slow scan mirror <NUM> and a fast scan control <NUM> for controlling a fast scan mirror <NUM>. The slow scan mirror <NUM> and the fast scan mirror <NUM> direct a laser beam <NUM> emitted from a laser beam emitter <NUM> towards a display region <NUM> for generating an image <NUM>. The fast scan control <NUM> comprises a mirror resonance control <NUM>, an open angle control <NUM>, and a lock resonance control <NUM>. The lock resonance control <NUM> can control the resonant frequency of the fast scan mirror <NUM> by the use of a detector <NUM> and a lock FS resonance <NUM>. The lock FS resonance <NUM> can receive data or a signal indicating a target resonant frequency <NUM>. The lock FS resonance <NUM> can generate an input signal for the fast scan mirror <NUM>. The lock FS resonance <NUM> can control a DC bias of the input signal, to vary the resonant frequency of the fast scan mirror <NUM> to minimize the difference between the resonant frequency of the fast scan mirror <NUM> and the target resonant frequency <NUM>. With this design, the mirror resonance control <NUM>, open angle control <NUM> and the slow scan control <NUM> can operate independently. Unlike existing systems, the slow scan control <NUM> is not required to receive any input from the fast scan control <NUM> to maintain the quality of the image <NUM>. With this design, these control loops can be optimized during a system start-up and there is no need for continuous real-time optimization during operation.

<FIG> schematically shows specific components and a feedback loop for a display device for dynamically controlling the resonant frequency of a mirror. In such an embodiment, a controller, such as the lock resonance control <NUM>, can receive a feedback signal from the FS mirror <NUM> indicating a current resonant frequency of the FS mirror <NUM>. The lock resonance control <NUM> can then control the DC bias of an input signal to the FS mirror <NUM> to increase or decrease the current resonant frequency of the FS mirror <NUM>. By receiving and analyzing the feedback signal indicating the current resonant frequency, the lock resonance control <NUM> can adjust the DC bias of the input signal to stabilize the current resonant frequency at a predetermined level, e.g., a target level.

In some embodiments, the feedback signal can indicate a position of the mirror. In such an embodiment, the lock resonance control <NUM> can determine a current resonant frequency from one or more suitable techniques using a number of mirror positions. In one example, the lock resonance control <NUM> can perform a fast Fourier transform on a number of data points indicating a mirror position to identify the current resonant frequency. Such techniques are described in more detail below in conjunction with <FIG>.

The fast scan mirror <NUM> includes a piezoelectric MEMS mirror not according to the claimed invention or an electrostatic MEMS mirror according to the claimed invention. For a piezoelectric MEMS mirror, the resonant frequency variation is proportional to the vertical electrical field, and can be expressed as:
<MAT>.

In Equation <NUM>, f<NUM>(<NUM>) is the initial resonant frequency at <NUM> V, S<NUM>E is the elastic compliance under constant electric field, d<NUM> is the nonlinear piezoelectric coefficient, and E<NUM> = σ/ε is the vertical electrical field. In terms of voltage, the resonant frequency can be expressed as:
<MAT>.

Equation <NUM> illustrates that the piezoelectric resonator can be tuned through applying a vertical voltage an input of the piezoelectric mirror. A piezoelectric mirror used as a fast-scan mirror is one resonator that follows this model. The piezoelectric mirror can work at its resonant frequency, but the resonant frequency can shift with time or under different environments, e.g., temperature changes, etc. The techniques disclosed herein can change the vertical bias voltage applied on the piezoelectric mirror and maintain the piezoelectric mirror resonant frequency at its original (e.g., a predetermined Target resonant frequency) value.

<FIG> shows a resonant frequency vs vertical voltage curve of a piezoelectric resonator. As shown, a controller, such as the lock resonance controller, can apply a positive voltage to increase the piezoelectric mirror resonant frequency or apply a negative voltage to decrease the piezoelectric mirror resonant frequency. For example, based on one or more environmental factors such as a change in temperature, the resonant frequency of the piezoelectric mirror can shift from <NUM>,<NUM> to <NUM>,<NUM>. Once the controller detects this shift, the controller can then and then apply a DC voltage to an input signal, such as -<NUM>. 02V, to the piezoelectric mirror. In response to the DC voltage change, the resonant frequency of the piezoelectric mirror shifts back to <NUM>,<NUM>. By giving a display device this type of control over the resonant frequency, each component, including but not limited to the SS mirror, and the whole system continually works in a desired status.

The resonant frequency of the FS mirror is coordinated with a SS mirror operation frequency. By applying positive/negative voltage to the piezoelectric mirror, the FS mirror resonant frequency can be controlled to be an integer multiplier of the SS mirror working frequency (e.g., a frequency based on a frame rate of input media). Since the SS mirror does not have to adjust for changes in the resonant frequency, this coordination avoids the need for a complicated filter and DSP design. By having the SS mirror operate independently without the need to adjust for changes in the resonant frequency, the overall robust and power consumption of a control system is improved.

In an embodiment according to the claimed invention, the fast scan mirror <NUM> comprises an electrostatic MEMS mirror. This embodiment helps achieve high speed scanning. The electrostatic resonator resonant frequency could be tuned through "spring constant softening" effects. The "spring softening" or "frequency pulling" nonlinearity arises because of an extra forcing function on the actuator that works against the restoring spring. This extra force creates extra displacement of the actuator, which makes the actuator move more than what is expected from the naively calculated electrostatic force.

<FIG> shows an electrostatic actuator capacitor model. In some embodiments, the electrostatic resonator can be modeled as the capacitor. This model comprises silicon structures <NUM> having oxide coatings <NUM>. In this model, the oxide coatings <NUM> can have a thickness (t ).

In addition to the resonant-frequency force that is applied to the resonator beams to actuate it, there is an additional force that results from the gradient of "DC" energy stored in the capacitor. The additional force can be modeled by the following equation:
<MAT>.

The tuned new resonance frequency, e.g., the controlled resonance frequency, can be represented as:
<MAT>.

Equation <NUM> illustrates that the resonant frequency of the electrostatic MEMS could be tuned by applying the bias voltage (Vbias). <FIG> illustrates a resulting chart showing a resonant frequency vs bias voltage curve. As shown, the electrostatic MEMS resonant frequency decreases as a result of a decrease in the DC bias that is applied to the input of the electrostatic MEMS. In addition, the electrostatic MEMS resonant frequency increases as a result of an increase in the DC bias that is applied to the input of the electrostatic MEMS.

In one embodiment, a controller can bias the electrical MEMS at voltage V1, to achieve a predetermined resonant frequency <NUM>. In such an embodiment, the controller can also decrease the bias to increase the electrostatic MEMS resonant frequency and increase the bias to decrease the electrostatic MEMS resonant frequency. In another embodiment, a controller can bias the electrical MEMS at voltage V2, to achieve another predetermined resonant frequency <NUM>. In such an embodiment, the controller can also increase the bias to increase the electrostatic MEMS resonant frequency or decrease the bias to decrease the electrostatic MEMS resonant frequency.

<FIG> illustrates a method <NUM> for controlling the resonant frequency of a mirror. Generally described, the method <NUM> adjusts the resonant frequency of the mirror by biasing an input signal to the mirror. A controller can determine a current resonant frequency detected in a feedback signal from the mirror. The controller can then dynamically control the resonant frequency of the mirror to minimize the difference between the current resonant frequency and a target resonant frequency.

The method <NUM> starts at step <NUM> where a controller receives a target resonant frequency. The target resonant frequency can be defined by any suitable data or a signal received by the controller. The target resonant frequency can be any predetermined value. In some configurations, the target resonant frequency can be an integer multiplier of the SS mirror working frequency. The SS mirror working frequency can be based on a frame rate of any media defining the content used as an input to generate the displayed image.

Next, at step <NUM>, the controller determines the current resonant frequency of the FS mirror can compares it with the target resonant frequency. The current resonant frequency of the FS mirror can be determined by any suitable technique. In some embodiments, a feedback signal produced by the FS mirror can define a position of the FS mirror. One or more methods can be performed using the position to determine the resonant frequency of the FS mirror. In one embodiment, the resonant frequency of the FS mirror can be determined by the techniques described below with respect to <FIG>.

As shown in <FIG>, in response to determining that the current resonant frequency is below the target resonant frequency, the method <NUM> proceeds to step <NUM> where the controller increases the bias that is applied to the input of the FS mirror. By increasing the bias, the controller increases the current resonant frequency. This example is provided for illustrative purposes it is not to be construed as limiting. As described herein, any adjustment of the bias can include increasing or decreasing the bias to move the current resonant frequency in a desired direction. For example, if the system starts at V1 as shown in <FIG>, in step <NUM>, the controller may actually decrease the bias to increase the resonant frequency of the FS mirror.

As shown in <FIG>, in response to determining that the current resonant frequency is above the target frequency, the method <NUM> proceeds to step <NUM> where the controller decreases the bias that is applied to the input of the FS mirror. By decreasing the bias, the controller decreases the current resonant frequency. This example is provided for illustrative purposes it is not to be construed as limiting. As described herein, any adjustment of the bias can include increasing or decreasing the bias to move the current resonant frequency in a desired direction. For example, if the system starts at V1 as shown in <FIG>, in step <NUM>, the controller may actually increase the bias of the input to the mirror to decrease the resonant frequency of the FS mirror.

After the bias is adjusted in step <NUM> or step <NUM>, the method <NUM> proceeds back to step <NUM> where the controller measures the current resonant frequency of the FS mirror. The method <NUM> continues to cycle through step <NUM> through step <NUM> to continuously monitor and control the resonant frequency of the FS mirror. The controller minimizes the difference between the current resonant frequency and the target resonant frequency.

<FIG> illustrate example techniques for determining the current resonant frequency of the FS mirror. <FIG> schematically shows a display device <NUM> that can be used to determine a resonant frequency of a mirror. The display device <NUM> may include a laser beam emitter <NUM> configured to emit a laser beam <NUM>. For example, the laser beam emitter <NUM> may be a laser diode. The laser beam <NUM> may impinge upon a display region <NUM> of the display device <NUM> to form a displayed image <NUM>. For example, the display region <NUM> may be a display of a head-mounted display device and the displayed image <NUM> may include one or more virtual objects.

The display device <NUM> may further include a slow-scan mirror <NUM> and a fast-scan mirror <NUM>. The slow-scan mirror <NUM> and the fast-scan mirror <NUM> may be configured to reflect the laser beam <NUM> onto the display region <NUM>, as shown in <FIG>. The displayed image <NUM> may be displayed in one or more frames in which the slow-scan mirror <NUM> and the fast-scan mirror <NUM> direct the laser beam <NUM> across the display region <NUM> to "draw" the displayed image <NUM>. In some embodiments, the slow-scan mirror <NUM> may be configured to complete a slow-scan period <NUM> during each frame. The slow-scan period <NUM> may include a display interval <NUM>. As shown in <FIG>, the slow-scan mirror <NUM> may linearly scan across the display region <NUM> from an initial scanning position <NUM> to a final scanning position <NUM>. The laser beam emitter <NUM> may be configured to emit the laser beam <NUM> during the display interval <NUM>. In the display interval <NUM>, the fast-scan mirror <NUM> may perform a plurality of scans across the display region <NUM> to "draw" the displayed image <NUM>.

The slow-scan period <NUM> may further include a non-display interval <NUM>. During the non-display interval <NUM>, the slow-scan mirror <NUM> may be configured to return from the final scanning position <NUM> to the initial scanning position <NUM>. This portion of the non-display interval <NUM> may also be referred to as the flyback. During the non-display interval <NUM>, the laser beam emitter <NUM> may be configured to not emit the laser beam <NUM>. In some embodiments, as shown in the example of <FIG>, the display region <NUM> may include one or more blank regions in which the displayed image <NUM> is not displayed. In addition to the flyback, the non-display interval <NUM> may include time during which the slow-scan mirror <NUM> is oriented toward the one or more blank regions. The initial scanning position <NUM> and the final scanning position <NUM> may be located in the one or more blank regions, as shown in <FIG>.

Returning to <FIG>, the fast-scan mirror <NUM> may be driven by a nonlinear driver <NUM>, which may be included in a fast-scan driver and sensor system <NUM>. The fast-scan driver and sensor system <NUM> may further include a fast-scan MEMS sensor <NUM>. The fast-scan MEMS sensor <NUM> may be configured to detect the motion and/or position of the fast-scan mirror <NUM>. For example, the fast-scan mirror <NUM> may be configured to transmit a fast-scan mirror output signal <NUM> to the fast-scan MEMS sensor <NUM>. The fast-scan mirror <NUM> and the nonlinear driver <NUM> may together have a MEMS resonant frequency <NUM>. In addition to the fast-scan MEMS sensor <NUM> and the nonlinear driver <NUM>, the display device <NUM> may further include a slow-scan MEMS driver <NUM> configured to drive the slow-scan mirror <NUM>.

The display device <NUM> may further include a signal generator <NUM>. In order to drive the fast-scan driver system <NUM>, the signal generator <NUM> may be configured to generate a periodic electrical signal <NUM> having a first frequency <NUM>. For example, the periodic electrical signal <NUM> may be a sine wave, a square wave, a triangle wave, a sawtooth wave, or some other type of periodic wave. The periodic electrical signal <NUM> may be used to drive the fast-scan mirror <NUM>. The signal generator <NUM> may be configured to generate the periodic electrical signal <NUM> based on periodic electrical signal instructions <NUM> received from a processor <NUM> included in the display device <NUM>, as discussed in further detail below. The periodic electrical signal instructions <NUM> may indicate the first frequency <NUM> at which the nonlinear driver <NUM> is configured to drive the fast-scan mirror <NUM>.

The slow-scan mirror <NUM> may be driven by the slow-scan MEMS driver <NUM> separately from the fast-scan mirror <NUM>. The slow-scan mirror <NUM> may be driven by another periodic electrical signal <NUM> received from the signal generator <NUM>. The other periodic electrical signal <NUM> may, in some embodiments, complete one period every slow-scan period <NUM>. For example, the other periodic electrical signal <NUM> may drive the slow-scan mirror with a sawtooth waveform, as shown in <FIG>.

The nonlinear driver <NUM> may be configured to receive the periodic electrical signal <NUM> from the signal generator <NUM>. The nonlinear driver <NUM> may be further configured to amplify the periodic electrical signal <NUM> to produce an amplified signal <NUM>. The nonlinear driver <NUM> may be nonlinear in that its gain may vary as a function of the frequency of the periodic electrical signal <NUM>. Using a nonlinear driver <NUM> rather than a linear driver may have the advantage of allowing the fast-scan driver system <NUM> to more efficiently drive the fast-scan mirror <NUM>. The nonlinear driver <NUM> may be further configured to transmit the amplified signal <NUM> to the fast-scan mirror <NUM> to drive the fast-scan mirror <NUM>.

The display device <NUM> may further include a signal detector <NUM>. The signal detector <NUM> may be configured to receive the periodic electrical signal <NUM> from the signal generator <NUM>. The signal detector <NUM> may be further configured to receive a fast-scan MEMS sensor output signal <NUM> from the fast-scan MEMS sensor <NUM>. The signal detector may be further configured to detect an amplitude difference <NUM> and/or a phase difference <NUM> between the periodic electrical signal <NUM> and the fast-scan MEMS sensor output signal <NUM>. The phase difference is detected, for example, from the interference between the periodic electrical signal <NUM> and the fast-scan MEMS sensor output signal <NUM>.

The display device <NUM> may further include a processor <NUM>, which may be operatively coupled to memory <NUM>. In some embodiments, the processor <NUM> may be configured to receive the amplitude difference <NUM> and/or the phase difference <NUM> from the signal detector <NUM>. In other embodiments, the processor <NUM> may receive the periodic electrical signal <NUM> and the fast-scan MEMS sensor output signal <NUM> rather than receiving the amplitude difference <NUM> from the signal detector <NUM>. The processor <NUM> may be further configured to determine, based on the amplitude difference <NUM>, the driver system resonant frequency <NUM> of the fast-scan driver system <NUM>. In some embodiments, as discussed above, the processor <NUM> may be configured to determine the driver system resonant frequency <NUM> at least in part by determining a phase difference <NUM> between the periodic electrical signal <NUM> and the fast-scan MEMS sensor output signal <NUM>. In such embodiments, the processor <NUM> may be configured to determine the phase difference <NUM> at least in part by performing a fast Fourier transform on the amplitude difference signal received from the signal detector <NUM>.

In some embodiments, the processor <NUM> may be further configured to generate modified periodic electrical signal instructions <NUM> based on the driver system resonant frequency <NUM>. The modified periodic electrical signal instructions <NUM> may include a second frequency <NUM> different from the first frequency <NUM>. The processor <NUM> may be further configured to transmit the modified periodic electrical signal instructions <NUM> to the signal generator <NUM>, as shown in <FIG>. In response to receiving the modified periodic electrical signal instructions <NUM>, the signal generator <NUM> may be configured to generate a modified periodic electrical signal <NUM> with the second frequency <NUM>. In some embodiments, the second frequency <NUM> may be the driver system resonant frequency <NUM>.

In some embodiments, the signal generator <NUM> may include a phase-locked loop <NUM>. In such embodiments, the phase-locked loop <NUM> may have a lock angle <NUM> between the periodic electrical signal <NUM> and the fast-scan MEMS sensor output signal <NUM>. <FIG> shows an example plot of the amplitude and phase of the fast-scan driver system <NUM> in an embodiment in which the lock angle is <NUM>°. If a linear driver were used rather than the nonlinear driver <NUM>, the phase difference <NUM> between the periodic electrical signal <NUM> and the fast-scan MEMS sensor output signal <NUM> would be <NUM>°. However, using the nonlinear driver <NUM> instead of a linear driver may result in a phase difference other than <NUM>° due to phase aliasing between the nonlinear driver <NUM> and the fast-scan mirror <NUM>. The lock angle <NUM> may be set to <NUM>° in the example of <FIG> in order to match the <NUM>° offset between the periodic electrical signal <NUM> and the fast-scan MEMS sensor output signal <NUM> and allow the fast-scan driver system <NUM> to operate at the MEMS resonant frequency <NUM>. It will be appreciated that these specific lock angles are merely exemplary and other lock angles are possible.

In embodiments in which the signal generator <NUM> includes a phase-locked loop <NUM>, the processor <NUM> may be configured to generate the modified periodic electrical signal instructions <NUM> at least in part by modifying a lock angle <NUM> of the phase-locked loop <NUM> to have a target lock angle <NUM>. Additionally or alternatively, the processor <NUM> may be further configured to determine the driver system resonant frequency <NUM> at least in part by determining a gain <NUM> of the signal generator <NUM>. when the fast-scan driver system <NUM> receives the periodic electrical signal <NUM> from the signal generator <NUM>. The processor <NUM> may determine the gain <NUM> of the phase-locked loop <NUM> in embodiments in which the processor <NUM> receives the periodic electrical signal <NUM> from the signal generator <NUM>. In such embodiments, the processor <NUM> may modify the first frequency <NUM> such that the signal generator <NUM> has a target gain <NUM>. The target gain <NUM> may be a minimum gain.

<FIG> shows a method <NUM> for searching for a resonance peak of the fast-scan driver system <NUM>. In some embodiments, the processor <NUM> may be configured to determine (e.g., "measure") the driver system resonant frequency <NUM> at least in part by detecting a plurality of amplitude differences <NUM> in a respective plurality of fast-scan periods <NUM>. The plurality of fast-scan periods <NUM> may occur during the non-display interval <NUM>. Alternatively, the plurality of fast-scan periods <NUM> may occur in both the display interval <NUM> and the non-display interval <NUM>, or only in the display interval <NUM>. The processor <NUM> may be further configured to iteratively update the first frequency <NUM> over the plurality of fast-scan periods <NUM> to determine the second frequency <NUM>. At step <NUM> of the flowchart <NUM>, the processor <NUM> may increase the first frequency <NUM> indicated in the periodic electrical signal instructions <NUM> by a change in frequency Δf. The modified periodic electrical signal instructions <NUM> with this increase in frequency may be transmitted to the signal generator <NUM> and executed to generate a modified periodic electrical signal <NUM>.

At step <NUM>, the processor <NUM> may determine the change in the amplitude of the fast-scan MEMS sensor output signal <NUM> resulting from the increase in the frequency. The change in amplitude may be a change in the amplitude difference <NUM> between the previous fast-scan period <NUM> and the current fast-scan period <NUM>. When the amplitude increases, the processor <NUM> may repeat step <NUM> and increase the frequency by Δf again. However, when the amplitude decreases, the processor <NUM> may decrease the frequency indicated in the periodic electrical signal instructions <NUM> by a change in frequency Δf. In other embodiments, the processor <NUM> may decrease the first frequency <NUM> by some other amount.

The processor <NUM> may be further configured to, at step <NUM>, determine the change in the amplitude of the fast-scan MEMS sensor output signal <NUM> following the decrease in the frequency. When the frequency increases, the processor <NUM> may repeat step <NUM>. When the frequency increases, the processor <NUM> may instead save the current frequency as the second frequency <NUM>. In embodiments in which the signal generator <NUM> includes a phase-locked loop <NUM>, saving the current frequency as the second frequency <NUM> may include modifying the lock angle <NUM> of the phase-locked loop <NUM>. Additionally or alternatively, at step <NUM>, the current frequency may be saved as the second frequency <NUM> in the memory <NUM> of the display device <NUM>.

Thus, via the method of <FIG>, the processor <NUM> may search for the resonance peak of the MEMS sensor output signal <NUM> by iteratively increasing and/or decreasing the frequency of the periodic electrical signal <NUM> and determining the change in the amplitude of the MEMS sensor output signal <NUM> to search for a peak in the amplitude. Although <FIG> shows the increase in frequency at step <NUM> prior to the decrease in frequency at step <NUM>, the method <NUM> may include decreasing the frequency of the periodic electrical signal <NUM> prior to increasing the frequency in other embodiments. In addition, in embodiments in which the processor <NUM> is configured to determine the change in the gain <NUM> of the signal generator <NUM> while the amplitude difference is kept constant, the searching method shown in <FIG> may be applied to the gain <NUM> rather than the amplitude of the fast-scan MEMS sensor output signal <NUM>. In addition, the search for the gain <NUM> is not limited to the non-display interval <NUM>.

<FIG> shows another method <NUM> for searching for a resonance peak of the fast-scan driver system <NUM>. In some embodiments, the processor <NUM> may be configured to determine (e.g., "measure") the driver system resonant frequency <NUM> at least in part by detecting a plurality of amplifier gain differences in a respective plurality of fast-scan periods <NUM>. The plurality of fast-scan periods <NUM> may occur during the non-display interval <NUM>. Alternatively, the plurality of fast-scan periods <NUM> may occur in both the display interval <NUM> and the non-display interval <NUM>, or only in the display interval <NUM>. The processor <NUM> may be further configured to iteratively update the first frequency <NUM> over the plurality of fast-scan periods <NUM> to determine the second frequency <NUM>. At step <NUM> of the method <NUM>, the processor <NUM> may increase the first frequency <NUM> indicated in the periodic electrical signal instructions <NUM> by a change in frequency Δf. The modified periodic electrical signal instructions <NUM> with this increase in frequency may be transmitted to the signal generator <NUM> and executed to generate a modified periodic electrical signal <NUM> as shown in <FIG>.

At step <NUM>, the processor <NUM> may determine the change in the amplifier gain resulting from the increase in the frequency. The change in the amplifier gain may be a change in the gain difference between the previous fast-scan period <NUM> and the current fast-scan period <NUM>. When the amplifier gain decreases, the processor <NUM> may repeat step <NUM> and increase the frequency by Δf again. However, when the amplifier gain decreases, the processor <NUM> may increase the frequency indicated in the periodic electrical signal instructions <NUM> by a change in frequency Δf. In other embodiments, the processor <NUM> may decrease the first frequency <NUM> by some other amount.

The processor <NUM> may be further configured to, at step <NUM>, determine the change in the amplifier gain following the decrease in the frequency. When the frequency decreases, the processor <NUM> may repeat step <NUM>. When the frequency decreases, the processor <NUM> may instead save the current frequency as the second frequency <NUM>. In embodiments in which the signal generator <NUM> includes a phase-locked loop <NUM>, saving the current frequency as the second frequency <NUM> may include modifying the lock angle <NUM> of the phase-locked loop <NUM>. Additionally or alternatively, at step <NUM>, the current frequency may be saved as the second frequency <NUM> in the memory <NUM> of the display device <NUM>.

Thus, via the method of <FIG>, the processor <NUM> may search for the resonance peak of the MEMS sensor output signal <NUM> by iteratively increasing and/or decreasing the frequency of the periodic electrical signal <NUM> and determining the change in the amplifier gain to search for a peak in the amplitude. Although <FIG> shows the increase in frequency at step <NUM> prior to the decrease in frequency at step <NUM>, the method <NUM> may include decreasing the frequency of the periodic electrical signal <NUM> prior to increasing the frequency in other embodiments.

<FIG> shows an example embodiment of the display device <NUM> in which the display device <NUM> is a head-mounted display device <NUM> having the form of wearable glasses or goggles, but it will be appreciated that other forms are possible. The head-mounted display device <NUM> may include an output device suite including a display <NUM>. In some embodiments, the head-mounted display device <NUM> may be configured in an augmented reality configuration to present an augmented reality environment, and thus the display <NUM> may be an at least partially see-through stereoscopic display configured to visually augment an appearance of a physical environment being viewed by the user through the display <NUM>. In some examples, the display <NUM> may include one or more regions that are transparent (e.g. optically clear) and may include one or more regions that are opaque or semi-transparent. In other examples, the display <NUM> may be transparent (e.g. optically clear) across an entire usable display surface of the display <NUM>.

The output device suite of the head-mounted display device <NUM> may, for example, include an image production system that is configured to display one or more virtual objects to the user with the display <NUM>. The processor <NUM> may be configured to output for display on the display <NUM> a mixed reality experience including one or more virtual objects superimposed upon the physical environment. In the augmented reality configuration with an at least partially see-through display, the virtual objects are visually superimposed onto the physical environment that is visible through the display <NUM> so as to be perceived at various depths and locations. In one embodiment, the head-mounted display device <NUM> may use stereoscopy to visually place a virtual object at a desired depth by displaying separate images of the virtual object to both of the user's eyes. Using this stereoscopy technique, the head-mounted display device <NUM> may control the displayed images of the virtual objects, such that the user will perceive that the virtual objects exist at a desired depth and location in the viewed physical environment.

Alternatively, the head-mounted display device <NUM> may be configured in a virtual reality configuration to present a full virtual reality environment, and thus the display <NUM> may be a non-see-though stereoscopic display. The head-mounted display device <NUM> may be configured to display virtual three-dimensional environments to the user via the non-see-through stereoscopic display. The head-mounted display device <NUM> may be configured to display a virtual representation such as a three-dimensional graphical rendering of the physical environment in front of the user that may include additional virtual objects. Displaying the virtual representation of the physical environment may include generating a two-dimensional projection of a three-dimensional model of the physical environment onto the surface of the display <NUM>. As another alternative, the computing system may include a portable computing device that is not head mounted, such as a smartphone or tablet computing device. In such a device, camera-based augmented reality may be achieved by capturing an image of the physical environment through a forward-facing camera and displaying the captured image on a user-facing display along with world locked graphical images superimposed on the captured image. While the computing system is primarily described in terms of the head-mounted display device <NUM> herein, it will be appreciated that many features of the head-mounted display device <NUM> are also applicable to such a portable computing device that is not head mounted.

The output device suite of the head-mounted display device <NUM> may further include one or more speakers <NUM> configured to emit sound. In some embodiments, the head-mounted display device <NUM> may include at least a left speaker 236A and a right speaker 236B situated such that the left speaker 236A may be located proximate the user's left ear and the right speaker 236B may be located proximate the user's right ear when the head-mounted display device <NUM> is worn. Thus, the one or more speakers <NUM> may emit stereo sound output. The output device suite may further include one or more haptic feedback devices <NUM> configured to provide tactile output (e.g. vibration).

The head-mounted display device <NUM> may include an input device suite including one or more input devices. The input device suite of the head-mounted display device <NUM> may include one or more optical sensors. In one example, the input device suite includes an outward-facing optical sensor <NUM> that may be configured to detect the real-world background from a similar vantage point (e.g., line of sight) as observed by the user through the display <NUM> in an augmented reality configuration. The input device suite may additionally include an inward-facing optical sensor <NUM> that may be configured to detect a gaze direction of the user's eyes. It will be appreciated that the outward facing optical sensor <NUM> and/or the inward-facing optical sensor <NUM> may include one or more component sensors, including an RGB camera and a depth camera. The RGB camera may be a high definition camera or have another resolution. The depth camera may be configured to project non-visible light and capture reflections of the projected light, and based thereon, generate an image comprised of measured depth data for each pixel in the image. This depth data may be combined with color information from the image captured by the RGB camera, into a single image representation including both color data and depth data, if desired.

The input device suite of the head-mounted display device <NUM> may further include a position sensor system that may include one or more position sensors <NUM> such as accelerometer(s), gyroscope(s), magnetometer(s), global positioning system(s), multilateration tracker(s), and/or other sensors that output position data as a position, orientation, and/or movement of the relevant sensor. The input device suite may further include one or more microphones <NUM> configured to collect sound data.

Optical sensor information received from the one or more optical sensors and/or position data received from position sensors <NUM> may be used to assess a position and orientation of the vantage point of head-mounted display device <NUM> relative to other environmental objects. In some embodiments, the position and orientation of the vantage point may be characterized with six degrees of freedom (e.g., world-space X, Y, Z, pitch, roll, yaw). The vantage point may be characterized globally or independent of the real-world background. The position and/or orientation may be determined by the processor <NUM> of the head-mounted display device <NUM> and/or by an off-board computing system.

Furthermore, the optical sensor information and the position sensor information may be used by the head-mounted display system to perform analysis of the real-world background, such as depth analysis, surface reconstruction, environmental color and lighting analysis, or other suitable operations. In particular, the optical and positional sensor information may be used to create a virtual model of the real-world background. In some embodiments, the position and orientation of the vantage point may be characterized relative to this virtual space. Moreover, the virtual model may be used to determine positions of virtual objects in the virtual space and add additional virtual objects to be displayed to the user at a desired depth and location. The virtual model is a three-dimensional model and may be referred to as "world space," and may be contrasted with the projection of world space viewable on the display <NUM>, which is referred to as "screen space. " Additionally, the optical sensor information received from the one or more optical sensors may be used to identify and track objects in the field of view of the one or more optical sensors. The optical sensors may also be used to identify machine recognizable visual features in the physical environment and use the relative movement of those features in successive frames to compute a frame to frame relative pose change for the head mounted display device <NUM> within the world space of the virtual model.

The head-mounted display device <NUM> may further include a communication system including one or more communication devices <NUM>, which may include one or more receivers 216A and/or one or more transmitters 216B. In embodiments in which the head-mounted display device <NUM> communicates with an off-board computing system, the one or more receivers 216A may be configured to receive data from the off-board computing system, and the one or more transmitters 216B may be configured to send data to the off-board computing system. In some embodiments, the head-mounted display device <NUM> may communicate with the off-board computing system via a network, which may be a wireless local- or wide-area network. Additionally or alternatively, the head-mounted display device <NUM> may communicate with the off-board computing system via a wired connection. The head-mounted display device <NUM> may be further configured to communicate with a server computing system via the communication system.

<FIG> shows a flowchart of a method <NUM> for use with a display device, according to one example embodiment. The display device may be the display device <NUM> of <FIG> or may alternatively be some other display device. The method <NUM> may include, at step <NUM>, scanning a display region with a slow-scan mirror driven by a slow-scan MEMS driver. The slow-scan mirror may scan the display region once per frame during a display interval. During the display interval, the slow-scan mirror may move from an initial scanning position to a final scanning position. While the slow-scan mirror scans the display region, a fast-scan mirror driven by a nonlinear driver may also scan the display region to "draw" a displayed image on the display region. The fast-scan mirror may scan the display region multiple times in a plurality of fast-scan periods during each slow-scan period. In some embodiments, scanning the display region may include, at step <NUM>, reflecting a laser beam off the slow-scan mirror and the fast-scan mirror onto the display region. At the end of the display interval, the slow-scan mirror may return from the final scanning position to the initial scanning position during a non-display interval.

The steps of the method <NUM> discussed below may be performed over one or more fast-scan periods of a fast-scan mirror driven by the nonlinear driver. At step <NUM>, the method <NUM> may include, at a signal generator, generating a periodic electrical signal. The periodic electrical signal may have a first frequency and may be generated based on periodic electrical signal instructions received from a processor. The method <NUM> may further include, at step <NUM>, transmitting the periodic electrical signal to a signal detector and a nonlinear driver. The nonlinear driver may be included in a fast-scan driver system along with a fast-scan MEMS sensor.

At step <NUM>, the method <NUM> may further include amplifying the periodic electrical signal at the nonlinear driver to produce an amplified signal. The nonlinear driver may amplify the periodic electrical signal with a gain that varies as a function of the amplitude of the periodic electrical signal. The method <NUM> may further include, at step <NUM>, driving the fast-scan mirror with the amplified electrical signal. The fast-scan mirror may transmit a fast-scan mirror output signal to the MEMS sensor, which may transmit a fast-scan MEMS sensor output signal to the signal detector.

At step <NUM>, the method <NUM> may further include receiving the periodic electrical signal and a fast-scan MEMS sensor output signal at the signal detector. At step <NUM>, the method <NUM> may further include, at the signal detector, detecting an amplitude difference between the periodic electrical signal and the fast-scan MEMS sensor output signal. For example, the periodic electrical signal may destructively interfere with the fast-scan MEMS sensor output signal at the signal detector. At step <NUM>, the method <NUM> may further include transmitting the amplitude difference to a processor. Alternatively, instead of detecting the amplitude difference at a signal detector, the periodic electrical signal and the fast-scan MEMS sensor output signal may be transmitted to the processor and the amplitude difference may be determined at the processor.

At step <NUM>, the method <NUM> may further include determining a driver system resonant frequency of a fast-scan driver system including the nonlinear driver and the fast-scan MEMS sensor. The driver system resonant frequency may be determined at the processor based on the amplitude difference. Step <NUM> may further include step <NUM>, at which the method <NUM> may include determining a phase difference between the periodic electrical signal and the fast-scan MEMS sensor output signal.

The method <NUM> of <FIG> may further include the following optional steps shown in <FIG>. At step <NUM>, the method <NUM> may further include generating, based on the driver system resonant frequency, a modified periodic electrical signal with a second frequency different from the first frequency. In some embodiments, the second frequency may be the driver system resonant frequency. The modified periodic electrical signal may be generated based on modified periodic electrical signal instructions received from the processor. In some embodiments, generating the modified periodic electrical signal may include, at step <NUM>, iteratively updating the first frequency over the plurality of fast-scan periods. In embodiments in which the signal generator includes a phase-locked loop, generating the modified periodic electrical signal may further include, at step <NUM>, modifying a lock angle of the phase-locked loop to have a target lock angle.

In some embodiments, determining the driver system resonant frequency at step <NUM> may include one of the following optional steps shown in <FIG>. At step <NUM>, the method <NUM> may further include detecting a plurality of amplitude differences in a respective plurality of fast-scan periods. Step <NUM> may be performed, for example, when step <NUM> shown in <FIG> is performed. In embodiments in which the signal generator is configured to vary the output gain to maintain constant MEMS output amplitude, determining the driver system resonant frequency may include, at step <NUM>, determining a gain over the frequency scan of the signal generator when the fast-scan driver system receives the periodic electrical signal from the signal generator. In some embodiments, at step <NUM>, step <NUM> may further include determining a minimum gain of the signal generator. It will be appreciated that at resonance for the fast-scan MEMS mirror, the fast-scan MEMS mirror gain is at a maximum, and thus the driver gain of the nonlinear driver can be kept to a minimum, thereby keeping the energy consumption of the system to a minimum.

The systems and methods described above may allow the processor to dynamically update a determination of the driver system resonant frequency as the driver system resonant frequency changes over time. Thus, by generating a modified periodic electrical signal with a frequency adjusted for changes in the driver system resonant frequency, the fast-scan mirror may be driven in a manner that is energy-efficient and results in a clearly displayed image.

Computing system <NUM> may embody the display device <NUM> described above and illustrated in <FIG>. Computing system <NUM> may take the form of one or more personal computers, server computers, tablet computers, home-entertainment computers, network computing devices, gaming devices, mobile computing devices, mobile communication devices (e.g., smart phone), and/or other computing devices, and wearable computing devices such as smart wristwatches and head mounted augmented reality devices.

According to one aspect of the present disclosure, a display device is provided, including a laser beam emitter configured to emit a laser beam. The display device may further include a fast-scan driver system including a nonlinear driver and a fast-scan microelectromechanical (MEMS) sensor. The display device may further include and a slow-scan MEMS driver. The nonlinear driver and the slow-scan MEMS driver may be respectively configured to drive a fast-scan mirror and a slow-scan mirror. The slow-scan mirror and the fast-scan mirror may be configured to reflect the laser beam onto a display region. The display device may further include a signal generator configured to generate a periodic electrical signal having a first frequency in response to receiving periodic electrical signal instructions. The nonlinear driver may be configured to receive the periodic electrical signal from the signal generator. The nonlinear driver may be further configured to amplify the periodic electrical signal to produce an amplified signal. The nonlinear driver may be further configured to drive the fast-scan mirror with the amplified electrical signal. The fast-scan MEMS sensor may be configured to detect a motion of the fast-scan mirror. The display device may further include a signal detector configured to receive the periodic electrical signal from the signal generator and a fast-scan MEMS sensor output signal from the fast-scan MEMS sensor. The signal generator may be further configured to detect an amplitude difference between the periodic electrical signal and the fast-scan MEMS sensor output signal. The display device may further include a processor configured to receive the amplitude difference from the signal detector and determine, based on the amplitude difference, a driver system resonant frequency of the fast-scan driver system.

According to this aspect, the processor may be configured to determine the driver system resonant frequency at least in part by determining a phase difference between the periodic electrical signal and the fast-scan MEMS sensor output signal.

According to this aspect, the second frequency may be the driver system resonant frequency. According to this aspect, the processor may be configured to determine the driver system resonant frequency at least in part by detecting a plurality of amplitude differences in a respective plurality of fast-scan periods that occur during the non-display interval.

According to this aspect, the processor may be configured to generate the modified periodic electrical signal instructions at least in part by iteratively updating the first frequency over the plurality of fast-scan periods.

According to this aspect, the signal generator may include a phase-locked loop. According to this aspect, the processor may be configured to generate the modified periodic electrical signal instructions at least in part by modifying a lock angle of the phase-locked loop to have a target lock angle. According to this aspect, the processor may be configured to determine the driver system resonant frequency at least in part by determining a gain of the signal generator when the fast-scan driver system receives the periodic electrical signal from the signal generator. According to this aspect, during the non-display interval, the slow-scan mirror may return from a final scanning position to an initial scanning position.

According to another aspect of the present disclosure, a method for use with a display device is provided. The method may include scanning a display region with a slow-scan mirror driven by a slow-scan microelectromechanical systems (MEMS) driver. In a fast-scan period of a fast-scan mirror driven by a nonlinear driver, the method may further include, at a signal generator, generating a periodic electrical signal. The periodic electrical signal may have a first frequency. The method may further include transmitting the periodic electrical signal to a signal detector and a nonlinear driver. The method may further include amplifying the periodic electrical signal at the nonlinear driver to produce an amplified signal. The method may further include driving the fast-scan mirror with the amplified electrical signal. At the signal detector, the method may further include receiving the periodic electrical signal and a fast-scan MEMS sensor output signal, detecting an amplitude difference between the periodic electrical signal and the fast-scan MEMS sensor output signal, and transmitting the amplitude difference to a processor. At the processor, the method may further include determining, based on the amplitude difference, a driver system resonant frequency of a fast-scan driver system including the nonlinear driver.

According to this aspect, determining the driver system resonant frequency may include detecting a plurality of amplitude differences in a respective plurality of fast-scan periods that occur during the non-display interval.

According to this aspect, generating the modified periodic electrical signal may include iteratively updating the first frequency over the plurality of fast-scan periods.

According to this aspect, the signal generator may include a phase-locked loop. Generating the modified periodic electrical signal may include modifying a lock angle of the phase-locked loop to have a target lock angle. According to this aspect, determining the driver system resonant frequency may include determining a gain of the signal generator when the fast-scan driver system receives the periodic electrical signal from the signal generator. According to this aspect, scanning the display region may include reflecting a laser beam off the slow-scan mirror and the fast-scan mirror onto the display region.

According to another aspect of the present disclosure, a display device is provided, including a laser beam emitter configured to emit a laser beam. The display device may further include a fast-scan driver system including a nonlinear driver and a fast-scan microelectromechanical (MEMS) sensor. The display device may further include and a slow-scan MEMS driver. The nonlinear driver and the slow-scan MEMS driver may be respectively configured to drive a fast-scan mirror and a slow-scan mirror. The slow-scan mirror and the fast-scan mirror may be configured to reflect the laser beam onto a display region. The display device may further include a signal generator that includes a phase-locked loop and is configured to generate a periodic electrical signal having a first frequency. The nonlinear driver may be configured to receive the periodic electrical signal from the signal generator, amplify the periodic electrical signal to produce an amplified signal, and drive the fast-scan mirror with the amplified electrical signal. The fast-scan MEMS sensor may be configured to detect a motion of the fast-scan mirror. The display device may further include a signal detector configured to receive the periodic electrical signal from the signal generator and a fast-scan MEMS sensor output signal from the fast-scan MEMS sensor. The signal detector may be further configured to detect an amplitude difference between the periodic electrical signal and the fast-scan MEMS sensor output signal. The display device may further include a processor configured to receive the amplitude difference from the signal detector and determine, based on the amplitude difference, a gain of the signal generator. From the gain, the system can determine a current resonant frequency, and from the determined resonant frequency, the system can bias an input signal to the mirror to control the resonant frequency of the mirror.

Claim 1:
A display device (<NUM>) comprising:
a laser beam emitter (<NUM>) configured to emit a laser beam (<NUM>);
a slow-scan mirror (<NUM>);
a slow-scan microelectromechanical, MEMS, control (<NUM>);
a MEMS fast-scan mirror (<NUM>), the MEMS fast-scan mirror (<NUM>) being an electrostatic MEMS fast-scan mirror;
a fast-scan MEMS control (<NUM>),
wherein the slow-scan MEMS control (<NUM>) and the fast-scan MEMS control (<NUM>) are respectively configured to drive the slow-scan mirror (<NUM>) and the MEMS fast-scan mirror (<NUM>) to reflect the laser beam (<NUM>) onto a display region (<NUM>) to generate an image (<NUM>);
wherein the fast-scan MEMS control comprises:
a first control loop (<NUM>) for maintaining a resonance actuation for the MEMS fast-scan mirror (<NUM>),
a second control loop (<NUM>) for maintaining an open angle for the MEMS fast-scan mirror (<NUM>), and
a third control loop (<NUM>) configured to receive a feedback signal from the MEMS fast-scan mirror indicating a current resonant frequency of the MEMS fast-scan mirror (<NUM>),
wherein the third control loop (<NUM>) adjusts a bias of a driving signal to the MEMS fast-scan mirror (<NUM>) to reduce an error between a target resonant frequency and the current resonant frequency; and
wherein adjusting the bias includes: decreasing the bias if the current resonant frequency is below the target resonant frequency and if the bias is positive;
increasing the bias if the current resonant frequency is below the target resonant frequency and if the bias is negative;
increasing the bias if the current resonant frequency is above the target resonant frequency and if the bias is positive; and
decreasing the bias if the current resonant frequency is above the target resonant frequency and if the bias is negative.