Patent Description:
<CIT> describes a scanning beam projection system including a scanning mirror having a fast-scan axis and a slow-scan axis. Movement on the slow-scan axis is controlled by a slow-scan scanning mirror control system. The control system receives position information describing angular displacement of the mirror. An outer loop of the control system includes least mean square (LMS) tone adders that determine harmonically related signals that when combined produce a scanning mirror drive signal. An inner loop of the control system compensates for a scanning mirror resonant vibration mode at a frequency within the frequency band occupied by the harmonically related signals.

Examples are disclosed herein related to controlling a scanning display system. One example provides a display device comprising a light source; a scanning mirror system configured to scan light from the light source in a first direction at a first, higher scan rate, and in a second direction at a second, lower scan rate; and a drive circuit configured to control the scanning mirror system to display video image data by providing a control signal to the scanning mirror system to control scanning in the second direction, and for each video image data frame of at least a subset of video image data frames, combining the control signal with an adjustment signal to adjust the scanning in the second direction, the adjustment signal comprising a low pass filtered signal with a cutoff frequency based on a lowest resonant frequency of the scanning mirror system in the second direction.

A scanning display system may utilize a microelectromechanical system (MEMS) mirror system to scan light from a light source to form an image for display. <FIG> shows a block diagram of an example MEMS scanning display device. Display device <NUM> comprises one or more light sources <NUM>, (e.g. lasers) that output light to a scanning mirror system <NUM>. The scanning mirror system <NUM> is configured to scan the light in a first scan direction <NUM> (e.g. horizontally) and in a second scan direction <NUM> (e.g. vertically). The scanning mirror system <NUM> may include a single mirror driven in both horizontal and vertical directions, or two mirrors separately driven in horizontal and vertical directions. The resulting image is provided to an output <NUM> for display. Output <NUM> may assume any suitable form, such as a display surface, projection optics, waveguide optics, etc. As examples, display device <NUM> may be configured as a virtual reality head-mounted display (HMD) device with output <NUM> configured as an opaque surface, or as an augmented reality HMD device with the output configured as a see-through structure that allows virtual imagery to be combined with a view of the surrounding real-world environment. Display device <NUM> may assume other suitable forms, such as that of a head-up display, mobile device screen, monitor, television, etc..

Display device <NUM> further comprises a controller <NUM> configured to control operation of the light source(s) <NUM>, scanning mirror system <NUM> and other device components. The controller <NUM> comprises a drive circuit <NUM> configured to provide signals to the scanning mirror system <NUM> to control scanning in each direction. Different scan rates may be used to scan in the first and second scan directions. For example, the display device <NUM> may scan in the first scan direction at a resonant frequency of the mirror, and in the second scan direction approximately at a frame rate of the video data. The one or more scanning mirrors may take any suitable form, such as resonant piezoelectric-actuated mirrors.

However, the resonant frequency of the mirror for the faster scan direction may not be an exact multiple of the frame rate of video data being displayed. Unless this issue is mitigated, the scan of images in sequential frames of image data will start at different locations in the image frames. One possible solution to this problem is to apply an adjustment signal to the slow scan mirror during a non-active portion of the scan, e.g. between completing the scan of one frame and starting the scan of the next frame, thereby shifting the position vertically and/or trajectory of the mirror by a suitable amount for the scan of the next frame to start at a correct location. Such an operation may be performed for each frame, or for only a subset of frames, to maintain a suitable level of synchronization between the video data and the scanning mirror system.

However, it may be challenging to achieve a precise shift in position within the duration of a non-active portion of a scan. For example, performing such an adjustment may require the position and velocity of the slow-scanning mirror to be precisely controlled at a frequency that is not the natural resonant frequency of the mirror. Even minute amounts of position error or velocity ripple during an active portion of a scan may create image artifacts. Such artifacts may arise from "ringing" due to harmonic oscillations in the slower scan direction that result from the shift. This ringing must be resolved sufficiently fast to allow the next frame scan to begin without producing visible artifacts.

One possible solution is to apply the adjustment signal as soon as the active portion of a current scan is completed, and then wait until the resonance-dependent "ringing" has decayed to an imperceptible level. However, such waiting may prolong the time when the next active can may begin, which may result in an unacceptable reduction of the scanned image size or frame rate.

Accordingly, examples are disclosed herein that relate to adjusting the scanning in the slower scan direction in a manner that may avoid such ringing. Briefly, the disclosed examples utilize a low pass filtered adjustment signal that includes a cutoff frequency based on a lowest resonant frequency of the vertical scanning mirror. Such an adjustment signal helps to avoid energizing the resonant frequencies of the scanning mirror system in the slow scan direction, and thereby helps to avoid ringing that leads to such artifacts. Further, the use of the low pass filtered adjustment signal also allows the signal to be applied prior to the end of scanning of a prior frame of data, as described in more detail below. The term "low pass filtered adjustment signal" as used herein includes signals that pass through a low pass filter prior to being combined with the control signal, recordings of such signals, and synthetic signals having the characteristics of a low pass filtered adjustment signal as disclosed herein.

<FIG> shows a block diagram of a MEMS mirror scanning system <NUM> that applies an adjustment signal without low pass filtering prior to summation with the control signal. Block <NUM> represents circuitry for producing a control signal for obtaining a steady-state scanning trajectory for the slow scan direction of mirror system <NUM>. The control signal, shown schematically as the upper trace in block <NUM>, has the form of a sawtooth in which the long slope represents mirror movement during image scanning and the short slope represents the mirror return between image frames. The control signal is amplified at a gain block <NUM> and passes through a low pass filter <NUM> prior to reaching the MEMS mirror system <NUM> (which may apply additional gain). The resulting mirror position <NUM> may be continuously provided as feedback to both an inner feedback loop at <NUM> and an outer feedback loop at <NUM>. The outer feedback loop <NUM> may be configured to monitor the quality of the steady state trajectory by comparing the received signal to a desired, ideal sawtooth waveform, generate an error signal based on this comparison, and provide the error signal to the inner feedback loop. The inner feedback loop <NUM> may be configured to apply changes to the control signal to correct for the error signal, helping to achieve a result that is closer to the desired trajectory. System <NUM> may further comprise a low pass filter <NUM> between the outer and inner feedback loops. A low pass filter <NUM> may be used to help the inner loop follow the requested scan trajectory specified by the outer loop.

MEMS mirror scanning system <NUM> further comprises an adjustment signal circuit <NUM> configured to add an adjustment signal to the control signal at summing block <NUM>. The adjustment signal, shown schematically as the lower trace in block <NUM>, takes the form of a step. As described above, applying such a step may result in an undesired "ringing" effect due to the high frequency components in the signal, which provide energy to the slow scan direction of the mirror system <NUM> at one or more resonant frequencies. Applying the adjustment signal upstream of low pass filter <NUM> may help to reduce some high-frequency components and thus mitigate the ringing to some degree. However, because low pass filter <NUM> has a relatively wide cutoff frequency region, such mitigation may be inadequate to reduce ringing below a perceptible amount.

<FIG> shows a plot <NUM> of a response of an example MEMS mirror to the application of an adjustment signal applied as a step change in the example of <FIG>. In this example, the adjustment signal is applied after scanning of the prior frame has completed. As illustrated, the step signal adjusts the mirror position to a desired position sufficiently fast; however, the resulting "ringing" of the mirror, shown as oscillations about the desired mirror position, continues at an undesirable magnitude (as shown by example perceptibility band <NUM>) for an unsuitably long duration.

As another possible strategy, instead of a step signal, a more gradual ramp signal may be used as an adjustment signal. However, such a signal still may have high frequency components that energize the mirror system at a resonant frequency. As a result, the use of a sufficiently slow ramp to avoid excessive ringing may result in the ramp being too slow to be performed in the time between image frame scanning. This is illustrated as plot <NUM> in <FIG>. While such a ramp signal may help to reduce the prolonged ringing of the response shown in plot <NUM>, the position of the mirror still goes outside the band <NUM> until the sixth approach to the desired position. Plot <NUM> reaches the desired position the first time more slowly than plot <NUM> but settles within the band <NUM> sooner, though still may not settle as quickly as desired.

As another possible solution to achieve a faster transition, an adjustment step of a larger magnitude than required (an "overshoot") may first be applied, and then a negative, dampening step may be applied to cancel out unwanted effects of the initial signal. Such an overshooting-then-dampening approach may be referred to as a "fine-tuning" method. However, with such a fine-tuning method, the overshoot and dampening steps must be specifically tuned to the system during manufacturing, and may require periodic re-tuning throughout the lifetime of the system to account for changes in the system that may affect the system's natural resonant frequencies (e.g. changes due to heat, age, etc.).

<FIG> shows a Bode magnitude plot of the frequency response of an example MEMS mirror scanning system (position behavior of the MEMS scanning system versus the input drive), excluding the closed-loop feedback system of <FIG> (which may change some of the resonances of the system). When applying a fine-tuning as described above, the goal of various specific overshoot and dampening signals applied would be to essentially form an inverse of the illustrated Bode plot to cancel out the peaks. However, as the system ages or changes, these peaks may change, requiring re-tuning over the device lifetime.

In contrast to the above solutions, the use of a low pass filtered adjustment signal does not require such fine-tuning. More specifically, instead of tuning an adjustment signal to dampen the specific peaks in a Bode magnitude plot, the use of a low pass filtered adjustment signal attenuates frequencies above a cutoff frequency that is based on a lowest anticipated resonant frequency of the slower scanning mirror. As such, no re-tuning of the adjustment signal is needed as the system changes over time (however the cutoff frequency of the low pass filter may be selected to allow for some variation in the resonant frequencies of the system over time to improve response time). The low pass filter applied to the adjustment signal may have any suitable cutoff frequency. In some examples, the low pass filter may be sufficiently close to the lowest resonant frequency of the slower scan direction system to allow some ringing to occur after application of the adjustment signal, as long as the ringing does not create perceptible artifacts (e.g. the magnitude of the artifacts is below a perceptible threshold level).

<FIG> shows a block diagram of an example MEMS scanning mirror system <NUM> configured to synchronize a scanning mirror with a video data frame using a low pass filtered adjustment signal. System <NUM> is configured similarly to system <NUM> described above. For example, system <NUM> comprises a drive circuit configured to generate a control signal for obtaining a steady-state scanning trajectory at <NUM> to control the MEMS mirror system <NUM> in the slow scan direction. The control signal passes through a gain block <NUM> and low pass filter <NUM> prior to reaching MEMS system <NUM>. MEMS system <NUM> may apply additional gain.

Further, system <NUM> comprises an adjustment signal circuit, two examples of which are illustrated at <NUM> and <NUM>, configured to sum the control signal with a low pass filtered adjustment signal at summing block <NUM>.

First referring to block <NUM>, in some examples the adjustment signal circuit may include a low pass filter for filtering a "step" signal <NUM>. In view of the very short time between image frame scans in which the mirror adjustment is to be performed, it may be desirable to utilize an aggressive low pass filter (e.g. a <NUM>-pole low pass filter), to achieve a narrow cutoff frequency region. This may allow the low pass filter cutoff frequency to be positioned closer to the lowest resonant frequency of the mirror system. In other examples, any other suitable low pass filter may be used. Suitable low pass filters include aggressive low pass filters that do not have any overshoot in the step response that exceeds the desired band size. Next referring to block <NUM>, in other examples the adjustment signal may take the form of a pre-recorded waveform that is intended to reproduce the output of the low pass filter, at <NUM>. In either case, the use of a signal that has been filtered as described herein may help to achieve ringing below a desired percentage of the shift magnitude within the time between frames.

In the example of <FIG>, the low pass filtered signal is shown as being combined with the control signal downstream of another low pass filter <NUM> in the signal path. <FIG> shows another example MEMS scanning mirror system <NUM> in which a low pass filtered signal and the control signal are combined upstream of a low pass filter <NUM>. In such a system, the control signal and/or the adjustment signal may be pre-distorted to proactively "undo" any effect of the downstream low pass filter <NUM>.

Returning to <FIG>, in some examples, the low pass filtered signal also may be combined with the feedback signal going to the outer feedback loop, at <NUM>. This may help prevent the adjustment in the control signal from being interpreted as a disturbance by the outer feedback loop.

<FIG> shows a plot <NUM> of an example response of a MEMS mirror to the application of a low pass filtered adjustment signal. <FIG> also shows magnifications <NUM> and <NUM> of selected regions of the plot of response <NUM>. First with reference to plot <NUM>, the low pass filtered adjustment signal permits a relatively fast transition between the unadjusted and adjusted signal states. Further, with reference to plot <NUM>, because the low pass filtered adjustment signal has a rounded, relatively gradual initial portion due to the filtering, the mirror response does not reach a perceptibility threshold for a fraction of a millisecond after application of the adjustment signal. As such, the adjustment signal may be applied prior to reaching a last active pixel of a current video image data frame, thereby allowing the adjustment signal to be applied at an earlier time relative to a sharp step or ramp. Once the last pixel of the previous frame is swept, the mirror response ramps up sufficiently fast to reach the desired starting position for the next scan before the start of the first pixel of the next scan. Further, as illustrated, any ringing after application of the signal is beneath the illustrated example perceptibility threshold, thereby allowing the scan of the next image frame immediately upon entering the example perceptibility band. This ringing may be determined based upon how close the cutoff frequency of the low pass filter is to the lowest resonant frequency of the mirror and on a sharpness of the cutoff frequency, and thus can be adjusted by filter design. <FIG> shows plot <NUM> in comparison to the plots caused by adjustment signals without low pass filtering characteristics as described herein.

As can be seen in magnified plot <NUM>, the ramp of the response before reaching the perceptibility band is not a straight incline, but instead becomes less steep as the band is approached due to ringing. In some examples, this may be mitigated by adjusting the input signal applied to the low pass filter. For example, the input signal applied to the low pass filter may include an initial overshoot portion to maintain a desired ramp profile, and thereby to help the mirror reach the desired position more quickly.

<FIG> shows a plot <NUM> of an output of a low pass filter in an adjustment signal path, and a plot <NUM> of an overall system output with the low pass filter applied.

While described in the context of a MEMS scanning display device, a low pass filtered adjustment signal as disclosed herein may be used to adjust any other suitable MEMS system.

The computing system <NUM> is shown in simplified form. The 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), wearable devices, and/or other computing devices.

The computing system <NUM> includes a logic subsystem <NUM> and a storage subsystem <NUM>. The computing system <NUM> may optionally include a display subsystem <NUM>, input subsystem <NUM>, communication subsystem <NUM>, and/or other components not shown in <FIG>.

The logic subsystem <NUM> includes one or more physical devices configured to execute instructions. For example, the logic subsystem <NUM> may be configured to execute instructions that are part of one or more applications, services, programs, routines, libraries, objects, components, data structures, or other logical constructs.

The logic subsystem <NUM> may include one or more processors configured to execute software instructions. Additionally or alternatively, the logic subsystem <NUM> may include one or more hardware or firmware logic machines configured to execute hardware or firmware instructions. Processors of the logic subsystem <NUM> may be single-core or multicore, and the instructions executed thereon may be configured for sequential, parallel, and/or distributed processing. Individual components of the logic subsystem <NUM> optionally may be distributed among two or more separate devices, which may be remotely located and/or configured for coordinated processing. Aspects of the logic subsystem <NUM> may be virtualized and executed by remotely accessible, networked computing devices configured in a cloud-computing configuration.

The storage subsystem <NUM> includes one or more physical devices configured to hold instructions executable by the logic machine to implement the methods and processes described herein. When such methods and processes are implemented, the state of the storage subsystem <NUM> may be transformed-e.g., to hold different data.

The storage subsystem <NUM> may include removable and/or built-in devices. The storage subsystem <NUM> may include optical memory (e.g., CD, DVD, HD-DVD, Blu-Ray Disc, etc.), semiconductor memory (e.g., RAM, EPROM, EEPROM, etc.), and/or magnetic memory (e.g., hard-disk drive, floppy-disk drive, tape drive, MRAM, etc.), among others. The storage subsystem <NUM> may include volatile, nonvolatile, dynamic, static, read/write, read-only, random-access, sequential-access, location-addressable, file-addressable, and/or content-addressable devices.

It will be appreciated that the storage subsystem <NUM> includes one or more physical devices.

Aspects of the logic subsystem <NUM> and the storage subsystem <NUM> may be integrated together into one or more hardware-logic components.

When included, the display subsystem <NUM> may be used to present a visual representation of data held by the storage subsystem <NUM>. As the herein described methods and processes change the data held by the storage machine, and thus transform the state of the storage machine, the state of the display subsystem <NUM> may likewise be transformed to visually represent changes in the underlying data. The display subsystem <NUM> may include one or more display devices utilizing virtually any type of technology. Such display devices may be combined with the logic subsystem <NUM> and/or the storage subsystem <NUM> in a shared enclosure, or such display devices may be peripheral display devices.

Claim 1:
A display device (<NUM>), comprising:
a light source (<NUM>);
a scanning mirror system (<NUM>) configured to scan light from the light source in a first direction at a first, higher scan rate, and in a second direction at a second, lower scan rate;
a controller (<NUM>) comprising a drive circuit (<NUM>) configured to control the scanning mirror system to display video image data by
providing (<NUM>) a control signal to the scanning mirror system to control scanning in the first and second directions, and
for each video image data frame of at least a subset of video image data frames, combining (<NUM>) the control signal with an adjustment signal to adjust the scanning in the second direction, the display device being characterized in that the controller comprises an adjustment signal circuit comprising:
a low-pass filter having a cut-off frequency based on a lowest resonant frequency of the scanning mirror system in the second direction and adapted to output the adjustment signal, and
a summing block adapted to sum the control signal and the adjustment signal;
wherein the display device is adapted to provide a step signal to the low-pass filter and to apply the adjustment signal to the second direction of the scanning mirror system during a non-active portion of the scan thereby shifting the position vertically and/or trajectory of the mirror by a suitable amount for the scan of the next frame to start at a correct location.