Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
  • G01S17/88—Lidar systems specially adapted for specific applications
  • G01S17/89—Lidar systems specially adapted for specific applications for mapping or imaging
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
  • G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
  • G01S17/06—Systems determining position data of a target
  • G01S17/08—Systems determining position data of a target for measuring distance only
  • G01S17/32—Systems determining position data of a target for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated
  • G01S17/36—Systems determining position data of a target for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated with phase comparison between the received signal and the contemporaneously transmitted signal
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
  • G01S17/87—Combinations of systems using electromagnetic waves other than radio waves
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
  • G01S7/481—Constructional features, e.g. arrangements of optical elements
  • G01S7/4811—Constructional features, e.g. arrangements of optical elements common to transmitter and receiver
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
  • G01S7/481—Constructional features, e.g. arrangements of optical elements
  • G01S7/4811—Constructional features, e.g. arrangements of optical elements common to transmitter and receiver
  • G01S7/4813—Housing arrangements

Definitions

• This invention relates to method and apparatus for obtaining measurements and, more particularly, to a method and apparatus of measuring with increased accuracy.
• Distance measurement devices usually measure short distances with good accuracy or long distances with poor accuracy. Currently devices that can measure with good accuracy or long range are expensive and complicated.
• the short range measurement devices i.e., microns to meters range
• the mechanical devices measure the distances directly and the optical devices measure optical fringes to count the distance to the measurement plane. Long distances are measured most commonly by time of flight and result in centimeter scale resolution.
• aspects of the present invention include a device comprising: a light source capable of transmitting an outgoing light beam toward a target;
• a frequency source coupled to the light source and capable of modulating the outgoing light beam; a first beam splitter configured to divide the outgoing light beam; a second beam splitter configured to combine a return ranging light beam from the target and a reference light beam from the first beam splitter; [0006] a first detector optically coupled to the second beam splitter and configured to produce a voltage signal; and a second detector coupled to the first detector and configured to determine target distance information from the voltage signal.
• Further aspects of the invention include a method comprising: transmitting an outgoing light beam toward a target; modulating the light beam with a first frequency; splitting the outgoing light beam; combining a first return ranging light beam with a first reference light beam; detecting the first return ranging light beam and the first reference light beam and producing a first voltage signal representing distance dependent phase information; and
• Figure 1 is a schematic diagram of a first embodiment measurement device having a frequency source which modulates a light source
• Figure 2 is a schematic diagram of a second embodiment measurement device
• Figure 3 is a cross-sectional diagram of a modular system incorporating a plurality of the second embodiment measurement devices.
• FIG. 1 illustrates a distance measurement system or device 100.
• the distance measurement device 100 includes a housing 100a having a frequency source 101 (e.g., oscillator) producing a sine wave output and capable of amplitude modulating a light source 102 with a modulation frequency f.
• the frequency source 101 may operate in the frequency range of approximately 1 MegaHertz (MHz) to approximately 2 GigaHertz (GHz).
• the light source 102 may typically be a laser diode, but in alternative embodiments, a broadband light source such as incandescent lighting or narrowband light source such as a light-emitting diode (LED) may be used.
• a broadband light source such as incandescent lighting or narrowband light source such as a light-emitting diode (LED) may be used.
• LED light-emitting diode
• the amplitude-modulated output from the light source 102 is incident on beam splitter 104.
• the output 103 of beamsplitter 104 is split into two beams 105 and 107.
• the splitting of the output 103 creates a ranging beam 105 and a reference beam 107.
• the ranging beam 105 is transmitted out of the housing 100a to a target 106.
• a return ranging beam is designated by reference numeral 105a and travels back from the target 106 to the housing 100a.
• the reference beam 107 is not propagated to the target 106, but rather is redirected in the device 100 by beam splitter 104 so as to be redirected by beam splitter 111 and combined with the return ranging beam 105a.
• the reference beam 107 optical propagation distance remains constant and thus allows for a constant reference distance to compare the distance to the target 106 that is transversed by return ranging beam 105a.
• the distance L between the housing 100a and the target 106 is the distance to be measured by the device 100.
• Return ranging beam 105a returns from the target 106 either by diffuse reflection from the target 106 itself, by a retroreflector 106a, or by another means of returning the light to the detector 110.
• a retroreflector 106a is a reflection source that may be used to obtain greater accuracy and is typically used when the distance to be measured by the device 100 is greater than approximately 50 feet.
• the return ranging beam 105a and light beam 107 output from the beam splitter 111 are then incident upon optical detector 110.
• the optical detector may also be referred to as the first detector for the purposes of this description.
• the optical detector 110 is a mixing detector which mixes the signals detected from the return ranging beam 105a and reference light beam 107.
• the optical detector 110 may be a square law detector which outputs a voltage signal proportional to the square of the electric field as a measurement of the optical intensity of the two incoherent beams 105a, 107.
• Detection electronics 115 are directly coupled to the optical detector 110. Detection electronics 115 may also be referred to as the second detector for the purposes of this description.
• Detection electronics 115 may include a phase detector and are designed to receive the voltage signal 114 and compare the phase of this voltage signal 114 with the phase of a modulation output signal 116 from light source 102. The detection electronics 115 are then configured to output a phase difference determination to controller 117.
• the optical detector 110 may be directly coupled to the detection electronics 115 with only an electrical line in between because the voltage signal 114 does not require amplification. Amplification is not required because the detection electronics 115 may be designed to detect minimum frequency values and, therefore, frequency difference signal 114 and modulation output signal 116 are not required to have substantially the same strength. Therefore, the modulation output signal 116 has substantially greater signal strength than the voltage signal 114.
• the detection electronics 115 are capable of determining the signal strength at the modulation frequency so as to obtain the important information from the optical detector 110 while discarding the unimportant information. After calculating the phase difference between the voltage signal 114 and the modulation output signal 116, the detection electronics 115 the phase difference will be used to determine the distance L.
• the detection electronics 115 may include, for example, a lock-in amplifier or a band-pass filter.
• the controller 117 is configured to receive the phase difference signal from the detection electronics 115 and calculate the one-way distance L from the housing 100a to the target 106.
• the controller may be, for example, a microprocessor.
• the controller 117 is connected through line 118 to control the frequency source 101.
• the controller 117 is designed to perform the following calculations to determine the distance L to the target 106.
• the value of and L are initially unknown, and therefore, a value to be measured may not be determined.
• the wave number of modulation, may be determined by automatically taking at least two readings at two different frequencies.
• the controller 117 is configured to control the frequency settings of the frequency source 101 in response to the operator's actions and to process the first and second readings at different frequencies.
• the two readings at two different frequencies allows the controller 117 to calculate the value of m using the phase difference signal from detector 115.
• a first difference signal will be compared to a first modulation output signal
• the second difference signal will be compared to a second modulation output signal.
• the first and second readings may be taken at frequencies in the range of approximately 1 MHz to approximately 2 GHz.
• the measuring frequencies may be digitally controlled frequencies that are controlled by the controller 117 directly or, in alternative embodiments, the frequencies may be obtained from a linear sweep that are read by the controller 117. Putting the value of m into the following equation yields the value of L and ⁇ L , a measure of uncertainty in the measured distance:
• f may be the frequency for either of the first or second readings and ⁇ f is the uncertainty in the modulating frequency.
• a third reading may be taken at a third frequency. This third minimum allows for the averaging of noise in the system and improves the accuracy of the distance measurement).
• the velocity of the object 106 at the distance L may also be measured by analyzing a Doppler shift in the detection electronics 115.
• the detection electronics 115 are constructed to perform this Doppler shift analysis using voltage signal 114. Since the return signal frequency is Doppler shifted in proportion to the velocity of the target 106, the output of the optical detector 110 will be shifted from the original frequency by this Doppler shift.
• the detection electronics 115 may detect this shift using standard frequency measurement techniques.
• the velocity of the object 106 will be proportional to this frequency shift.
• the velocity calculation may be done simultaneously with the distance measurement.
• An advantage of the embodiment disclosed in Figure 1 is that inexpensive optical components may be used to measure distances with greater accuracy than is normally found in commercial surveying equipment and at substantially lower cost. Specifically, measurements may be obtained by the device 100 with an accuracy in the range of approximately 10 micrometers ( ⁇ m) to approximately 100 // m at approximately 5 meters from the target 106 and the accuracy varying by approximately 1 micrometer per meter as measured to the target 106. Therefore, this embodiment may be used effectively in the range of approximately 1 millimeter (mm) to approximately 5 kilometers.
• Figure 2 illustrates a second embodiment of a measurement device 200 which may determine the distance D to an object or target 202.
• the device 200 may be designed to be a small package having a length X which may be less than approximately 5 centimeters (cm), height Y which may be less than approximately 2 cm and depth (not shown) which may be less than approximately 2 cm.
• a light source e.g., laser diode, LED, incandescent light
• 212 either coherent (i.e., light waves all in phase with one another) or incoherent may be modulated in external modulator 214 by a signal generator (or frequency source) 210 (e.g., oscillator).
• the light output from light source 212 may be directly modulated without the use of an external modulator 214.
• Modulator 214 may be, for example, from the group consisting of an acousto-optical (AO) modulator, electro-optical (EO) modulator, Mach-Zender modulator, peizoelectric switch, and a liquid crystal light valve.
• AO acousto-optical
• EO electro-optical
• Mach-Zender modulator Mach-Zender modulator
• peizoelectric switch e.g., peizoelectric switch
• a liquid crystal light valve e.g., a periodic signal (e.g., sine wave, pulsed format) from signal generator 210 is modulated with the light beam from the light source 212 in the modulator 214.
• the periodic signal may be approximately 2 GHz or greater.
• the modulated light beam may then be transmitted through a fiber optic cable 215 to a remote head assembly 216.
• the remote head assembly 216 includes beam shaping optics 218, an acousto-optical (AO) deflector 222, beam shaping optics 220 and a receiving lens 224.
• Radio frequency (RF) signal generator 226 may optionally be mounted inside or outside the remote head assembly.
• the first set of beam shaping optics 218 focus the light signal from the fiber optic cable 215 onto the AO deflector 222.
• the AO deflector 222 may contain a crystal and a piezoelectric transducer.
• the AO deflector 222 may be actuated by the voltage controlled or digitally controlled radio frequency (RF) signal generator 226 (e.g., oscillator) coupled to the AO deflector 222.
• RF radio frequency
• the RF signal generator 226 may operate in the range of approximately 1 MHz to approximately 10 GHz and while shown outside the remote head assembly 216, the RF signal generator 226 may also be mounted inside the remote head assembly 216.
• the AO deflector 222 is constructed to diffract light off of the sine wave transmitted from the first beam shaping optics 218. This diffraction by the AO deflector 222 controls the light and causes the light to shift to a predetermined angle proportional to the driving frequency produced by the RF signal generator 226.
• the controller 233 may be used to control the RF signal generator 226 frequency (connection shown by reference numeral 250 in Figure 2).
• the diffracted light is transmitted to the second beam shaping optics 220 and a light beam 227 is created which forms a light spot 227a on the target 202 and follows a predetermined pattern as it scans the target 202.
• the reflected or diffused light 228 from the object 202 is captured by a receiving lens 224 (which also may be a fiber) and transmitted through a fiber optic cable 229 to a detector 230.
• Detector 230 may be a fast detector (e.g., capable of detecting signals approximately less than 2 nanoseconds in frequency).
• the output of the fast detector 230 is then amplified in amplifier 231.
• the output of this amplifier 231 is kept at a constant level for frequencies that are low compared to the original modulation frequency produced by the signal generator 210. This may be accomplished by using an automatic gain controlled amplifier with feedback from the output signal 231a or by controlling the transmit output level using feedback from the amplifier 231.
• the amplified output 231a of the amplifier 231 is transmitted to a phase detector 232 as a RF input.
• a second input to the phase detector 232 is a signal from the original modulating signal source, signal generator 210, is used as a reference signal by the phase detector 232.
• the output of the phase detector 232 is transmitted to a controller 233.
• Controller 233 may be a microprocessor or circuitry configured to control the frequency setting of the signal generator 210, receive readings from the phase detector 232 and perform calculations to determine the one-way distance D. Controller 233 may also be connected to a workstation 234 to display the distance measurements. In alternative embodiments, the controller 233 may be removed and the workstation 234 may be used to control the device 200. [0025] In operation, phase detector 232 measures the phase shift of the output signal 231a with respect to the reference signal produced by signal generator 210 and forwards the result to controller 233. The distance D may be calculated in the controller 233. The equations used in determining the distance D and the measure of uncertainty in the measured distance ⁇ D are the following: c( + 2 ⁇ m)
• ⁇ is the phase difference between the output signal 231a and the reference signal from signal generator 210; ⁇ ⁇ is the uncertainty in the phase difference; and ⁇ f is the uncertainty in the modulation frequency.
• r ⁇ may be determined by taking at least two readings at difference frequencies and then using this information to determine the distance D.
• the light spot 227a from the light beam 227 may be scanned around on the object 202 in a one-dimensional or two-dimensional pattern depending on the specific application. This one-dimensional and two-dimensional scanning capability allows for full three-dimensional object profiling of the target 202.
• the device 200 may be configured to scan in at least two directions which are substantially perpendicular allowing for an area scan without moving the object 202.
• the light spot 227a from the light beam 227 may be used to conduct both substantially transverse measurement and substantially vertical measurement.
• the substantially transverse measurement direction may be defined as that direction that is approximately perpendicular to the optical propagation direction of light beam 227.
• the substantially vertical measurement direction may be defined as that direction that is approximately parallel to the optical propagation direction of the light beam 227.
• the target 202 may also be translated in any direction (e.g., substantially horizontal, substantially vertical) with respect to the light beam 228 or rotated to facilitate the scanning process.
• the device 200 may be used to measure the distance to each measured point on the target 202 with an accuracy of less than approximately 1 ⁇ rsx and, typically, less than approximately 100 nanometers (nm).
• the transverse measurement accuracy is determined by the spot size 227a and is nominally in the range of 1 ⁇ m to 1 mm. In operation, a linear scan of over approximately 5 to approximately 15 centimeters (cm) per detector 200 and, typically, approximately 8 cm per detector may be possible.
• the device 200 may be self-calibrated using symmetrical detectors. Two optical detectors 200 may be used to measure the distance between each other by pointing the two at each other. T en a thin, well-calibrated target 202 may be placed in between the two detectors 200. The exact position and thickness of this target 202 may be calculated and verified with these measurements. Then all other targets 202 placed inside the measurement area will be measured to the same calibration accuracy as was made in the previous measurements.
• the device 200 is modular in format it is possible to combine a plurality of devices 200 into one modular system 300 that may scan many positions of the target 202 mounted in a target chamber 330 as shown in Figure 3.
• the modular system 200 design allows for a plurality of devices 200 to be operated simultaneously to improve the speed of measurement.
• a plurality of devices 200 may be mounted on an upper mounting section 302a and below the target 202 on a lower mounting section 302b.
• the number of devices on each of the mounting sections 302a, 302b may range in number from one on each mounting section to at least ten on each mounting section.
• Each of the mounting sections 302a, 302b may have a substantially arcuate shape as shown by Figure 3.
• Each of the modulation frequencies of the devices 200 may be different to avoid crosstalk between the devices 200 and allow for simultaneous operation.
• the target 202 may be rotated on a mounting device 320 or the upper and lower sections 302a, 302b may be rotated together or separately to increase the speed of the scanning operation.
• Each device 200 may measure areas of up to and including approximately 5 cm by 5 cm of the target 202 with substantially transverse measurement accuracies less than approximately 50 ⁇ m and substantially vertical measurement accuracies of less than approximately 500 ran.
• the measuring device 200 disclosed herein may be used in measuring small parts accurately (e.g., parts less than approximately 1 mm). Examples include computer disk parts, electrical assemblies, microchip inspection, circuit board inspection and general factory line inspection that require tight tolerances. However, the measuring device 200 is not limited to small size applications, and may be used in large automotive parts, industrial machinery, building inspection, Amplitude Modulated Light Detection and Ranging (AM LIDAR), distance measurement, modulated optical distance measurement, and survey instruments.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Electromagnetism (AREA)
• Computer Networks & Wireless Communication (AREA)
• General Physics & Mathematics (AREA)
• Radar, Positioning & Navigation (AREA)
• Remote Sensing (AREA)
• Optical Radar Systems And Details Thereof (AREA)

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• 2001
  • 2001-03-06 US US09/801,144 patent/US6570646B2/en not_active Expired - Fee Related
• 2002
  • 2002-03-01 AU AU2002306611A patent/AU2002306611A1/en not_active Abandoned
  • 2002-03-01 WO PCT/US2002/006085 patent/WO2002071097A2/en not_active Application Discontinuation
• 2003
  • 2003-03-17 US US10/391,378 patent/US6750960B2/en not_active Expired - Fee Related

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