WO1999010706A1

WO1999010706A1 - Digital 3-d light modulated position measurement system

Info

Publication number: WO1999010706A1
Application number: PCT/US1998/017921
Authority: WO
Inventors: John E. Lasala; David M. Zuk; Robert Dewar
Original assignee: Perceptron, Inc.
Priority date: 1997-08-29
Filing date: 1998-08-28
Publication date: 1999-03-04

Abstract

An apparatus for locating an object (28) within a working volume (30) via amplitude modulated light. An optical transmitter (22) transmits and scans an amplitude modulated beam (24) with working volume (30). An optical signal receiver (32) is connected to the object (28) and receives scanned optical beam (24). The received signal is processed through a digital signal process. A phase difference calculator, located within a digital signal processor, determines the location of object (28) based upon the phase difference between the transmitted and received optical beams (24).

Description

DIGITAL 3-D LIGHT MODULATED POSITION MEASUREMENT SYSTEM

This application bases priority upon United States Application Serial No. 60/057,236 filed August 29, 1997.

Background of the Invention

The present invention relates generally to object location systems and more particularly to systems which utilize amplitude modulation of optical signals to locate objects.

In a variety of industrial, commercial, transportation and other applications it is often desirable to be able to measure the three-dimensional position of one or more points on objects that may either move about or be variously positioned in a predefined volume of space. In addition, it is often desirable to measure the three- dimensional orientation of objects in space, which can be accomplished by measuring the position of at least three different points on such an object whose positions relative to each other is already known and fixed. To be practicable in many applications, devices to accomplish such measurements must be economical, miniature, stable in not requiring recalibration or realignment, and capable of producing measurement samples at a high rate, such as tens or hundreds of measurements per second.

Examples of applications for such three-dimensional position and orientation measurement include, but are not limited to, industrial metrology, robot control and tracking, vehicle control and tracking in bounded environments, calibration of other measurement systems that involve multiple fixed sensors, and collision avoidance systems .

Numerous means have been reported and employed previously for three-dimensional measurement of range. Many of these have employed triangulation methods in which the angle of a point is measured from multiple fixed sensors to each point of interest . In such an implementation, the point of interest may emit a signal detected by an angle-sensitive sensor (such as a sensor utilizing an array of photosensitive elements) . Angle measurements used in triangulation systems inherently produce position measurements errors which increase as the range from the sensor to the point begin measured increases, due to the increasing "lever arm" that amplifies any angular errors. Other prior art has utilized range measurement instead of triangulation. In one form, this art has employed imaging sensors in which a modulated laser beam is scanned angularly in two dimensions in a non-cooperative system (i.e., no corresponding receiver for the transmitter) across a field of view in order to obtain information about the transmitter's environment. The reflected signal from sequentially illuminated points in the field of view is processed for each point in the scan to measure the phase shift of the modulation relative to the phase of the transmitted signal, called the "reference channel". This phase shift is proportional to the range traveled by the light signal from the sensor to the point of interest and back to the sensor. Other prior art has eliminated the use of a scanning sensor. This prior art, like that previously described, also performs measurement of the phase shift of the sinusoidal amplitude modulation superimposed upon a light beam. But in this art, the signal is transmitted directly, without reflection, from a transmitter at one three- dimensional position, and detected by a sensor at another three-dimensional position. The phase shift is, as before, measured relative to the phase of the transmitted signal or "reference channel" . Either the transmitter or the sensor may be fixed and the other may be moving. By employing multiple transmitters or multiple receivers or both, the three dimensional position of points, and the position and orientation of their associated objects, may be estimated. Prior art has utilized analog electronic signal processing to estimate the phase shift of the light signal modulation. Measurements derived from such analog electronic processing suffer from multiple practical deficiencies which include but are not limited to:

1. The susceptibility of analog electronic components to short-term and long-term changes in characteristics including drift, component aging effects, and sensitivity to environmental conditions.

2. The necessity to utilize components and interconnections, such as microwave guides or cables that are not susceptible to miniaturization to a degree that is adequate for practical measurement systems. 3. The vulnerability of analog signal processing to electronically or otherwise induced random effects, often referred to as "noise", that degrades the ability to accurately measure phase shift values. Analog electronics are also more vulnerable to amplitude/phase cross-talk leading to systematic errors induced by variations in input optical signal intensity. 4. The inability of analog electronic processing components to perform complex logic and mathematical analysis operations upon the measured signal values to remove anomalous signal samples so as to improve the accuracy of phase measurement . Additional disadvantages include, for example, multiple optical reflections which occur from various surfaces within the field of illumination of each transmitter and the field of view of each sensor. Such multiple or secondary reflections introduce multipath phase errors as the composite signal from an unknown number of multiple reflections plus the direct signal are simultaneously received by a receiver.

Summary of the Present Invention

The present invention avoids these disadvantages and others. The invention employs a digital processing system employing novel and beneficial phase-shift extraction calculations and component interconnections. Moreover, the present invention eliminates several adjustments and error corrections that are required in the prior analog signal processing art and which are undesirable due to their cost and lack of reliable repeatability. It entails a novel implementation of a system utilizing a digital signal processor (DSP) to accomplish accurate, economical, reliable, and miniturizable phase measurement. The DSP- based phase measurement system produces, as its output data, both phase shift and amplitude measurements of the modulation signal received by the sensor.

The present invention in a preferred embodiment includes an optical signal transmitter for transmitting and scanning an amplitude modulated optical beam within a working volume. An optical signal receiver which is connected to an object to be measured receives the scanned optical beam. A phase difference calculator which is connected to the optical signal receiver determines the location of the object based upon the phase difference between the transmitted and received optical beams.

Brief Description of the Drawings Figure 1 is a perspective view depicting the operation of the present invention in an exemplary application;

Figure 2 is a block diagram depicting the top-level location determining components of the present invention;

Figures 3a-3c are perspective view of exemplary scanning patterns used within the present invention;

Figure 4 is a block diagram depicting the components of the digital phase-shift measurement system input signal generator of the present invention; Figure 5 is a block diagram depicting the components of the digital phase-shift determinator; and

Figure 6 is a block diagram depicting an alternate embodiment for the components of the digital phase-shift determinator .

Detailed Description of the Present Invention

The following discussion presents an exemplary description of the present invention. Therefore the examples should not be taken as limiting the scope of the invention to any particular set of values for frequencies, bandwidths or any other parameters.

Figure 1 depicts the object location detection system 20 of the present invention. The present invention uses modulated light signals from transmitter 22, such as may be generated from lasers or other light sources, to measure ranges between points in space. In the example of Figure 1, transmitter 22 emits a fan beam 24 to determine the range 25 to arm 26 of robot 28. Transmitter 22 scans fan beam 24 across working volume 30 in order to strike optoelectronic receiver 32 with fan beam 24. It should be noted that the term "beam" is to include any type of relatively focused optical signal being emitted from transmitter 22.

Receiver 32 concentrates the received light energy and converts the optical signal into an electrical signal . At a signal and data processing unit 34, the electrical signal is amplified and converted to a lower Intermediate Frequency (IF) , where the phase measurements are made. Unit 34 preferably employs a digital signal processing system for phase measurement. However, an alternate embodiment includes unit 34 employing an analog signal processing system for phase measurement. Within such an analog embodiment, unit 34 includes an analog phase difference calculator for performing phase difference operations directly upon analog signals of said transmitted and received optical beams.

Figure 2 is a block diagram of the components depicted in Figure 1. In the preferred embodiment, transmitter 22 scans fan beam 24 across working volume 30 via an actuator 50 connected to mirrors 52. Mirrors 52 are actuated in order scan the beam across the working volume 30. An alternate embodiment includes using a light modulator to perform the scanning of the beam. However, it should be understood that the present invention is not limited to these embodiments, but also includes such embodiments as actuator 50 moving transmitter 22 so as to produce a scanning fan beam.

In the preferred embodiment, a multi-transmitter multi-receiver optical trilateration system is utilized in which the modulated transmitters emit fan beam 24 as a ninety degrees wide by five degrees thick beam. Fan beam 24 is preferably scanned through an angle of ninety degrees across working volume 30. Each opto-electronic receiver 32 detects the phase of the fan beam' s modulation signal . The scanning of fan beam 24 significantly reduces the probability and strength of corrupting multipath signals, relative to a floodlight transmitted signal covering the same total solid angle.

Each receiver 32 is preferably controlled via gated circuitry 54 to only measure phase when the fan beam is approximately pointed at receiver 32. Gated circuitry 54 is preferably provided with a control signal from actuator 50 in order to accurately perform the gating function for receiver 32. The control signal from actuator 50 is based upon when the actuator performs its actuation function so as to make the transmitted beam proximate to receiver 32. Also, in the preferred embodiment, transmitter 22 is provided with gated circuitry 51 in order to indicate to transmitter 22 to not transmit until receiver 32 is within the field of view of transmitter 22.

An alternate embodiment wherein the location of the receiver is even approximately known includes the receivers 32 constantly measuring the detected modulation phase, including occasional multipath signals and then only using the phase values corresponding to the highest detected signal strengths during each scan period which is the measurement from the direct non-multipath signals. Signal and data processing unit 34 includes a phase difference calculator system 56 and a location and orientation computational system 58 in order to determine the range and orientation of object 28.

As an example, fan beam 24 is swept through ninety degrees in a millisecond which results in eighteen beam thicknesses per sweep or fifty-six microseconds as the fan beam moves one beam thickness. In this example, each receiver 32 makes two phase measurements every two milliseconds as the beam is swept. With a 1 GHz modulation signal, a single phase measurement averages 56,000 modulation cycles. Over a period of, for example, ten milliseconds, the receiver makes a total of ten such measurements which when averaged provides a high precision phase measurement.

Each such phase measurement is corrupted by secondary reflections only if a multipath exists within the instantaneous intersection of fan beam 24 with a material surface within working volume 30 and the orientation of an intersected portion of such a surface is such as to redirect the transmitted beam into the aperture of, and from the field of view of receiver 32.

It should be understood that the present invention is not limited to the system configuration described above since the dimensions, shape and scan time sequence of the scanned transmitter beam are all subject to design in order to optimize their values for a particular application.

Figures 3a-3c depict alternate beam shape and scanning patterns. Figure 3a depicts transmitter 22 emitting fan beam 24 with its width and thickness being indicated respectively by reference numerals 80 and 82. Reference numeral 84 depicts the scanning motion of fan beam 24.

Figure 3b depicts transmitter 22 emitting spot beam 90 as it is scanned in a spiral pattern 92. The diameter of spot beam 90 is indicated by reference numeral 94.

Figure 3c depicts transmitter 22 emitting spot beam 90 as it is scanned in a raster pattern 100. The diameter of spot beam 90 is indicated by reference numeral 94. It should be understood that other patterns may be selected to optimize scan times and measurement rates for any application.

Figures 4-6 depict the internal components of signal and data processing unit 34 depicted on Figure 2. Design of the digital phase measurement system entails two considerations. The first consideration is the selection of IF center frequency and IF bandwidth. The second consideration is the specific design of the DSP-based phase detector. Two alternative design variations for this second design consideration are disclosed below. One variation uses two analog-to-digital (A-to-D) converters

(no multiplexing is assumed; as shown in Figure 5) while the alternate embodiment uses only a single A-to-D converter and reduces the number of required DSP computations (as shown in Figure 6) .

Figure 4 depicts the digital phase-shift measurement system input signal generator 160 of the present invention. The present invention employs a 1.00 gigahertz (GHz) selected frequency for the modulation signal to be superimposed upon the transmitted light beam. The modulation signal detected by the conventional photo- detector element of the sensor is first preamplified by conventional electronic amplifier device 162. Then the output of the preamplifier is translated down to an intermediate frequency (IF) by conventional electronic mixing device 164 using a local oscillator (LO) 166. A power splitter 168 divides the output of local oscillator 166 (which preferably operates at 1.07 GHz) to mixing devices 164 and 170. Mixing device 170 is used to translate down to an intermediate frequency (IF) the output of modulation oscillator 172 which operates preferably at 1 GHz. Modulation oscillator 172 represents the modulated reference signal for the transmitter. The output signals of mixers 164 and 170 are respectively filtered by filters 174 and 176. Filters 174 and 176 are preferably 70 MHZ IF filters with a bandwidth of 10 kHz.

For the preferred embodiment of the digital system of the present invention, the IF is further translated down to an even lower IF through local oscillator 178, power splitter 180 and mixers 182 and 184 so that the digitization and signal processing can take place. The output signals from mixers 182 and 184 are further respectively filtered by filters 186 and 188. Filters 186 and 188 are preferably 20 kHz IF filters with a 10 kHz bandwidth. The output IF signal of filter 186 is the signal received by the receiver (i.e., the received signal) . The output IF signal of filter 188 is the reference signal of the transmitter.

The IF bandwidth preferably is large enough to handle the signal's information bandwidth. For example, consider an information bandwidth of 10 kHz ± 5 kHz. A guard band of 5 kHz may be added to this minimum required bandwidth, which indicates use of a second IF of 20 kHz. With a 70 MHZ IF bandwidth of 10 kHz, and the local oscillator (LO) 178 at 70.02 MHZ, the noise from the image sideband would be at 70.04 MHZ. The additional noise from the image sideband is negligible. The 70 MHZ IF frequency is commonly used in communications so that standard filters can be readily employed.

With reference to Figure 5, conventional A-to-D converters 200 and 202 are employed that convert the IF signal directly to 16-bit data at a 100 kHz rate which is five times the 20 kHz second IF. With respect to A-to-D converters 200 and 202, both the signal received by the sensor and the transmitted signal ("reference signal") include an A-to-D converter. Both converters' conversion operations are synchronized and approximately identical. The reference and received I & Q signals are then each calculated in DSP 204 (i.e., a digital phase difference calculator) , of which many suitable models are commercially available as conventional single-chip integrated circuit devices. Since the I & Q signal generation is digital, there is much less error associated with the quadrature phase shift as would occur with the analog methods of the prior art .

The DSP phase measurement method can be explained using a phasor representation of the modulation signal. The term "phasor" , as commonly used in the field of electronic engineering, refers to a representation of a sinusoidal signal as a vector which includes, using the notation of complex mathematics, a real and an imaginary component. That is, a signal presented as a phasor is graphically represented by a vector rotating around the origin in the complex plane . The phasor rotates through a full 360 degrees in one cycle of the sinusoidal waveform. At any instant of time, the vector's projection onto the real axis corresponds to the signal's instantaneous amplitude. The phasor' s phase is measured from the positive real axis.

Computation of phase can be performed from a signal's real and imaginary components. The inverse tangent or arctangent of the absolute value of the ration of the real component of the signal to the imaginary component of the signal defines that phase angle. The absolute value of the ratio restricts the phase angle to the first quadrant. Examination of the sign of the real and imaginary components provides the information necessary to define the proper quadrant for the calculated phase angle. A variation of phasor signal representation is commonly employed in communications and Radar systems. In those systems, rather than referring to real and imaginary components of the signal, the two components are commonly referred to as the In-phase and Quadrature or I & Q components. Measurement of the I & Q components is analogous to measuring the real and imaginary components of a signal.

More specifically, generation of the I & Q signals in DSP 204 is as follows. The digitized received and reference signals are respectively multiplied by I & Q calculators 206 and 208 by cos to produce the Q signal, where the quantity is equal to the product 2* π*f*t*n. In this calculation, f is the value of the second IF, t is the time between successive A-to-D samples, and n is an integer index value. This value repeatedly increments, at each sample time, from 0 to one less than the number of A- to-D samples taken per second IF cycle, resets to 0 and repeats its incrementing action cyclically. In the present example of 100 kHz, sampling of the 20 kHz signal "n" cycles between 0 and 4. By the process just described, the generation of the I & Q signals produces four output digital data streams from two input data streams . Each of the four output data streams are then digitally filtered by 5 kHz filters 210, 212, 214 and 216 to remove signal frequency components that are outside of the sensor's actual information bandwidth (5 kHz in the example above) . After the filtering, the data streams may optionally be re-sampled by resamplers 218, 220, 222, and 224 at a lower rate, since all of the valid information is contained within the information rate bandwidth (5 kHz in the example presented above) . A 10 kHz re-sample rate is sufficient for the 5 kHz information rate used in the example .

Once the filtered I & Q signals are computed, amplitude calculator 226 and phase calculators 228 and 230 compute the amplitude and phase of both the received signal and the reference signal in the manner described above.

The amplitude measurements can be used to implement any required amplitude modulation-to-phase modulation

(AM/PM) conversion correction. The AM/PM conversion correction may be applied to the data streams either before or after the phase difference between the reference signal and the received signal is calculated by phase difference calculator 232. The resulting phase (range) signal can be filtered or processed as required, but further filtering of the phase (range) signal is preferably performed using modulo arithmetic in order to properly deal with the ambiguity interval inherent in any range measurements based upon phase shift. An alternate embodiment of DSP 204 is shown in Figure 6. The alternate DSP 250 produces an additional constraint on the phase shift measurement system operation as it utilizes only a single A-to-D converter 252. The over-all procedure for the single A-to-D converter configuration is essentially the same as the dual A-to-D configuration. The exception in the procedure for the single A-to-D converter 252 is that subtraction of the reference phase is unnecessary because the reference phase is zero by definition, as is further explained below. The additional required constraint is that the A-to-D conversion operations (that is, the strobe signals that define the instant when the signal is sampled for conversion) must occur with a fixed and known phase relationship to the phase of the reference signal. Establishment of the phase relationship enables the DSP to effectively "know" the instantaneous amplitude of the reference signal. "Knowing" the A-to-D conversion result without making the measurement or even before it happens is possible because the reference signal's amplitude and frequency are essentially constant. In order to maintain a fixed phase relationship between the reference signal and the received signal, the value n (n was defined in the dual A-to-D converter discussion) is an input to DSP 250 instead of the reference signal. By sampling the received signal with a conversion command that is phase lock multiplied from the reference signal, and synchronizing a value n with respect to the positive zero crossing of the reference (or, alternatively, any other uniquely defined point on the reference waveform) , a fixed and known phase relationship between the I & Q computation and the reference is established.

In order to generate an A-to-D conversion command that has a fixed and known phase relationship to the reference signal, a phase-locked relationship can be established by either a conventional phase-locked loop or by conventional multiplication of the reference frequency by frequency multiplier 254. In the example presented above, the conversion command occurs at 5 times (100 kHz in the example) the reference signal frequency (20 kHz in the example) .

Synchronization of the DSP parameter n to the reference is accomplished by counter 256 that counts the conversion commands and is reset by the positive zero crossing of the reference. The value n is delivered to DSP 250 along with each digital sample of the received signal. DSP 250 then uses I & Q calculator 258 to compute the I and Q signals of the input signal to DSP 250. The two output data streams are then digitally filtered by 5 kHz filters 260 and 262 to remove signal frequency components that are outside of the sensor's actual information bandwidth (5 kHz in the example above) . After the filtering, the data streams may optionally be re-sampled by resamplers 264 and 266 at a lower rate, since, by definition, all of the valid information is contained within the information rate bandwidth (5 kHz in the example presented above) . A 10 kHz re sample rate is sufficient for the 5 kHz information rate used in the example. Once the filtered I & Q signals are produced, amplitude calculator 268 and phase calculator 270 compute the amplitude and phase of the input signal to DSP 250. The phase and amplitude values are then used to determine the location of the receiver relative to the transmitter. With multiple receivers on an object and with multiple transmitters, the orientation of the object can be ascertained through the aforementioned amplitude and phase calculation method.

The embodiments which have been set forth above were for the purpose of illustration and were not intended to limit the invention. It will be readily recognized that choice of signal modulation frequency, number and values of intermediate frequencies, bandwidths and sampling rates are not limited to those illustrated above but will depend on the application itself and the cost and availability of commercial components used in the design. It will also be recognized that, depending on the frequencies chosen and the cost and availability of commercial components, it may be possible to replace mixers and other analog components used in frequency translation with purely digital devices (ADCs) for direct digitization or sub-sampling of the signal or IFs.

It will also be readily recognized that through the use of multiple transmitters or multiple sensors or both and the digital phase-shift measurement system described herein applied to modulation signals on light beams, multiple transmitter-to-sensor distance measurements can be produced. Taken together, and employing well-known principles of classical trigonometry and geometry, these multiple one-dimensional measurements can be combined to determine the position of points, and the orientation of surfaces and solid objects in three-dimensional space. Accordingly, it will be appreciated by those skilled in the art that other various changes and modifications may be made to the embodiments discussed in this specification without departing from the spirit and scope of the invention defined by the appended claims.

Claims

It Is Claimed:

1. An object location system for detecting an object within a working volume, comprising: an optical signal transmitter for transmitting and scanning an amplitude modulated optical beam within said working volume; an optical signal receiver connected to said object for receiving said scanned optical beam; and a phase difference calculator connected to said optical signal receiver for determining the location of said object within said working volume based upon the phase difference between said transmitted and received optical beams .

2. The object location system of Claim 1 wherein said phase difference calculator is a digital phase difference calculator which performs phase difference operations upon digitized representations of said transmitted and received optical beams.

3. The object location system of Claim 2 further comprising: an intermediate frequency determining device connected to said optical signal receiver for generating transmitted and received intermediate frequency signals respectively of said transmitted and received optical beams ; at least one analog-to-digital convertor connected to said intermediate frequency determining device for digitizing said transmitted and received intermediate frequency signals; said digital phase difference calculator determining the phase difference between said transmitted and received intermediate frequency signals based upon said digitized transmitted and received intermediate frequency signals.

4. The object location system of Claim 2 further comprising: a first intermediate frequency determining device connected to said optical signal receiver for generating first transmitted and first received intermediate frequency signals respectively of said transmitted and received optical beams; a second intermediate frequency determining device connected to said first intermediate frequency determining device for generating second transmitted and second received intermediate frequency signals respectively of said first transmitted and first received optical beams; at least one analog-to-digital convertor connected to said second intermediate frequency determining device for digitizing said second transmitted and said second received intermediate frequency signals; said digital phase difference calculator determining the phase difference between said transmitted and received intermediate frequency signals based upon said digitized second transmitted and said digitized second received intermediate frequency signals.

5. The object location system of Claim 1 further comprising: a plurality of optical signal transmitters for transmitting and scanning a plurality of amplitude modulated optical beams within said working volume; a plurality of optical signal receivers connected to said object for receiving said scanned optical beams; said phase difference calculator being connected to said optical signal receivers for determining the location of said object within said working volume based upon the phase difference between said transmitted and received optical beams.

6. The object location system of Claim 1 further comprising: at least one mirror for reflecting said optical beam of said transmitter within said working volume; and an actuator for actuating said mirror so as to scan said reflected optical beam within said working volume.

7. The object location system of Claim 6 further comprising: a piezoelectric actuator for actuating said mirror so as to scan said reflected optical beam within said working volume.

8. The object location system of Claim 1 further comprising: an actuator for actuating said transmitter so as to scan said optical beam within said working volume.

9. The object location system of Claim 1 further comprising: gated circuitry connected to said receiver for gating said receiver as to when to receive said scanned optical beam.

10. The object location system of Claim 1 further comprising: at least one mirror for reflecting said optical beam of said transmitter within said working volume; an actuator for actuating said mirror so as to scan said reflected optical beam within said working volume; and gated circuitry connected to said actuator for gating said receiver as to indicate when to receive said scanned optical beam based upon a signal from said actuator, said signal from said actuator being indicative of said the transmitted optical beam being approximate to the position of said receiver.

11. The object location system of Claim 1 further comprising: gated circuitry connected to said transmitter for gating said transmitter as to when to transmit said scanned optical beam.

12. The object location system of Claim 1 wherein said receiver substantially continuously monitors for said scanned optical beam.

13. The object location system of Claim 12 wherein said receiver utilizes the optical beams which have a predetermined intensity.

14. The object location system of Claim 1 wherein said optical signal transmitter scans said optical beam in a sweeping fan pattern.

15. The object location system of Claim 1 wherein said optical signal transmitter scans said optical beam in a spiral pattern.

16. The object location system of Claim 1 wherein said optical signal transmitter scans said optical beam in a raster pattern.

17. The object location system of Claim 1 wherein said optical beam is scanned through an angle of ninety degrees within said working volume.

18. The object location system of Claim 1 wherein said optical beam is scanned through an angle of approximately ninety degrees within said working volume.

19. The object location system of Claim 1 wherein said optical beam is approximately five degrees thick.

20. The object location system of Claim 1 wherein said phase difference calculator is an analog phase difference calculator which performs phase difference operations upon analog signals of said transmitted and received optical beams.

21. An objection location system for locating an object within a working volume via an amplitude modulated light signal, comprising: an optical signal transmitter for transmitting an amplitude modulated optical beam within said working volume ; an optical signal receiver for receiving said amplitude modulated light, said receiver being physically coupled to said object; an intermediate frequency determining device connected to said optical signal receiver for generating transmitted and received intermediate frequency signals respectively of said transmitted and received signals; at least one analog-to-digital convertor connected to said intermediate frequency determining device for digitizing said transmitted and received intermediate frequency signals; a digital phase difference calculator for determining the phase difference between said transmitted and received intermediate frequency signals based upon said digitized transmitted and received intermediate frequency signals; said determined phase difference being used to locate said object within said working volume.

22. The object location system of Claim 21 wherein said digital phase difference calculator determines the inphase and quadrature characteristics associated with said transmitted and received signals.

23. The object location system of Claim 21 further comprising: a first intermediate frequency determining device connected to said optical signal receiver for generating first transmitted and first received intermediate frequency signals respectively of said transmitted and received optical beams; a second intermediate frequency determining device connected to said first intermediate frequency determining device for generating second transmitted and second received intermediate frequency signals respectively of said first transmitted and first received optical beams; at least one analog-to-digital convertor connected to said second intermediate frequency determining device for digitizing said second transmitted and said second received intermediate frequency signals; said digital phase difference calculator determining the phase difference between said transmitted and received intermediate frequency signals based upon said digitized second transmitted and said digitized second received intermediate frequency signals.

24. The object location system of Claim 23 wherein said first intermediate frequency determining device includes an intermediate frequency filter for filtering said transmitted and received signals to an intermediate frequency less than one hundred megahertz.

25. The object location system of Claim 24 wherein said filtered transmitted and received signals having a bandwith of approximately ten kilohertz.

26. The object location system of Claim 23 wherein said first intermediate frequency determining device includes a seventy megahertz intermediate frequency filter for filtering said transmitted and received signals.

27. The object location system of Claim 23 wherein said second intermediate frequency determining device includes an intermediate frequency filter for filtering said first transmitted and first received intermediate frequency signals to an intermediate frequency less than 100 kilohertz.

28. The object location system of Claim 27 wherein said filtered first transmitted and first received signals having a bandwith of approximately ten kilohertz.

29. The object location system of Claim 23 wherein said second intermediate frequency determining device includes a twenty kilohertz intermediate frequency filter for filtering said first transmitted and first received intermediate frequency signals.

30. The object location system of Claim 21 wherein said transmitter produces said optical beam by modulating a carrier wave, said carrier wave being approximately about one gigahertz.

31. The object location system of Claim 21 further comprising: a plurality of optical signal transmitters for transmitting and scanning a plurality of amplitude modulated optical beams within said working volume; a plurality of optical signal receivers connected to said object for receiving said scanned optical beams; said phase difference calculator being connected to said optical signal receivers for determining the location of said object within said working volume based upon the phase difference between said transmitted and received optical beams.

32. The object location system of Claim 21 further comprising: only one analog-to-digital convertor connected to said intermediate frequency determining device for digitizing said transmitted and received intermediate frequency signals.

33. The object location system of Claim 32 further comprising: a frequency multiplier for multiplying said transmitted intermediate frequency signal; an n counter connected to said frequency multiplier for counting the number of conversions associated with said transmitted intermediate frequency signal, said digital phase difference calculator determining the inphase and quadrature characteristics associated with said transmitted and received signals based upon said counted number supplied by said n counter.

34. The object location system of Claim 21 wherein said optical signal transmitter transmits and scans an amplitude modulated optical beam within said working volume.

35. The object location system of Claim 34 further comprising: at least one mirror for reflecting said optical beam of said transmitter within said working volume; and an actuator for actuating said mirror so as to scan said reflected optical beam within said working volume .

36. The object location system of Claim 34 wherein said optical signal transmitter scans said optical beam in a sweeping fan pattern.

37. The object location system of Claim 34 wherein said optical signal transmitter scans said optical beam in a spiral pattern.

38. The object location system of Claim 34 wherein said optical signal transmitter scans said optical beam in a raster pattern.

39. The object location system of Claim 34 wherein said optical beam is scanned through an angle of ninety degrees within said working volume .

40. The object location system of Claim 34 wherein said optical beam is scanned through an angle of approximately ninety degrees within said working volume.

41. An object location method for detecting an object within a working volume, comprising the steps of: transmitting and scanning an amplitude modulated optical beam within said working volume; receiving said scanned optical beam; and determining the location of said object within said working volume based upon the phase difference between said transmitted and received optical beams.

42. The object location method of Claim 41 further comprising the steps of: performing phase difference operations upon digitized representations of said transmitted and received optical beams.

43. The object location method of Claim 42 further comprising the steps of: determining transmitted and received intermediate frequency signals respectively of said transmitted and received optical beams; digitizing said transmitted and received intermediate frequency signals; determining the phase difference between said transmitted and received intermediate frequency signals based upon said digitized transmitted and received intermediate frequency signals.

44. The object location method of Claim 42 further comprising: determining first transmitted and first received intermediate frequency signals respectively of said transmitted and received optical beams; determining second transmitted and second received intermediate frequency signals respectively of said first transmitted and first received optical beams; digitizing said second transmitted and said second received intermediate frequency signals; and determining the phase difference between said transmitted and received intermediate frequency signals based upon said digitized second transmitted and said digitized second received intermediate frequency signals.

45. The object location method of Claim 41 further comprising the step of : scanning said optical beam in a sweeping fan pattern.

46. The object location method of Claim 41 further comprising the step of : scanning said optical beam in a spiral pattern.

47. The object location method of Claim 41 further comprising the step of : scanning said optical beam in a raster pattern.

48. The object location method of Claim 41 further comprising the step of : scanning said optical beam through an angle of ninety degrees within said working volume.

49. The object location method of Claim 41 further comprising the step of: scanning said optical beam through an angle of approximately ninety degrees within said working volume.

50. The object location method of Claim 41 further comprising the step of: transmitting said optical beam with an approximate thickness of five degrees within said working volume .