Adaptive thresholding for metal detection

• US 5,600,942 A
• Filed: 03/31/1995
• Issued: 02/11/1997
• Est. Priority Date: 03/31/1995
• Status: Expired due to Term

• Current Assignee: Blue Leaf IP Incorporated (CNH Industrial N.V.)
• Original Assignee: New Holland North America Incorporated (CNH Industrial N.V.)
• Inventors: Strosser, Richard P.
• Primary Examiner(s): Melius, Terry Lee
• Assistant Examiner(s): SHACKELFORD, HEATHER CHUN
• Application Number:
  US08/414,329
• Time in Patent Office: 683 Days
• Field of Search: 56/10.25, 56/10.2 R, 56/10.3, 56/DIG. 15, 460/2, 460/3, 340/679, 340/684
• US Class Current: 56/10.20J
• CPC Class Codes: G01V 3/08 operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices with electromagnetic waves G01V3/12
• CPC Class Codes: Y10S 56/15 Condition responsive

Abstract

A two-channel metal detector system disposed adjacent the crop feed path of a harvester to detect ferrous metal objects in the crop material is provided with both positive and negative adaptive detection thresholds for each channel, the thresholds being independently varied according to the average positive and negative peaks of the noise component in the channel output signals. A channel output signal is sampled and digitized and the positive and negative peaks of the noise component occurring over fixed time intervals are filtered to provide average positive peak and average noise peak values that are stored in an E² PROM and updated according to long term variations in the noise component. The positive and negative adaptive detection thresholds are set according to the average positive and negative peak values and the adaptive detection thresholds are compared with the sample channel output signal to determine if a ferrous metal object has been detected. By storing the positive and negative average peak values of the noise component in an E² PROM memory, the adaptive detection thresholds and the filters may be initialized at system start-up to the values existing at the time of system shut-down.

Claims

1. A metal detector system for detecting the passage of metal objects through a magnetic detection field, said metal detector system including at least one channel comprising a detection coil disposed in said magnetic detection field and a detection circuit for producing a channel output signal having a noise component therein, said channel output signal including a detection component upon the passage of a ferrous metal object through said magnetic detection field;

means for sampling said channel output signal and producing digitized channel signals, means for comparing said digitized channel signals with positive and negative adaptive thresholds and producing an output indication when one of said digitized channel signals exceeds either said positive or said negative adaptive threshold, and means for producing positive and negative peak averages and independently setting said positive and negative adaptive thresholds according to the positive peak average and the negative peak average, respectively, of said noise component.

2. A metal detector as claimed in claim 1 wherein said means for setting said positive and negative adaptive thresholds comprises positive and negative peak detectors for detecting positive and negative peaks of said noise component, filter means for developing positive and negative average peak noise values from said positive and negative peaks, and means for setting said positive and negative adaptive thresholds according to said positive and negative average peak noise values, respectively.

3. A metal detector system for detecting the passage of metal objects through a magnetic detection field, said metal detector system including at least one channel comprising a detection coil disposed in said magnetic detection field and a detection circuit for producing a channel output signal having a noise component therein, said channel output signal including a detection component upon the passage of a ferrous metal object through said magnetic detection field;

means for sampling said channel output signal and producing digitized channel signals, means for comparing said digitized channel signals with positive and negative adaptive thresholds and producing an output indication when one of said digitized channel signals exceeds either said positive or said negative adaptive threshold, means for independently setting said positive and negative adaptive thresholds according to the positive peak average and the negative peak average, respectively, of said noise component, said means for setting said positive and negative adaptive thresholds comprising positive and negative peak detectors for detecting positive and negative peaks of said noise component, filter means for developing positive and negative average peak noise values from said positive and negative peaks, and means for setting said positive and negative adaptive thresholds according to said positive and negative average peak noise values, respectively, and an E² PROM for storing said positive and negative average peak noise values and means operative at system wake-up for setting said positive and negative adaptive thresholds according to the positive and negative average peak noise values in said E² PROM at system shut-down.

4. A metal detector as claimed in claim 3 wherein said filter means comprises first and second filters, having first and second registers, and means operative at system wake-up for setting said first and second registers according to the positive and negative average peak noise values, respectively, in said E² PROM at system shut-down.

5. A method of operating a metal detector system that detects the passage of metal objects through a magnetic detection field, the metal detector system including at least one channel comprising a detection coil disposed in the magnetic detection field, and a detection circuit for producing a channel output signal having a noise component therein, said channel output signal including a detection component upon the passage of a ferrous metal object through the magnetic detection field, said method comprising:

sampling said channel output signal and producing digitized channel signals;

comparing the digitized channel signals with positive and negative adaptive thresholds and producing an indication when one of the digitized channel signals exceeds either the positive or the negative adaptive threshold; and

,

producing positive and negative peak averages and independently adjusting the positive and negative thresholds according to the positive peak average and negative peak average, respectively, of the noise component.