Temperature compensation for ground piercing metal detector

• US 6,529,007 B2
• Filed: 12/01/2000
• Issued: 03/04/2003
• Est. Priority Date: 08/04/1999
• Status: Expired due to Fees

• Current Assignee: Ellen Ott, James H. Ott
• Original Assignee: Ellen Ott, James H. Ott
• Inventors: Ott, Ellen, Ott, James H.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US09/727,563
• Publication Number:
  US 20020027438A1
• Time in Patent Office: 823 Days
• Field of Search: 324/67, 324/225, 324/236, 324/239, 324/326-329, 324/202, 324/207.12
• US Class Current: 324/327
• CPC Class Codes: G01V 3/15 specially adapted for use during transport, e.g. by a person, vehicle or boat

Abstract

A hand tool has a ground-piercing probe, which is manually insertable in the soil. The probe has a chamber in which an inductor is positioned and connected to metal detection circuitry. A pulse generator applies current pulses to an inductor for inducing eddy currents in a buried target object. After the pulse terminates, the decaying coil voltage is sampled at different times to detect both the presence and range of the object, as well as the type of metal in the object. The coil voltage samples are applied to signaling circuitry, which provides an audible signal which is a series of bursts of an audio tone. The pitch of the audio tone signals the presence, range and bearing of the buried metal object, while the pulse rate of the bursts signals the metal type. Signal variations resulting from temperature changes when the probe is inserted into soil are compensated for by detecting a signal proportional to the coil current during the pulse and subtracting a scaled portion of that signal from voltage samples.

Claims

1. A hand tool for detecting buried objects, the tool having a hand grip attached to a ground piercing probe which is manually insertable into soil to displace the soil radially outwardly from the probe, the tool comprising:

(a) at least one inductor positioned in a chamber formed in the probe and connected to a pulse induction metal detection circuit including a source of inductor current;

(b) a current detecting and scaling circuit connected to the inductor to detect a signal representing the current through the inductor; and

(c) a summing circuit connected to the output of the scaling circuit for subtracting the current signal from a metal detection signal derived from the coil by the metal detection circuit to provide a temperature compensated metal detection signal.

2. A tool in accordance with claim 1 wherein a resistor is connected in series with the inductor and the current detecting and scaling circuit is a sample and hold circuit having its input connected between the inductor and the resistor.

3. A tool in accordance with claim 2 wherein the sample and hold circuit includes a resistor network having at least one resistor which is alternately switchable into and out of the circuit for providing alternate scaling factors.

4. A method for compensating for temperature variation in signals generated by an inductor mounted in a probe of a ground piercing metal detector and connected to a metal detector circuit, the metal detector circuit including a source of inductor current, to apply metal detection signals to the metal detector circuit, the method comprising:

(a) detecting a signal representing the current through the inductor by the pulse induction method including application of a pulse to the inductor and wherein the current through the inductor is detected by sampling and holding a voltage which is proportional to the inductor current at a time interval near the end of the pulse; and

(b) subtracting a scaled portion of the current signal from the metal detection signal, said scaled portion representing thermal error, to provide a temperature compensated metal detection signal.

5. A metal detection circuit for detecting the metal type of a metal target object, the circuit comprising:

(a) a pulse generator;

(b) a coil connected to the pulse generator for generating a time changing magnetic field in response to electrical pulses applied to the coil;

(c) a resistive energy damper coupled to the coil for attenuating the energy in the magnetic field;

(d) a first sampling and storing circuit connected to the coil for sampling a signal representing the magnitude of the coil current at a sampling time within a time interval beginning after termination of a pulse and ending before the time at which inductor current in the presence of a high conductivity metal target object decays to a current which exceeds the current to which the inductor current decays in the absence of a metal target object;

(e) a temperature compensation circuit including a second sampling and storing circuit connected to the coil for sampling a signal representing coil current at a time interval during the pulse, a scaling circuit and a summing circuit for subtracting a scaled portion of the coil current sample from the sample obtained by the first sampling and storing circuit to provide a temperature compensated metal detection signal; and

(f) a signaling circuit connected to the first sampling and storing circuit for signaling changes in the detected coil current at the sampling time as the coil is moved, the signaled changes representing the metal type of the metal target object.

6. A method for detecting the metal type of a metal target object buried below the surface of a soil, the method comprising:

(a) attempting to induce eddy currents in the target object by applying a pulse to an inductor spaced from the target object to generate a time changing magnetic field about the inductor;

(b) detecting a signal representing the current through the inductor near the end of the pulse;

(c) detecting a signal which is a function of coil apparent inductance resulting from mutual coupling with the eddy currents;

(d) compensating for temperature variation in the inductor by subtracting a scaled portion of the current signal from the apparent inductance signal; and

(e) signaling the direction of change in the apparent inductance of the inductor as the probe is moved.