High resolution measuring device for time difference

• US 5,075,878 A
• Filed: 04/18/1991
• Issued: 12/24/1991
• Est. Priority Date: 06/29/1988
• Status: Expired due to Term

• Current Assignee: Kabushiki Kaisha Topcon
• Original Assignee: Kabushiki Kaisha Topcon
• Inventors: Ohishi, Masahiro, Ohtomo, Fumio
• Primary Examiner(s): Dixon, Joseph L.
• Application Number:
  US07/687,198
• Time in Patent Office: 250 Days
• Field of Search: 364/550, 364/551.01, 364/561, 364/569, 364/167.01, 364/182, 356/4, 356/5, 356/358, 356/400, 328/109, 328/133, 328/129.1, 307/234, 342/125, 342/127, 250/206.1, 250/231.16, 250/231.18, 250/560
• US Class Current: 702/176
• CPC Class Codes: G01S 17/10 using transmission of interrupted, pulse-modulated waves determination of distance by phase measurements G01S17/32
• CPC Class Codes: G04F 10/06 by measuring phase G04F10/005 takes precedence

Abstract

High resolution measuring systems use a continuous series of laser pulses in measuring distance to an object. An approximate time difference between transmitted and reflected pulses is derived. A second, more accurate, time difference is derived through use of a reference signal sampled in correlation to the pulse signal in order to derive a sampled waveform signal having a period which is long relative to the period of the pulse signal. Phase difference measurements of this sampled waveform provide delay time measurements of high accuracy. The high accuracy and approximate time difference signals are combined to determine time difference between measurement pulses with increased resolution. Time difference is converted to precision distance measurments, with greater accuracy available by averaging several measurements. Several embodiments are described.

Claims

1. A high resolution measuring device, for measuring time difference between continuous pulses, comprising:

measurement signal means for supplying a measurement signal M having a period and comprising a continuous series of spaced pulses, including a series of reference pulses and a series of delayed pulses;

reference signal means for supplying a periodic waveform reference signal S which is asynchronous with measurement signal M over a plurality of said periods of signal M;

approximate measuring means, responsive to said signal M, for deriving a first approximate time difference between separated pulses of said signal M;

precision measuring means including sampling means, responsive to said signal M and said signal S, for sampling said signal S in correlation with said reference pulses of said signal M for deriving a first sampled waveform signal having a period which is at least twice said period of said signal M and for sampling said signal S in correlation with said delayed pulses of said signal M for deriving a second sampled waveform signal having a period which is at least twice said period of said signal M, and phase detecting means, coupled to said sampling means, for detecting phase differences between said first and second sampled waveform signals; and

processing means, coupled to said phase detecting means and said approximate measuring means, for deriving a second time difference based on said phase differences between said first and second sampled waveform signals and for utilizing said second time difference and said first approximate time difference for determining the time difference between pulses with increased resolution.

2. A high resolution measuring device in accordance with claim 1, wherein said reference signal means comprises means for supplying a reference signal S having a period which is smaller than the period of said measurement signal M, whereby said sampled waveform signals derived by said sampling means is a periodic waveform with a relatively long period enabling detection of phase differences with high resolution.

3. A high resolution measuring device in accordance with claim 1, wherein said reference signal means comprises means for supplying a continuous reference signal having a period and synchronism such that effective synchronism between said S and M signals occurs on approximately every eighth M signal pulse, whereby said sampled waveform signal derived by said sampling means is a periodic waveform with a relatively long period enabling detection of phase differences with high resolution.

4. A high resolution measuring device in accordance with claim 1, wherein said processing means is additionally configured for averaging a plurality of such time differences determined from said first and second time differences, so as to further increase resolution.

5. A high resolution measuring device in accordance with claim 1, wherein said measurement signal means and reference signal means respectively comprise means for supplying an M signal and an S signal having a relationship such that the frequency f_M of the M signal and the frequency f_S of the S signal are related as follows:

##EQU22## where m is an integer representing the relationship between f_M and the higher frequency f_S, and m is an integer representing the relationship between the frequency of said sampled waveform signal and the higher frequency f_M.

6. A high resolution measuring device, for measuring the distance to an external object, comprising:

measurement signal means for supplying a measurement signal M having a period and comprising a continuous series of spaced pulses;

means for transmitting, in the form of pulses of light, said measurement signal M over a distance to an external object and for receiving such signal M pulses after reflection from said object;

reference signal means for supplying a periodic waveform reference signal S which is asynchronous with measurement signal M over a plurality of said periods of signal M;

approximate measuring means, responsive to said transmitted and received signals M, for deriving a first approximate time difference between separated pulses of said signals M;

precision measuring means including sampling means, responsive to said signal M and said signal S, for sampling said signal S in correlation with pulses of said transmitted signal M for deriving a first sampled waveform signal having a period which is at least twice said period of said signal M and for sampling said signal S in correlation with pulses of said received signal M for deriving a second sampled waveform signal, having a period which is at least twice said period of said signal M, and phase detecting means, coupled to said sampling means, for detecting phase differences between said first and second sampled waveform signals; and

processing means, coupled to said phase detecting means and said approximate measuring means, for deriving a second time difference based on said phase differences between said first and second sampled waveform signals, for utilizing said second time difference and said first approximate time difference for determining the time difference between pulses with increased resolution, and for converting said time difference determinations to precision distance measurements;

whereby the distance to an external object can be measured with a high degree of accuracy.

7. A high resolution measuring device in accordance with claim 6, wherein said reference signal means comprises means for supplying a continuous reference signal having a period and synchronism such that effective synchronism between said S and M signals occurs on approximately every eighth M signal pulse, whereby said sampled waveform signals derived by said sampling means is a periodic waveform with a relatively long period enabling detection of phase differences with high resolution.

8. A high resolution measuring device in accordance with claim 6, wherein said measurement signal means and reference signal means respectively comprise means for supplying an M signal and an S signal having a relationship such that the frequency f_M of the M signal and the frequency f_S of the S signal are related as follows:

##EQU23## where m is an integer representing the relationship between f_M and the higher frequency f_S, and m is an integer representing the relationship between the frequency of said sampled waveform signal and the higher frequency f_M.