LASER RADAR PROJECTOR

A laser radar projection system is provided. The system includes a laser projector that projects a light beam. A beam splitter is arranged to receive the light beam from the projector and divides the light beam into a signal light beam and a reference light beam. A steering system changes the direction of the signal light beam and scans the light beam over at least a portion of the surface. An optical signal detector is arranged to receive a feedback light beam and the reference light beam. The optical signal detector generates a feedback signal in response to the feedback light beam and a reference signal in response to the reference light beam. One or more processors determine the distance to one or more points on the at least a portion of the surface based at least in part on the feedback signal and the reference signal.

BACKGROUND

The subject matter disclosed herein relates to a laser radar projector, and in particular to a laser radar projector with a reference channel having about the same optical power as the measurement channel.

Many of today's advanced production processes require in-line quality control and in-process verification. This is especially important, for example, in aircraft manufacturing, where most of assembly operations are manual. In these types of applications, one hundred percent quality assurance is often desired. Hence, in-process measurement of 3-dimensional structures, parts, and assemblies is frequently used for validation. In a number of applications, especially involving composite materials, the non-contact methods are used for inspection purposes.

Further, laser systems commonly referred to as laser projectors are also widely used in contemporary manufacturing. Laser scanning technique in the form of laser projection is often utilized in production processes as a templating method in manufacturing of composite parts, in aircraft and marine industries or other large machinery assembly processes, truss building, painting, and other applications. It gives the user ability to eliminate expensive hard tools, jigs, templates, and fixtures. Laser projectors utilize computer-assisted design (CAD) data to generate glowing templates on a 3D object surface. Glowing templates generated by laser projection are used in production assembly processes to assist in the precise positioning of parts, components, and the like on any flat or curvilinear surfaces. Laser projection technology brings flexibility and full CAD compatibility into the assembly process. In the laser assisted assembly operation, a user positions component parts by aligning some features (edges, corners, etc.) of a part with the glowing template. After the part positioning is completed, the user fixes the part with respect to the article being assembled. However, the accuracy of laser projection, and, consequently, of the assembly process, is only adequate if the object is built exactly up to its CAD model. This is not the case for all applications, and as such there are a number of non-trivial issues associated with such applications. The combination of the laser projector with laser light detection and ranging (“LIDAR”) provides a system that performs both placement and verifying functions.

Accordingly, while existing laser radar projectors are suitable for their intended purposes the need for improvement remains, particularly in providing a laser radar projector having features described herein.

BRIEF DESCRIPTION

According to one aspect of the disclosure a laser radar projection system is provided. The laser radar projection system includes a laser projector that projects a light beam. A beam splitter is arranged to receive the light beam from the laser projector, wherein in operation the beam splitter divides the light beam into a signal light beam and a reference light beam. A steering system is provided that in operation changes the direction of the signal light beam onto the surface of an object and in operation scan the light beam over at least a portion of the surface, wherein the projected light beam is diffusely reflected from the surface as a feedback light beam. An optical signal detector is arranged to receive the feedback light beam and the reference light beam, the optical signal detector generating a feedback signal in response to receiving the feedback light beam and a reference signal in response to receiving the reference light beam. One or more processors that are responsive to executable computer instructions are provided for determining the distance to one or more points on the at least a portion of the surface based at least in part on the feedback signal and the reference signal.

According to another aspect of the disclosure a method of determining three-dimensional coordinates of at least one point on a surface of an object is provided. The method includes emitting a beam of light from a laser projector. The beam of light is divided with a beam splitter into a signal light beam and a reference light beam. The signal light beam is directed onto at least one point on a surface of an object and diffusely reflecting the signal light beam as a feedback light beam. The feedback light beam is received and the feedback light beam directed along a first path to an optical signal detector. The reference light beam is transmitted along a second path onto the optical signal detector.

According to yet another aspect of the disclosure a laser radar projection system is provided. The laser radar projection system including a laser projector that projects a light beam. A beam splitter is arranged to receive the light beam from the laser projector, wherein in operation the beam splitter divides the light beam into a signal light beam and a first reference light beam. An optical modulator is arranged to receive the first signal light beam and operable to bifurcate the first signal light beam into a zero-order light beam and a first-order light beam, the optical modulator being controlled by an input voltage. An attenuator is arranged to receive the first reference light beam and output a second reference beam, the second reference beam having a reference optical power level that is less than an optical power of the first reference beam. A steering system is provided that in operation changes the direction of the first order light beam onto the surface of an object and in operation scan the light beam over at least a portion of the surface, wherein the projected light beam is diffusely reflected from the surface as a feedback light beam. An optical signal detector is arranged to receive the feedback light beam and the reference light beam, the optical signal detector generating in operation a feedback signal in response to receiving the feedback light beam and a reference signal in response to receiving the reference light beam. One or more processors are provided that are responsive to executable computer instructions for determining the distance to one or more points on the at least a portion of the surface based at least in part on the feedback signal and the reference signal. The one or more processors are further responsive to applying a synchronized periodic waveform control signal to the input voltage to change the intensity of the first-order light beam.

DETAILED DESCRIPTION

Embodiments of the present invention provide for a laser radar projector having a feedback beam of light and a reference beam of light measured by a single optical sensor. Further embodiments of the present invention provide for a laser radar projector wherein an optical power level of a reference light beam is reduced to have an optical power level at the optical sensor that is substantially equal to the optical power level of the feedback light beam.

Referring now toFIG. 1, an embodiment is shown of a laser radar projector system700. The system700emits a light beam onto a surface of an object in the environments. As discussed herein, in some embodiments, the light beam is traced over a predetermined path at a rapid rate to generate a template or a light pattern on the surface205(FIG. 2) of object200. The system700is also operable to measure the distance from the system700to the surface205of object200and determine the three-dimensional coordinates of points on the surface205. The laser radar projector system700includes a projection subsystem720and a feedback subsystem730. The projection subsystem720includes a light source701, such as a laser light source that is operable to emit pulses of laser light at a rate of 50 kHz to 100 kHz. The light of laser source701is used in both functions of the system700: laser projection and non-contact 3D measurement by scanning and ranging. In an embodiment, the light source701emits a light712at a green wavelength of 532 nanometers and has a pulse duration of about 250-500 picoseconds. The light beam712emitted from laser701may have a diameter of about 0.4 to 1.0 millimeters. In an embodiment, the average output power of the laser701is about 25 to 30 milliwatts that is adequate, after power losses in the system, to provide the average power of the output beam715up to 5 milliwatts. Output laser beam average power of the system700within 5 milliwatts corresponds to the Laser Safety Class 3R according to the International Standard IEC 60825-1. In another embodiment, the system700could have an output average beam power within 1 milliwatt that corresponds to the Laser Safety Class 3R of International Standard IEC 60825-1.

The light source701emits a pulsed light beam712that strikes a beam splitter740. The beam splitter740reflects a reference light portion742of the light beam712towards an attenuator744. In an embodiment, some of the light742reflected by the beam splitter740passes through a lens743that focuses the light into the optical fiber747. In the exemplary embodiment, the beam splitter740reflects about 1% of the light712towards the attenuator744. In the exemplary embodiment, the beam splitter may be a Beam Sampler manufactured by THORLABS, INC. of Newton, N.J. The optical fiber747is preferably a single mode type of fiber. A single mode fiber, for example, for a green light, has the fiber core about 4 micrometers in diameter. The light from the fiber747travels through the attenuator744, through an output fiber745and is launched into the detector body760via an opening764. In the exemplary embodiment, the attenuator744is a variable micro-electromechanical-system (MEMS) such as that manufactured by DICON FIBEROPTICS, INC. of Richmond, Calif. for example. It should be appreciated that other types of optical attenuators may also be used provided that they allow to reduce the optical power of the reference light742, these attenuators include but are not limited to different kind of variable attenuators, such as loopback attenuators, liquid crystal variable attenuators, electro-optical and acousto-optical attenuators and alike. As will be discussed in more detail herein, the attenuator744changes the optical power of the reference light742to be similar or substantially equal to the optical power of the feedback light beam that is reflected from the surface205. This provides advantages in maintaining a similar dynamic range of signals at the optical sensor710between the reference light beam and the feedback light beam. As will be discussed in more detail herein, the output of the attenuator744is a fiber optic cable745that routes the reference light beam to an opening764in the detector body that allows the light to strike an optical sensor770(FIG. 3).

The light that passes through the beam splitter740is directed toward an acousto-optical modulator (AOM)703. The AOM703serves as a beam shutter and attenuator thus adjusting the power of the output beam715directed toward the object200. In an embodiment, the AOM703works similar to that described by Xu, Jieping and Stroud, Robert, Acousto-optic Devices: principles, design and applications, John Willey & Sons, Inc., 1992, the contents of which are incorporated by reference herein. In the preferred embodiment, the AOM703is an AO Frequency Shifter Model 1205-1118 manufactured by ISOMET CORP. of Springfield, Va. USA. The AOM703splits the incoming laser light beam into a first order beam746and a zero-order beam748. The intensity or optical power of the first order beam746depends on a control signal transmitted from the controller400(FIG. 4) to the AOM703. Depending on a control signal, part of the incoming light is redirected from zero-order748to the first order746. Therefore, the intensity of the first order beam746may be varied based on the control signal. In an embodiment, the zero-order beam748is directed into a plate750. In an embodiment, when no control signal is provided to the AOM703, substantially all of the incoming light beam is being blocked by the plate750.

The first order beam746further passes through a beam expander/collimator702which outputs a light beam752. The beam expander702typically consists of two lenses (not shown inFIG. 1) collimating the beam746and expanding its diameter about 10 to 15 times. The output lens of the beam expander702may be moved in the directions indicated by arrow713to allow adjustment of the size and convergence of the beam752(and, therefore, the beam715) thus focusing the output beam715as a cone of light115(FIG. 2) into a focused laser spot210on the surface205of the object200. In an embodiment, the beam expander702is coupled to a motor (not shown). This allows the signal light beam715to be focused onto a desired focusing point as a cone115. The light beam752that is coming out of the beam expander702is directed toward the beam splitter704. The beam splitter704reflects a portion of the light beam752as light beam716. The light beam716strikes a beam dump711and it dissipated. The remainder of the light beam752passes through the beam splitter and proceeds as signal light beam714. The signal light beam714proceeds to steering system754. The steering system754directs the signal light beam715from the system700towards the object200. In the exemplary embodiment, the steering system754includes a first mirror705and a second mirror706. As discussed in more detail herein, the mirrors705,706each are coupled to a galvanometer403,404(FIG. 4) that allows the selective changing of the angle of the mirror relative to the incoming light beam to allow the changing of the direction of the signal light beam715. It should be appreciated that the use of mirrors with galvanometers is for exemplary purposes and the claims should not be so limited. In other embodiments, the steering system754may include a rotating mirror that rotates about an axis that is substantially collinear with the optical axis of the light beam714. In still other embodiments, the steering system754includes a gimbal arrangement that is rotatable about a pair of orthogonal axes. In this arrangement, the signal light beam715may be emitted directly from the beam splitter704. In yet another embodiment the beam steering system may be based on electro-optical phase array.

In operation the signal light beam715is emitted from the system700converges into a cone115and strikes the surface205on the object200. In this embodiment, the signal light beam715is focused on a spot210. Typically, the surface205reflects the light diffusely, and the reflected light211is directed widely back towards the system700. It should be appreciated that a portion of this reflected light211, referred to herein as the feedback light beam215, is directed back towards the system700. In the embodiment ofFIG. 1, the feedback light beam enters the system700via the mirrors706,705and into the optical feedback subsystem730. The feedback light beam is transmitted towards the beam splitter704along the same optical path as light beam714. The feedback light beam is reflected off of the beam splitter704as light beam717towards mirror707which decouples the feedback light beam from the shared path with light beam714. The light beam717further passes through a focusing lens708and spatial filter709. The feedback light beam717then passes through a beam size lens756before passing through an opening762in the detector body760and striking the optical sensor710. In an embodiment, the optical sensor710is a photomultiplier tube or a hybrid photo detector such as Model R10467U-40 or Model R11322U-40 high speed compact hybrid photo detector manufactured by HAMAMATSU PHOTONICS K.K. of Iwata City, Japan. In an embodiment, a neutral density filter757is movable in the direction758into or out of the optical path of feedback light beam between the beam size lens756and the opening762. In an embodiment, insertion of the neutral density filter757into the optical path based on the brightness of the feedback light beam.

In an embodiment, the lens708, spatial filter709and beam dump711cooperate to suppress undesired background light. In an embodiment, the background light suppression may be accomplished in the manner described in co-owned U.S. Pat. No. 8,582,087, the contents of which is incorporated herein by reference. In an embodiment, the spatial filter709contains centrally located pinhole formed in a disk-shaped mask as described in the above reference '087 patent. Since the background light that goes through the lens708is not collimated it is not concentrated on the pinhole but rather over an area of the mask. The arrangement of the pinhole and the mask thus substantially blocks the undesired background light from striking the optical sensor710.

In an embodiment, the output of fiber optical cable745emits the reference light beam774towards the diffuser766as shown inFIG. 3. The diffuser766diffuses the incoming light and has been found, in combination with the imaging lens768, to reduce speckle on the optical sensor effective area770. It should be appreciated that since the reference light beam774is on an angle relative to the surface of diffuser766(and the optical axis of feedback light beam772), the diffuser766and lens768redirect the reference light beam774to allow the reference light beam774to strike the optical sensor effective area770. Thus, the reference light beam774and feedback light beam772both strike the same effective area770of the detector710. This provides advantages in reducing or eliminating signal errors that occurred in prior art systems that utilized separate and discrete optical sensors for the reference and feedback light beams. The system700uses time-of-flight principles to determine the distance to the object200based on the time difference between the reference light beam pulse and the feedback light beam pulse striking the optical sensor.

Referring now toFIG. 4, an embodiment is shown of the control and signal processing electronics block diagram800for the system700. In an embodiment, the signal light beam715is directed towards the object200by a pair of orthogonal mirrors705,706. The mirrors705,706are mounted on shafts of galvanometers403,404. The galvanometers include servo motors containing angular position sensors. In an embodiment, the galvanometers may by a type of Model GM1010 manufactured by CANON U.S.A. of Melville, N.Y. The galvanometer403rotates the mirror705to steer the light beam714in a first plane, such as the horizontal or azimuth plane for example. The azimuth beam steering angle is referenced in the description below as H. Galvanometer404rotates the mirror706to steer the light beam715in a second plane, such as the vertical or elevation plane. In an embodiment, the first plane and the second plane are substantially orthogonal. The elevation beam steering angle is referenced in the description below as V. The mirrors705,706cooperate in a coordinated manner to project the light beam715toward a desired point on the object200that is within the angular range of the galvanometers403,404with mirrors705,706. In an embodiment, the mirrors705,706operate within a range of angles H, V of +/−30°. The galvanometers403,404are activated by servo drivers401,402respectively. In an embodiment, each servo driver has an input interface to receive command signals from a controller400.

The connection between the controller400and the components of the system700may be a wired-connection/data-transmission-media or a wireless connection. The controller400is a suitable electronic device capable of accepting data and instructions, executing the instructions to process the data, and presenting the results. Controller400may accept instructions through user interface410, or through other means such as but not limited to electronic data card, voice activation means, manually-operable selection and control means, radiated wavelength and electronic or electrical transfer.

Controller400uses signals act as input to various processes for controlling the system700. The digital signals represent one or more system700data including but not limited to signals from optical sensor710, operator inputs via user interface410and the like.

Controller400is operably coupled with one or more components of system700by data transmission media. Data transmission media includes, but is not limited to, twisted pair wiring, coaxial cable, and fiber optic cable. Data transmission media also includes, but is not limited to, wireless, radio and infrared signal transmission systems. Controller400is configured to provide operating signals to these components and to receive data from these components via the data transmission media.

In general, controller400accepts data from optical sensor710, and is given certain instructions for the purpose of determining the distance and direction to the object200and 3D coordinates of points on surfaces being scanned. The controller400may compare the operational parameters to predetermined variances and if the predetermined variance is exceeded, generates a signal that may be used to indicate an alarm to an operator or to a remote computer via a network. Additionally, the signal may initiate other control methods that adapt the operation of the system700such as changing the operational state of laser light source701, the position of galvanometers403,404, the setting of AOM703, the position of neutral density filter757, and the gain of optical sensor710to compensate for the out of variance operating parameter.

The data received from optical sensor710may be displayed on a user interface410. The user interface410may be an LED (light-emitting diode) display, an LCD (liquid-crystal diode) display, a touch-screen display or the like. A keypad may also be coupled to the user interface for providing data input to controller400. In an embodiment, the controller400displays in the user interface410a point cloud to visually represent the acquired 3D coordinates.

In addition to being coupled to one or more components within system700, controller400may also be coupled to external computer networks such as a local area network (LAN) and the Internet via a communications interface412. A LAN interconnects one or more remote computers, which are configured to communicate with controller400using a well- known computer communications protocol such as TCP/IP (Transmission Control Protocol/Internet Protocol), RS-232, ModBus, and the like. Additional systems700may also be connected to LAN with the controller400in each of these systems700being configured to send and receive data to and from remote computers and other systems700. The LAN is connected to the Internet. This connection allows controller400to communicate with one or more remote computers connected to the Internet.

Controller400includes a processor414coupled to a random-access memory (RAM) device416, a non-volatile memory (NVM) device418, a read-only memory (ROM) device420, one or more input/output (I/O) controllers, and a communications interface device412.

Communications interface412provides for communication between controller400and a network in a data communications protocol supported by the network. ROM device420stores an application code, e.g., main functionality firmware, including initializing parameters, and boot code, for processor414. Application code also includes program instructions as shown inFIG. 8for causing processor414to execute any system700operation control methods, including starting and stopping operation, changing operational states of the laser light source701, galvanometers403,404and optical sensor710, monitoring predetermined operating parameters, and generation of alarms. In an embodiment, the application code creates an onboard telemetry system may be used to transmit operating information between the system700and one or more remote computers or receiving locations. The information to be exchanged remote computers and the controller400include but are not limited to computer-aided-design (CAD) data and 3D coordinate data.

NVM device418is any form of non-volatile memory such as an EPROM (Erasable Programmable Read Only Memory) chip, a disk drive, or the like. Stored in NVM device418are various operational parameters for the application code. The various operational parameters can be input to NVM device418either locally, using a user interface410or remote computer, or remotely via the Internet using a remote computer. It will be recognized that application code can be stored in NVM device418rather than ROM device420.

Controller400includes operation control methods embodied in application code such as that shown inFIG. 8. These methods are embodied in computer instructions written to be executed by processor414, typically in the form of software. The software can be encoded in any language, including, but not limited to, assembly language, VHDL (Verilog Hardware Description Language), VHSIC HDL (Very High Speed IC Hardware Description Language), Fortran (formula translation), C, C++, Visual C++, C#, Objective-C, Java, Javascript ALGOL (algorithmic language), BASIC (beginners all-purpose symbolic instruction code), visual BASIC, ActiveX, HTML (HyperText Markup Language), Python, Ruby and any combination or derivative of at least one of the foregoing. Additionally, an operator can use an existing software application such as a spreadsheet or database and correlate various cells with the variables enumerated in the algorithms. Furthermore, the software can be independent of other software or dependent upon other software, such as in the form of integrated software.

As will be discussed in more detail herein, the controller400may be configured to determine three-dimensional coordinate data for one or more points located on the surface205of object200.

In an embodiment, the controller400further includes an energy source. In an embodiment, the energy source may be a battery that is an electrochemical device that provides electrical power for the controller400. In an embodiment, the battery may also provide electrical power to the light source701, optical sensor710and galvanometers403,404. In some embodiments, the battery may be separate from the controller (e.g. a battery pack). In an embodiment, a second battery may provide electrical power to the light source701, optical sensor710and galvanometers403,404. In still further embodiments, the light source701may have a separate energy source (e.g. a battery pack).

It should be appreciated that the controller400may be arranged in a housing (not shown) with the light source701, optical sensor710and galvanometers403,404, or may be spaced apart (separate). Further, while embodiments herein illustrate the controller400as being coupled with a system700, this is for exemplary purposes and the claims should not be so limited. In other embodiments, the controller400may be coupled to and combine three-dimensional coordinate data from multiple systems700.

In operation, the controller400causes the light source701to generate light pulses which are triggered by a master clock802. In the exemplary embodiment, the light pulses are generated at a frequency of about 100 kHz. The controller400generates a scan pattern and trajectory for the signal light beam as a series of beam steering commands that are transmitted to the servo-drivers401,402. In an embodiment, the beam steering commands are transmitted at equal time increments as defined by the master clock802. The master clock802synchronizes the stream of position commands to the galvanometer servo drivers401,402during both modes of operation, namely projection scan and object scan. In the projection operation, the AOM703is controlled in an on/off mode of operation by controller400. This allows the system700to generate piece-wise trajectories thus creating glowing templates (when the light beam715is in the visible spectrum) by repeating the same trajectory at a high rate on the surface205of the object200. If the repetition rate is more than 25-30 Hz than the user perceives the glowing template as a steady image. In the object scan operation, the system700generates a raster scan pattern, and collects the light feedback signal by the subsystem730. A raster scan pattern, such as that shown with respect toFIG. 6, will be discussed in more detail in the description below. In the object scan mode, the system700processes the feedback signals and derives a three-dimensional point cloud of the object. As used herein, a point cloud is a collection or a set of data points in a coordinate system. In a three-dimensional coordinate system, these points are usually defined by X, Y, and Z coordinates, and represent the digitized surface of an object200that is scanned with the system700.

In an embodiment, the electrical signal obtained from the optical sensor710during raster scan of the object200is amplified by amplifier405and transmitted to the ADC407to digitize this analog signal. The electrical analog output signal of the sensor710is generated in response to the striking of the effective area770of the optical sensor710by the reference light pulse in the beam764or the feedback light pulse in the beam717. The ADC407is controlled by a sampling clock804. In an embodiment, the sampling rate is about 10 billion samples per second (10 Gigasamples per second) and the resolution of the ADC407is 10 bits. It should be appreciated that due to the distance travelled by the light beam715to the object200and back, the reference light pulse in the beam764will strike the optical sensor710before the feedback light pulse in the beam717. The optical sensor710generates an electrical reference pulse signal when the reference light pulse in the beam764strikes the optical sensor710and an electrical feedback pulse signal when the feedback light pulse in the beam717strikes the optical sensor710. Thus, determining the time difference between the reference pulse signal and the feedback pulse signal allows the determination of the distance to the point210on the object200using time-of-flight principles and knowing the speed of light in air.

For a distance of 100 feet, the delay between the reference pulse signal and the feedback pulse signal is about 200 nanoseconds or less. When the system700is operated to generate light pulses at a rate of 100 kHz, the time period between a first pair of pulse signals and the following pair of pulse signals is about 10 microseconds.

The output of ADC407is connected to the controller400that processes the signals to determine the pulse amplitude for the object feedback signal that corresponds to the feedback light intensity, and the time delay between the feedback signal pulse and the reference signal pulse. Each signal pulse is represented in memory416as a sampled and recorded waveform of the electrical signal digitized by the ADC407. In an embodiment, the feedback signal pulse amplitude values (e.g. the peak values of the recorded waveform) are utilized to construct a pixelized intensity image during raster scan of the object200as discussed in more detail in the description below.

It should be appreciated that the object200may have a variety of surface conditions, such as different surface reflectivity for example. The object may have shiny metal surfaces, retroreflective targets and black carbon-fiber materials. Further, the intensity of the light received by the optical sensor710varies reciprocally to the squared distance between the system700and the point on the surface210. It has been found that in a typical scanning application, the dynamic range of the reflectivity variations may be about of 100,000, and, sometimes, may be as large as 500,000.

Due to this large variation in reflectivity, the controller400may reduce intensity of the outgoing light pulse so that the feedback light intensity from shiny surfaces (e.g. highly reflective) is brought to values within an acceptable signal range for the sensor710and the ADC407. To change the intensity/optical-power of the outgoing light beam715, the controller400transmits a command signal to DAC806. The DAC806controls the voltage applied to the AOM703to vary intensity of the first order beam746transmitted by the AOM703.

In one embodiment, the AOM703is controlled to improve system700stability and accuracy. According to its principal of operation described in the aforementioned publication “Acousto-optic Devices: principles, design and applications”, a typical acousto-optical modulator changes the intensity of the beam746(transmitted through it as a first diffraction order) following variation of the AOM's internal ultrasound acoustic power. Typically, the ultrasound acoustic power is being applied to the AOM crystal via an internal transducer driven by a sinusoidal electrical signal in a radio frequency (RF) range. In an embodiment, the RF power applied to the AOM ultrasound transducer is proportional to the control signal voltage applied to the internal RF power driver (not shown) of the AOM703from the DAC806. In an embodiment, the output voltage of DAC806is changing between 0 and 1 Volts. The higher this voltage the more intensity of the first order beam713is transmitted through the AOM703. However, if the AOM is controlled by an arbitrary variable DC voltage it can unpredictably change direction of its output beam746thus introducing errors in scanning and in determining point cloud coordinates X, Y, Z. The source of beam directional errors is in the arbitrary RF power heat dissipation inside the AOM crystal originated from the arbitrary variable DC voltage control needed for AOM usage as the beam shutter and/or attenuator in projection and scanning modes of operation. It has been found that variable RF power heat dissipation inside the AOM crystal leads to beam directional errors because it causes variations of the ultrasound wave periodicity that changes the diffraction angle for the first order beam746.

In an embodiment, the elimination of beam directional errors is is performed by applying a synchronized periodic square waveform control signal from the output of the DAC806to the AOM module703. In an embodiment, the magnitude of the control signal is symmetrically variable within the range, for example, from 0 to 1 volts while the average level of the control signal is equal to the half of the maximum voltage control, e.g. 0.5 volts in the exemplary embodiment. The frequency and the phase of the square waveform control signal are synchronized with the laser light pulses as it shown inFIGS. 7A-7D. The level of the control signal1000,1001,1002,1003during each half of the period without laser pulse1005always complements the level of the control signal during every half of the period with laser pulse. As the result, the RF power heat dissipation inside the AOM crystal stays constant at the level corresponding to the average level of the control voltage 0.5 volts regardless of the arbitrary varying output beam intensity by varying magnitude of the square waveform control signal. Consequently, the AOM crystal will be at a thermal equilibrium at all times, and an undesired directional wander of the output beam746will be reduced or eliminated.

As shown inFIGS. 7A, the control signal1000drives the AOM703fully opened (e.g. maximum intensity of the light beam746) because the control signal level1010is equal 1 volt at every half of the period when the laser pulse1005appears. The average level1004of the control signal1000is equal to the half of its maximum, e.g. 0.5 volts. Another embodiment, shown inFIG. 7Bshows the control signal1001with such a magnitude that the AOM transmission is reduced to about three quarters of the maximum because the control signal level1011is equal 0.75 volt at every half of period when the laser pulse1005appears. The average level1006of the control signal1001stays at 0.5 volts because the level1012of the signal1001at every half of period without laser pulse is equal 0.25 volts, e.g. complementing the level1011symmetrically around the level1006. Similarly, in another embodiment shown inFIG. 7C, the magnitude of the control signal1002reduces the AOM transmission to about a quarter of the maximum because the control signal level1013is equal 0.25 volt at every half of period when the laser pulse1005appears. The average level1007of the control signal1002stays at 0.5 volts because the level1014of the signal1002at every half of period without laser pulse is equal 0.75 volts, e.g. complementing the level1013symmetrically around the level1007. In yet another embodiment shown inFIG. 7D, control signal1003causes the AOM703to be fully closed (no transmission of the first order light beam746) because the control signal level1015is equal 0 volts at every half of the period when the laser pulse1005appears. The average level1008of the control signal1003stays at 0.5 volts because the level1016of the signal1003at every half of period without laser pulse is equal 1 volt, e.g. complementing the level1015symmetrically around the average level1008. It should be appreciated that, in this way of operation, the average level of the AOM control signal stays exactly at the same value equal to the half of the maximum control voltage (1 volt in this exemplary embodiment) regardless of an arbitrary varying the control signal level required for a given transmission of the AOM703.

It should be appreciated that while embodiments described herein refer to a particular voltage or waveform, this is for exemplary purposes and the claims should not be so limited. In other embodiments, other voltages or waveforms may be used.

Further, in an embodiment the variable gain amplifier405may be controlled by the DAC406to mitigate variations in signal strength. The dynamic range of the amplifier405is typically between 10-20. Finally, the gain of the optical sensor710may be adjusted by changing its power source voltage through DAC808. By combining the signal strength adjustment capabilities of the AOM703, the amplifier405, the neutral density filter757, and the optical sensor710, a dynamic range of 500,000 or greater may be achieved.

It should be appreciated that while the beam splitter740diverts less than1% of the light beam712for use as a reference light beam, the optical power of this reference light beam, without variable attenuation, would be constant and, in many cases, would substantially exceed in many times that of the optical power of the feedback light beam717that varies. The variations and losses in optical power of the feedback light beam717may be due to surface conditions, distance and diffusion upon striking the surface205. It should further be appreciated that it is desirable to have the optical power of the reference light beam764to be adjustable to become about the same as that of the feedback light beam717. This allows to substantially narrow the required dynamic signal range of the sensor710and the ADC407. To accomplish this, the controller400transmits a command signal to the DAC810that controls the variable attenuator744. Typically, the weaker the signal from the feedback beam717, the deeper attenuation is by the variable attenuator744to equalize the feedback signal and the reference signal, and the higher the gain that is needed for the sensor710and the amplifier405to process them together.

Typically, a preliminary object scan is performed to evaluate a signal strength of the feedback beam717and to establish proper gain control levels for the amplifier405, sensor710, and AOM703through their DACs406,808, and806, as well as attenuation control level for the variable attenuator744through its DAC810. In an embodiment shown inFIG. 5, the system700performs an initial scan of an area500around a part502of the surface205of object200. The controls for sensor710, amplifier405, AOM703, and attenuator744are set to their initial values. The light beam715is steered via galvanometers403,404and mirrors705,706at a constant velocity and varying azimuth angle H along a pattern504. The pattern504begins along trace line506. At the end of line506, the galvanometer403stops and the galvanometer404steers the beam to vary the elevation of the signal light beam715along line508. The galvanometer404then stops and the galvanometer403steers the signal light beam715along retrace line510. This scan process continues in this bi-directional manner to cover the area500. It should be appreciated that during each trace and retrace, the galvanometer403is driven by a stream of digital command signals from controller400via servo driver401. In an embodiment, the command signals are transmitted at substantially equal time increments as defined by the master clock802. At each time increment, controller400processes the output of ADC407to determine the pulse amplitude for the object feedback signal that corresponds to the feedback light intensity. In an embodiment, the controller400constructs a two-dimensional image array comprised of a series of rows. Each row representing a digitized signal intensity along the trace or retrace line.

In an embodiment, after the completion of the preliminary scan, the controller400analyzes a captured digital image (based at least in part of the image array) and determines the high or maximum value of the image array. That value corresponds to a large or maximum amplitude of the amplified feedback signal pulses. Based on the result, the controller400may determine adequate levels of controls for sensor710, amplifier405, AOM703, and attenuator744that could be used for the next detailed object scan to keep the pulse signals amplitudes within an acceptable signal range for the sensor710and the ADC407. It should be appreciated that multiple successive preliminary scans could be performed to establish proper levels of controls for sensor710, amplifier405, AOM703, and attenuator744.

The detailed object scan that is being performed after one or more preliminary scans is illustrated inFIG. 6. It shows a scan trajectory that follows a bi-directional scan pattern600. In contrast to the preliminary scan, in an embodiment, the final scan includes a trace602and a retrace604that are superimposed or collinear. It should be appreciated that lines602,604are illustrated slightly separated inFIG. 6for clarity purposes only. The controller400then proceeds to perform the scan line by line, as described herein with respect to the preliminary scan, with the trace and retrace lines being separated by a vertical segment606. In an embodiment, the trace and retrace line segment606(V pixel size) and the sampling interval608(H pixel size) are each typically between 30 to 50 micro radians. In an embodiment, the resolution is user definable.

In an embodiment, an array of pixel data is being constructed by the controller400as the result of the detailed object scan. Each element of the array is associated with the H and V pixel locations and contains the values of the feedback light intensity and the time-of-flight represented as the time delay between the reference signal pulse and the feedback signal pulse. The light intensity values are utilized to construct a pixelized two-dimensional intensity image for object feature detection. This feature detection may be the same as that described in the aforementioned U.S. Pat. No. 8,582,087. The time-of-flight represented as the time delay is used to calculate the distance between the system700and the pixel point by multiplying the value of time delay by the speed of light in air. The time delay is determined as being the difference between the timing locations of the reference signal waveform and the feedback signal waveform with respect to the train of sampling pulses generated by sampling clock804. An exemplary method of extracting the timing location of the pulse waveform independently from the pulse's amplitude is described in Merrill Scolnik, “Introduction to Radar Systems”, McGraw-Hill, International Editions, 2002, the contents of which are incorporated herein by reference.

As the distance to each pixel point is determined during the detailed object scan, the controller400derives the X, Y, Z coordinates of this point based on its distance and the H and V values in a projector coordinate frame of reference (sometimes referred to as galvanometer space). Controller400further displays a three-dimensional point cloud array X, Y, Z on the user interface410as representation the digitized surface205of the object200. It should be appreciated that the point cloud data may be also sent to an external computer network via communications interface412.

Referring now toFIG. 8, a method900is shown for operating the system700. The method900starts with block902where the user selects a scan area containing the object200, such as area500(FIG. 5) for example. The method900then proceeds to block904where the laser radar projection system700performs a preliminary scan, such as using the raster scan pattern504for example, within the scan area. This preliminary scan is performed to ensure that valid signals are obtained from the scan pixels. As discussed herein the feedback signal may vary based on a variety of factors, such as but not limited to surface conditions and distance to the object for example. In the exemplary embodiment, the preliminary scan is an iterative process having a sequence of scans performed at different optical power. For the initial scan, the optical power is set to a low or minimum optical power, such as by adjusting the AOM703for example. The scan is performed for each trace and retrace of the pattern504with the pixels recorded that return a valid feedback signal. In one embodiment, the pixels are valid if the feedback signal strength is within a predetermined range for the ADC407for example. The method900then proceeds to query block906where it is determined whether a sufficient number of valid pixels have been recorded. This may be determined by assigning a pixel density for example. When the query block906returns a negative, the method900proceeds to block908where the optical power is increased, the gain of the optical sensor710is increased or a combination thereof. The method900then loops back to block904and the process is repeated until a desired number of pixels is achieved. When the query block906returns a positive, the system700has an array of pixel locations and system700parameters (e.g. AOM setting, amplifier gain, power source voltage) to achieve a desired feedback signal strength.

The method then proceeds to block910where the optical power and feedback light for each pixel are determined. In block912the detailed object scan is performed within the selected area. In an embodiment, the detailed object scan follows a raster pattern, such as pattern500(FIG. 6) for example. For each pixel location within the pattern500, the system performs control levels adjustments in block914, these adjustments may include adjusting916the transmission of the AOM703, adjusting918the gain of amplifier405, adjusting920the voltage of power source of optical sensor710, adjusting922the reference attenuator744, emitting924the light pulse712from light source701, and then adjusting926the position of the galvanometers403,404. The adjustments916-922may be based at least in part on the preliminary scan904to provide a feedback signal strength that is within a desired range for the optical sensor710. It should be appreciated that in another embodiment the detailed scan may be an iterative process having a sequence of scans performed at common sets of adjustments916-922defined by the controller400for the next successive scan based on the results of the previous one. Finally, for each pixel, the distance to the measured point is determined and stored along with the H and V values in block928.

Once the object scan is completed, the method900then proceeds to block930where the three-dimensional coordinates X, Y, Z for each pixel are determined based on the distance, H and V values, and the point cloud data array is generated. The method900then stops in block932.

It should be appreciated that while the exemplary embodiments illustrates portions of the light beams being transmitted through free air, this is for exemplary purposes and the claims should not be so limited. In other embodiments, other devices, such as fiber optic devices may be used to transfer the light beams from a first portion of the system to a second portion of the system. Further, components such as fiber optic couplers may be used in place of a beam splitter for example.

It should also be appreciated that while raster scan patterns are described herein, the claims should not be so limited. In other embodiments, other scan patterns may be used.