Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
1998-06-30
2001-02-20
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207250, C324S207120, C327S510000
Reexamination Certificate
active
06191576
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to position sensing apparatus and more particularly to magnetic effect sensing apparatus including linear position sensing as well as the commonly known rotary position “geartooth sensors” wherein a magnetically sensitive device senses a ferrous object or objects generally projecting from a rotating target and resembling the teeth of a gear.
2. Discussion of the Prior Art
Various sensors are known in the magnetic effect sensing arts. Examples of common magnetic effect sensors may include Hall effect and magnetoresistive technologies. Generally described, these magnetic sensors will respond to the change of magnetic field as influenced by the presence or absence of a ferromagnetic target object of a designed shape passing by the sensory field of the magnetic effect sensor. The sensor will then give an electrical output which can be further modified as necessary by subsequent electronics to yield sensing and control information. The subsequent electronics may be either onboard or outboard of the sensor package ital.
For example, geartooth sensors are known in the automotive arts to provide information to an engine controller for efficient operation of the internal combustion engine. One such known arrangement involves the placing of a ferrous target wheel on the crank shaft of the engine with the sensor located proximate thereto. The target objects, or features, i.e. tooth and slot, are of course properly keyed to mechanical operation of engine components.
Examples of United States patents in the related art include: U.S. Pat. Nos. 5,650,719; 5,694,038; 5,44,283; 5,414,355; 5,497,084 and 5,500,589.
It is well known in the art that the waveforms produced by the magnetic sensor change in response to varying airgap between the target and sensor faces. Also, differences among the biasing magnets used in the magnetic sensor, temperature, mechanical stresses, irregular target feature spacing, etc., can vary the sensor output. Therefore, the point at which the sensor changes state, i.e. the switch point, varies in time, or drifts, in relation to the degree of rotation of the target. But the mechanical action of the engine as represented by the target does not change. That is, there is a “true point” on the target in angle, or degrees of rotation, related to a hard-edge transition, which represents the point at which the sensor should change state to indicate a mechanical function of the engine. But, due to inherent limitations of the sensing system, the point at which the sensor changes state will vary by some amount from this true point. Therefore, the sensor is losing accuracy, e.g. not really giving a timing signal accurately representing piston travel. Therefore, the system controlled by the sensor can be inefficient. Several schemes are known in the art to reduce this sensor drift by providing an adaptive threshold of waveform voltage at which to switch the sensor. The adaptive threshold seeks to switch the sensor at a nearly constant angle in order to decrease switch point drift and increase accuracy of the sensor and efficiency of the engine.
Various known systems for producing an adaptive threshold (AT) include setting the adaptive threshold at a fixed level above a measured minimum magnetic bias signal. However, this function does not convey information proportional to air gap, therefore high accuracy is not achievable. Another method is setting the threshold at the average value of magnetic bias by using a time based integrator such as an RC circuit. While this method can yield high accuracy, the accuracy is not achieved until considerable amount of target rotation has taken place. It is more desirable to achieve the adaptive threshold point very quickly in the target rotation.
Other proposals, such as that proposed by U.S. Pat. No. 5,650,719, include digital schemes for tracking the voltage peak and voltage minimum of the output waveforms and selecting a point therebetween for the adaptive threshold and updating these peak and minimum values on a regular basis determined by a selected passage of target features.
However, all the known schemes for setting a threshold to compensate for the sensor drift to minimize switch point deviation suffer drawbacks. Such drawbacks may include increased circuit complexity, leading to increased expense; extensive target rotation before the adaptive threshold is determined; and lessened overall accuracy of the determined adaptive threshold for the waveform variance. Compromises among these negatives are inherent in any design. The present invention seeks to minimize the deleterious tradeoffs and provide a magnetic sensor which is an adequate balance of low cost, fast threshold acquisition time, and high accuracy.
SUMMARY OF THE INVENTION
The present invention discloses a method for operating a geartooth sensor. In another embodiment, the present invention discloses a method for constructing a geartooth sensing system. An empirically derived first constant (M1) is derived to account for anticipated output voltage fluctuations inherent in the target and the anticipated airgap tolerance range. The M1 constant is applied to the measured peak value (B
peak
) or values of selected waveforms to obtain a value B
max.
Using this value alone as the AT point eliminates the majority of drift in the sensor.
A second empirically derived constant (M
2
) is derived and applied to a measured or time integrated average value (B
avg
) of, e.g., each wave to obtain a low value (B
min
). The average value referred to may be either a calculated arithmetical value or a time based value taking the form of median or mean values. B
max
and B
min
are then added to obtain the final adaptive threshold value (AT) which is up-dated at the sensor circuitry to eliminate another portion of drift and define accurate sensor output switch points.
M
1
and M
2
are empirically derived or modeled constants which are adapted to a specific target configuration and duty cycle as measured or modeled over the anticipated airgap tolerances of the specific application of the Hall sensor
11
with respect to target
13
.
Because B
max
is derived from quickly acquired peak values, and because B
max
is the much larger value in the algorithm, the present invention synchronizes quickly while obtaining very good accuracy when the B
min
value is added at the slightly later time taken to acquire it.
Further, by utilizing the algorithm with its two empirically derived constants applied to the two waveform values, the need for performing calibration on the sensor and, e.g. adjusting its circuitry by laser trimming or the like, to improve sensor accuracy is minimized.
The AT point is held in the sensor circuitry, whether analog or digital, and may be updated at any chosen frequency to minimize the drift of the sensor switch points, thereby minimizing sensor inaccuracy and increasing engine efficiency.
REFERENCES:
patent: 5442283 (1995-08-01), Vig et al.
patent: 5497084 (1996-03-01), Bicking
patent: 5500589 (1996-03-01), Sumcad
patent: 5650719 (1997-07-01), Moody et al.
patent: 5694038 (1997-12-01), Moody et al.
patent: 0036950A (1981-10-01), None
patent: 0759313A (1997-02-01), None
patent: 0844736A (1998-05-01), None
patent: 2309311A (1997-07-01), None
Hancock Peter G.
Lamb Wayne A.
Ricks Lamar F.
Abeyta Andrew A.
Honeywell Inc.
Norris Roland W.
Patidar Jay
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