Electricity: measuring and testing – Electrical speed measuring – Including speed-related frequency generator
Reexamination Certificate
2002-06-11
2004-09-28
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Electrical speed measuring
Including speed-related frequency generator
C324S173000, C324S207250
Reexamination Certificate
active
06798192
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to methods and apparatus whereby the speed of a moving entity is measured by a magnetic pickup coil subject to variable magnetic fields as ferromagnetic discontinuities on the moving entity move past the magnetic pickup coil and, more specifically, to apparatus and methods for the generation of a clean square-wave pulse train from a noisy signal received from the pickup coil. The ferromagnetic discontinuities generally have no magnetism of their own. They are detected by a passive magnetic sensor which includes a permanent magnet and the magnetic pickup coil.
When a ferromagnetic discontinuity on the moving entity approaches or recedes from the passive magnetic sensor, magnetic flux inside the coil changes and, by Gauss's law, a variable electromotive force (emf) or voltage is generated in the coil. Systems for measuring the speed of a rotating or translating entity are known wherein ferromagnetic discontinuities, which generally have no magnetism of their own, are disposed on the moveable entity, spaced apart in a direction of movement of the entity. One or more passive magnetic sensors are placed adjacent the ferromagnetic discontinuities so that movement of the entity causes electrical pulses to be induced in the sensors as the ferromagnetic discontinuities move past the sensor(s).
For measuring rotary motion, the discontinuities are generally formed in a circular array. For example, a ferromagnetic gear may be placed on a shaft, and the teeth of the gear constitute the ferromagnetic discontinuities. Alternatively, slots or flutes may be formed in a ferromagnetic shaft to delineate the required ferromagnetic discontinuities, which are uncut portions of the shaft between the slots or flutes. In either case, a passive magnetic detector is placed adjacent the ferromagnetic discontinuities so that, as the shaft rotates, the ferromagnetic discontinuities cause variable magnetic flux inside the coil of the detector, and hence generate variable emf's in the coil.
For measuring linear motion, the ferromagnetic discontinuities are generally formed as parallel ridges spaced apart laterally in the direction of motion. The ridges, preferably, lie perpendicular to the direction of motion. A type of gear known as a “rack” may be employed for this purpose.
Since the emf generated by such a coil depends on the rate of change of magnetic flux, such a coil generates a signal that alternates between negative and positive values. If the ferromagnetic discontinuities are uniformly sized and spaced, the emf from the coil will comprise periodic alternating positive and negative segments. It is known to generate a train of clean square-wave pulses from the coil emf. A zero crossing detector is employed for this purpose. When the coil emf crosses zero in the positive direction, the output of the zero crossing detector is set to one level. When the coil emf crosses zero in the negative direction, the output of the zero crossing detector is set to another level. For example, the output may go high for a positive crossing and low for a negative crossing.
In order to reduce the sensitivity of the zero crossing detector to noise, it is known to employ an upper threshold to indicate zero crossing in the positive direction, and a lower threshold to indicate zero crossing in the negative direction. In prior art systems for measuring speed of rotating or translating entities, these thresholds are generally set at fixed, constant values.
It is noted that the signal generated by the pickup coil increases linearly in strength with the speed of the moving entity. Therefore, in prior art systems, as the velocity of the moving entity increases, the signal gets stronger, and the threshold becomes a smaller and smaller fraction of the signal strength. This is inconsistent with a rule of thumb known in the art, namely, that the thresholds should have a magnitude of about one fifth to one eighth of the peak signal strength.
Two prior art patents teach pulse detection systems employing zero crossing detectors with variable thresholds. Both of these patents are for computer disk drive data reading, not for sensing speed of a translating or a rotating body, and neither patent adjusts the threshold as a function of measured signal strength in real time.
U.S. Pat. No. 5,287,227 teaches a manufacturing system in which several points are tested on the disk surface during disk drive manufacture. Thresholds for the zero crossing detector that will later be used to process a signal from the disk reading head are determined based on the quality of the surface points for accepting and retaining a digital test signal. Extrapolation is done for points in between the tested points. The thresholds are stored in memory and written onto the disk during manufacture. Later, when the disk is started up, the thresholds are read from the disk and stored in memory. Then, when data is read from the disk, thresholds are obtained from memory and employed in the zero crossing detector. This invention does not adjust the thresholds in accordance with an ambient noise level or the strength of the signal actually obtained by the read head during reading of the disk.
U.S. Pat. No. 5,150,050 teaches a manufacturing system in which tests are made at points on the disk surface during disk drive manufacture. Thresholds for a zero crossing detector are varied to determine whether a spot on the disk can reliably be written to and read from. If no such threshold can be found for a spot on the disk, the location of the spot is stored and is written onto the disk. Later, when the disk is in use, the bad spots on the disk are not used for storing information. This patent also teaches a system whereby, when a disk data read fails, the threshold is changed by a predetermined amount and a data re-read attempted, in order to recover the data.
It is noted that the use of a zero crossing detector in the references cited differs greatly from the use in a speed sensor employing a pickup coil. The mechanism for detecting a pulse from the pickup head of a disk drive is not a zero crossing detector, but a peak detector. The variable-threshold zero-crossing detector merely enables the peak detector, such that only the first peak after a zero-crossing is counted. The peak detector is mechanized via a zero-crossing detector with a fixed zero-volt threshold acting on a time derivative of the sensed signal.
It is further noted that the references cited do not dynamically compensate for degradation of either the write head or the read head, or a change in signal strength from any other cause, such as height of the heads above the disk, or special misalignments. They furthermore do not dynamically compensate for variations in electrical noise. U.S. Pat. No. 5,287,227 does not compensate at all, while U.S. Pat. No. 5,150,050 compensates only after a data read has failed, adjusting the threshold by pre-determined amounts, rather than constantly adjusting the thresholds in real time to prevent the data read failure in the first place.
As can be seen, there clearly is a need for a speed sensing system employing a passive magnetic sensor which detects pulses of a signal from the sensor by a zero crossing detector having a variable threshold. Such a system would allow use of detectors with weak outputs at low speeds (which are less failure prone due to larger wire sizes) without undue sensitivity to noise at higher speeds, would compensate for degradation of the sensor magnet, variations in spacing of the detector to the ferromagnetic discontinuities which cause the signal, and would compensate for detector failure modes and for ambient noise.
SUMMARY OF THE INVENTION
In one aspect of the present invention, a turbofan engine comprises a gas turbine engine and a bypass fan driven by the gas turbine engine. At least one shaft of the turbofan engine has a circular array of ferromagnetic discontinuities either formed as a portion of the shaft or attached to the shaft to rotate with the shaft. A passive magne
Faymon David K.
Mulera Tom G.
Shah Mansoor
Stevens Paul M.
Desmond, Esq. Robert
Honeywell International , Inc.
Patidar Jay
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