Electricity: measuring and testing – Electrical speed measuring – Including speed-related frequency generator
Reexamination Certificate
1998-11-17
2001-05-08
Strecker, Gerard R. (Department: 2862)
Electricity: measuring and testing
Electrical speed measuring
Including speed-related frequency generator
C324S165000, C324S166000, C324S207200, C340S672000, C702S147000
Reexamination Certificate
active
06229299
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention relates to sensing the velocity of a rotational body and, more particular, to a novel method and apparatus for sensing shaft rotation.
2. Background Art
It is important in many different control applications to sense the angular velocity of a shaft or other rotating object. Such applications include engine speed sensing, transmission speed sensing, and anti-skid brake systems, among others. There are a number of known devices for measuring angular position and rotational speed of a rotating object that have been used in these applications with varying degrees of success.
One such prior art system relies on passive variable reluctance sensing elements that are positioned proximate to a toothed gear or wheel. As the gear passes close to such a sensor, the reluctance changes and there is a small increase in the voltage output of the sensor. Although these devices generally perform satisfactorily, they suffer from two primary disadvantages: (1) the quality of the output signal is largely dependent on the distance between the sensing element and the gear or wheel (i.e., the distance should be small for best signal characteristics); and (2) the amplitude of the output signal decreases as the rotational speed decreases. The first of these disadvantages results in higher manufacturing costs associated with more accurate placement and closer tolerances of the sensing element adjacent to the outer circumference of the rotating gear. Since the space is small, a slight misalignment during manufacturing or resulting from use may cause the sensing element to enter the path of the rotating gear and cause the rotating gear to damage the sensing element. As can be appreciated, sensor failure may cause equipment downtime and should be avoided. The second of the above-noted disadvantages, i.e., the decreasing magnitude of the sensor output voltage at lower rotational speeds, renders the passive variable reluctance sensing devices of limited use in measuring low rotational velocities.
There are still other shortcomings of variable reluctance sensors. At low speeds there is tendency for some gears to vibrate as the gear rotates. This can cause a high frequency signal to be superimposed on the low frequency signal produced by the revolution of the gear. It is possible for the controller to improperly interpret the high frequency signal as a high speed reading while the shaft is in fact turning at low speeds.
Because of the shortcomings of the variable reluctance sensing devices, some prior art systems position Hall effect sensors adjacent to the path of a magnet on the rotating body. As a rotating body turns around an axis, the Hall effect sensors sense the magnet passing near the sensor and produce signals. One such device is shown in U.S. Pat. No. 5,670,877 issued to Scheiber. The Scheiber reference discloses a system including two directional magnetic sensors located adjacent a rotating magnetic source. The sensors produce a signal in response to the magnetic source passing adjacent to the sensor. As disclosed in the Scheiber reference, the magnetic sensors are preferably flux gate magnetometers located proximate to an annular magnet which is attached to the surface of an axle. The magnetometers are preferably located 90 degrees apart with respect to the axis of the axle and produce signals in response to the changing magnetic flux generated by rotation of the annular magnet. One particular aspect of the invention is to eliminate erroneous readings resulting from magnetic interference or magnetic fields other than the rotating magnet. To do this, the reference discloses connecting the sensor outputs in series.
The sensors of systems such as the one disclosed in the Scheiber reference typically produce a sinusoidal output wave form. Such systems typically calculate the rotational speed of the rotating object by sampling the output signal and measuring the elapsed time from one peak of the sinusoid wave to the next peak. These systems generally work well when measuring higher rotational speeds. As the speeds decrease, however, the amount of time between peaks increases and can be a relatively long period of time as the speed approaches zero. This long period of time between speed readings is undesirable when using the speed signal in real time control applications. For example, if a control loop is several milliseconds long, as the speed of the rotational body decreases, it is possible for there to be no peak to peak speed readings during that control loop. Then, the control would have to rely on an old speed reading to implement the control.
Perhaps even a greater disadvantage of these systems is that it is difficult to measure zero rotational velocity. Because the output of the sensor depends on the changing magnetic flux generated by the passing gear teeth or magnet, the signal output will be a DC voltage at zero speed. Such systems cannot perform the peak to peak measurements required to calculate speed. In these instances, the speed reading would be indeterminate and unable to produce a reliable speed signal.
Accordingly, it is an object of the present invention to provide a method and apparatus for sensing the rotational velocity of a rotating body that is relatively accurate and not dependent on the speed of the object being measured and especially at low and zero speeds. Other objects of the invention as well as particular features, embodiments, and advantages thereof will be apparent to those skilled in the art upon reading the following description in connection with the drawings and claims.
DISCLOSURE OF THE INVENTION
In one aspect of the invention a system for sensing the rotational velocity of an object about a rotational axis is disclosed. The system includes a magnet fixed to the object whose rotational velocity is to be measured; a non-contacting sensor, senses the strength of the magnetic field; and an electronic controller, said controller receiving said signal calculating a derivative of said signal and calculating a rotational velocity of said object in response to said derivative.
In another aspect of the invention, a method for sensing the rotational velocity of an object is disclosed. The method includes the steps of determining a derivative of first and second signals produced by first and second non-contacting sensors; and calculating a velocity of said object as a function of the selected derivative.
These and other aspects and advantages of the present invention will become apparent upon reading the specification in connection with the drawings and claims.
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Caterpillar Inc.
McPherson III W. Bryan
Strecker Gerard R.
Wilbur R. Carl
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