Abstract:
A simplified system and method for synchronizing a three dimensional digitizers is disclosed. Various three dimensional digitizers utilize detected light sequences received from a probe as a synchronization signal negating the need for complex synchronization circuitry and communication signals. One embodiment utilizes no transmitted synchronization signal, but relies on embedded, high-stability clocks to maintain synchronization after initial one-time synchronization of the clocks. In this manner the design of the three dimensional digitizer may be simplified.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of priority to U.S. Provisional Application No. 61/370,720, filed Aug. 4, 2010, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The present invention relates generally to three-dimensional digitizers, and more particularly to three-dimensional digitizers with optical sensors. 
     BACKGROUND 
     Various systems and methods exist for using optical instruments to measure position in a three-dimensional space. These systems and methods may convert the position measurements into digital form and plot the measurements over time to trace various shapes and forms. For example, these systems may operate as a digitizing pointer held by hand and used to trace structures for reverse-engineering, sizing purposes, medical procedures, or motion tracking. 
     SUMMARY 
     The need exists for optical digitizing systems that employ lower-cost probes and trackers, which in some cases can be disposable. In an embodiment, optical digitizing systems and methods utilize a controller having increased capabilities in conjunction with probes and/or trackers which are simplified in order to reduce cost. Increased intelligence and cost in the controller can be tolerated in return for simplified probes and trackers. 
     With such a simplified probe or tracker, the synchronization that is normally achieved by means of built-in intelligence in the probe or tracker is achieved in some other way, preferably by means of increased intelligence in the controller. 
     In accordance with one aspect of the invention, systems and methods for determining spatial coordinates include a probe that flashes one or more light emitters autonomously, an optical sensor that receives the flashing of the probe&#39;s light emitter(s), and a controller that synchronizes the operation of the sensor system with the light flashes from the probe. 
     In one embodiment, the controller captures and processes images from the sensor continuously with a frame sequence period approximately equal to the period of the probe&#39;s emitter flashing sequence. Synchronization is accomplished by shifting the start of the controller&#39;s frame sequence until the image height or intensity of one or more emitters on one or more sensors is maximized. As used herein, the image “height” refers to the maximum pixel value in the image, and the “intensity” refers to the total energy of the image, i.e., the area under the curve of the image. Either metric can be used to determine optimum image capture to maximize the image. The controller then identifies the position of each emitter within the sequence. This is easily done if a gap is left in the probe&#39;s flashing sequence (the gap may be used for a background capture frame). 
     In an alternate embodiment, one of the emitters can be flashed twice to allow the controller to identify that emitter. In the event that switches or buttons are attached to a probe, information on button presses and switch closures can be communicated to the controller by flashing emitters in gaps in the flashing sequence where imaging emitter flashes are not expected. In the absence of any gaps in the flashing sequence the emitter identities can still be determined by analyzing the x, y, z coordinates of each emitter in terms of the probe geometry. In the case of more than one emitter being on at the same time, the known geometry of the probe (emitter pattern) can be used to identify each individual emitter or the probe tip directly. 
     In a further embodiment, the controller is enhanced by adding a photo sensor of a type that is sensitive to the flashing light emanating from the probe. This is in addition to the photo-sensitive elements, typically charge coupled devices (“CCD”), that are sensitive to the same light and uses it for the actual tracking. The photo sensor detects the light flashes from the probe, which the controller uses to synchronize itself to the probe. 
     In a further embodiment, the controller is enhanced by adding a photo sensor of a type that is sensitive to light at a different electromagnetic frequency than the light that is emitted in the probe flashing sequence. In this embodiment, the probe includes a transmitter that emits light at this different frequency. This photo transmitter is fired in sync with the flashing imaging emitters to signal the start or any other portion of the flashing sequence. 
     In a further embodiment, synchronization is achieved by utilizing high stability clocks in both the controller as well as in the probe or tracker. After the clocks have been synchronized at a point in time, they keep running in sync for the duration of the measurement procedure, with the probe flashing the markers at the time expected by the controller. 
     In a further embodiment a wireless sync signal is sent from the controller to all probes and trackers to signal the start of the marker flashing sequence. The sync sent out from the controller would preferably be of infrared or radio frequency in nature. 
     In a further embodiment, the controller is reduced in size by utilizing modern highly integrated circuits and is incorporated into the sensor housing. A complete system so constructed can consist of the sensor unit, with built-in controller, and a probe or tracker. The user would only need a computer, laptop or other computing device to display the results or otherwise process the results for use in a software application, such as a computer aided design (CAD) program or custom-designed medical or tracking application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate example embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the features of the invention. 
         FIG. 1  is a perspective schematic view of an optical system for determining special coordinates. 
         FIG. 2  is a perspective view of a probe having a plurality of emitters. 
         FIG. 3  is a process flow diagram of an example of a method for synchronization of a controller and an autonomously flashing probe. 
         FIG. 4  is a perspective schematic view of an optical system including multiple controllers and optical sensor systems. 
         FIG. 5  is a perspective schematic view of an optical system including multiple probes. 
         FIG. 6  is a component block diagram of an example computer suitable for use with various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes, and are not intended to limit the scope of the invention or the claims. 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other implementations. 
     Existing optical digitizing systems typically include a controller, optical sensors, and a probe or tracker. The controller contains the processing engine and generates signals that are sent to the probe. The signals cause the probe to flash emitters of light in time slots expected by the sensor and controller. The flashing probe is then tracked by the optical sensors. Optical sensor systems typically contain two or more photo sensitive instruments, such as cameras or charge coupled devices (“CCD”). The sensors may receive an image of the emitter&#39;s light. The images from each of the sensors may be contrasted. The position of the light may be determined in x, y, z coordinates through mathematical processing of the various emitter images and the related geometry of the sensors and/or the probe that the emitters are attached to. 
     In some systems, the probe contains passive reflectors, which reflect light coming from timed flashes of a separate emitter controlled by the controller. In some cases, where flashing visible light from an emitter or probe would cause disturbance to human operators, infrared light is used. 
     Various examples of optical systems used for determining coordinates in three-dimensional space are described in, for example, U.S. Pat. No. 6,608,688 to Faul et al., U.S. Pat. No. 5,923,417 to Leis, and U.S. Pat. No. 5,828,770 to Leis et al., each of which is incorporated by reference in its entirety. 
     An embodiment optical system  100  for determining spatial coordinates and orientation in three-dimensional space, indicated at  10 , is illustrated in  FIG. 1 . In general, the system  100  uses optical instruments to measure position (including location and/or orientation) in a three-dimensional space. In exemplary embodiments, the system  100  is a three-dimensional digitizer that converts the position measurements into digital form and plots the measurements over time in order to, for example, trace various shapes or forms or to track the motion of objects, including humans, instruments and such, in 3D space. 
     The optical system  100  illustrated in  FIG. 1  includes a probe  112  having one or more emitters  118  that emit electromagnetic radiation, a sensor system  130  including at least two optical sensors  131 , such as cameras or charge coupled devices (CCDs), that are photo-sensitive to the radiation from emitter(s)  118 , and a controller  132  that is electronically coupled to and controls the operation of the sensor system  130 . A computer  138  is configured to receive input of data from at least one of the controller(s)  132  and the sensor system  130 , and from these data, the computer  138  can calculate the x, y, z coordinates of the location of each emitter  118  that appears as a substantial point source of radiation. In some cases, the computer  138  can receive x, y, z and vector data calculated by the controller  132 , and use these data for further processing and/or display. In certain embodiments, the computer  138  can use the computed coordinates of each emitter and a known geometry of the probe  112  to compute the location and orientation of the probe, and any point on the probe, such as the probe tip  114 . The computer  138  can also determine the unit 3D vector describing the longitudinal direction of the probe (which is one aspect of the orientation of the probe). If more than two non-collinear electromagnetic ray emitters are disposed on the probe, a transverse 3D vector can also be computed to describe the rotational orientation of the probe or its yaw-pitch-and-roll angles. 
     For clarity, the controller  132  and computer  138  in this embodiment are illustrated as two separate components, although it will be understood that any of the functions described as being performed by the controller  132  could alternatively or in addition be performed by the computer  138 , and vice versa. Moreover, in some embodiments, the controller  132  and computer  138  can be combined in a single device. 
     In certain embodiments, the controller  132  and the sensor system  130  are combined in a single device. The controller  132  can be made highly-compact by utilizing modern highly integrated circuits, for example, and incorporating them into a common housing with the sensor system  130 . A complete system so constructed may comprise the sensor system  130 , with built-in controller  132 , and one or more probe(s)  112  or tracker(s). The probe(s)  112  can be simplified low-cost probe(s), or even disposable probe(s), as described below. The user can utilize a computer  138 , which could be a laptop, tablet, or other computing device, to display the results or otherwise process the results for use in a software application, such as a CAD program or custom-designed medical or tracking application. 
     Three dimensional digitizers enjoy a wide range of uses. In some of these uses, a need exists for a simplified probe. An exemplary embodiment of a probe  114  is illustrated in  FIG. 2 . The probe  114  includes a plurality of energy emitters  118 . The emitters  118  may generally be active light emitting elements, such as light emitting diodes (LEDs), although they may also be passive reflecting elements that emit reflected light from an external light source. The probe  114  further includes a power source  113 , which can be a battery, and a pulse generating circuit  115  that causes the emitter(s)  118  to flash light in a particular, generally periodic, sequence. In certain embodiments, the probe  114  flashes its emitters autonomously, meaning that the probe&#39;s emitter flash sequence is not determined or controlled by commands from an external control system such as controller  132 . In this way, the probe  114  can be greatly simplified relative to conventional probes. In certain embodiments, the probe  114  includes a simplified circuit that includes a power source  113  (e.g., a battery), a straightforward pulse generating circuit  115  that may be as simple as a free-running oscillator  117 , and light emitters  118 . 
     The simplified probe  114  of this embodiment does not need to include the advanced electronic circuitry that enables conventional probes of this type to receive and interpret commands from a controller and flash each of its light emitters in precise time-slots prescribed by the controller. A drawback of these conventional probes is that they can be expensive to manufacture. By contrast, the present simplified probe is generally cheaper to produce and easier to replace. These simplified probes can be advantageously used in hazardous applications, for example, where the probe is likely to be damaged. In other embodiments, the probe is used in medical applications or other applications that require a sterile probe. In such applications, the probe can be a disposable probe designed for a single use. The present simplified probe can also be used in low cost consumer applications. 
     The controller  132  can be configured to detect and synchronize its own control circuitry with the autonomous flashing of the probe  112 . Added intelligence to the controller provides the ability for it to seek and synchronize to the probe emitters. Thus embodiments may include a more complex and expensive controller, but the probes become cheaper and simpler. Therefore, various embodiments of an optical system can include multiple probes, disposable probes, and/or multiple disposable probes. 
     Controllers may synchronize with an autonomously flashing probe in different ways. In one embodiment of a synchronization method  300 , illustrated in the process flow diagram of  FIG. 3 , the controller captures and processes images from optical sensors, such as cameras or charge coupled devices (“CCD”). At step  301 , these images are processed continuously with a frame sequence period approximately equal to the period of the probe&#39;s emitter flashing sequence. Synchronization is accomplished by shifting the start of the controller&#39;s frame sequence (step  303 ) and measuring the image height or intensity of one or more emitters on one or more probes. When the image height or intensity is maximized (step  302 ), the sensor and probe are synchronized (step  305 ). Each emitter&#39;s location in the flash sequence is determined (step  306 ) based on the start of the sequence. The sequence start can be determined by leaving a gap, during which time no emitter is flashed, at the end of the sequence. 
     An alternative to leaving a gap in the emitter flash sequence is to flash one emitter twice. Identifying the position of each emitter  306  is facilitated by detecting the double emitter-flash. In the event that switches or buttons are attached to a probe, information on button presses and switch closures can be communicated to the controller by flashing emitters in gaps in the flashing sequence where imaging emitter flashes are not expected. In the absence of any gaps in the flashing sequence the emitter identities can still be determined by analyzing the x, y, z coordinates of each emitter in terms of the probe geometry. In the case of more than one emitter being on at the same time, the known geometry of the probe or emitter pattern may be used to identify each individual emitter or the probe tip directly. 
     In an embodiment, the optical system  100  can include an additional photo sensor  133  that is coupled to the controller  132 , as shown in  FIG. 1 . The controller  132  uses the additional photo sensor  133  to synchronize with an autonomously-operating probe  112  or emitter  118 . In an embodiment, the photo sensor  133  detects a flash of light from emitter  118  and generates a pulse that is received at the controller  132 . The pulse is used by the controller  132  to start the optical sensors or CCDs  131  clocking, thus timing the sensors  131  to the flashing of the emitters  118  on the free-running probe  112 . 
     The additional photo sensor  133  can be sensitive to the light flashed by the emitter(s)  118  on probe  112 . In an alternative embodiment, the photo sensor  133  is sensitive to light having a different electromagnetic frequency than the light from the probe emitters  118 . In this case, the probe  112  includes a transmitter that emits light at this different frequency. The transmitter is fired in sync with the imaging emitters  118  to signal the start or any other portion of the flashing sequence. 
     The controller  132  and the probe  112  can each include high-stability clock circuitry, so that once the controller  132  and the autonomously-flashing probe  112  are synchronized at one point in time, they will remain synchronized over an extended duration, preferably over the duration of one or more measurement/digitization procedures. 
     Further embodiments may include multiple controllers  132 ,  232 , as shown in  FIG. 4 . In order to cover measurement volumes larger than what a single controller and associated optical sensors are able to observe, multiple systems  100 ,  200  can be cascaded to receive signals from the same autonomously flashing probe  112  or probes. These multiple systems  100 ,  200  may each include a controller  132 ,  232  and an optical sensor system  130 ,  230 . The respective controllers  132 ,  232  sync to the same probe  112  using their individual synchronizing circuitry and/or algorithms. This is highly simplified compared to existing systems in which controllers must be synchronized with each other to generate a uniform signal to a probe. 
     Some embodiments may include multiple probes.  FIG. 5  illustrates an embodiment in which two probes  112 ,  212 , each having a plurality of emitters  118 ,  218 , are both within the field-of-view of sensor system  130 . When more than one probe is used, the controller  132  may have to deal with more than one light flash at the same time. A controller  132  can overcome this problem and synchronize with multiple probes by implementing one or more embodiment methods. In one embodiment, the probes  112 ,  212  are configured to flash their respective emitters  118 ,  218  at different flash rates. Thus, for example, the controller  132  would detect images with multiple flashes and images with only one flash, and use these images to identify the respective emitters  118 ,  218  and synchronize with each of the multiple probes  112 ,  212  using the different flash rates to distinguish the probes. In another embodiment, the controller  132  includes a transmitter  141  that sends out a periodic sync signal. This signal can be, for example, a radiofrequency or optical signal (e.g., infrared, visible light, etc.) that is received by a receiver  142 ,  242  on the probe(s)  118 ,  218 . This signal can tell the probes to restart or re-sync their individual, autonomous flashing sequences. In another embodiment, the probes  112 ,  212  can send short signals to each other (e.g., via radio frequency or optical signaling). In an embodiment, the probes  112 ,  212  can include receivers/detectors  142 ,  242  that allow them to receive signals from neighboring probes. In this embodiment, the probes  112 ,  212  can sync with each other by detecting signals from other probes, including for instance the light flashes of other probes. 
     In further embodiments, synchronization is achieved by utilizing high stability clocks in both the controller  132  and the probe(s)  112 ,  212 . After the clocks have been synchronized at a point in time, they keep running in sync for the duration of the measurement procedure, so the probes flash the markers at the time expected by the controller. 
     The embodiments discussed above may be combined in various ways to create further embodiment systems, such as a system with both multiple controllers and multiple disposable probes. 
     A number of the embodiments described above may also be implemented using a variety of commercially available computers, such as the computer  700  illustrated in  FIG. 6 . Such a computer  700  typically includes a processor  701  coupled to volatile memory  702  and a large capacity nonvolatile memory, such as a disk drive  703 . The computer  700  may also include a USB memory device and/or a compact disc (CD) drive  706  coupled to the processor  701 . The computer  700  may also include network access ports  704  coupled to the processor  701  for establishing data connections with receiver devices and/or a network  705 , such as a local area network for coupling to the receiver devices and controllable elements within a digitizing or tracking system. 
     Computers and controllers used in the digitizing system for implementing the operations and processes described above for the various embodiments may be configured with computer-executable software instructions to perform the described operations. Such computers may be any conventional general-purposes or special-purpose programmable computer, server or processor. Alternatively, some steps or methods may be performed by circuitry that is specific to a given function. 
     In one or more exemplary aspects, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. The steps of a method or algorithm disclosed herein may be embodied in a processor-executable software module which may reside on a tangible computer-readable storage medium. Computer-readable storage media may be any available media that may be accessed by a computer. By way of example, and not limitation, such computer-readable media may comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that may be used to store desired program code in the form of instructions or data structures and that may be accessed by a computer. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. Additionally, the operations of a method or algorithm may reside as one or any combination or set of codes and/or instructions on a machine readable medium and/or computer-readable medium, which may be incorporated into a computer program product. 
     The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.