Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
  • G01S5/16—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using electromagnetic waves other than radio waves

Definitions

• the present invention is directed to apparatus for locating a target.
• Such apparatus for accurately measuring the instantaneous location of a desired object in space is important for studying the operational characteristics of various mechanisms, especially biological systems like human limbs, and is a critical requirement for many control systems, wherein control of an object ' s movements often rel ies on cont inuous feedback of position and orientation with respect to the environment of the object.
• control systems requiring such positional information are prevalent in, for example, biological, mechanical and electrical environments.
• manipulators Similarly, as robotic manipulators become more popular and beneficial in our society, control of these machines must also become more sophisticated.
• the use of manipulators requires precise knowledge of the manipulator's position in 3-D space for positional control purposes in order to avoid damage to equipment, operators, and the manipulator itself.
• Present designs combine several types of electromechanical devices such as encoders, tachometers, and resolvers to obtain the location and control movements of the machine. Joint angles and velocities of machine segments are fed back via these electromechanical devices to the main processor which determines the machine's next movement. These transducers are fairly reliable and are adequate in many situations.
• rangefinders have been proposed over the years for tracking objects in space where no markers, detectors or other devices are attached.
• Odenthal et al "A Linear Photodiode Array Employed in a Short Range Laser Triangulation Obstacle Avoidance Sensor", M.S. Thesis, Rensselaer Polytechnic Institute, December 1980, have employed lasers and photodetectors in a system for terrain sensing. Pulsed lasers are used to scan the immediate area ahead of the application vehicle, the Martian Roving Vehicle. Photodiode arrays are used to sense reflected laser light from obstacles ahead. Although adequate for the needs of the Martian Rover, the accuracy, approximately 25 cm, for most applications would be poor. These results are typical, however, of systems which do not use markers of some sort attached.
• the present invention is directed to an apparatus and method for locating a target, target.
• the apparatus includes a source mechanism for sweeping across a predetermined space a plurality of energy planes from different known apparent locations.
• Known location reference mechanism detects each of the energy planes as said energy planes sweep across the predetermined space.
• the reference mechanism also measures and stores the sweep times for each of the planes.
• Movable target mechanism in the predetermined space detects said energy planes at the target.
• the target mechanism obtains a traget time for each of the planes with respect to the beginning of the sweep in the predetermined space.
• the apparatus further includes mechanism for transferring the sweep time information and the target time information to the computer.
• the preprogrammed computer calculates target location as a function of the known locations, the sweep times and the target times.
• the method for locating a target in accordance with the present invention includes the steps of firstly, sweeping energy planes across a predetermined space from known locations. Secondly, starting timing mechanism for each particular plane when the particular plane contacts detecting mechanism at a first known reference location as the particular plane enters the . predetermined space.
• the present invention is directed to the use of all types of energy. That is, the preferred embodiment as described hereinafter uses noncoplanar planes of light to sweep a predetermined space so that when the planes are superimposed at the target which detected them, the target location may be calculated. Although light is used, however, in the preferred embodiment, the present invention is just as applicable for any plane of electromagnetic radiation or of some other type of energy, such as propagating pressure in solids, liquids or gases.
• a laser scanning system is used. More particularly, spacially separated, low power lasers are used to scan a field of, for example, six feet by six feet by four feet wide. Coverage of the entire predetermined space or field is accomplished with each of three lasers by directing the beam of the particular laser through a lens system capable of producing a plane of laser light. The plane of light is focused along a line in the predetermined space as well as directed at a multiple sided, mirrored scanner rotating at a constant rate. Each of the three scanners is phased to allow only one plane of light in the target field at any given instant in time. A reference photodetector is placed at each side of the field.
• photodetectors capable of sensing the laser light are attached as targets to the object being tracked through the predetermined field. Since three points determine the position of a rigid body in space, it is preferable to attach three detectors to each body being located or tracked. In the case of a person walking through the space, a group of three detectors is attached to each limb segment so as to track each segment. In the case of a robot, a group of detectors is attached to each segment between joints so as, again, to track the various segments.
• An electronic pulse is generated each time a photodetector is hit by any of the planes of light.
• a measure of the swept angle may be derived.
• the coordinate of the target detector may be calculated. As long as at least one of the three planes of light is nonparallel to the others and all three planes are noncoplanar in the predetermined target space, the superimposed intersection at the target detector of the three planes will identify the target location.
• three-eight sided scanners rotating at 3600 rpm produce a scan rate of 1440 scans per second.
• Each target and reference detector generates a pulse a intervals of 1/1440 each second.
• These signals are amplified, filtered and converted to a TTL compatable level.
• the timing circuit includes a pulse generator and individual counters controlled by the generated TTL pulse.
• the data are multiplexed directly into the memory of a computer. The data is processed on the fly by the computer producing 480 three-dimensional location coordinates from the 1440 angle measurements every second.
• Calculated locations or data obtained from the computer may be used in a variety of applications. As previously intimated, for example, resulting location data may be displayed on a graphics or video display screen for realtime animation of the moving target or may be utilized for controlling the position of the target being measured.
• the present apparatus and method are usable to find target location wherever there is relative motion among various segments of a target body, such as a human or robot, even when there is no fixed instantaneous center of rotation between segments. For example, in the application to robots, this would signify the ability to not only sense relative motion between robot manipulator members but also the ability to detect the position and orientation of any desired portion of the robot including the gripper and its payload.
• the present target location and tracking method and apparatus overcomes the limitations of the various camera systems needing special lighting, high storage capacity, limited resolution, etc.
• the present system provides much greater capability than any XY detector apparatus which senses light emitting diodes.
• range finders of the type used on the Martian Roving Vehicle are limited and are far surpassed in accuracy by the present system.
• the present system is relatively simple, uses known electronics and has data requirements which are easily handled by a relatively small computer and having calculated output easily used for feedback to control systems or displayed on readily available video equipment.
• FIGURE 1 is an illustration of representative geometry for apparatus in accordance with the present invention.
• FIGURE 2 is an illustration of a typical light plane generating and scanning device
• FIGURE 3 is an illustration of three light planes superimposed to intersect at a target detector
• FIGURE 4 is an illustration of a person with target detectors attached to him walking through target space
• FIGURE 5 is an illustration of a robotic manipulator with target detectors attached thereto;
• FIGURE 6 is an illustration showing how triangulation error may result from the present apparatus
• FIGURE 7 is a block diagram of signal conditioning and data reduction electronics for use with apparatus in accordance with the present invention.
• FIGURE 8 is a block diagram of the counter and register array of FIGURE 7;
• FIGURE 9 is an illustration of the geometry needed for calibrating apparatus in accordance with the present invention.
• FIGURE 10 is an illustration of typical geometry showing total angular scan for two light sources
• FIGURE 11 is an illustration similar to FIGURE 10 showing angular scan between a target detector and a reference detector.
• FIGURE 12 is an illustration of typical geometry for apparatus in accordance with the present invention showing the one light source having a light plane orthogonal with respect to the other two light sources.
• FIGURE 1 an illustrated apparatus for locating a target in accordance with the present invention is designated generally by the numeral 10.
• the apparatus 10 in FIGURE 1 and the other figures is representative of the type of apparatus disclosed herein and claimed hereafter.
• Apparatus 10 with three laser sources as illustrated is a logical extension of a two laser system and locates a target in three dimensional space.
• other configurations and other types of energy than laser light may also function in accordance with the present invention and as defined by the claims and, consequently, are covered as well by the present invention.
• a predetermined space 12 comprises the field within which a target 14 may be accurately located. Spacially separated therefrom and from each other, low powered lasers L 1 , L 3 , L 2 are used to scan the target field 12. A pair of reference detectors R 1 , R 2 provide entry and exit data for a scan of the target field 12 by each of the lasers. Detectors 14, R 1 , R 2 communicate with a computer, for example, and a display terminal through electronic circuitry as represented by box 26 and explained more fully hereinafter.
• Coverage of the entire target field 12 by the various lasers is accomplished by passing the beams of each of the three lasers L 1 , L 3 , L 2 through separate optical lens 27, 28 to produce planes of laser light in three dimensional space as illustrated for L 1 in FIGURE 2. That is, laser source L 1 directs a beam 30 of laser light toward lens combination 27 which focuses the beam on a line within the predetermined target space, The beam then passes through a semicylindrical lens 28. Lens 28 spreads the beam 30 of monochromatic light into a plane of light 32. It is understood that lens 28 may be also represent combination of optical elements which function to spread a beam into a plane. The plane of light 32 is then directed at a multiple-sided, mirrored scanner 34 rotating at a constant speed. Scanner 34 is driven by motor 36.
• L 3' L 2 is phased so as to allow only one plane of light 32 in the target field 12 at any instant in time.
• apparatus 10 could be comprised of, for example, laser sources having different wavelengths which would allow all of the multiple laser planes to sweep through the target field at the same time.
• the light planes whether superimposed or instantaneously convergent define a point location 38 within the target field 12. It is noted that one of the axes of the scanners reflecting the three laser planes 32 must be nonparallel with respect to the other two, while all three light planes must be noncoplanar within the predetermined field 12.
• photodetectors 42 capable of sensing the laser light are attached to moving parts of the target of interest 40. Since three locations determine the position and orientation of a rigid body in space, a minimum of three photodetectors 42 are attached to each segment of the target. It is understood that the photodetectors 42 may be much closer together than shown and may in fact be a part of a single detector element as known to those skilled in the art. It is cautioned that the three photodetectors must be located and oriented in the path of the laser planes at all times. For that reason, in some systems it may be preferable to include more than three photodetectors 42 on each segment of the target 40 or to use as a part of apparatus 10 more than three lasers yielding planes of light.
• At least three planes of light as indicated must pass across the predetermined space and be detected by a detector group on each segment of a target. It is noted that three target photodetectors are not needed if the orientation of a particular segment of the target is unnecessary. In fact, one photodetector is sufficient if only location of a target is needed.
• FIGURE 5 illustrates a robotic target 40'.
• the usefulness of the present invention is immediately apparent when the actual location of load 42 after arm deflections is observed in reference to its likely location before deflections of arm segments.
• the broken lines illustrate nondeflected locations of particular segments.
• a plurality of detector groups are attached at various locations on a leg of a person in order to track the various segments of the leg. The subject walks through the laser scanned area.
• a plurality of detector groups are attached at various locations on each segment of the robotic manipulator 41 in order to track the various segments of the manipulator as it moves through a laser scanned area.
• the target receivers 42 are triggered as the laser light planes pass by.
• Two fixed reference photodetectors R 1 , R 2 are placed at the perimeter of the target field 12. An electronic pulse is generated each time a photodetector is hit by moving light.
• a measure of the swept angle By measuring the lapsed time between pulses from the stationary reference detectors R 1 , R 2 and the moving target detectors 42 a measure of the swept angle, given that the angular velocity for the moving light is constant or known between the stationary detectors R 1 , R 2 , is derivable. A similar swept angle is obtained for each laser source. Using trigonometric relations, as described hereinafter, the coordinate location of the target group may then be calculated.
• the present system may be constructed to provide excellent resolution at a reasonably low cost.
• five milliwatt continuous wave helium-neon lasers manufactured by Coherent, Inc., Box 10321, 3210 Porter Drive, Palo Alto, CA 94304, with a 632.8 nanometer wavelength are appropriate. Human safety requirements are easily met for clinical and other environments with such laser.
• the lens 28 is semi-cylindrical as indicated, or is a combination of optical elements which provide the same result as a semi-cylindrical lens.
• Lens 28 is used in combination with focusing optics 27 for expanding a line of light into a plane and focusing it appropriately for reflection from scanner 34 into the desired predetermined target space 12.
• a typical assembly having lens 27, 28 is Model 501/Laser Line Generator from Tropel, a division of Coherent, Inc., 1000 Fairport Park, Fairport, NY 14450.
• the preferred embodiment is octogonal and rotates to sweep the light planes across the target field 12, it is understood that the light planes could be swept across the field using other types of scanners including single mirrored scanners which translate, vibrate or oscillate.
• the number of faces 44 determines the data rate and target field size. In order to preserve the uniformity of the plane in a sweep across target space 12, all points along a wave front must be reflected at substantially the same time. Consequently, the axis of scanner 34 must be substantially perpendicular to the axis of semicylindrical lens 28. The rotational speed of the scanner must be considered since it establishes a triangulation error associated with a moving target for a system like the preferred embodiment wherein laser planes are phased to serially sweep across the target field 12. In order to provide maximum power to the photodetectors in the target field, scanner faces 44 must reflect laser plane 32 with minimal power loss and defusion. That is, the mirrored surfaces must be highly reflective and the tolerances for surface flatness and facet to facet angularity must be relatively small.
• the data rate increases and the dead time between light planes sweeping the field decreases.
• the dead time reaches zero, a system like the preferred embodiment would have to be phased exactly. That is, one light plane would immediately follow the next thereby allowing zero slack time between them.
• Such a configuration would be difficult to achieve without very sophisticated electronics for controlling each motor. Therefore, it is preferred to allow for a less rigid requirement on facing and to select a scanner with the number of faces less than the maximum resulting in zero slack time.
• the preferred embodiment has 8 faces 44 on each scanner 34.
• a configuration using serial sweeps of phased light planes like the preferred embodiment necessarily results in a triangulation error, as indicated.
• This error is a function of the scan rate of the light planes.
• light plane 46 emitted from source L 1 is detected by detector 14 at time t 0 .
• detector 14 has moved to a location represented by numeral 14' and light plane 48 from source L2 is detected at that location.
• the intersection of light planes 46 and 48 is located at point 50 which represents the apparent location of detector 14.
• the location of point 50 representing target 14 at t 0 in relation to the actual location of target 14 at t 0 yields the triangulation error.
• the magnitude of the triangulation error determined by the scanning motor speed and the system geometry for the preferred embodiment is very small, however, compared to present measurement systems.
• the triangulation error decreases.
• the speed of the electronics associated with the detector, the signal conditioner, the register array and the computer must also increase as appropriate.
• a motor speed of 3600 rpm has been found to be preferable for the preferred embodiment and has resulted in a triangulation error of less than 0.1 inch.
• the motors 36 must keep scanners 34 out of phase with one another during system operation. If light planes would overtake one another, detected target information would be useless since a target detector would not be triggered by the same laser that started the counter triggered by a reference detector. Therefore, the various motors 36 must maintain a constant phase relationship. Additionally, the angular measurement between reference detector R 1 and target detector 14 is based on the assumption that motor speed is constant during the angular sweep. That is, the motor speed must remain constant during the time interval that it takes to sweep the distance between reference detectors R 1 and R 2 . A reluctance synchronous motor is preferred because not only is its rotational speed constant but its phase angle is constant. Thus, the phase relationship among the motors remains constant.
• various photodetectors may be applied directly to the target or various transmission techniques may be used to direct the light to a location removed from the target location.
• fiber optics may direct light from a desired target location to another location where it may be detected.
• retroflectors may be used to pass light back through the scanner to a detector or detector array (with or without a light splitter following the scanner on the return path).
• the light plane should consist of multiple known wavelengths and the reflector must abe wavelength specific in reflective character in order to distinguish which retroflector horrt position is being detected and thus meansured.
• Photodetectors With respect to photodetectors, many kinds are available. Photoemissive and photoconductive detectors. however, require relatively high voltage levels for operation and use on human targets is not advisable. Unless fiber optics were used to direct light from the target 14 to a photodetector, photovoltaic type detectors are preferred. A detector with a wide field of view is most appropriate in order to detect laser light from relatively large angles with respect to the perpendicular. Clearly, the detectors must have a spectral response matched to the spectral properties of the laser sources. For the five milliwatt helium-neon lasers with a wavelength of 632.8 nanometer mentioned earlier, an appropriate detector is Part No. FIL-20V with a 120 degree field of view available from United Detector Technology, 3939 Landmark Street, Culver City, CA 90230.
• the reference and target detectors (i.e., R 1 and 42) provide appropriate electronic signals which are communicated by known electronics to a computer 74 for data reduction as explained hereinafter.
• the signals from the reference or target detectors are conditioned in an identical fashion up to the counter and register array 70. That is, using reference detector R 1 and target detector 42 as examples of other reference and target detectors, the signals via lines 52 go to amplifiers 54 generally located on a board in proximity with the photodetectors. Then, through line 56 the signals go to a filter gain adjustment 58 and on to filter 62 through line 60 before passing through comparator threshold adjustment 66 via line 64.
• the signal from comparator 66 passes through lines 68 to counter and register array 70 shown in block form in greater detail in FIGURE 8.
• the plurality of lines 68 illustrate that the signals from all reference detectors and target detectors are conditioned similarly for input to the counter and register array 70.
• data passes through line 72 to computer 74.
• computer 74 After reduction, an appropriate control signal may be sent to control mechanism for a robotic device or, as illustrated, the data may be sent through line 76 to a display 78. It is understood that use of the reduced data is merely illustrative and that a particular application of the present invention may result in uses other than display 78 or control of a robotic device.
• FIGURES 7 and 8 are representative of one type of known electronics for conditioning and producing information for input to a preprogrammed computer. It is understood in addition that the preprogrammed computer may actually be an electronic chip which functions the same as a mainframe computer loaded with appropriate FORTRAN programs as presented in the appendices.
• reference detector R1 includes three detectors, one for each light plane.
• each light plane may be specifically and individually detected, and the time information associated with a particular light plane may be specifically manipulated by the electronics without possibility of data contamination by an unsynchronized light plane.
• there are three lines 68' associated with reference detector R 1 while there is only one line 68 associated with all other detectors, including all target detectors (one or more of which may be reference detectors).
• the signals being communicated on lines 68' are connected into main counter 80 and all logic circuits 82 for the various other reference and target detectors. Circuits 82 include the logic for determining which of three latch circuits to use to store time information for the particular light plane detected.
• Lines 68 from the various target and reference detectors other than reference detector R 1 are connected to different and only one of the various logic circuits 82 as illustrated.
• a clock 84 provides continuous intermittent pulses via line 86 to counter 80.
• Counter 80 counts pulses from clock 84 on initiation by reference detector R 1 , and the count is latched or fixed by the appropriate register circuit and the appropriate level latch 1, 2 or 3 of 16 bit latch register 88. The count is provided from main counter 80 to all of latch registers 88 via line 90. The count is latched on receipt of a signal via line 92 from the appropriate logic circuit 82.
• the latched information corresponding to the angular sweep value of the various light planes is polled by computer 74 via line 72 which represents address in line 94 and data out line 96 to an address selection and data asserted circuit 97.
• the data information from registers 88 is communicated to circuit 97 via lines 98.
• the FORTRAN program that performs the data collection by computer 74 is listed in Appendix I.
• Calculation of a target location in accordance with the present invention depends upon knowing the locations of the reference detectors R 1 , R 2 and the apparent locations of lasers L 1 , L 3 and L 2 . (From the perspective of photodetectors in the predetermined or target space 12, it is an imaginary point projected through the axis of a scanner which appears to be the source of the laser light.) The present calculation is for the case where the light beams have a constant velocity as they travel across the target space 12 and the light plane projecting from the scanner of laser L 3 is perpendicular to the light planes from lasers L 1 and L 2 . It is understood that the present calibration procedure is representative of other procedures which could be used in accordance with the claims to obtain the appropriate target location with the apparatus of the present invention.
• calibration may be obtained by utilizing a grid of five photodetectors having known locations with respect to a defined origin. Locations with respect to the origin may be physically measured using large calibers or other measuring devices. The origin is necessary but wil not appear in the equations. It is also necessary for the present calibration procedure that the grid of photodetectors be sufficiently large to allow two of its photodetectors to be reference detectors R 1 and R 2 . Additionally, the present calibration procedure requires that the four nonorigin photodetectors form a square or rectangle.
• two pair of the photodetectors must be vertically aligned and can be done so with a plumb bob, and two pair of the photodetectors must be horizontally aligned and can be done so with a carpenter's square or with other known devices.
• the axes of scanners 34 for laser sources L 1 and L 2 may be aligned vertically, while the axis of scanner 34 for laser source L 3 may be aligned horizontally. That is, by monitoring the grid photodetectors with an oscilloscope, the light planes from sources L 1 and L 2 may be separately swept across the predetermined space 12. The planes must be detected simultaneously by the vertical pairs of photodetectors. If they are not, the scanners for sources L 1 and L 2 must be aligned until such is the case. Similarly, the axis of scanner 34 for laser source L 3 must be aligned horizontally.
• the grid origin is located at a photodetector having coordinates (0,0).
• a second photodetector has known coordinates with respect to the origin at
• L 1 scans an angle ⁇ between the origin and the known grid detector and laser source L 2 scans an angle ⁇ between the origin and the known grid detector.
• the grid detector location at point (X gi , Y gi ) represents the intersection of lines B and D.
• Line B represents the projection of the scanner plane projected from laser source L 2 in the XY plane after sweeping the angle and hitting the point (X gi , Y gi ).
• Line D represents the same for laser source L 1 after sweeping through an angle ⁇ .
• An equation can then be written representing line B with knowledge of two points on the line.
• the first point is (X L2 , Y L2 ).
• a second point may be determined by calculating a new point 100 on line B which represent point (0, 0) after being rotated by angle ⁇ .
• the coordinates of point 100 are;
• a point 102 may be defined by rotating the point (0, 0) through an angle ⁇ to be located on line D. Its coordinates are then:
• the present calibration method determines the two-dimensional coordinates of two of the laser sources
• the methodology considers first the problem of locating the target with lasers L 1 and L 2 in the XY plane and second the problem of locating the Z coordinate of the target along the line identified by the first calculation.
• the timing information is converted to angular measures as follows:
• t R1,T be the time between trigger of reference detector R1 and the trigger of the target detector for a particular laser scan
• t R1,R2 be the time between trigger of reference detector R1 and the trigger of reference R2 for a particular scan.
• the angular rotation fro reference detector R1 to the target may be calculated as (se FIGURE 11):
• the plane locations when the target is hit may be calculated as:
• intersection of the two planes in the XY plane is defined by two points on each line representing the light planes in the XY plane.
• the three dimensional target location is a simple extension of the above two dimensional case.
• the calibration procedure assures that one of the three laser planes is orthogonal to the other two in XYZ space.
• the Z location of the target is then the 2-D location of the target and the angular rotation of the third laser. That is, X target , Y target are known from laser L 1 and laser L 2 scans. This defines a line in 3-D space that is parallel to the Z-axis. Therefore, with the YZ plane as illustrated in FIGURE 12 and using the angular coverage equations as before, if
• the three dimensional location of any target detector may be calculated based on the known locations of the laser sources and the reference detectors.
• a computer program for calculating target location for the twodimensional case is given herein as Appendix III.
• a program for the three-dimensional case is readily derivable by those skilled in the art based on the equations for the target location given hereinbefore.
• motors 36 must be synchronized and apparatus 10 must be calibrated before target location may be calculated by preprogrammed computer 74. Since motors 34 are of a type which maintain constant phase, they are easily synchronized with respect to one another simply by angulary fixing them with respect to each other. More particularly, a detector in the target field 12 is monitored with an oscilloscope. All three sources and scanners are functioned. Motors 34 are physically rotated slightly and fixed until the light planes are detected at equal time intervals. As indicated hereinbefore, calibration requires a grid of four detectors having known coordinates with respect to an origin detector.
• Pairs of the four detectors must be vertical with respect to one another, while other pairs must be horizontal with respect to one another thereby creating a square or rec tangle having horizontal and vertical sides. This may be done using a plumb bob and a carpenter's level or other known equipment.
• Two of the five photodetectors used for calibration are designated reference detectors R 1 , R 2 and, consequently, the locations of detectors R 1 , R 2 are known by physical measurement with respect to the origin detector.
• apparatus 10 is functioned to obtain the time information from which the preprogrammed computer may calculate target location based upon the earlier derived equations for the three coordinates of a target detector. Since many locations may be calculated .each second, they may be displayed on a video display for an animated viewing of various targets with respect to each other or they may be used in other ways depending on the application of apparatus 10.
• Apparatus 10 functions as follows.
• Laser sources L 1 , L 3 and L 2 pass a beam of light through focusing optics 27 and a semicylindrical lens 28, the latter of which spreads the beam into a plane.
• the planes of light 32 are reflected from one of the mirrored faces 44 of rotating scanner 34 so as to intermittently travel across the predetermined target space 12.
• the scanners 34 are synchronized so that only one of the light planes traverses target space 12 at a time.
• one of reference detectors R 1 , R 2 detects the particular light plane as it enters target space 12 and starts a counter 80. As the light plane continues to travel across the target space, detection by one of target detectors 42 causes timing information to be latched in a register 88.
• Detection by the second of the reference detectors R 1 , R 2 causes a second time to be latched in a second register.
• the time information is read into computer 74 for use in the target location equations as preprogrammed in computer 74.
• the location equations essentially superimpose the planes of light as shown in FIGURE 3 to calculate target location 38.
• the calculated location may then be used as feedback to a control system or it may be displayed on a video screen or used in some other fashion.
• apparatus 10 overcomes the limitations of prior art systems including camera systems needing special lighting, requiring high storage capacity, having lens distortion, etc., and XY detector apparatus which senses light emitting diodes,, as well as range finder equipment of the type used on the Martian Roving Vehicle which results in limited accuracy.
• the present invention is a novel departure from present methods and machines.
• the disclosure given, however, with the advantages and details of structure and function as set forth herein, must be considered exemplary. Although it has been presented as a preferred embodiment and various geometries, equations and programs given, the present disclosure is only representative of the concept.

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