Radial movement capacitive torque sensor

A radial movement capacitive torque sensor includes a pair of concentric capacitor plate rings lying in a common plane, a capacitor plate ring facing the pair of concentric capacitor plate rings, and a paddle assembly disposed between the pair of concentric capacitor plate rings and the capacitor plate ring. The paddle assembly includes a first rotor having a circular opening and having at least one pair of spaced apart bearings mounted thereon, a second rotor having a circular opening and having at least one pivot point located thereon, and at least one paddle having a dielectric head, a curved neck and a body. The curved neck is disposed between a corresponding one of said at least one pair of spaced bearings, and the body is pivotally coupled to a corresponding one of said at least one pivot point. The sensor can be mounted on a shaft having two shaft portions interconnected by an embedded torsion rod to measure the torque applied to the torsion rod when the shaft portions are rotated one with respect to the other.

FIELD OF THE INVENTION

The present invention is related to a sensor for automotive applications including steering, and more particularly to a radial movement capacitive torque sensor for a rotating shaft.

BACKGROUND

Recent requirements from the automotive industry for reduced engine power consumption have dictated the replacement of the power steering hydraulic pump with a more efficient electric motor geared to the steering shaft to assist the steering effort. The main difficulty has been with sensing the effort being applied by the driver so as to know how much to assist in the steering effort.

One sensor for sensing such effort applied by the driver is disclosed in U.S. Pat. No. 6,564,654 entitled “Vertical Movement Capacitive Torque Sensor.” This patent discloses using capacitive sensing technology, in which capacitances formed using two concentric ring plates are varied depending on the location of a dielectric material. The sensor has a plurality of dielectric vanes that move perpendicularly to an axis of rotation of the shaft on which the sensor is mounted. In one exemplary embodiment of this patent, each vane is biased by a biasing member to realize such movement in the perpendicular direction. Any such biasing member may introduce undesirable and/or unpredictable forces or torques into the sensor.

Therefore, it is desirable to provide an apparatus and method for sensing the driver's effort without biasing the dielectric material.

SUMMARY

In an exemplary embodiment of the present invention, a radial movement capacitive torque sensor is provided. The sensor includes a pair of concentric capacitor plate rings lying in a common plane, a capacitor plate ring facing the pair of concentric capacitor plate rings, and a paddle assembly disposed between the pair of concentric capacitor plate rings and the capacitor plate ring. The paddle assembly includes a first rotor having a circular opening and having at least one pair of spaced apart bearings mounted thereon, a second rotor having a circular opening and having at least one pivot point located thereon, and at least one paddle having a dielectric head, a curved neck and a body. The curved neck is disposed between a corresponding one of said at least one pair of spaced apart bearings, and the body is pivotally coupled to a corresponding one of said at least one pivot point.

In another exemplary embodiment of the present invention, is provided a radial movement capacitive torque sensor for a rotating shaft having an axis of rotation and having two shaft portions for which torque applied to one shaft portion causes an angular displacement with respect to the other shaft portion. The sensor includes a pair of concentric capacitor plate rings lying in a common plane and encircling said one shaft portion, a capacitor plate ring facing the pair of concentric capacitor plate rings and encircling the other shaft portion, and a paddle assembly disposed between the pair of concentric capacitor plate rings and the capacitor plate ring. The paddle assembly includes a first rotor having a circular opening for engaging said one shaft portion and having at least one pair of spaced apart bearings mounted thereon, a second rotor having a circular opening for engaging the other shaft portion and having at least one pivot point located thereon, and at least one paddle having a dielectric head, a curved neck and a body. The curved neck is disposed between a corresponding one of said at least one pair of spaced bearings, and the body is pivotally coupled to a corresponding one of said at least one pivot point.

In yet another exemplary embodiment of the present invention, a method of measuring torque between two shaft portions for which applied torque to one shaft portion causes an angular displacement with respect to the other shaft portion, is provided. A dielectric head of a paddle is moved in a generally radially outward direction between a pair of concentric capacitor plate rings lying in a common plane and encircling said one shaft portion and a capacitor plate ring facing the pair of concentric capacitor plate rings and encircling the other shaft portion, when the one shaft portion is rotated in a first direction with respect to the other shaft portion. The dielectric head of a paddle is moved in a generally radially inward direction between the pair of concentric capacitor plate rings and the capacitor plate ring, when the one shaft portion is rotated in a second direction with respect to the other shaft portion. The paddle has a body and a curved neck disposed between the body and the dielectric head. The neck is disposed between a pair of spaced apart bearings substantially fixed with respect to the one shaft portion. The body is pivotally coupled to a pivot point which is substantially fixed with respect to the other shaft portion.

These and other aspects of the invention will be more readily comprehended in view of the discussion herein and accompanying drawings.

DETAILED DESCRIPTION

Capacitive sensing technology is well known. The value of a capacitor depends on the permitivity of the dielectric material between the capacitor plates, the area of the plates and the distance between the two plates. By way of example, the value of the capacitance C in a capacitor having two parallel identical metal plates, is given by C=εA/d, where ε is the dielectric constant, A=the area of one plate, and d=the distance between the two plates. Any one of these parameters may be used in the sensing process.

FIGS. 1A,1B and2illustrate metal plates that form the capacitors in exemplary embodiments of the present invention without the inserted dielectric.FIG. 1Ashows an annular or ring-type capacitor plate10encircling an axis of rotation12. The axis of rotation may be a rotation axis of a shaft on which the sensor is mounted. Capacitance is formed between the capacitor plate10and a pair of concentric ring-type capacitor plates14and16, shown inFIG. 1B, lying in the same plane with each other and encircling the shaft axis12.

As can be seen inFIG. 2, a capacitor Cl is formed by the outer capacitor plate16and the capacitor plate10, and a capacitor C2is formed by the inner capacitor plate14and the capacitor plate10. The capacitor plate10and the concentric capacitor plates14and16are separated by a distance d. In the gap between the capacitor plate10and the concentric capacitor plates14and16, a movable dielectric material may be disposed such that the capacitances of the C1and C2capacitors can be adjusted by moving the dielectric material in a generally radial direction between the capacitor plates14and16.

The capacitor plates14and16should have equal areas in order to provide a balanced capacitive output (i.e., C1and C2have an identical capacitance) at zero torque. Equal areas can easily be provided by simple geometry. Referring toFIG. 1Bwhere the three pertinent radii are shown for equal area capacitive plates, the following formula is applicable: r2=r12+r322,
where r1is the inner radius of the inner ring14, r2is the outer radius of the inner ring14, which is approximately equal to the inner radius of the outer ring16, and r3is the outer radius of the outer ring16.

Referring now toFIGS. 3 and 4, when the capacitances of the capacitors C1and C2are compared, they produce the output voltage VT and as illustrated inFIG. 4when there is a balanced condition at, for example, 2.5 volts, this indicates zero torque. The output voltage VT corresponding to the maximum clockwise torque may be slightly less than 5.0 volts and the output voltage VT corresponding to the maximum counter-clockwise torque may be slightly above 0.0 volts. Of course, in other embodiments, a capacitive sensor may be configured such that the output voltage VT decreases as the clockwise torque is applied, and increases as the counter clockwise torque is applied.

A typical off-the-shelf capacitive sensor driver based on Application Specific Integrated Circuit (ASIC) as shown inFIG. 3is readily available and provides a suitable signal conditioning circuit. This circuit is based on a charge compensation feedback loop, and converts the difference of two capacitances, relative to their sum, into an analog voltage. The output characteristic of this signal conditioning circuit is:V⁢⁢τ=(1+G·C1-C2C1+C2).Vcc2
where G is the gain of the amplifier and Vcc is the supply voltage of the ASIC chip. Since the sensor is based on a ratiometric arrangement, environmental effects such as humidity, temperature, etc. will have a minimal effect on the accuracy due to the fact that the value of C1and C2will track (i.e., increase or decrease proportionally to) each other.

FIG. 5illustrates a radial movement capacitive torque sensor in an exemplary embodiment of the present invention mounted on a shaft formed by joining a smaller shaft40(“upper shaft”) with a larger shaft42(“lower shaft”) using a torsion rod44, which is embedded in the shafts40and42. When a torque is applied to the torsion rod, the twist on the torsion rod results in an angular displacement between the upper and lower shafts. The applied torque is detected by the torque sensor and is then converted into a voltage that represents the torque applied. The angular displacement is directly proportional to the amount of torque applied. The torsion rod44, for example, may provide a movement of 0.3 to 0.5 degrees per Newton-Meter. The torsion rod in other embodiments may have other suitable movement versus torque characteristics. In other embodiments, the diameters of the shafts interconnected by a torsion rod may be substantially identical to each other.

The shaft40has formed thereon a groove46to which a protrusion30of a first rotor22is engaged. The shaft42has formed thereon a groove48to which a protrusion32of a second rotor24is engaged. Thus engaged, the first rotor22is locked to and rotates together with the smaller shaft40, whereas the second rotor24is locked to and rotates together with the larger shaft42. At zero torque, the grooves46and48should be aligned as shown inFIG. 5. In other embodiments, the rotors may be rigidly attached to the respective shafts using any other suitable method such as via friction, spline, epoxy or the like.

In addition to the first and second rotors22and24, the sensor includes printed circuit boards (PCBs)50and52. Each PCB has a generally circular portion, and may also have a rectangular portion attached to the generally circular portion. The PCB board50has formed thereon concentric metal rings14and16that are used to form capacitors C2and C1, respectively. The diameter of the smaller concentric ring14is larger than the diameter of the rotor22, such that the capacitance of the capacitor C2is substantially not affected by the rotor22. The PCB board52has formed thereon a single plate ring10, which has a diameter greater than that of the second rotor24.

The first rotor22and the second rotor24form a paddle assembly20, which also includes a plurality of paddles26aand26c. While only two paddles are shown inFIG. 5, the paddle assembly may include more than two (e.g., four) paddles in practice. Each of the paddles has a head (i.e.,28aor28c), which is moved in a generally radial direction between the concentric rings14and16, such as to adjust the capacitances of the capacitors C1and C2. The paddles are made of dielectric material such as FR-4, G10 or any other suitable material, or just the heads may be made of dielectric material.

Inherently in the symmetry of the geometry of the paddle assembly20is the cancellation of error due to radial run-out in the shaft. As one paddle moves toward one capacitor ring, another paddle 180 degrees away is moving in the opposite direction, thereby canceling the error. By way of example, if the head of one paddle is moved in an inward direction because of radial run-out, the head of another paddle 180 degrees apart is moved in an outward direction. Further, if the head of one paddle is moved in an outward direction because of radial run-out, the head of another paddle 180 degrees apart is moved in an inward direction.

FIG. 6is an enlarged view which illustrates the movement of the head28aof the paddle26ain a generally radial direction. By way of example, if the upper shaft40is rotated in a counter clockwise (CCW) direction with respect to the lower shaft42, the head section28amoves outward toward the outer ring16, thereby increasing the capacitance of the capacitor C1. On the other hand, if the upper shaft40is rotated in a clockwise (CW) direction with respect to the lower shaft42, the head section28amoves inward toward the inner ring14, thereby increasing the capacitance of the capacitor C2. In other embodiments, the head may move inward when the upper section is rotated in a CCW direction, whereas the head may move outward when the upper section is rotated in a CW direction.

As can be seen inFIG. 7, the first rotor22has a plurality of pairs of spaced apart bearings70a,70b,70cand70dformed thereon. In one exemplary embodiment, each bearing is a pin or a post such as a head of an Allen screw. In other embodiments, each bearing may be a cylindrical roller which substantially freely rotates about its center axis. In still other embodiments, each bearing may be a ball bearing or any other suitable structure that can guide the movement of another component that slidingly engages (or rides on) it.

Each pair of spaced apart bearings is located 90 degrees apart from two adjacent pairs of spaced apart bearings. While four pairs of spaced apart bearings are illustrated inFIG. 7, a different number of pairs of spaced apart bearings may be used in other embodiments. The first rotor22has an opening at its center for mounting onto a shaft, such as the upper shaft40ofFIG. 5. The first rotor22has a low cylindrical wall60around the periphery of the center opening. A protrusion30is formed on the inner surface surrounding the opening such that it can be used to engage the upper shaft40at the groove46.

InFIG. 8, a plurality of paddles26a,26b,26cand26dare pivotally coupled to the second rotor24at pivot pins (“pivot points”)80a,80b,80cand80d, respectively. While the pivot pins formed on the second rotor24are used to make pivot connections in the illustrated embodiment, any other suitable pivot connections may be used in other embodiments. The paddles26a,26b,26cand26dhave heads28a,28b,28cand28d, respectively, formed on their respective ends away from the respective pivot points. While four paddles are illustrated inFIG. 8, the number of paddles may be different in other embodiments, and would correspond to the number of pairs of spaced apart bearings on the first rotor22.

In the illustrated embodiment, the second rotor24has an opening formed at its center for engaging the lower shaft42. The opening of the second rotor24is larger than the opening of the first rotor22because the lower shaft42is larger in diameter than the upper shaft40. Hence, the low cylindrical wall60of the first rotor22may fit into the center opening of the second rotor24when the paddle assembly is assembled. The second rotor24also has a low cylindrical wall90around the periphery of the opening. Also, a protrusion32for engaging the groove48of the lower shaft42is formed on the inner surface surrounding the opening.

The second rotor24has also formed thereon on the surface facing the first rotor22a plurality of depressed areas82a,82b,82cand82d. The depressed areas are located such that they engage the corresponding pairs of spaced apart bearings mounted on the first rotor22. As will be discussed in more detail below in reference toFIGS. 9A–9C, a neck of each paddle is placed between a corresponding pair of spaced apart bearings on the first rotor22. Since each pair of spaced apart bearings engage the corresponding depressed area, when the first and second rotors are rotated with respect to each other, the bearings and the depressed areas together prevent the rotors from rotating beyond a predetermined rotation angle range. By way of example, the rotation angle range may be ±8 degrees in one exemplary embodiment. In other embodiments, the rotation angle range may vary from ±1to ±10 degrees, or may be any other suitable range of degrees.

The shape and operation of the paddle26cand the spaced apart bearings70awill be described below in reference toFIGS. 9A,9B and9C, with the understanding that the other pairs of spaced apart bearings and the paddles have substantially the same structure and operate in substantially the same manner as the paddle26cand the spaced apart bearings70aillustrated inFIGS. 9A–9C.

FIG. 9Ashows only one of the paddles, namely, the paddle26c, of the paddle assembly20. It can be seen inFIGS. 7 and 8, when the first and second rotors22and24are assembled together, the pair of spaced apart bearings70awould engage the paddle26cand the depressed area82c.

In addition to the head28c, the paddle26cincludes a neck102and a body104. The neck may also be referred to as an “arm”. Near the end of the body104away from the neck102is formed a hole100, which is used to pivotally couple the paddle26cto the pivot pin80con the first rotator22. As indicated previously, the pivot point in other embodiments may be formed by any suitable pivot connection, which may be different from the pivot pin and the hole. The body104is curved with a curvature that generally tracks the curvature of the circular opening at the center of the first rotor22. Between the neck102and the body104is formed a cove106, which engages one of the spaced apart bearings70awhen the paddle26cis at a most outward-extended position.

It can be seen inFIG. 9Athat the neck102is between the pair of spaced apart bearings70a. The neck102has a gentle curvature having a radius of curvature R, such that the head28cmoves outwardly not only in a radial direction, but also in a circumferential direction, as the neck having the radius R rides on the bearings. The neck has an outline of two curved lines that are substantially equidistant from each other through the length of the curves. The distance between the curves are substantially the same as the spacing between the spaced apart bearings70asuch that the neck slidably engages the bearings on both sides as the paddle26cis moved between the bearings. Hence, a desmodronic approach has been taken for the design of the paddle and the spaced apart bearings, where the inner and outer radii (i.e., inwardly curved and outwardly curved curves) of the paddle neck are captured between the spaced apart bearings, and the paddle is forced to travel in a predetermined direction between the same. Using this technique, substantially no rotational bias is applied to the sensor in either direction.

In the illustrated embodiment, when the rotor24is rotated in a counter clockwise direction with respect to the rotor22, the neck exerts force mainly on the bearing on the convex (“outwardly curved”) side of the neck, such that the neck can be said to ride on the bearing on its convex side. On the other hand, when the rotor24is rotated in a clockwise direction with respect to the rotor22, the neck exerts force mainly on the bearing on the concave (“inwardly curved”) side of the neck, such that the neck can be said to ride on the bearing on its concave side.

By adjusting the radius R and/or the pivot point on the second rotor24, a linear relationship between the rotation of the rotors and the radial movement of the head28cmay be realized, as those skilled in the art would appreciate. Those skilled in the art would also appreciate that other desired (e.g., non-linear) relationship between the rotational movement of the rotors and the radial movement of the head28cmay be realized by varying the radius R, the concavity/convexity of the neck and/or the location of the pivot point.

InFIG. 9A, the portion108of the head opposite the cove106is proximate to one of the pair of spaced apart bearings70a, and the head28cmainly covers the inner ring14, such that the capacitance of the inner capacitor C2is higher than the capacitance of the outer capacitor C1.

When a counter clockwise torque is applied to the shaft assembly, the pivot point (i.e., the hole100) of the paddle26cis moved closer to the bearings70a(and away from the bearings70b). This causes the paddle to move outward as the neck102rides along the bearings70a. This motion causes the value of the capacitor C1to increase and the value of the capacitor C2to decrease.

It can be seen inFIG. 9B, by the movement of the hole100which is pivotally coupled to the pivot pin80c, that the second rotor24(not shown) has rotated in a counter clockwise direction with respect to the first rotor22. The head28chas moved outward in a generally radial direction. Also, because of the curvature of the neck102disposed between the spaced apart bearings70a, the paddle26chas pivotally rotated in a clockwise direction about the pivot pin80c.

When the spaced apart bearings70aare placed at substantially the middle of the neck102having the curvature radius R, the head is substantially symmetrically positioned over the outer and inner rings as shown inFIG. 9B. In other words, the head28cinFIG. 9Bcovers approximately equal amount of the outer ring16and the inner ring14. At this point, the capacitance of the capacitor C1is substantially equal to the capacitance of the capacitor C2.

InFIG. 9C, the second rotor24(not shown) has further rotated counter clockwise with respect to the first rotor22. It can be seen that the hole100has moved further away from an adjacent pair of spaced apart bearings70bon the first rotor22. Accordingly, the head28chas moved further outwardly as the neck102has moved outward through the pair of spaced apart bearings70a. It can be seen inFIG. 9Cthat one of the spaced apart bearings70ais disposed substantially within the cove106. In addition, the paddle26chas further rotated in a clockwise direction with respect to the pivot pin80c(i.e., the pivot point at which the hole100is pivotally coupled).

At this point, when a clockwise torque is applied to the shaft assembly, the pivot point is moved away from the bearings70a(and toward the bearings70b), thus allowing the head28cto move inward as the neck having radius R rides along the bearings70a. This motion allows the value of the capacitor C2to increase and the value of the capacitor C1to decrease. By changing the configuration/shape of the paddles, more particularly the design of the neck, the paddle and its head can be made to move in different directions at different rates as the rotors22and24rotate one with respect to the other.

A major contributor to hysteresis in the radial movement capacitive torque sensor as described above is any free play in the movement of the paddles. To reduce this movement to a reasonable level would require that mechanical dimensions be held to very exacting tolerances, and this translates to an increase to the cost of the sensor.

One method of making up for looser tolerances is to spring load the paddle radius (i.e., neck of the paddle) against one of the bearings. The problem with a spring, however, is that the force is always in one direction. This force will aid movement of the paddle in one direction while hindering the movement in the opposite direction. Such spring bias would also show up as hysteresis.

In another exemplary embodiment of the present invention, the inside radius (“inwardly curved curve”) and the outside radius (“outwardly curved curve”) of the paddle neck is equally spring loaded against their respective bearings without favoring movement in any direction. A paddle26′ ofFIG. 10is one such paddle having an integral spring in a neck102′ in another exemplary embodiment of the present invention.

The paddle26′ may be used to replace each of the paddles26a–26dinFIG. 8, and has components corresponding to the paddle26cillustrated in FIGS.8and9A–9C. Hence, the paddle26′ has a head28′, a hole100′, a neck (“arm”)102′, a body104′, a cove106′, and a portion108′ of the head opposite the cove106′. In addition, the paddle26′ has a slot111cut in the neck to form the integral spring. The slot111is a short distance away from, and generally tracks the curvature of the inside radius of the neck102′. The slot111is slightly longer than the traveling distance of the spaced apart bearings, as it starts approximately at the junction between the head28′ and the neck102′, and runs all the way down along the neck102′ to approximately the junction between the neck102′ and the body104′ This causes the surface riding on the inside bearing to be slightly springy. Then, by machining the inside radius for a 0.001 inch (“0.00254 cm”) interference, for example, the neck102′ is always in intimate contact with both bearings and the source of free play is substantially eliminated. In other words, the width between the curves of the neck is made slightly larger (e.g., by 0.001 inch) than the distance between the spaced apart bearings.

FIG. 11is an exploded view of the radial movement capacitive torque sensor in an exemplary embodiment of the present invention. The sensor has covers120and150that can be fixed to each other through a plurality of holes121and151formed along their respective peripheries. The first cover120also has a plurality of guide pins122formed thereon for aligning the other components when they are installed between the covers.

The PCB52has formed thereon a plurality of openings123for engaging the guide pins122. The PCB52has also formed thereon a capacitor ring plate10, and a rectangular portion on which one or more ASIC/logic chips130are mounted. A paddle assembly140is placed between the PCB52and the PCB50. The PCB50has formed thereon a pair of concentric ring plates14and16. When installed, the concentric ring plates14and16should face the ring plate10such that capacitance is formed therebetween. A spacer ring54is inserted between the PCB52and the PCB50such that the heads of the paddles in the paddle assembly140are substantially free to rotate between the metal ring plates on the PCBs. The PCB50also has a plurality of guide holes125for engaging the guide pins122. Finally, a cover150is placed on top of the PCB50to form the sensor package.

While certain exemplary embodiments of the present invention have been described above in detail and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive of the broad invention. It will thus be recognized that various modifications may be made to the illustrated and other embodiments of the invention described above, without departing from the broad inventive scope thereof. In view of the above it will be understood that the invention is not limited to the particular embodiments or arrangements disclosed, but is rather intended to cover any changes, adaptations or modifications which are within the scope and spirit of the invention as defined by the appended claims.