Methods and apparatus for magnetic article detection
Apparatus and methods for detecting passing magnetic articles using a first, True Power On State (TPOS) detector during a first time interval and transitioning to a second, running mode detector after the first time interval, when the output of the second detector is accurate. A phase comparator is responsive to the output signal of the first and second detectors and provides a control signal indicative of a change in the phase relationship between the two output signals. An output switch controlled by the phase comparator provides, as the detector output signal, the first detector output signal during the first time interval and the second detector output signal thereafter. A threshold signal associated with the TPOS detector is at a fixed level during a first portion of the first time interval and is adjusted during a second portion of the first time interval in response to an automatic gain control circuit in order to reduce the phase error associated with transitioning from the TPOS detector to the running mode detector. Also described are apparatus and methods for updating a running mode threshold signal that is a percentage of the peak-to-peak magnetic field sensor signal, so that unnecessary update events are reduced.
1. Apparatus for detecting passing magnetic articles and providing a magnetic article detector output signal indicative of the passing magnetic articles comprising:
a magnetic field sensor providing a signal proportional to an ambient magnetic field;
a first circuit responsive to the magnetic field sensor signal to provide a first output signal indicative of the passing magnetic articles, wherein the first output signal provides the magnetic article detector output signal during a first time interval;
a second circuit responsive to the magnetic field sensor signal to provide a second output signal indicative of the passing magnetic articles, wherein the second output signal provides the magnetic article detector output signal during a second time interval following the first time interval;
a phase comparator responsive to the first output signal and the second output signal to provide a control signal indicative of a change of a phase relationship between the first output signal and the second output signal; and
a switch having a first input responsive to the first output signal, a second input responsive to the second output signal, and an output at which the magnetic article detector output signal is provided, wherein the switch is controlled by the control signal.
2. The apparatus of claim 1 wherein the first circuit comprises a comparator having a first input responsive to the magnetic field sensor signal, a second input responsive to a first threshold signal, and an output at which the first output signal is provided.
3. The apparatus of claim 2 wherein the first threshold signal is adjustable.
4. The apparatus of claim 3 wherein the first threshold signal is at a predetermined level during a first portion of the first time interval and is adjusted during a second portion of the first time interval.
5. The apparatus of claim 4 wherein the first portion of the first time interval ends after a predetermined number of transitions occur in the first detector output signal.
6. The apparatus of claim 4 wherein the first threshold signal is adjusted by increments determined by an automatic gain control circuit.
7. The apparatus of claim 1 wherein the second circuit comprises a comparator having a first input responsive to a gain-controlled version of the magnetic field sensor signal, a second input responsive to a second threshold signal, and an output at which the second output signal is provided.
8. The apparatus of claim 7 further comprising a positive digital to analog converter (PDAC) operative to provide a PDAC signal that tracks positive peaks of the magnetic field sensor signal, and a negative digital to analog converter (NDAC) operative to provide an NDAC signal that tracks negative peaks of the magnetic field sensor signal, wherein the second threshold signal is a percentage of the difference between the PDAC signal and the NDAC signal.
9. The apparatus of claim 8 wherein the PDAC signal increases in response to increasing positive peaks of the magnetic field sensor signal and wherein the PDAC signal decreases in response to a predetermined number of positive peaks of the magnetic field sensor signal occurring that are less than a positive update threshold signal and wherein the NDAC signal decreases in response to decreasing negative peaks of the magnetic field sensor signal and wherein the NDAC signal increases in response to a predetermined number of negative peaks of the magnetic field sensor signal occurring that are greater than a negative update threshold signal.
10. Apparatus for detecting passing magnetic articles and providing a magnetic article detector output signal indicative of the passing magnetic articles comprising:
a magnetic field sensor providing a signal proportional to an ambient magnetic field;
a first circuit responsive to the magnetic field sensor signal to provide a first output signal indicative of the passing magnetic articles, wherein the first output signal provides the magnetic article detector output signal during a first time interval, wherein the first circuit comprises a comparator having a first input responsive to the magnetic field sensor signal and a second input responsive to a threshold signal and an output at which the first output signal is provided, wherein the threshold signal is adjustable; and
a second circuit responsive to the magnetic field sensor signal to provide a second output signal indicative of the passing magnetic articles, wherein the second output signal provides the magnetic article detector output signal during a second time interval following the first time interval.
11. The apparatus of claim 10 wherein the threshold signal is at a predetermined level during a first portion of the first time interval and is adjusted during a second portion of the first time interval.
12. The apparatus of claim 10 wherein the first portion of the first time interval ends after a predetermined number of transitions occur in the first output signal.
13. The apparatus of claim 10 wherein the apparatus further comprises an automatic gain control circuit for adjusting the gain of the magnetic field sensor signal and wherein the threshold signal is adjusted by increments determined by the automatic gain control circuit.
14. The apparatus of claim 10 further comprising a phase comparator responsive to the first output signal and to the second output signal to provide a control signal indicative of a change in the phase relationship between the first output signal and the second output signal and wherein the first time interval ends in response to a transition in the control signal.
15. A method for detecting passing magnetic articles and providing a magnetic article detector output signal indicative of the passing magnetic articles, comprising:
providing a magnetic field signal proportional to an ambient magnetic field;
detecting the passing magnetic articles with a first detector that is responsive to the magnetic field signal to provide a first detector output signal, wherein the first detector output signal provides the magnetic article detector output signal during a first time interval;
detecting the passing magnetic articles with a second detector that is responsive to the magnetic field signal to provide a second detector output signal, wherein the second detector output signal provides the magnetic article detector output signal during a second time interval following the first time interval; and
transitioning from the first time interval to the second time interval in response to a change in the phase relationship between the first detector output signal and the second detector output signal.
16. The method of claim 15 wherein detecting the passing magnetic articles with a first detector comprises comparing the magnetic field signal to a threshold signal to provide the first detector output signal.
17. The method of claim 16 further comprising providing the threshold signal at a predetermined level during a first portion of the first time interval and adjusting the threshold signal during a second portion of the first time interval.
18. The method of claim 17 further comprising ending the first portion of the first time interval after a predetermined number of transitions occur in the first detector output signal.
19. The method of claim 17 further comprising adjusting a gain of the magnetic field signal and adjusting the threshold signal by increments determined by the gain adjustment of the magnetic field signal.