Proximity detector

Details Proximity detector Proximity detector

2002-05-28

2004-02-17

Snow, Walter E. (Department: 2862)

Electricity: measuring and testing

Magnetic

Displacement

C324S207200, C324S207250, C324S166000, C327S511000

Reexamination Certificate

active

06693419

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable.
FIELD OF THE INVENTION
This invention relates to proximity detectors and more particularly to a proximity detector providing close tracking of a magnetic field signal.
BACKGROUND OF THE INVENTION
Proximity detectors for detecting ferrous, or magnetic articles are known. One application for such devices is in detecting the approach and retreat of each tooth of a rotating ferrous gear. The magnetic field associated with the ferrous article is detected by a magnetic field-to-voltage transducer, such as a Hall element or a magnetoresistive device, which provides a signal proportional to the detected magnetic field (i.e., the magnetic field signal). The proximity detector processes the magnetic field signal to generate an output signal which changes state each time the magnetic field signal crosses a threshold signal.
In one type of proximity detector, sometimes referred to as a peak-to-peak percentage detector, the threshold signal is equal to a percentage of the peak-to-peak magnetic field signal. One such peak-to-peak percentage detector is described in U.S. Pat. No. 5,917,320 entitled DETECTION OF PASSING MAGNETIC ARTICLES WHILE PERIODICALLY ADAPTING DETECTION THRESHOLD and assigned to the assignee of the present invention. In another type of proximity detector, sometimes referred to as a slope-activated or a peak-referenced detector and described in U.S. Pat. No. 6,091,239 entitled DETECTION OF PASSING MAGNETIC ARTICLES WITH A PEAK REFERENCED THRESHOLD DETECTOR which is assigned to the assignee of the present invention, the threshold signal differs from the positive and negative peaks (i.e., the peaks and valleys) of the magnetic field signal by a predetermined amount. Thus, in this type of detector, the output signal changes state when the magnetic field signal comes away from a peak or valley by the predetermined amount.
In order to accurately detect the proximity of a ferrous article, the detector must be capable of closely tracking the magnetic field signal. Typically, one or more digital-to-analog converters (DACs) are used to generate a signal which tracks the magnetic field signal. For example, in the above-referenced U.S. Pat. Nos. 5,917,320 and 6,091,239, two DACs, a PDAC and an NDAC, are used; one to track the positive peaks of the magnetic field signal and the other to track the negative peaks of the magnetic field signal.
Referring to
FIG. 1
, a peak-referenced proximity detector
10
which uses a single DAC
28
to track a magnetic field signal, DIFF, is shown. A Hall element
14
generates a differential signal proportional to the ambient magnetic field, which signal is amplified by an amplifier
16
to provide the DIFF signal. The DIFF signal is coupled to a non-inverting input of a tracking comparator
20
which receives, at the inverting input, the output signal, PEAKDAC, of the DAC
28
, as shown. The DIFF signal is further coupled to a non-inverting input of a comparator
40
which receives at the inverting input, the PEAKDAC signal and which generates a POSCOMP output signal. The comparator
40
has hysteresis, here on the order of 100 mV, so that the POSCOMP signal changes state when the DIFF signal exceeds the PEAKDAC signal by approximately 100 mV. The output signal of the comparator
20
, COMPOUT, is coupled to an exclusive OR (XOR) gate
36
which additionally receives the POSCOMP signal and which provides a HOLD input signal to an up/down counter
24
. Counter
24
is further responsive to a clock signal, CLK, and to the POSCOMP signal for controlling whether counter
24
counts up or down. The output of the counter
24
is converted into the analog tracking PEAKDAC signal by the DAC
28
.
As is illustrated in
FIG. 2
, whenever the DIFF signal exceeds the PEAKDAC signal by the hysteresis level of comparator
20
, such as by 100 mV, the COMPOUT signal transitions to a logic high level, thereby causing the counter
24
to count if the POSCOMP signal is also high. Once the counter
24
counts up one step, the COMPOUT signal goes low causing the count value to be held until the DIFF signal exceeds the PEAKDAC signal by 100 mV again. When the DIFF signal reaches a positive peak, as occurs at time t
1
, the PEAKDAC signal stays above the DIFF signal, thereby causing the HOLD input to the counter
24
to be asserted until the hysteresis of the comparator
40
has been overcome, as occurs when the POSCOMP signal goes low, just before time t
2
.
When the DIFF signal experiences high frequency fluctuations, as occurs beginning at time t
3
, the PEAKDAC signal is not able to keep up with the fast changing DIFF signal. More particularly, the DAC
28
counts at its maximum rate (i.e., the PEAKDAC signal experiences its maximum slope, dV/dt) after the POSCOMP signal transitions, such as at time t
0
, t
2
, and t
3
. Between times t
4
and t
5
, the DIFF signal has a slope faster than the maximum dV/dt of the DAC and the PEAKDAC signal does not catch up with the falling DIFF signal until time t
5
when the DIFF signal is rising. In this case, the DIFF signal valley occurring between times t
4
and t
5
is not detected, thereby causing an output transition of the POSCOMP signal to be skipped and a passing magnetic article to go undetected. It will be appreciated that the same potential problem of skipping POSCOMP signal transitions can occur when the DIFF signal has a small amplitude, since the DAC signal will not have time to catch the DIFF signal before it changes direction.
SUMMARY OF THE INVENTION
A proximity detector comprises a magnetic field-to-voltage transducer providing a magnetic field signal indicative of an ambient magnetic field, a peak detector responsive to the magnetic field signal for providing a tracking signal which substantially follows the magnetic field signal, and a comparator for providing a detector output signal which changes state when the magnetic field signal varies from the tracking signal by a predetermined amount. According to the invention, at least one of the tracking signal and the magnetic field signal is forced towards the other one of the tracking signal and the magnetic field signal in response to changes in state of the detector output signal. With this arrangement, the tracking signal closely follows the magnetic field signal, even in response to high frequency and/or low amplitude variations in the magnetic field signal.
Various embodiments are described for forcing at least one of the tracking signal and the magnetic field signal towards the other one of the tracking signal and the magnetic field signal. In some embodiments, the tracking signal is brought to substantially the same level as the magnetic field signal upon transitions of the output signal and in other embodiments, the magnetic field signal is brought to substantially the same level as the tracking signal upon output signal transitions. Alternatively, the tracking signal is brought to a level which is at a fixed offset with respect to the magnetic field signal or the magnetic field signal is brought to a level which is at a fixed offset with respect to the tracking signal.
The predetermined amount by which the magnetic field signal must differ from the tracking signal in order to cause a change of state in the detector output signal may be established by generating a threshold signal, which differs from the tracking signal by the predetermined amount, for use by the comparator or may be established by hysteresis of the comparator. In one embodiment in which a threshold signal is generated, the magnetic field signal and the tracking signal are forced towards each other by interchanging the threshold signal level and the tracking signal level upon transitions of the output signal.
Also described is a method for detecting a ferrous article including the steps of generating a magnetic field signal indicative of an ambient magnetic field, generating a tracking signal which substantially follows the magnetic field signal, generating an output si

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