Metal detector coil resistance testing

A metal detector comprises a detection coil disposed in a magnetic field, the ends of the coil being connected through a balanced resistor network to the inputs of a differential amplifier. A test signal is injected into the metal detector at a point such that the coil is effectively in series with one branch of the balanced resistor network. The injected signal produces a potential difference at the inputs of the differential amplifier, the magnitude of the potential difference being dependent on the resistance of the detection coil. The test signal is generated by a circuit including a microcomputer which successively feeds digital values corresponding to the sine function to a digital-to-analog converter, so that the converter generates the test signal in the form of a sine wave. The output signal from the differential amplifier is applied to an analog-to-digital converter. The microcomputer controls the analog-to-digital converter to repetitively sample the output signal from the amplifier and convert each sample to a digital value. The digital values are compared with two threshold values corresponding to the maximum and minimum values the amplifier output signal may have when the coil resistance is within an acceptable range and when the amplifier output signal exceeds the maximum or is less than the minimum value an error indication is produced.

RELATED APPLICATIONS 
This application discloses and claims subject matter disclosed in the 
concurrently filed application of Strosser et al., Ser. No. 08/414,330 
entitled Metal Detector Coil Inductance Testing. 
FIELD OF THE INVENTION 
The present invention relates to a method and apparatus for testing the 
resistance of a coil in a metal detector by injecting a test signal into 
the coil and determining the magnitude of the output signal produced by 
the metal detector detection circuit connected to the coil. 
BACKGROUND OF THE INVENTION 
Agricultural machines such as forage harvesters are generally provided with 
metal detectors for detecting the presence of metal objects in crop 
material picked up from a field. Upon detection of a metal object the 
metal detector produces an output signal to stop the crop feed mechanism 
before the metal object can reach the cutter knives and cause damage. As 
shown in FIG. 2, the metal detector is frequently located within a housing 
70 that is in turn located within a rotatable lower front feed roll 72. 
Crop material 73 is picked up from a field by a pick-up mechanism (not 
shown) and fed between lower and upper front feed rolls 72, 71 and lower 
and upper rear feed rolls 76, 74 to a cutter mechanism comprising a 
rotating reel 78 having peripheral cutter knives 75 cooperating with a 
stationary cutter bar 77 to cut the crop material. Obviously, metal 
objects fed between knives 75 and cutter bar 76 can severely damage the 
cutter mechanism. The metal detector prevents such damage by sensing metal 
objects and, upon sensing such an object, producing an output signal which 
is applied to a stop mechanism 98 to stop the feed rolls. 
Because the metal detector coils are located within housing 70 and also 
within the feed roll 72, they are difficult to access. Furthermore, the 
coils are usually encased in a potting material and the electronic 
detection circuits to which the outputs of the coils are connected are 
also enclosed within housing 70 as described in U.S. Pat. No. 4,433,528, 
thereby making access to coil test points even more difficult. On the 
other hand, the resistance of the coils should be checked because changes 
in the resistance of a coil affects the output signal from the coil and 
thus the sensitivity of the metal detector to metal objects passing in 
proximity to the coil. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method and apparatus for 
testing an electrical characteristic of a metal detector coil by injecting 
a signal into the coil and sensing the magnitude of the output signal 
produced by a detection circuit connected to the coil. 
Another object of the invention is to provide a method for testing the 
resistance of a metal detector coil connected to a detection circuit for 
amplifying an output signal produced across the coil when it detects a 
metal object, the method comprising injecting a signal into the coil and 
determining the magnitude of the resulting output signal from the 
detection circuit. 
A further object of the invention is to provide an apparatus for testing 
the resistance of a detection coil in a magnetic metal detector wherein 
the detection coil has first and second ends coupled to first and second 
inputs of a detection circuit which produces an output signal having a 
magnitude dependent on the difference in potential at its inputs, the 
apparatus comprising: means for injecting a test signal into the detection 
coil to produce across the coil and the inputs of the detection circuit a 
potential difference that is dependent on the resistance of the coil; 
means, operative while the test signal is being injected, for comparing 
the magnitude of the output signal produced by the detection circuit with 
a first and a second reference signal value representing a maximum and a 
minimum magnitude, respectively, that the output signal may have when the 
resistance of the detection coil is within a normal range; and means for 
producing an indication that the resistance of the detection coil is 
outside the normal range of resistance when the magnitude of the output 
signal is greater than the first reference signal value or less than the 
second reference signal value. 
Yet another object of the invention is to provide an apparatus as described 
above wherein the means for injecting the test signal includes a 
microcomputer for supplying digital values to a digital to analog 
converter to generate the test signal and means connecting the digital to 
analog converter to the detection coil. Preferably, the test signal is a 
sine wave. 
Other objects and advantages of the invention and the manner of making and 
using it will become obvious upon consideration of the following 
description and the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
As shown in FIG. 1A, a typical metal detector of the prior art comprises 
first and second detector coils 10, 12 connected respectively to first and 
second channels or detection circuits 14, 16. It will be understood that 
coils 10, 12 are disposed in magnetic detection fields generated by 
suitable means (not shown) so that metal objects passing through the 
magnetic detection fields perturb the flux of the fields thereby inducing 
an emf in the coils. The arrangement of coils 10, 12 and the means for 
generating the magnetic fields may, for example, be as shown in U.S. Pat. 
No. 4,433,528. 
The detection circuits 14, 16 are identical hence only the details of the 
detection circuit 16 are shown in FIG. 1A. Each detection circuit includes 
an RFI filter section 18, a balanced resistor network 20 feeding first and 
second inputs 21, 23 of a differential amplifier 22, and a low pass audio 
filter 24. 
The ends of coil 12 are connected to inputs of the RFI filter section 18 
which serves to filter out any radio frequency interference picked up by 
coil 12. The output leads 26, 28 of filter section 18 are connected to the 
balanced resistor network 20 which comprises four resistors R.sub.1, 
R.sub.2, R.sub.3 and R.sub.4. Resistors R.sub.1 and R.sub.2 have equal 
resistances (about 1K). A bias voltage V2 (+2.5 V) is connected to a 
junction 30 between first ends of R.sub.1 and R.sub.2. The second ends of 
R.sub.1 and R.sub.2 are connected to leads 26, 28 at junctions 32 and 34, 
respectively. Resistors R.sub.3 and R.sub.4 have equal resistances (about 
250K). One end of resistor R.sub.3 is connected to the first input 21 of 
differential amplifier 22 and the other end is connected to junction 32. 
One end of resistor R.sub.4 is connected to the second input 23 of 
differential amplifier 22 and the other end is connected to junction 34. 
Preferably, the detection coil 12 is located in a static magnetic detection 
field so that in the absence of movement of a metal object through the 
detection field there is no potential difference between the ends of coil 
12 and V2 determines the voltages at the inputs of differential amplifier 
22 and thus the steady state output of the amplifier. Moving parts of the 
agricultural machine distort the detection field and the field is further 
distorted each time a tramp metal object passes through the field. As the 
detection field is distorted, an emf is induced in coil 12. Since the ends 
of the coil are coupled to second ends of resistors R.sub.1, R.sub.2 the 
induced emf first adds to V2 at one of junctions 32, 34 and opposes V2 at 
the other junction as a metal object enters the detection field and then 
reverses polarity as the metal object leaves the detection field. This 
results in unequal voltages being applied to the inputs of the 
differential amplifier and it produces a bipolar output signal that varies 
about the steady state reference according to the difference in potential 
at inputs 21, 23. 
The output signal from differential amplifier 22 is applied to the filter 
24. Filter 24 filters out the high frequency "noise" caused by cyclic 
movement of parts of the agricultural machine in the detection field. The 
filtered bipolar signal CH1 at the output of filter 24 is applied via a 
lead 36 to one input of a multi-channel analog to digital converter (ADC) 
40 (FIG. 1B). 
The purpose of ADC 40 is to convert the magnitudes of the analog signals 
CH0 and CH1 to digital values representing the magnitudes of the signals. 
The ADC is controlled by a conventional microcomputer 42 having a CPU and 
RAM, ROM and E.sup.2 PROM memories. A serial link 41 interconnects the 
microcomputer and ADC. The microcomputer executes a program during which 
it sends signals to the ADC to enable the ADC, select one of the input 
channels, and transfer to the microcomputer a digital signal representing 
the magnitude of the output signal of the selected channel at the time the 
ADC is enabled. The ADC is controlled to sample and digitize each of the 
signals CH0 and CH1 every 2.5 ms. The ADC has a resolution of 256 steps 
(0-255) and is biased at 128. That is, when a detector channel output 
signal CH0 or CH1 is sampled by the ADC and has a value of zero, the ADC 
produces the digital value 128. 
A positive and a negative threshold or reference value is stored in the 
E.sup.2 PROM memory of the microcomputer 42 for each of the metal detector 
channels. During normal operation of the metal detector, that is, during 
the time the metal detector is being operated to sense tramp metal 
objects, the microcomputer compares each digital value transferred from 
the ADC with the positive and negative threshold values for that channel. 
The threshold values define the upper and lower limits within which the 
magnitude of the output signal from a respective channel will fall as long 
as the coil connected to the channel does not detect the passage of a 
tramp metal object. If a comparison shows that a value produced by ADC 40 
is greater than the positive threshold value or less than (i.e. more 
negative than) the negative threshold value with which it is compared, 
thereby indicating the detection of tramp metal, the microcomputer 
produces an output signal that is applied via serial data link 43, which 
may be a Controller Area Network (CAN), and a further microcomputer 45 to 
the stop mechanism 98 to stop the crop feed rolls. 
According to the present invention, apparatus for testing the resistance of 
detector coils 10, 12 comprises a digital to analog converter (DAC) 44, an 
attenuator 46, a latch register 48 and FET switches 50 in addition to the 
ADC 40 and microcomputer 42. 
DAC 44 is connected to microcomputer 42 via the serial data link 41. During 
a test of detection coil resistance, the microcomputer enables DAC 44 
every 2.5 ms and transfers a digital value to a holding register in the 
DAC. The DAC converts the digital value to an equivalent analog voltage 
which is applied over a lead 52 to attenuator 46 and a buffer 54. The 
buffer 54 is used for inductance testing of coils 10 and 12 as described 
in the aforementioned copending application of Strosser et al. Inductance 
testing requires a larger test signal than resistance testing and since 
DAC 44 is used to generate the test signal for both tests, attenuator 46 
serves to reduce the magnitude of the test signal during resistance 
testing. 
The output of attenuator 46 is connected to inputs 1B and 4B of two FET 
switches 50 and the output of buffer 54 is connected to inputs 2B and 3B 
of two further FET switches 50. Only one of the switches is turned on at a 
time, the active switch being determined by which of the addressing or 
selection signals SW1-SW4 is active. 
When signal SW1 is active, the output of attenuator 46 is passed through a 
first switch to output 1A and when SW2 is active the output of buffer 54 
is connected through a second switch to output 2A. The outputs 1A and 2A 
are tied together and connected through a resistor 56 to the junction 32. 
The signals SW3 and SW4 enable third and fourth switches, respectively so 
that the output of buffer 54 is passed through the third switch to output 
3A or the output of attenuator 46 is passed through the fourth switch to 
the output 4A. Outputs 3A and 4A are connected together and are further 
connected through a resistor 58 to a point in detector channel 0 
corresponding to the junction 32 in channel 1. 
The selection signals are applied to switches 50 from the register 48. 
Register 48 is an 8-bit serial input, parallel output register with 
latches. The register receives data from microcomputer 42 via the serial 
data link 41. 
When the resistance or inductance of a detection coil 10, 12 is to be 
tested, microprocessor 42 sends a code word to latch register 48 to select 
which detection coil 10, 12 is to receive the test signal and which test 
is to be performed. The test performed and the coil to which the test 
signal is applied are determined by a 1-bit in one of four bit positions 
of the code word as indicated in the following table: 
TABLE I 
______________________________________ 
Code Register Test 
Word Output Performed 
______________________________________ 
1000 0000 
SW1 Resistance of coil 12 
0100 0000 
SW2 Inductance, signal applied to coil 12 
0010 0000 
SW3 Inductance, signal applied to coil 10 
0001 0000 
SW4 Resistance of coil 10 
______________________________________ 
Assume that the resistance of coil 12 is to be tested. Register 48 is 
loaded with the code word 1000 0000 so that the register produces the 
signal SW1. The signal SW1 enables one of switches 50 to connect switch 
input 1B to switch output 1A so that the output voltage of attenuator 46 
is applied through resistor 56 to coil 12 via the junction 32. FIG. 3 
shows the equivalent circuit for the detection coil 12 and resistance 
network 20 when the attenuator output voltage (assumed to be larger than 
bias voltage V2) is applied and the detection coil is not open. The total 
current I.sub.T flowing from the attenuator through switch 50 and resistor 
56 divides at junction 32 with a first portion I.sub.1 flowing through the 
detection coil 12, having a resistance R.sub.C, and resistor R.sub.2 to 
junction 30. The second part I.sub.2 of the total current I.sub.T flows 
through resistor R.sub.1 to junction 30. 
The potential difference between the ends of coil 12, that is between 
junctions 32 and 34, is 
##EQU1## 
This potential difference is applied to the inputs of differential 
amplifier 22 hence the resistance of the detection coil determines the 
magnitude of output signal from the amplifier. 
If coil 12 should be shorted out completely so that it has no resistance 
then in FIG. 3 R.sub.C will be zero and there will be no potential drop 
between junctions 32 and 34. Therefore, when the coil is shorted the 
differential amplifier will produce no output signal. 
If coil 12 should be open then the current I.sub.1 cannot flow. Junction 34 
assumes the same potential as junction 30 hence the full potential 
difference between junctions 32 and 30 is applied across the inputs of 
amplifier 22 and it produces a maximum output signal. 
The resistance of one of the detection coils 10, 12 is tested under control 
of a program routine executed by microcomputer 42. The routine may be part 
of a diagnostic routine called when power is turned on or when an operator 
initiates the routine by actuation of a control on a control panel (not 
shown) 
The microcomputer 42 first loads into register 48 a code word designating 
the detection coil whose resistance is to be tested. This causes register 
48 to produce the signal SW1 if the resistance of coil 12 is to be tested 
or the signal SW4 if the resistance of coil 10 is to be tested. This 
enables one switch 50 so that the output of attenuator 46 is connected to 
one of the coils 10, 12. 
The microcomputer 42 stores a table of sine wave values in ROM memory and 
every 2.5 ms the microcomputer enables DAC 44 and transfers one of these 
values to a holding register in the DAC. The DAC converts the sine wave 
values to an output voltage which is applied through the attenuator 46 and 
switch 50 to the designated detection coil 10, 12. Assuming the output of 
the attenuator is applied to detection coil 12 channel 1 produces an 
output signal CH1 on lead 36 having a magnitude dependent on the 
resistance of coil 12. 
The microcomputer 42 then turns ADC 40 on and every 2.5 ms transfers from 
the ADC to the microcomputer the digital signals produced by the ADC and 
representing the magnitude of the signal on lead 36. 
A peak detector 66 and a comparator 68 are implemented by programming in 
the microcomputer 42. The peak detector detects the peak positive value of 
the values received from ADC 40. The ROM in the microcomputer holds first 
and second threshold or reference signal values representing a maximum 
magnitude and a minimum magnitude, respectively, that an output signal on 
lead 36 or 38 may have when the resistance of coil 10 or coil 12 is within 
an acceptable range. 
The magnitude of the test signal injected into the detection circuits is 
known and from the design and configuration of the detection circuits and 
detection coils the magnitudes of the output signals from the detection 
circuits may be calculated. The reference signal values are offset above 
and below the calculated magnitude depending on how much variation in coil 
resistance may be tolerated. 
The microcomputer 42 compares the positive peak value produced by the peak 
detector with each of the reference signal values. If the comparison 
indicates that the peak value derived from the ADC signals has a magnitude 
greater than the reference maximum magnitude or less (i.e. more negative) 
than the reference minimum magnitude then the resistance of the coil being 
tested is outside the acceptable range. In this case the microcomputer 
sends to a display 60, via the serial data link 43 and microcomputer 45, a 
code indicating that a coil is defective. 
The resistance of coil 10 is tested in a similar manner except that 
register 48 is loaded with the code 0001 0000 and the microcomputer 
controls ADC 40 to sample the resulting detector output signal CH0 on lead 
38. 
Since successive sine wave values are transferred to DAC 44 during a test, 
the signal injected into the coil being tested is a sine wave. The 
frequency of this sine wave is quite low so that the signal produced at 
the output of the differential amplifier will pass through the low pass 
filter 24. 
A specific preferred embodiment of the invention has been described in 
detail to illustrate the principles of the invention. It will be 
understood that various modifications and substitutions may be made in the 
described embodiment without departing from the spirit and scope of the 
invention as defined by the appended claims. For example, the test signal 
need not be a sine wave signal although a sine wave is preferred. 
Furthermore, the invention is not limited to use with the specific metal 
detector described herein but may be used with many types of metal 
detectors.