Time multiplexed fringe counter

• US 5,526,114 A
• Filed: 07/20/1994
• Issued: 06/11/1996
• Est. Priority Date: 07/20/1994
• Status: Expired due to Fees

• Current Assignee: Steven A. Eselun
• Original Assignee: Steven A. Eselun
• Inventors: Eselun, Steven A.
• Primary Examiner(s): Turner, Samuel A.
• Application Number:
  US08/277,739
• Time in Patent Office: 692 Days
• Field of Search: 356/345, 356/363, 356/356, 356/358, 250/237 G
• US Class Current: 356/450
• CPC Class Codes: G01B 2290/45 Multiple detectors for detecting interferometer signals
• CPC Class Codes: G01B 9/02083 characterised by particular signal processing and presentation

Abstract

A method and apparatus for making measurements on light fringes such as are produced by interferometers or gratings wherein such measurements can include measurements of position and velocity of an object such as a step and repeat carriage or microscope stage

The fringe pattern is sensed with a linear array detector whose elements are repetitively time multiplexed. The resultant output signal is filtered, yielding an AC signal with a phase indicative of fringe position. Rapid, digital, submicron counting is made possible using a phase lock loop to control the multiplexor clock. The advantages of AC interferometry are realized without optical modulation.

Claims

1. An apparatus for converting a light pattern having a spatially distributed intensity to a time dependent signal which comprises:

an array of a plurality of photodetectors positioned to intercept said light pattern;

a multiplexor means for generating a plurality of signals responsive to intensity of said light pattern at each photodetector having a plurality of input terminals, one said input terminal connected to one of said plurality of detectors, respectively;

a multiplexor clock means for transferring successively and repeatedly each one of said plurality of signals at each input terminal of said multiplexor means to an output terminal of said multiplexor means;

said multiplexor clock means being operably connected to said multiplexor means;

a band pass filter having a center frequency substantially equal to a frequency of said multiplexor clock means divided by a number equal to said plurality of detectors, said band pass filter having an input terminal connected to said output terminal of said multiplexor means and an output terminal where said time dependent signal is elicited.

2. An apparatus for generating a position signal responsive to a position of an object which comprises:

means attached to said object for producing a pattern of light fringes;

an array of a plurality of photodetectors positioned to intercept said pattern of light fringes;

a multiplexor means for generating a plurality of signals responsive to intensity of said pattern at each photodetector having a plurality of input terminals, one said input terminal connected to one of said plurality of detectors, respectively;

a multiplexor clock means for transferring successively and repeatedly each one of said plurality of signals at each input terminal of said multiplexor means to an output terminal of said multiplexor means;

said multiplexor clock means being operably connected to a clock terminal of said multiplexor means;

a band pass filter having a center frequency substantially equal to a frequency of said multiplexor clock means divided by a number equal to said plurality of detectors, said band pass filter having an input terminal connected to said output terminal of said multiplexor means and an output terminal where said time dependent signal is elicited;

a phase comparator having one input terminal connected to said output terminal of said filter and another input terminal connected to said multuplexor clock means such that a phase difference signal is generated at an output terminal of said phase comparator

responsive to a position of said object.

3. A device as in claim 2 which comprises means for displaying said phase difference signal.

4. A device as in claim 2 which comprises means for differentiating said phase difference signal with respect to time such as to produce a velocity signal responsive to a velocity of said object.

5. An apparatus for generating a position signal responsive to a position of an object which comprises:

means adapted to be attached to said object for producing a pattern of light fringes;

an array of a plurality of photodetectors positioned to intercept said pattern;

a multiplexor having a multiplexor clock terminal, an output terminal and a plurality of input terminals, one said input terminal connected to one of said plurality of detectors, respectively;

a reference clock means for emitting a signal having a reference clock frequency and reference clock phase;

a first divider means connected to said reference clock means for generating a timing clock signal having a timing clock frequency equal to said reference clock frequency divided by a selected first integer;

a band pass filter having a center frequency substantially equal to said timing clock frequency;

said band pass filter having an input terminal connected to said output terminal of said multiplexor such that a filtered multiplexor signal is generated at an output terminal of said band pass filter having a multiplexor signal phase responsive to a position of said pattern of light fringes on said array of photodetectors;

a first phase comparator means having one input terminal connected to an output terminal of said filter and another input terminal connected to an output terminal;

of said first divider for generating a phase difference signal at an output terminal of said first phase comparator means;

a voltage controlled oscillator means having an input terminal connected to said output terminal of said first phase comparator for generating a voltage controlled oscillator signal having a phase difference clock frequency proportional to said phase difference signal;

a second divider means connected to an output terminal of said voltage controlled oscillator means for generating a muktiplexor clock signal having a multiplexer clock frequency equal to said phase difference clock frequency divided by a second integer;

said second integer being equal to said first integer divided by said plurality;

said multiplexer clock terminal being connected to an output terminal of said second divider means;

a second phase comparator means having one input terminal connected to said output terminal of said voltage controlled oscillator means and a second input teriminal connected to said output terminal of said reference clock means for generating said position signal being responsive to a difference in phase between signals applied at said first and second input terminals of said second phase comparator means where by a displacement signal is generated at an output terminal of said second phase comparator means which is proportional to a displacement of said fringe pattern multiplied by said first integer;

said first integer selected to provide a desired sensitivity to changes of said displacement.

6. An apparatus as in claim 5 which comprises a means for displaying said position signal connected to said output terminal of said second phase comparator.

7. An apparatus as in claim 6 wherein said means for displaying comprises an up and down counter means having an input terminal connected to an output terminal of said second phase comparator means.

8. A method for converting a spatially dependent light pattern to a time dependent signal which comprises the steps:

(a) shining said light pattern on a linear array of detectors such as to generate a series of signals, one signal from each detector;

(b) passing said series of signals successively and repeatedly at a frequency of a clock signal through a band pass filter such as to produce a continuous time dependent signal having a time varying amplitude corresponding to an intensity of said light pattern at each one of said detectors in succession such that a phase of said time dependent signal depends on spatial distribution of intensity of said pattern;

(c) determining a difference signal being a difference in phase between said time dependent signal and said clock signal;

(d) displaying said difference signal such as to indicate a position of said pattern on said array of detectors.

9. A method as in claim 8 wherein said filter is a band pass filter having a center frequency equal to said clock frequency.

10. A method as in claim 8 wherein step (b) includes the steps;

(i) connecting each inputm terminal of a multiplexor to one of said detectors, respectively;

(ii) connecting an output terminal of said multiplexor to an input terminal of said band pass filter;

(iii) applying a clock signal having said frequency to a clock terminal of said multiplexor.

11. A method as in claim 8 wherein step (b) includes the steps

(i) connecting each input terminal of a multiplexor to one of said detectors, respectively;

(ii) connecting an output terminal of said multiplexor to an input terminal of said bandpass filter so as to generate a filtered multiplexor signal at an output terminal of said band pass filter when a multiplexor clock terminal is connected to a clock signal;

(iii) connecting said multiplexor clock terminal of said multiplexor to a clock output terminal of a phase locked loop having a reference clock generating a reference signal;

(iv) connecting an output terminal of said band pass filter to an input terminal of said phase lock loop;

and, step (c) includes the step;

measuring the phase difference between said multiplexor signal and said reference signal being said difference signal.