Hence, a light transmission technique includes transmitting the beams of light into the field of view from one or two transmission mirrors that use two resonant scanning axes to transmit according to a Lissajous scanning pattern. The transmission mirrors continuously oscillate in resonance about each scanning axes such that the beams of light are projected into the field of view that moves across the field of view as the transmission mirror(s) changes the transmission direction. Moreover, additional conditions are set by the system controller 23 in order to synchronize the oscillations about the two scanning axes for generating a Lissajous scanning pattern as a repeatable pattern.

To make the Lissajous pattern reproduce itself periodically with a frame rate FR frequency [Hz], the two resonant scanning axes are each driven at a target scanning frequency f1, f2 with a defined frequency ratio/difference therebetween that forms a repeatable Lissajous pattern (frame) with a frame rate FR. A new frame begins each time the Lissajous pattern restarts, which occurs when a phase difference between a mirror phase about scanning axis 13x and a mirror phase about scanning axis 13y is zero. The system controller 23 synchronizes the oscillations about the two resonant scanning axes to ensure this defined frequency ratio/difference is maintained based on measurements acquired of the oscillations.

In the examples described herein, frequency f1 is the driving frequency in the time domain of a MEMS mirror (e.g., MEMS mirror 12x or 12xy) about the scanning axis 13x and frequency f2 is the driving frequency in the time domain of a MEMS mirror (e.g., MEMS mirror 12y or 12xy) about the scanning axis 13y. The oscillations about the two scanning axes may drift out of synchronization and must be brought into synchronization by the system controller 23.

FIG. 2 illustrates a signal diagram of various signals generated by a MEMS driver 25 based on the mirror angle θ and/or position, including a position signal (Position_L). For example, the position signal (Position_L) may be a pulsed signal during which a first pulse transition (e.g., falling-edge transition) is triggered at a zero-crossing as the mirror oscillates in a first rotation direction (e.g., from left to right) and a second pulse transition (e.g., rising-edge transition) is triggered at a zero-crossing as the mirror oscillates in a second rotation direction (e.g., from right to left). Furthermore, the signal is “high” when the mirror points in one direction (e.g., points left) and the signal is “low” when the mirror points in a second direction (e.g., points right). Thus, the position signal not only indicates a zero-crossing event by triggering a pulse transition, but also indicates absolute phase information by indicating the directional tilt of the mirror. As the intervals between zero-crossing events increase, the frequency of the position signal decreases. Thus, the position signal is also representative of a measured frequency of a MEMS mirror about a scanning axis.

Alternatively, a short pulse may be generated by each MEMS driver 25x and 25y at the each zero-crossing event such that a pulsed position signal (Position_L) is output to the system controller 23. That is, the signal remains low (or high) between zero-crossing pulses. In this case, the absolute phase information indicating in which direction the mirror is moving would be absent. Based on this position signal a phase and/or a frequency of two or more position signals can be compared.

Based on this position signal, at least one of phase or frequency information of the MEMS mirror can be extracted from the position signal as a measurement of an actual state of the MEMS mirror. Accordingly, driver 25x is configured to receive a measurement signal representative of an entire angular trajectory traversed by the its MEMS mirror throughout its oscillation, detect zero-crossing events at which a value of the measurement signal is detected to be equal to predefined value that corresponds to a zero displacement angle of the MEMS mirror structure with respect to the axis 13x, and generate a position signal that indicates each of the detected first zero-crossing events with a signal transition or a signal pulse. Likewise, driver 25y is configured to receive a measurement signal representative of an entire angular trajectory traversed by the its MEMS mirror throughout its oscillation, detect zero-crossing events at which a value of the measurement signal is detected to be equal to predefined value that corresponds to a zero displacement angle of the MEMS mirror structure with respect to the axis 13y, and generate a position signal that indicates each of the detected first zero-crossing events with a signal transition or a signal pulse.

A measurement signal may be provided to a driver in the following manner. As the mirror oscillates, the capacitance between the finger electrodes changes according to the mirror's rotation position. A MEMS driver is configured to measure the capacitance between the interdigitated finger electrodes, and determine a rotation position or angle position of the MEMS mirror therefrom. By monitoring the capacitance, the MEMS driver can detect the zero-crossing events and a timing thereof, and can determine the tilt angle of the MEMS mirror. It can also generate the position signal by monitoring the capacitance. The MEMS driver can also use the measured capacitance to determine a mirror frequency, and record the information in memory at the MEMS driver or at the system controller.

The sensing of the position of a MEMS mirror is performed based on a detector that is configured to measure the capacitance. For example, as the MEMS mirror moves, the geometry of the finger structure changes, resulting in a change in the geometry of the capacitance. As the geometry of the capacitance changes, the capacitance itself changes. Thus, a specific capacitance corresponds directly with a specific position (i.e., tilt angle) of the MEMS mirror. By sensing the capacitance of the finger structure, the MEMS driver can monitor and track the oscillations of the mirror, and determine a specific position of the MEMS mirror, including the zero-crossing.

One way to measure the capacitance is to measure a current flowing through the finger structure, convert the measured current into a voltage, and then further correlate the voltage to a capacitance and/or a rotation angle. However, any method to measure the capacitance may be used. A rotation direction (e.g., positive or negative, left-to-right or right-to-left, clockwise or counter-clockwise, etc.) is also detected by measuring a change in capacitance over time, where a positive or a negative change indicates opposing rotation directions. The MEMS driver can also record the currents and voltages measured during the measuring of the capacitance. Thus, increasing the accuracy of position sensing of the mirror may improve the overall accuracy of the LIDAR system.

Each MEMS driver 25x and 25y may send respective position information to the system controller 23 so that the system controller 23 can use the position information to control the triggering of the laser pulses of the illumination unit 10. The position information may also be used by the system controller as feedback information such that the system controller 23 can maintain a stable operation of the MEMS mirror 12xy via control signals provided to the MEMS drivers 25x and 25y and also maintain synchronization with other MEMS mirrors.

Hence, a scanning technique includes transmitting the beams of light into the field of view from a transmission mirror that uses two resonant scanning axes to transmit according to a Lissajous scanning pattern. The transmission mirror continuously oscillates in resonance about each scanning axis such that the beams of light are projected into the field of view that moves across the field of view as the transmission mirror changes the transmission direction. Moreover, additional conditions are set by the system controller 23 in order to generate a Lissajous scanning pattern. The following conditions are used to synchronize the driving about the two scanning axes according to the Lissajous pattern.

FIGS. 3A and 3B illustrate a schematic block diagram of a scanning system 300 according to one or more embodiments. FIGS. 3A and 3B represent a single schematic block diagram that share a common border (A) and are intended to be extensions of each other. Thus, FIG. 3A illustrates a first portion (e.g., a left side) of the scanning system 300 and FIG. 3B illustrates a second portion (e.g., a right side) of the scanning system 300.

The scanning system 300 includes a MEMS mirror 12x, a MEMS driver 25x operably coupled to the MEMS mirror 12x, a MEMS mirror 12y, a MEMS driver 25y operably coupled to the MEMS mirror 12y, and a system controller 23 (i.e., a synchronization controller) operatively coupled to the MEMS driver 25x and the MEMS driver 25y. While two 1D MEMS mirrors are illustrated in this example, it will be appreciated that the two MEMS mirrors 12x and 12y can be replaced with a single 2D MEMS mirror, as illustrated in FIG. 1B.

The scanning system 300 employs both phase and frequency synchronization control for both scanning axes 13x and 13y. Both scanning axes are operated in an open loop phase-locked loop (PLL) control mode. A respective target frequency is imprinted on each scanning axis and is therefore rock-solid stable. The phase between expected zero-crossings (ZC) and measured zero-crossings is controlled via change in actuation of the driving signal.

The scanning system 300 includes a reference signal generator 90 that generates two reference signals Ref1 and Ref2 having reference frequencies (i.e., target frequencies) f1 and f2, respectively. Both reference signals Ref1 and Ref2 can be generated by any suitable clock source such as crystal oscillator 91 or an FPGA (not illustrated). In this example, the reference frequencies f1 and f2 are set at 2200 Hz and 2300 Hz, respectively, but are not limited thereto. Since reference frequency f1 is lower than reference frequency f2, reference frequency f1 may be referred to as the slow axis frequency and reference frequency f2 may be referred to as the fast axis frequency. However, it will be appreciated that reference frequency f1 could be set to be higher than reference frequency f2 and could therefore be the faster axis frequency.

In this example, the reference signal generator 90 uses a crystal oscillator 91 to generate a clock signal, a fractional n-divider 92 that divides the clock signal by integer X, a divider 93 that divides by integer M, and a divider 94 that divides by integer N to generate two reference signals Ref1 and Ref2 that have a defined fractional relationship at which mirrors 12x and 12y operate about their respective axes 13x and 13y. Thus, the frequency of mirror 12x is synchronized to frequency f1 via the f1 frequency controller 40 and the frequency of mirror 12y is synchronized to frequency f2 via the f2 frequency controller 60. In addition, dividers 93 and 94 are configured to maintain the defined fractional relationship between the oscillation frequencies of the two axes based on integers N and M (N M, with X, M, and N being positive integers).

Thus, the reference signal generator 90 establishes the fixed frequencies f1 and f2 according to a predefined fractional relationship, and the frequency controllers 40 and 60 are configured to independently maintain the oscillation frequency of their respective mirrors based on respective reference signals Ref1 and Ref2 and respective position signals Position_L1 and Position L2.

The system controller 23 further includes a phase detector 71 that receives both reference signals Ref1 and Ref2 and measures a phase difference therebetween. Because frequencies f1 and f2 are different, the phase difference changes in a periodic manner from 0° to 360°. The phase detector 61 includes logic that evaluates the phase of both reference signals and generates a voltage signal which represents the difference in phase between two reference signals. Alternatively, the phase detector 71 could be configured to receive the two position signals Position_L1 and Position_L2 instead of reference signals Ref1 and Ref2 and measure a phase difference therebetween.

A Lissajous frame start detector 72 receives the phase difference from the phase detector 61 and determines a start of a next Lissajous frame based thereon. In particular, the Lissajous frame start detector 72 detects a start of a Lissajous frame when the phase difference between the reference signals (or the position signals) determined by the phase detector 61 is zero or at some other predetermined value. The next frame starts at this predetermined phase difference.

Thus, the Lissajous frame start detector 72 is configured to generate and output a frame start detection signal upon detecting the predetermined phase difference based on the reference signals Ref1 and Ref2. The next frame signal may be output to a signal processor that processes image data received by LIDAR sensors that utilize the scanning mirrors 12x and 12y. In particular, the signal processor may use the next frame signal to identify a start of a new frame of image data, and thus an end of a current frame.

MEMS driver 25x provides two output signals: a phase error signal PE1 and a position signal (Position_L1) as previously described in reference to FIG. 2. The phase error signal PE1 is a value representative of a timing difference between an expected zero-crossing and a measured zero-crossing of the oscillation about scanning axis 13x. An error value is output for each detected zero-crossing. The phase error signal PE1 is provided to a phase controller 30 of the system controller and the position signal Position_L1 is provide to a frequency controller 40 of the system controller 23.

The phase controller 30 includes a loop filter 31 (e.g., a proportional-integral (PI) controller) that receives the phase error signal PE1 and changes the actuation phase of the driving signal generated by the MEMS driver 25x. The loop filter 31 may change the actuation phase of the driving signal by changing the duty cycle, the HV level, and/or the LV level of the driving signal. Consequently, the phase error will be controlled to a minimum to drive the phase error signal PE1 to zero without affecting the reference frequency.

Specifically, the loop filter 31 is configured to receive the phase error signal PE1 and generate an actuation phase value SAP1 based on the phase error signal, where the loop filter 31 adjusts the actuation value based on the phase error signal to adjust an actuation phase of the MEMS mirror 12x about the axis 13x to minimize the phase error. The actuation phase value SAP1 may be a duty cycle value, an HV level value, or an LV level value for changing the duty cycle, the HV level, or the LV level of the driving signal, respectively, without affecting the actuation frequency of the MEMS mirror 12x about axis 13x.

For example, the driving signal comprises an ON duration and an OFF duration that define a duty cycle of the driving signal, a first signal level set for the ON duration, and a second signal level set for the OFF duration. The actuation phase value SAP1 may control at least one of the ON duration or the OFF duration for controlling the first duty cycle of the first driving signal. Alternatively, the actuation phase value SAP1 may control the first signal level or the second signal level of the driving signal. The second signal level may be a zero or a non-zero voltage.

The frequency controller 40 includes a phase frequency detector (PFD) 41 that is configured to receive position signal Position_L1 from the MEMS driver 25x and a reference signal Ref1, whose frequency is equal to the (target) driving frequency f1. In particular, the PFD 41 receives both signals and determines whether the actual (measured) frequency of mirror 12x is faster, slower, or equal to the target frequency f1 provided by the reference signal Ref1. The PFD 41 includes logic that evaluates the frequency of both signals, determines how the frequency of position signal Position_L1 compares to frequency f1 of the reference signal Ref1, and adjusts a frequency error signal FE1 based on the determination.

The frequency error signal FE1 may be representative of (e.g., proportional to) a difference between reference frequency f1 and the actual MEMS axis' f1. This error value feeds into a loop filter 43 (e.g., a PI controller) that changes the actuation frequency at MEMS axis 13x. Consequently, this actuation frequency is imprinted into the MEMS axis 13x and the oscillation about MEMS axis 13x strictly follows this frequency.

For example, the PFD 41 may generate either an “up” control pulse or a “down” control pulse and provides the respective control pulse to a frequency error signal generator 42. In response to receiving an up-control signal, the frequency error signal generator increments a value of the frequency error signal. Conversely, in response to receiving a down-control signal, the frequency error signal generator decrements the value of the frequency error signal. The frequency error signal FE indicates the frequency relationship between the measured frequency and the reference frequency.

For instance, the frequency error signal FE1 may be a positive value if mirror 12x is operating slower than the reference frequency f1, thus indicating that the frequency of mirror 12x should be increased. Alternatively, the frequency error signal FE1 may be a negative value if mirror 12x is operating faster than the reference frequency f1, thus indicating that the frequency of the mirror 12x should be decreased. Alternatively, the frequency error signal may be a zero if the frequency of the mirror 12x is matched with the frequency of the reference signal Ref1, thus indicating that the current frequency of the mirror 12x should be maintained unchanged.

The frequency controller 40 further includes a loop filter 43 that is configured to receive the frequency error signal FE, and sets a frequency value SAF1 (e.g., a digitally-controlled oscillator (DCO) value) based on the frequency error signal FE. The frequency value SAF1 serves as a configuration setting to a DCO of the MEMS driver 25x. For example, the frequency value SAF1 may adjust the counter speed of the DCO, which in turn adjusts the actuation frequency of the driving signal. For example, the higher the counter speed of the DCO, the higher the actuation frequency. The loop filter 43 then provides the frequency value SAF1 to the MEMS driver 25x, which generates the driving signal based on the frequency value received from the loop filter 43. In turn, the MEMS mirror 12x either oscillates faster or slower in order to match the reference frequency ft Thus, the loop filter 43 changes the actuation frequency of the MEMS mirror 12x with respect to axis 13x based on the frequency of the position signal Position_L1 such that the actual frequency of the MEMS mirror 12x follows the reference frequency f1. The driver 25x generates the driving signal based on the actuation frequency.

Both SAP1 and SAF1 are control values that are independently generated by the system controller 23 to independently control either an actuation phase or an actuation frequency of MEMS mirror 12x about axis 13x without affecting the other actuation parameter. The driving signal is provided to the MEMS mirror 12x to control the phase/frequency of the mirror oscillation such that the phase error and frequency error are zero or substantially zero.

Similarly, MEMS driver 25y provides two output signals: a phase error signal PE2 and a position signal (Position_L2) as previously described in reference to FIG. 2. The phase error signal PE2 is a value representative of a timing difference between an expected zero-crossing and a measured zero-crossing of the oscillation about scanning axis 13y. An error value is output for each detected zero-crossing. The phase error signal PE2 is provided to a phase controller 50 of the system controller and the position signal Position_L2 is provide to a frequency controller 60 of the system controller 23.

The phase controller 50 includes a loop filter 51 (e.g., a PI controller) that receives the phase error signal PE2 and changes the actuation phase of the driving signal generated by the MEMS driver 25y. The loop filter 51 may change the actuation phase of the driving signal by changing the duty cycle, the HV level, and/or the LV level of the driving signal. Consequently, the phase error will be controlled to a minimum to drive the phase error signal PE2 to zero without affecting the reference frequency.

Specifically, the loop filter 51 is configured to receive the phase error signal PE2 and generate an actuation phase value SAP2 based on the phase error signal, where the loop filter 51 adjusts the actuation value based on the phase error signal to adjust an actuation phase of the MEMS mirror 12y about the axis 13y to minimize the phase error. The actuation phase value SAP2 may be a duty cycle value, an HV level value, or an LV level value for changing the duty cycle, the HV level, or the LV level of the driving signal, respectively, without affecting the actuation frequency of the MEMS mirror 12y about axis 13y.

For example, the driving signal comprises an ON duration and an OFF duration that define a duty cycle of the driving signal, a first signal level set for the ON duration, and a second signal level set for the OFF duration. The actuation phase value SAP2 may control at least one of the ON duration or the OFF duration for controlling the first duty cycle of the first driving signal. Alternatively, the actuation phase value SAP2 may control the first signal level or the second signal level of the driving signal. The second signal level may be a zero or a non-zero voltage.

The frequency controller 60 includes a phase frequency detector (PFD) 61 that is configured to receive position signal Position_L2 from the MEMS driver 25y and a reference signal Ref2 whose frequency is equal to the (target) driving frequency f2. In particular, the PFD 61 receives both signals and determines whether the actual (measured) frequency of mirror 12y is faster, slower, or equal to the target frequency f2 provided by the reference signal Ref2. The PFD 61 includes logic that evaluates the frequency of both signals, determines how the frequency of position signal Position_L2 compares to frequency f2 of the reference signal Ref2, and adjusts a frequency error signal FE2 based on the determination.

The frequency error signal FE2 may be representative of (e.g., proportional to) a difference between reference frequency f2 and the actual MEMS axis' f2. This error value feeds into a loop filter 63 (e.g., a proportional-integral (PI) controller) that changes the actuation frequency at the MEMS axis 13y. Consequently, this actuation frequency is imprinted into the MEMS axis 13y and the oscillation about MEMS axis 13y strictly follows this frequency.

For example, the PFD 61 may generate either an “up” control pulse or a “down” control pulse and provides the respective control pulse to a frequency error signal generator 62. In response to receiving an up-control signal, the frequency error signal generator increments a value of the frequency error signal. Conversely, in response to receiving a down-control signal, the frequency error signal generator decrements the value of the frequency error signal. The frequency error signal FE indicates the frequency relationship between the measured frequency and the reference frequency.

For instance, the frequency error signal FE2 may be a positive value if mirror 12y is operating slower than the reference frequency f2, thus indicating that the frequency of mirror 12y should be increased. Alternatively, the frequency error signal FE2 may be a negative value if mirror 12y is operating faster than the reference frequency f2, thus indicating that the frequency of the mirror 12y should be decreased. Alternatively, the frequency error signal may be a zero if the frequency of the mirror 12y is matched with the frequency of the reference signal Ref2, thus indicating that the current frequency of the mirror 12y should be maintained unchanged.

The frequency controller 60 further includes a loop filter 63 that is configured to receive the frequency error signal FE, and sets a frequency value SAF2 (e.g., a digitally-controlled oscillator (DCO) value) based on the frequency error signal FE. The frequency value SAF2 serves as a configuration setting to a DCO of the MEMS driver 25y. For example, the frequency value SAF2 may adjust the counter speed of the DCO, which in turn adjusts the actuation frequency of the driving signal. For example, the higher the counter speed of the DCO, the higher the actuation frequency. The loop filter 63 then provides the frequency value SAF2 to the MEMS driver 25y, which generates the driving signal based on the frequency value received from the loop filter 63. In turn, the MEMS mirror 12y either oscillates faster or slower in order to match the reference frequency f2 without affecting the mirror phase. Thus, the loop filter 63 changes the actuation frequency of the MEMS mirror 12y with respect to axis 13y based on the frequency of the position signal Position_L2 such that the actual frequency of the MEMS mirror 12y follows the reference frequency f2. The driver 25y generates the driving signal based on the actuation frequency.

Both SAP2 and SAF2 are control values that are independently generated by the system controller 23 to independently control either an actuation phase or an actuation frequency of MEMS mirror 12y about axis 13y without affecting the other actuation parameter. The driving signal is provided to the MEMS mirror 12y to control the phase/frequency of the mirror oscillation such that the phase error and frequency error are zero or substantially zero.

FIGS. 4A and 4B illustrate a schematic block diagrams of control systems of the scanning system 300 according to one or more embodiments. The control systems includes the phase controller 30 for slow axis phase control, the frequency controller 40 for slow axis frequency control, the phase controller 50 for fast axis phase control, the frequency controller 60 for fast axis frequency control. The phase controller 50 includes similar components to that of phase controller 30 and the description of the phase controller 30 can be similarly extended to the phase controller 50. Likewise, the phase controller 60 includes similar components to that of phase controller 40 and the description of the phase controller 40 can be similarly extended to the phase controller 60.

The phase controller 30 incudes a programmable delay unit 32 and a phase detector 33 that may reside in the MEMS driver 25x. The programmable delay unit 32 receives a zero-crossing (ZC) reference signal and applies a programmable delay thereto to generate a delayed ZC reference signal. The ZC reference signal indicated expected zero-crossings of the MEMS mirror 12x about axis 13x. The programmable delay may be imposed to account for the amount of time it takes for the MEMS driver 25x to measure actual zero-crossing events at the MEMS mirror 12x and generate a measurement signal 0° ZC Comp that is indicative of measured (actual) zero-crossing events. The phase detector 33 is configured to receive the delayed ZC reference signal and the measurement signal 0° ZC Comp, measure a phase difference therebetween, and output the phase error signal PE1 representative of the measured phase difference. Loop filter 31 then generates an actuation value SAP1 that regulates the actuation phase of MEMS mirror 12x based on the phase error signal PE1. The loop filter 31 may be a PI controller that includes two gain factors k1 and k2, an integrator 31i