FIELD OF THE INVENTION
The present application is based upon and claims the benefit of U.S. Provisional Application Ser. No. 60/183,270 filed Feb. 17, 2000.
The invention generally relates to position sensors, and more particularly, to a magnetically sensitive sensor that detects a positional parameter of a rotating element.
Rotary sensors using contacting technology, such as potentiometers, suffer from various disadvantages that have limited their use. In applications requiring prolonged use over many years or requiring many cycles, the contacting sensors develop dead zones and non-uniform electrical behavior. Additionally, contacting sensors, even when in good condition, exhibit a relatively high degree of electrical noise during operation. This noise is a problem in sensitive electronic circuits.
It has been suggested to use non-contacting angular sensors to overcome the disadvantages of contacting sensors. Such sensors are not as susceptible to wear and exhibit reduced electrical noise. One barrier to the widespread use of non-contacting transducers, however, is the restricted angular range of those devices. Examples of such devices are shown in the U.S. Pat. Nos. 3,777,273; 3,988,710; 4,425,557 and 5,789,917. All of these devices have an angular operating range of less than one-half turn.
Another barrier to the widespread use of non-contacting sensors is the use of magnetic devices in such sensors. Magnetic fields are short acting, thus magnetic sensors have limited range and they exhibit undesirable signal-to-noise ratios (SNR) due to outside magnetic disturbances. The problems with non-linearity and SNR have been somewhat offset by the use of pole pieces, or flux directors. Flux directors attempt to extend the usable linear range of a magnetic field by advantageously shaping the field. The need to improve the SNR has been cited in various publications, including U.S. Pat. Nos. 5,444,369; 5,789,917; and 5,757,179.
- SUMMARY OF THE INVENTION
There are contacting rotary sensors that exhibit extended range, but these sensors suffer from the disadvantages discussed above and are generally too complex and too costly for commercial acceptance. Moreover, these multi-turn sensors provide only a relative position indication, having no absolute reference.
BRIEF DESCRIPTION OF THE DRAWING
The advantages of magnetic, non-contacting sensors over contacting potentiometric types include virtually unlimited operating life, owing to the fact that there are no physical contacts. Non-contacting sensors according to the principles of the invention do not suffer from the wear degradation and electrical noise exhibited by contacting sensors. A sensor according to the principles of the invention also offers a useful range of many full revolutions. Such a range makes the present invention a suitable replacement for absolute rotary encoders, angular sensors, potentiometers, tuners, and robotic joint sensors. A sensor according to the principles of the invention reduces SNR by converting a relatively large rotational mechanical input to a smaller linear mechanical translation of the magnet in the measurement circuit. This technique allows the sensor to use a very small portion of the magnetic field that is close to the magnet. This field portion offers the highest magnetic flux density for a given magnet and thus results in improved SNR. Also, the small size of this field portion reduces dependence on linearity.
A more complete understanding of the invention may be obtained from consideration of the following description in conjunction with the drawing in which:
The FIGURE is a perspective view, partially cut away, showing the rotary shaft position sensor constructed in accordance with the present invention.
Referring to FIG. 1, there is shown a perspective view, partially cut away, of a rotary shaft position sensor 10 constructed in accordance with the present invention. As can be seen, there is a housing 12 that may be of any variety of materials, such as metals or plastic compositions. Within the housing 12, there is formed a specially configured pocket 14 within which is located a magnetic element 16. The magnetic element 16 may be a permanent magnetic that emits a magnetic flux therefrom and the purpose of the magnetic field flux will be later appreciated. The exterior portion 18 of the magnetic element 16 is also specially configured, and as shown, that exterior portion 18 may be formed in a prismatic shape, it being of importance that the exterior portion 18 of the magnetic element 16 be specially configured so as to fit with the pocket 14 such that the magnetic element 16 is prevented from rotating with respect to housing 12 but can move along a linear path within the housing 12 indicated by the arrow A.
Extending outwardly from the magnetic element 16 is threaded shaft 20, shown as externally threaded and the threaded shaft 20 extends external of the housing 12. The threaded shaft 20 may be a separately formed part that is inserted into the magnetic element 16 or insert molded in the production of the magnetic element 16, however, in any event, the threaded shaft 20 is firmly affixed to that magnetic element 16.
An input shaft 22 is threadedly affixed to the threaded shaft 20 external of the housing 12 by means of internal threads 24. Thus, the input shaft 22 and the threaded shaft 20 are threadedly engaged together and, as stated, in the preferred embodiment, the threaded shaft 20 has external threads and the input shaft 22 has internal threads 24. The threaded shaft 20 and the input shaft 22 are threadedly engaged together and, as can be seen, the external and internal locations of the respective threads on each of those components could readily be reversed and the rotary shaft position sensor 10 still function in the manner to be explained.
The input shaft 22 is rotatable affixed to the housing 12 such that the input shaft 22 can freely rotate with respect to the housing 12 but is restrained from any axial movement with respect thereto. In the FIGURE, the mounting of the input shaft 22 is accomplished by means of a mounting bushing 26 that is fitted to the housing 12 and that mounting bushing 26 has an inner end 28, an outer end 30 and an exterior flange 32 that seats against the housing 12. Mounting bushing 26 thus surrounds the input shaft 22 and provides a seal against the input shaft 22 while allowing the input shaft 22 to freely rotate within the mounting bushing 26.
To prevent the input shaft 22 from moving axially with respect to the housing 12, there is formed an inner flange 34 at the inner end of the input shaft 22 and which rotatably engages the inner end 28 of the mounting bushing 26 to prevent the input shaft 22 from moving axially outwardly from the housing 12. A further device, such as a C-clip, not shown, can be inserted into a groove 36 formed in the input shaft 22 just proximate the outer end 30 of the mounting bushing 26 such that the axial movement of the input shaft 22 toward the housing 12 is also prevented, thus restraining the input shaft 22 with respect to the housing 12 along either axial direction while, at the same time, allowing the input shaft 22 to freely rotate within the housing 12.
A magnetically sensitive sensor 38 is mounted proximate the magnetic element 16 and that mounting is preferably by means of a recess 40 that is formed in the housing 12 and into which the sensor 38 is fitted, thus retaining that sensor 38 firmly in a fixed position with respect to the housing 12 so that the finite distance between the magnetically sensitive sensor 38 and the magnetic element 16 is affected only by the movement of the magnetic element 16. As can be noted, however, it is important that the magnetically sensitive sensor 38 be positioned within the magnetic field of the magnetic element 16 and the precise finite distance from that magnetic element 16 is dependent, of course, on the strength of the magnetic field of that magnetic element 16.
The actual magnetically sensitive sensor 38 may be of a variety of sensors provided that such sensors are capable of detecting the changing magnetic field of the magnetic element 16 as the distance between the magnetic element 16 and the sensor changes. Typical of such magnetically sensitive sensors is a Hall Effect sensor or a Giant Magneto Resistive (GMR) sensor, however other sensors sensitive to magnetic flux may be used consistent with the principles of the invention.
Accordingly, with the elements of the present invention described, the operation of the rotary shaft position sensor 10 can be described. As the input shaft 22 is rotated, that rotation causes the magnetic element 16 to translate in the direction along the axis of the threaded shaft 20 and thus the magnetic element 16 moves axially within the housing 12 dependent upon the amount of angular rotation of the input shaft 22. The amount of such translational movement per angular rotation of the input shaft 22 is, of course, dependent upon the threads per inch, thread pitch or other parameter of the threaded interengagement between the input shaft 22 and the threaded shaft 20 of the magnetic element 16. In the design of a particular rotating shaft position sensor 10, therefore, the amount of linear translation of the magnetic element 16 per the amount of angular rotation of the input shaft 22 can be designed in accordance with the particular rotation of the input shaft 22 depending upon the particular use of the device such that the use of the rotary shaft position sensor 10 is applicable to a wide variety of differing applications.
In any event, as the input shaft 22 is rotated, the magnet element 16 is linearly translated by the interengement of the threaded shaft 20 and the internal threads 24 of the input shaft 22 to change the distance between that magnetic 16 and the magnetically sensitive sensor 38 such that the effect of the magnetic flux field on sensor 38 changes in proportion thereto and that change in magnetic field can be measured in order to determine the rotation of the input shaft 22.
It should be noted here that the rotation of the input shaft 22 being measured by this invention is not limited to a 90, 180 or even 360 degree rotation of the input shaft 22 but may be applicable readily to multiple full rotations of the input shaft 22.
As indicated, the magnetically sensitive sensor 38 experiences the change in the magnetic field flux radiated by the magnet element 16 and the sensor itself may be any one of a variety of sensors, among such typical sensors is a Linear Hall Effect sensor where the sensor produces an electrical change in a voltage signal analogous to the angular position of the input shaft 22, or, alternatively, the sensor 38 may be a Giant Magneto Resistive (GMR) sensor that experiences a change in internal resistance responsive to the change in the magnetic field and, in such case, that change of resistance can be measured and which is also analogous to the angular position of the input shaft 22. Other sensors may also be utilized as long as the particular sensor is sensitive to a changing of the magnetic flux field emitted by the magnetic element 16. In this way, a varying magnetic influence is imposed upon the magnetically sensitive sensor, the magnetic influence being dependent on the angular position of the input shaft.
As can now be seen, the rotary shaft position sensor 10 according to the principles of the invention can be used to detect and measure various parameters of the rotational movement of the input shaft 22 including, but not limited to, the angular position of that input shaft 22, the amount of rotation of the input shaft 22 and even the angular velocity of the rotation by means of the sensing and interpretation of the changing effect of the magnetic field upon the magnetically sensitive sensor 38. The present invention can even be used to determine the direction of rotation of the input shaft 22, that is, whether it is moving in the clockwise or counterclockwise directions by determining whether the influence of the magnetic flux is increasing or decreasing upon the magnetically sensitive sensor 38.
It is to be understood that the invention is not limited to the illustrated and described forms of the invention contained herein. It will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not considered limited to what is shown in the drawing and described in the specification.