Direct reading inductance meter

• US 4,045,728 A
• Filed: 04/15/1976
• Issued: 08/30/1977
• Est. Priority Date: 04/15/1976
• Status: Expired due to Term

• Current Assignee: Fletcher James C Administrator of The National Aeronautics and Space Administration With Respect To An Invention of, Richard B. Kolbly
• Original Assignee: Fletcher James C Administrator of The National Aeronautics and Space Administration With Respect To An Invention of, Richard B. Kolbly
• Inventors: Fletcher, James C. Administrator of the National Aeronautics and Space Administration, with respect to an invention of, Kolbly, Richard B.
• Primary Examiner(s): Krawczewicz, Stanley T.
• Application Number:
  US05/677,351
• Time in Patent Office: 502 Days
• Field of Search: 324/59, 324/57 H, 324/79 R, 324/79 D, 324/81
• US Class Current: 324/655
• CPC Class Codes: G01R 27/2611 Measuring inductance

Abstract

A direct reading inductance meter is comprised of a crystal oscillator for generating a reference frequency f_r and an LC tuned oscillator for generating an initial frequency f_o which, when mixed with the reference, produces a difference equal to zero. Upon connecting an inductor of small unknown value in the LC circuit to change its resonant frequency to f_x, a difference frequency (f_r -f_x) is produced that is very nearly a linear function of the inductance of the inductor. The difference frequency is measured and displayed on a linear scale in units of inductance.

Claims

1. Apparatus for direct measurement of the inductance exhibited by an element which is essentially inductive in a range below 3μ

H, comprising

means for generating a first alternating signal at a stable reference frequency, f_r,

means for generating a second alternating signal at an initial frequency, f_o, substantially equal to said reference frequency, said second signal generating means including an LC circuit tuned to resonance at said initial frequency f_o to control the frequency of said second signal, said LC circuit having an inductance at least approximately five times greater than said element,

switching means for connecting said element to be measured to said LC circuit to change the resonant frequency thereof to a frequency f_x,

means for mixing said first and second signals to obtain a third signal of a frequency equal to the difference between the frequencies f_r and f_x, and

means for measuring the frequency of said third signal and for displaying the value of said element as a linear, function of the frequency of said third signal.

2. Apparatus as defined in claim 1 wherein said frequency measuring means is comprised of means for generating a pulse of predetermined pulse width for each cycle of said third signal, and

a lossy-integration and display means for so integrating each of said pulse of predetermined pulse width as to produce a display of the value of said element as a linear function of the rate of said pulses integrated.

3. Apparatus as defined in claim 2 wherein said lossy-integration and display means is comprised of an ammeter.

4. A direct reading inductance meter comprising

a reference oscillator for generating a reference signal at a stable frequency,

an LC tuned oscillator for generating a second signal at a frequency equal to said reference frequency as determined by an included LC tuned circuit,

a mixer for mixing said second signal with said reference signal to obtain an output signal at a difference frequency equal to the difference between said reference frequency and said second frequency,

means for connecting an element of unknown inductance of some unknown value less than about 3μ

H to said LC tuned circuit having an inductance at least approximately five times greater than the unknown element thereby to shift said second frequency by an amount approximately in linear proportion to said unknown value of inductance, and

means for measuring the frequency of said output signal with said element connected to said LC tuned circuit for determining said unknown value of inductance as a linear function of said difference frequency.

5. Apparatus as defined in claim 4 wherein said frequency measuring means is comprised of

a separate pulse shaping amplifier for each of said reference and second signals to produce a reference train of pulses and a second train of pulses from said reference signal and second signal, respectively, one pulse in each train for each cycle of the signal being amplified,

a flip flop adapted to be set in response to a particular point in each pulse of one of said trains of pulses to a state determined by the voltage level of the other of said trains of pulses,

a multivibrator responsive to the output of said flip-flop to produce an output pulse of predetermined duration for each cycle of said flip-flop, and

means for integrating output pulses from said multivibrator with a lossy integrator, thereby producing a signal the average amplitude of which is proportional to the unknown inductance of said element.

6. Apparatus as defined in claim 5 wherein said last named means is comprised of means for rectifying said output pulses, and an ammeter connected to measure the current output of said rectifying means.

7. Apparatus as defined in claim 4 wherein said LC tuned circuit is comprised of a circuit of inductance and capacitance in parallel, and said unknown element is connected by said connecting means in series with the inductance of said parallel circuit.

8. Apparatus as defined in claim 7 wherein said frequency measuring means is comprised of

a separate pulse shaping amplifier for each of said reference and second signals to produce a reference train of pulses and a second train of pulses from said reference signal and second signal, respectively, one pulse in each train for each cycle of the signal being amplified,

a flip-flop adapted to be set in response to a particular point in each pulse of one of said trains of pulses to a state determined by the voltage level of the other of said trains of pulses,

a multivibrator responsive to the output of said flip-flop to produce an output pulse of predetermined duration for each cycle of said flip-flop, and

means for integrating output pulses from said multivibrator with a lossy integrator, thereby producing a signal the average amplitude of which is proportional to the unknown inductance of said element.

9. Apparatus as defined in claim 8 wherein said last named means is comprised of means for rectifying said output pulses, and an ammeter connected to measure the current output of said rectifying means.