EP0156858A1 - Target locating apparatus - Google Patents

Abstract

A target locating device (10) and method for locating a target. The device (10) comprises a plurality of laser sources (L1, L2, L3) which pass their beams through focusing optical elements (27) and a semi-cylindrical lens (28) for diffusing the beams. by light planes (32). The light planes (32) are reflected by one of the faces (44) of rotary scanning members (34) to scan the light planes through a predetermined target space (12). The light planes are put in phase in order to sweep the space in a serial way. Taking into account the known locations of reference photodetectors (22, 24) and the known locations of the apparent sources of the light planes, the time taken to scan a light plane from a reference detector to the target detector (42) and to the Another reference detector for the three light planes is used to calculate the location of the target. The calculation essentially superimposes the three light planes, which are all not coplanar with the target space (12) to define a point of intersection at the level of the target.

Description

TARGET LOCATING APPARATUS

Field of the Invention 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. Such control systems requiring such positional information are prevalent in, for example, biological, mechanical and electrical environments.

Background of the Invention Consideration of a couple of typical applications for an apparatus in accordance with the present invention will provide perspective for understanding the necessity for the invention.

When analyzed, the seemingly simple act of walking down a hallway becomes a very complex action. A constant update of the location of each limb, the movement of the center of gravity for balance, and the position of the body with respect to the horizon and any objects in the environment are all fed back via the vestibular. and muskuloskeletal systems to the motor control center of the brain. This control center in turn innervates muscles with correct sequencing and amplitudes to allow a smooth controlled motion as we walk.

The requirements for implementation of orthotic systems capable of electrically stimulating paralyzed muscle in such a manner as to allow paraplegics and quadraplegics to walk again are immense. Certainly feedback is required for position and velocity of the limb segments. In order to achieve this type of control, a better understanding must be developed of the biomechanical interrelationships for walking and more specifically, of the need to track the limb trajectories in three dimensional (3-D) space during walking.

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. However, if the manipulator picks up a sizeable load, the resolvers cannot account for the resulting link deformations due to stress and the subsequent dynamic deflections at the load tip. In order to implement sufficient control of the gripper under such deformation causing loads, position information in 3-D space is required.

Many location measurement systems are presently known. These systems differ depending primarily on the intended application. For example, sports medicine applications require portable systems that may be transported to remote sporting events. Clinical applications require reasonably high data reduction rates so that patient testing may be completed in a single session. Robotic applications must have real-time data available for control purposes.

The clinical and sports medicine environments commonly use high speed television camera systems. A system disclosed in "Television Computer Analysis of Kinematics of Human Gait", Computers and Biomedical Research, Vol. 5, pp 498-504, 1972, is currently in use at the University of Manitoba utilizing a one camera system. The subject is fitted with large circular reflective markers and the camera system moves along a guided rail as the patient moves. Since a picture matrix must be stored for each frame, the system produces a large amount of data . most of which is merely scene data and not relevant target data. As result, memory requirements are inordinately high to handle the data storage and image processing task.

Multiple camera systems are currently being investigated to determine 3-D data. Typical systems are discussed in Chen, H. J., A Minicomputer-Multiple Microprocessor System for Gait Analysis Using Television and Force Plate Data, Phd Thesis, Ohio State University, August 1977, and in Taylor, K.D., et al, "An Automated Motion Measurement System for Clinical Gait Analysis", Journal of Biomechanics, Vol. 15, pp 505-516, 1982. Both use a three camera system for measurement of total body movement instead of only considering one side of the body, the current limitation of most video systems.

All of these systems, however, suffer from similar limitations. For example, lighting must be carefully arranged; the markers must be very large; the data storage capacity required for multiple picture frames is very high; each picture matrix must be analyzed in order to extract the points of interest; and overall resolution is limited by the spatial resolution of the image array size. In addition, nonlinearities are introduced by the camera optics. These factors, as well as the high cost of camera equipment and the complexity of synchronizing the frame rates, make television systems difficult to use.

In "New Possibilities for Human Motion Studies by Real-Time Light Spot Position Measurement", Biotelemetry, Vol. 1, pp. 132-146, 1974, a system utilizing XY detectors and light emitting diodes is analyzed. Two XY detectors sense the location of time multiplexed LED's that are attached to the subject of interest. The system is being used in several laboratory situations.

Many rangefinders have been proposed over the years for tracking objects in space where no markers, detectors or other devices are attached. For example, 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.

Additionally, the literature describes other approaches for measuring motion. Chen and Taylor, referred to above, provide very good descriptions of several available systems. Later systems suggest a movement towards optoelectronic techniques for measurement. The systems described, however, require the use of sophisticated electronics to acquire the signals generated by the optical devices, and, unfortunately, as the sophistication increases, the expense also increases.

Thus, the need for a low cost 3-D measurement system with good resolution is evident. A new approach to the measurement problem is disclosed herein, with the aim of simplicity to achieve low cost, but without sacrificing resolution, sampling rate, or versatility and with no upper limit on the number of targets that can be handled while maintaining the same resolution, accuracy, sampling rate, etc. Summary of the Invention In a general sense, 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.

Thirdly, storing a first time from the timing mechanism for each particular plane when the particular plane contacts a movable target within the predetermined space. Fourthly, storing a second time from the timing mechanism for each particular plane when said particular plane contacts a second known reference location as the particular plane leaves the predetermined space. Fifthly, communicating the first and second times for all of the energy planes to a preprogrammed computer. And, lastly, calculating target location with the computer from the known locations and the first and second times.

It is observed that in the most general sense, 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.

In the preferred embodiment, 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. Also, 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. By measuring the elapsed time between pulses from the stationary reference detectors and the moving target detectors, a measure of the swept angle may be derived. Using trigonometric relations, 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.

As an example, 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. In general, 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. In addition, this ability would still exist even if members of the robot bend under load, if joints are loose and nonrigid, if links are bent, if various dynamic effects are imposed such as coriolis, gravitational, centrifugal, inertial and frictional effects. This ability would enable the calibration of robots and the compensation for inaccuracies in robot construction and for the deterioration and wear of robots with time.

Thus, 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. Also, range finders of the type used on the Martian Roving Vehicle are limited and are far surpassed in accuracy by the present system. Additionally, 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.

Brief Description of the Drawings

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; and

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.

Detailed Description of the Preferred Embodiment Referring now to the illustrative drawings wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIGURE 1, an illustrated apparatus for locating a target in accordance with the present invention is designated generally by the numeral 10. It is understood that the apparatus 10 in FIGURE 1 and the other figures is representative of the type of apparatus disclosed herein and claimed hereafter. For example, an apparatus similar to apparatus 10 utilizing two spacially separated lasers will locate a point in a two dimensional plane. 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. In addition, it is understood that 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.

As shown in FIGURE 1, 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 L₁, L₃, L₂ are used to scan the target field 12. A pair of reference detectors R₁, R₂ provide entry and exit data for a scan of the target field 12 by each of the lasers. Detectors 14, R₁ , R₂ 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 L₁, L₃, L₂ through separate optical lens 27, 28 to produce planes of laser light in three dimensional space as illustrated for L₁ in FIGURE 2. That is, laser source L₁ 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.

Each scanner 34 for the various laser sources L₁,

^L3' ^L2 is phased so as to allow only one plane of light 32 in the target field 12 at any instant in time. Although such sychronization is preferable, it is understood that 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. In any case, as shown in FIGURE 3, 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.

As shown in FIGURE 4, 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. In any case, for locating a target in 3-D space 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.

As shown in FIGURE 4, 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. Similarly, as shown in FIGURE 5, 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. During the time in the scan volume, the target receivers 42 are triggered as the laser light planes pass by. Two fixed reference photodetectors R₁, R₂ are placed at the perimeter of the target field 12. An electronic pulse is generated each time a photodetector is hit by moving light. By measuring the lapsed time between pulses from the stationary reference detectors R₁ , R₂ 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 R₁, R₂, 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.

As indicated hereinbefore, the present system may be constructed to provide excellent resolution at a reasonably low cost. With this in mind, 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.

Although 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.

A number of variables must be considered with respect to scanner 34. 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.

With respect to the number of faces 44 of scanner 34, as the number of faces increases, the data rate increases and the dead time between light planes sweeping the field decreases. When 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. As shown in FIGURE 6, light plane 46 emitted from source L₁ is detected by detector 14 at time t₀. A short time later, at time t₁ 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 t₀ in relation to the actual location of target 14 at t₀ 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.

As indicated previously, as the scanner speed increases, the triangulation error decreases. As scanner speed increases, however, the speed of the electronics associated with the detector, the signal conditioner, the register array and the computer must also increase as appropriate. As a compromise, 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.

As indicated previously, for the preferred embodiment 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 R₁ 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 R₁ and R₂. 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.

With respect to detectors, 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. For example, fiber optics may direct light from a desired target location to another location where it may be detected. Alternately, 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). For a retroreflector arrangement, 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 traget position is being detected and thus meansured.

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.

As shown in FIGURES 7 and 8, the reference and target detectors (i.e., R₁ and 42) provide appropriate electronic signals which are communicated by known electronics to a computer 74 for data reduction as explained hereinafter. In any case, as shown in block form in FIGURE 7, on detection of a light plane, 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 R₁ 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. In the form of a TTL compatable pulse, 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. From array 70, data passes through line 72 to computer 74. From 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. It is understood also that the present discussion of block diagrams shown in FIGURES 7 and 8 is 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.

In the preferred embodiment, reference detector R1 includes three detectors, one for each light plane. In this fashion, even though the scanners 34 are phased, 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. Thus, in FIGURE 8, there are three lines 68' associated with reference detector R₁, 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 R₁ 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 R₁, 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.

Calibration and Target Location Equations

Calculation of a target location in accordance with the present invention depends upon knowing the locations of the reference detectors R₁, R₂ and the apparent locations of lasers L₁, L₃ and L₂. (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 L₃ is perpendicular to the light planes from lasers L₁ and L₂ . 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.

In accordance then with the present preferred set of calculations, 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 R₁ and R₂. Additionally, the present calibration procedure requires that the four nonorigin photodetectors form a square or rectangle. More particularly, 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. With such a grid, the axes of scanners 34 for laser sources L₁ and L₂ may be aligned vertically, while the axis of scanner 34 for laser source L₃ may be aligned horizontally. That is, by monitoring the grid photodetectors with an oscilloscope, the light planes from sources L₁ and L₂ 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 L₁ and L₂ must be aligned until such is the case. Similarly, the axis of scanner 34 for laser source L₃ must be aligned horizontally.

As shown in FIGURE 9, the grid origin is located at a photodetector having coordinates (0,0). A second photodetector has known coordinates with respect to the origin at

(X_gi, Y_gi) where i = 1, 2, 3, 4 or 5. The unknown coordinates of apparent laser source L₁ and apparent laser source L₂ are (X_L1, Y_L1) and (X_L2, Y_L2). Laser source

L₁ scans an angle ∅ between the origin and the known grid detector and laser source L₂ scans an angle θ 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 L₂ in the XY plane after sweeping the angle and hitting the point (X_gi, Y_gi). Line D represents the same for laser source L₁ 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 θ . The coordinates of point 100 are;

(-X_L2 cos θ + Y_L2 sin θ + X_L2,

-X_L2 sin θ - Y_L2 cos θ + Y_L2)

Similarly, 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:

(-X_L1 cos ∅ + Y_L1 sin ∅ + X_L1,

-X_L1 sin ∅ - Y_L1 cos ∅ + Y_L1)

Using these coordinates, there are two lines for which equations can be written which intersect at the grid detector (X_gi, Y_gi). Therefore, the line defined as gi gi

Y - Y_L1 [x - X_L2] intersects the line

y - Y_L2 [x - X_L2] at the point (X_gi, Y_gi) where sp = sin ∅ cp = cos ∅

St = sin θ ct = cos θ.

To further simplify notation, let

^XL1 = w

^XL1 = x

^XL2 - Y

Y_L2 = z

Combining these equations a single equation corresponding to one grid location may be written as:

(c₁-c₄)wy + (c₃-c₂)xy - (c₂-c₃) wz - (c₄+c₁)xz - c₅w²y

+c₆w ²z - c₇wxy + c₈wxy + C₈Wy² - c₆xy² + c₇wyz - c₅xyz

- z + x = 0 where c₁ = x_gi .sp.ct c2 = x_gi.st.sp c₃ = x_gi.cp.ct c₄ = x_gi.cp.st c₅ = ct^.sp c₆ = st.sp c₇ = cp.ct c₈ = cp.st

Given four grid locations, four equations as above may be determined, Since the equations are functions of four unknowns, the Newton-Raphson method for nonlinear systems may be used to identify the points (-S_{L 1}' Y_L1)' (X _L2 Y_L2)

A computer program for solving the four equations using the

Newton-Raphson method is given as Appendix II.

Thus, the present calibration method determines the two-dimensional coordinates of two of the laser sources

L₁ and L₂. For the three-dimdensional case, that is, to locate three-dimensional coordinates for the three laser sources L₁, L₂ and L₃, the methodology just presented is expanded to incorporate the third dimension Z and the third laser L₃. A similar, albeit larger, set of non-linear equations results and must be resolved using similar appropriate iterative techniques.

Next, it is appropriate to present the methodology for calculating target location given the known reference detector and light source apparent locations as determined by the indicated or some other calibration method. The methodology considers first the problem of locating the target with lasers L₁ and L₂ in the XY plane and second the problem of locating the Z coordinate of the target along the line identified by the first calculation.

With respect to FIGURE 10, the above calibration procedure provides then:

(X_R1, Y_R1), location of reference detector R₁;

(X_{R 2} , Y_{R 2} ) , location of reference detector R₂;

(X_L1 , Y_L1), location of laser source L₁; and

(X_{L 2} , Y_L2), location of laser source L₂.

From these parameters the angular coverage for each laser may be calculated as follows:

Next, the angular coverage between a reference detector and a target detector may be obtained. Since apparatus 10 generates timing information, the timing information is converted to angular measures as follows:

Let ^tR1,T be the time between trigger of reference detector R1 and the trigger of the target detector for a particular laser scan, and t_R1,R2 be the time between trigger of reference detector R1 and the trigger of reference R2 for a particular scan. Assuming a constant laser scan rate between reference detec tor R1 and reference detector R2, the angular rotation fro reference detector R1 to the target may be calculated as (se FIGURE 11):

when scanned by laser L1; and

when scanned by laser L2. These angular measures define the location of the lase planes at the time of target trigger. Therefore, the inter section of the two planes (laser L1 and laser L2) will identify the location of the target in the XY plane.

The plane locations when the target is hit may be calculated as:

Laser plane L1 location after angular rotation

laser plane L2 location after angular rotation β ,

The 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.

Let X₁, Y₁ = X_L1, Y_L1 location of laser source L1;

X₂, Y₂ = X_rotated, Y_rotated location of the rotated reference point in plane L1; X₃, Y₃ = X_L2, Y_L2 location of laser source L2; and X_4' Y₄ = X_rotated, Y_rotated location of the rotated reference point in plane L2. Then the intersection of the two beams is:

^Xtarget=

^Ytarget

where X₂ = (_R1-X_L1) cosα - (Y_R1-Y_L1) sinα+ X_L1' Y₂ = (Y_R1-Y_L1) cos c + (X_R1-X_L1) sino + Y_L1;

X₄ = (X_R1-X_L2) cosβ + (Y_R1Y_L2) Sinβ + X_L2; Y₄ = (_R1-Y_L2) cosβ - (X_R1-X_L2) sinβ+ Y_L2.

As indicated, the three dimensional target location is a simple extension of the above two dimensional case. The calibration procedure, as indicated, 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 L₁ and laser L₂ 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

. θ_{L 3} , for scan by laser L3 then, the Z location of the target may be calculated as: z _target = z_L3 - (Y_target - Y_L3) tan

Thus, 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.

Operation

With apparatus 10 generally configured as shown in FIGURE 1, 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 R₁, R₂ and, consequently, the locations of detectors R₁, R₂ are known by physical measurement with respect to the origin detector.

While monitoring the output of the vertical detectors with an oscilloscope, first the light plane from source L1 and then the light plane from source L2 is functioned. They will be vertical when a pair of vertical calibration detectors detects each sweeping light plane at the same time. Similarly, the light plane from source L3 is operated and a pair of horizontally located calibration detectors are monitored to assure that the light plane is horizontal and, therefore , orthogonal with respect to the other two light planes.

Then, the calibration as specified hereinbefore is performed, and the known locations obtained from the calibration procedure are loaded into the computer. Next, 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 L₁, L₃ and L₂ 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. Within target space 12, one of reference detectors R₁, R₂ 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 R₁, R₂ 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. Although the three planes of light serially sweep across target space 12, 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.

As indicated hereinbefore, 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. Thus, 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. Consequently, changes made, especially in matters of shape, size, arrangement, combinations of known components, assemblies, geometries, equations and computer programs to the full extent extended by the general meaning of the terms in which the appended claims are expressed, are considered within the principle of the present invention.

Claims

WHAT IS CLAIMED IS:

1. A method for locating a target comprising the steps of: sweeping energy planes across a predetermined space from known apparent locations; starting timing means for each particular plane when said particular plane contacts detecting means at a first known reference location as said particular plane enters a predetermined space; storing a first time from said timing means for each particular plane when said particular plane contacts a movable target within the predetermined space; storing a second time from said timing means for each particular plane when said particular plane contacts a second known reference location as said particular plane leaves the predetermined space; communicating said first and second times for all said energy planes to a preprogrammed computer; and calculating target location with said computer from the known locations and said first and second times.

2. The method in accordance with claim 1 including displaying on a video screen the location of said target with respect to a reference system.

3. The method in accordance with claim 2 including repeating said steps of claim 2 many times each second thereby tracking said target through the predetermined space.

4. The method in accordance with claim 1 including controlling future position of said target using calculated target location information.

5. The method in accordance with claim 4 including repeating said steps of claim 4 many times each second thereby controlling movement of said target through the predetermined space.

6. A method for locating a target comprising the steps of: spreading first, second and third monochromatic beams of light into first, second and third light planes for first, second and third light sources; sweeping from known, spaced-apart apparent locations at substantially constant rates said first, second and third light planes across a predetermined space, said first, second and third light planes being non-coplanar within the predetermined space, said first, second and third light planes sweeping from first, second and third axes, said third axis being non-parallel with said first and second axes, only one of said first, second and third light planes being in the predetermined space at a time; detecting at a first known reference location the sweeping light plane as said sweeping light plane enters the predetermined space and simultaneously starting timing means; detecting at a movable target within the predetermined space the sweeping light plane and simutaneously storing a first time obtained from the first timing means; detecting at a second known reference location the sweeping light plane as said sweeping light plane leaves the predetermined space and simultaneously storing a second time obtained from the timing means; communicating said first and second times for said first, second and third light planes to a preprogrammed computer; and calculating target location with said computer based on the known locations and the detected time information.

7. Target locating apparatus, comprising: source means for sweeping across a predetermined space a plurality of energy planes from different known apparent locations; known location reference means for detecting said energy planes as said energy planes sweep across said predetermined space, said reference means including first means for obtaining a sweep time for each of said planes, said sweep time having a beginning time; movable target means in said predetermined space for detecting said energy planes at a target, said target means including second means for obtaining a target time for each of said planes to reach said target with respect to the beginning time; a preprogrammed computer including means for calculating location of said target means, as a function of the known apparent locations, the sweep times and the target times; and means for transferring the sweep time information from said reference means and the target time information from said target means to said computer.

8. Apparatus in accordance with claim 7 wherein said source means for each energy plane includes means for creating a laser light beam and means for spreading said beam into an ever expanding plane of light, said source means further including means for intermittently directing at a constant angular rate said plane of light across said predetermined space.

9. Apparatus in accordance with claim 8 wherein said spreading means includes a semicylindrical lens.

10. Apparatus in accordance with claim 8 wherein said directing means includes a movable element with a mirror along an outer surface and a power source for moving said element.

11. Apparatus in accordance with claim 8 wherein said reference means includes at least a pair of spaced apart photodetectors.

12. Apparatus in accordance with claim 7 wherein said target means includes a plurality of groups of detectors whereby multiple bodies may be tracked by said apparatus.

13. Apparatus in accordance with claim 7 including means for displaying calculated target location.

14. Apparatus in accordance with claim 7 including means for controlling future motion of said target using the target location calculated by said computer.

15. Target locating apparatus, comprising: first, second and third means for generating electromagnetic energy; first, second and third means for spreading said electromagnetic energy from said first, second and third generating means into planes, the plane of said third spreading means being everywhere noncoplanar with respect to the planes of said first and second spreading means, the planes of said first and second spreading means being noncoplanar within a predetermined space; first, second and third means for projecting said energy planes of said first, second and third spreading means into the predetermined space, said first, second and third projecting means having known apparent locations; first, second and third means for moving said energy planes across the predetermined space at substantially constant speeds, said energy planes being synchronized to move in a phased relationship with respect to one another across the predetermined space; known location reference means for obtaining time information of a movement of each of said energy planes across said predetermined space, each said movement having a beginning time; movable target means in said predetermined space for detecting said energy planes at a target and obtaining time information with respect to the beginning time for each of said energy planes as said planes move across said predetermined space; a preprogrammed computer including means for calculating location of said target means as a function of the known apparent locations of said first, second and third projecting means and of the known locations of said reference means, the times obtained by said reference means and the target times obtained by said target means; and means for transferring the time information from said reference means and the target time information from said target means to said computer.

16. Target locating apparatus, comprising: first, second and third means for generating a laser light beam; first, second and third means for spreading said light beams into light planes; first, second and third means for projecting said light planes into the predetermined space, said first, second and third projecting means each including a plurality of plane mirrors disposed perpendicular to radial planes regularly spaced about first, second and third axes, respectively, said first, second and third projecting means having known apparent locations, said first, second and third pro jecting means being spaced apart, said third axis being perpendicular to said first and second axes, said first and second axes being parallel; first, second and third constant velocity and constant phase motors for rotating each of said first, second and third projecting means, said motors being synchronized causing said first, second and third projecting means to serially sweep said light planes across the predetermined space, only one of said light planes being in the predetermined space at a time; first reference means for detecting at a known location each of the light planes as they begin to cross the predetermined space, said first reference means including means for starting to time each of the light planes as said light planes cross the predetermined space, said first reference means including a first photodetector; second reference means for detecting at a known location each of said light planes as they leave the predetermined space, said second reference means including a second photodetector, the second reference means including first means for obtaining and storing sweep time between said first and second photodetectors as the light plane detected by said first reference means leaves the predetermined space; means, attached to a movable target in the predetermined space, for detecting each of the light planes, said detecting means including first, second and third photodetectors for detecting said first, second and third light planes, respectively, said detecting means including second means for obtaining and storing a target sweep time between the first photodetector and the target for each of said light planes as the light plane detected by said first reference means is detected by said target detecting means; a preprogramed computer including means for calcu lating location of the target as a function of the known apparent locations of the first, second and third projecting means, the known locations of the .photodetectors of the first and second reference means, and reference and target sweep times from the first and second storing means for each of the light planes; and means for communicating times from the first and second storing means to said computer.