Electronic apparatus and method for measuring length of a communication cable

• US 6,646,454 B2
• Filed: 05/07/2002
• Issued: 11/11/2003
• Est. Priority Date: 01/07/2002
• Status: Expired due to Term

• Current Assignee: Viavi Solutions, Inc. (Lumentum Holdings Inc.)
• Original Assignee: Test-Um Inc. (Lumentum Holdings Inc.)
• Inventors: Watkins, Lee A.
• Primary Examiner(s): Le, N.
• Assistant Examiner(s): Dole, Timothy J.
• Application Number:
  US10/141,368
• Publication Number:
  US 20030128039A1
• Time in Patent Office: 553 Days
• Field of Search: 324/674, 324/662, 340/870.37
• US Class Current: 324/674
• CPC Class Codes: G01B 7/02 for measuring length, width or thickness G01B7/004, G01B7/12 take precedence
• CPC Class Codes: G01B 7/026 for measuring length of cable, band or the like, which has been paid out, e.g. from a reel measuring length of objects while moving G01B7/04
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

The length of an electrical cable is measured from only one of its ends, even though much of the cable may not be visible and its remote end may not be accessible. The ends of a conductor pair are selected to serve as contacts for a test capacitor. A reference capacitor having known capacitance value, and the test capacitor, are alternately inserted into an oscillator circuit while measuring the oscillator frequency. The two oscillator frequencies are compared to provide a ratio, which is then mathematically combined with a number indicative of the capacitance per unit length of the cable so as to produce an output number representing cable length.

Claims

1. The method of measuring from only one of its ends the length of a cable whose capacitance per unit length is previously known, comprising the steps of:

selecting an oscillator circuit into which a desired capacitor may be selectively inserted, the oscillator circuit being characterized by an operating frequency substantially proportional to the capacitance value of the inserted capacitor;

inserting into the oscillator circuit a reference capacitor whose capacitance value is known;

then operating the oscillator circuit a predetermined period of time while counting a first number of oscillations it generates;

thereafter inserting the one end of the cable into the oscillator circuit in lieu of the reference capacitor, and again operating the oscillator circuit for the same predetermined period of time while counting a second number of oscillations it then generates;

comparing the first and second numbers of oscillations to obtain a ratio;

combining the ratio with the known capacitance to produce a result indicating cable capacitance; and

then combining the cable capacitance with the previously known capacitance per unit length of the cable to produce a result indicating the length of the cable.

2. The method of measuring from only one of its ends the length of a cable whose capacitance per unit length has been previously known, comprising the steps of:

selecting an oscillator circuit into which a desired capacitor may be selectively inserted, the oscillator circuit being characterized by an operating frequency substantially proportional to the capacitance value of the inserted capacitor;

inserting into the oscillator circuit a reference capacitor whose capacitance value is known, to thereby control the frequency of the oscillator;

while the reference capacitor is operatively coupled therein, operating the oscillator circuit so as to determine an operating frequency associated with the reference capacitor;

inserting one end of the cable into the oscillator circuit to thereby control the frequency of the oscillator;

while the cable is operatively coupled therein, operating the oscillator circuit so as to determine an operating frequency associated with the cable;

comparing the two thus-measured operating frequencies to obtain a frequency ratio;

combining the frequency ratio with the known capacitance to produce a result indicating cable capacitance; and

then combining the cable capacitance with the previously known capacitance per unit length of the cable to produce a result indicating the length of the cable.

3. The measurement method of claim 2 wherein the oscillator is operated throughout the same known period of time for each of the inserted capacitors, and the number of its oscillations during the known time period is counted to determine its operating frequency corresponding to each inserted capacitor.

4. The measurement method of claim 2 wherein the known time period is adjusted to improve the accuracy of the count.

5. Test apparatus for measuring from only one of its ends the length of a cable whose capacitance per unit length is already known, the apparatus comprising:

a reference capacitor having a known capacitance value;

an oscillator circuit adapted for the insertion therein of a selected capacitor such that the operating frequency of the oscillator circuit will then be substantially proportional to the capacitance value of the inserted capacitor;

means for selectively inserting the reference capacitor into the oscillator circuit to control its operation;

means for selectively inserting the one end of the cable into the oscillator circuit to providing a capacitor for controlling its operation;

means for measuring the oscillating frequency of the oscillator circuit when either of the selected capacitors has been inserted therein;

means for comparing the two measured oscillation frequencies to obtain a ratio;

means for combining the ratio with the known capacitance to produce a result indicating cable capacitance; and

means for combining the cable capacitance with the previously known capacitance per unit length of the cable to produce a result indicating the length of the cable.

6. The test apparatus of claim 5 which includes timing means associated with the oscillator circuit for timing its operation during a predetermined period of time;

and

counting means associated with the oscillator circuit and cooperating with the timing means for counting the number of oscillations of the oscillator circuit during the predetermined period of time.

7. Test apparatus as in claim 6 which includes means for adjusting the duration of the predetermined period of time to accommodate length of a particular cable that is being tested.

8. Test apparatus for measuring from only one of its ends the length of a cable whose capacitance per unit length is already known, the apparatus comprising:

a reference capacitor having a known capacitance value;

an oscillator circuit adapted for the insertion therein of a selected capacitor such that the operating frequency of the oscillator circuit will then be substantially proportional to the capacitance value of the inserted capacitor;

timing means associated with the oscillator circuit for timing its operation during a predetermined period of time;

counting means associated with the oscillator circuit and cooperating with the timing means for counting the number of oscillations of the oscillator circuit during the predetermined period of time;

means for inserting a selected one of the reference capacitor and the cable end into the oscillator circuit and maintaining it there for the predetermined period of time;

means for storing a first number of pulses generated by the oscillator circuit during that predetermined period of time;

means for then inserting the other of the reference capacitor and the cable end into the oscillator circuit and maintaining it there for the predetermined period of time;

means for counting the number of pulses generated by the oscillator circuit during each aforesaid time period;

means for storing first and second numbers of pulses, respectively, that are generated by the oscillator circuit during the two respective predetermined periods of time;

means for comparing the two stored numbers of pulses to obtain a ratio;

means for combining the ratio with the known capacitance to produce a result indicating cable capacitance; and

means for combining the cable capacitance with the previously known capacitance per unit length of the cable to produce a result indicating the length of the cable.

9. Circuitry for measuring the capacitance value of an unknown capacitor, comprising:

an oscillator circuit adapted for insertion of a selected capacitor and whose operating frequency is then substantially proportional to the inserted capacitance value;

a reference capacitor having a known capacitance value;

means for selectively inserting either the reference capacitor or the unknown capacitor into the oscillator circuit;

timing means associated with the oscillator circuit for timing its output during a predetermined period of time;

counting means associated with the oscillator circuit for counting its output pulses during that period of time;

means for storing a first number of pulses generated by the oscillator circuit when the reference capacitor has been inserted therein;

means for storing a second number of pulses generated by the oscillator circuit when the unknown capacitor has been inserted therein;

means for comparing the two numbers of pulses to obtain a ratio; and

means for combining the ratio with the known capacitance to produce a result representing the value of the unknown capacitance.

10. The method of measuring the length of an electrical cable, comprising the steps of:

selecting a reference capacitor having a known capacitance value;

selecting at one end of the cable, the ends of a conductor pair to serve as contacts for a test capacitor;

alternately inserting the reference capacitor and the test capacitor into a stable oscillator while measuring the two associated operating frequencies of the oscillator;

comparing the two oscillator frequencies to provide a ratio;

combining the ratio with the known capacitance to produce a result indicating cable capacitance; and

then combining the cable capacitance with a number that is indicative of the capacitance per unit length of the cable to produce an output number representing the cable length.