AC Resistance measuring instrument

• US 4,408,282 A
• Filed: 04/22/1981
• Issued: 10/04/1983
• Est. Priority Date: 04/22/1981
• Status: Expired due to Fees

• Current Assignee: United States Department of Energy
• Original Assignee: United States Department of Energy
• Inventors: Hof, Peter J.
• Primary Examiner(s): Chin, Gary
• Application Number:
  US06/256,372
• Time in Patent Office: 895 Days
• Field of Search: 364/480, 364/481, 364/482, 364/483, 364/550, 364/551, 364/571, 324/57 R, 324/59, 324/62, 324/64, 324/65 R, 324/DIG. 1
• US Class Current: 702/65
• CPC Class Codes: G01N 27/223 for determining moisture content, e.g. humidity rain detectors on vehicle windows B60S1/0825
• CPC Class Codes: G01R 27/00 Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
• CPC Class Codes: G01R 27/02 Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant by measuring phase angle only G01R25/00

Abstract

An auto-ranging AC resistance measuring instrument for remote measurement of the resistance of an electrical device or circuit connected to the instrument includes a signal generator which generates an AC excitation signal for application to a load, including the device and the transmission line, a monitoring circuit which provides a digitally encoded signal representing the voltage across the load, and a microprocessor which operates under program control to provide an auto-ranging function by which range resistance is connected in circuit with the load to limit the load voltage to an acceptable range for the instrument, and an auto-compensating function by which compensating capacitance is connected in shunt with the range resistance to compensate for the effects of line capacitance. After the auto-ranging and auto-compensation functions are complete, the microprocessor calculates the resistance of the load from the selected range resistance, the excitation signal, and the load voltage signal, and displays of the measured resistance on a digital display of the instrument.

Claims

1. An instrument for measuring the impedance of an electrical device through a transmission line having one end connected to said electrical device and the other end connected to said instrument comprising:

signal generating means for generating an excitation signal having a particular wave shape;

compensation circuit means for compensating for the capacitance of the transmission line coupled between said signal generating means and said other end of the transmission line, said compensation circuit means, the transmission line, and said electrical device being responsive to said excitation signal to develop an output signal at the other end of the transmission line corresponding to the voltage across the transmission line and said electrical device; and

signal processing means coupled to said other end of the transmission line for sampling said output signal and comparing the wave shape of the said signal with said particular wave shape, said signal processing means further being connected to said compensation circuit means and acting to control the impedance of said compensation circuit means to bring the wave shape of the output signal within a preselected correspondence with said particular wave shape wherein the output signal is indicative of the impedance of the transmission line and the device.

2. The instrument of claim 1 wherein said signal processing means further acts to measure said sampled output signal for calculating the impedance of the device.

3. The instrument of claim 2 further comprising delay means coupled to said signal processing means for displaying said calculated impedance.

4. The instrument of claim 1 further comprising range select means including a plurality of resistors connected in parallel with said compensation circuit means for limiting the amplitude of the output signal, said signal processing means further being connected to said range select means and acting to control said range select means wherein an appropriate resistor range is selected to bring the amplitude of the output signal within a preselected value.

5. The instrument of claim 4 wherein said compensating circuit means comprises a plurality of capacitors and a plurality of switching devices, each associated with a different one of said capacitors and each individually operable by said signal processing means.

6. The instrument of claim 5 wherein said capacitors are binary weighted in value.

7. The instrument of claim 4 wherein said range select means comprises a plurality of resistors and a plurality of switching devices, each associated with a different one of said resistors and each individually operable by said signal processing means.

8. The instrument of claim 7 further comprising an additional range resistor connected between said signal generating means and said range select means.

9. The instrument of claim 8 wherein said signal generating means comprises a microprocessor.

10. The instrument of claim 8 wherein said particular wave shape is a bipolar square wave.

11. The instrument of claim 10 wherein said preselected value is 50% to 90% of the amplitude of the excitation signal.

12. The instrument of claim 11 wherein said signal processing means further comprises timing means for generating a timing signal which establishes the frequency of the excitation signal and which establishes output signal sampling rates.

13. The instrument of claim 12 wherein the output signal is sampled near the leading and trailing edges of the positive and negative half cycles of the bipolar square wave signal.

14. The instrument of claim 13 wherein said signal processing means averages the signal samples for a plurality of cycles of the excitation signal.

15. The instrument of claim 14 wherein the frequency of the excitation signal is 50 Hz.

16. The instrument of claim 1 wherein the instrument power source is a battery.