Position sensor

Abstract: A position sensor having a shaped dual magnet structure (212, 214) carried upon a pole piece (210) having a generally "C"-shaped cross-section, wherein the magnet and pole piece define a generally circular linear field which is concentric about the axis of rotation of the complete rotor (220). A Hall effect device (510) is located in the open portion or gap between the two magnets and is exposed to a well defined field, so that, by the use of selected geometries and particular magnet materials, a precise, compact and yet tolerant magnetic circuit is produced.

Description: This invention relates generally to a position sensor, and more especially to a position sensor which is compact, durable and precise for use in rugged and demanding environments. Electronic devices are advancing technologically at phenomenal rates. The production cost of such devices decreases continually, and is accompanied by almost simultaneous increases in capability. These more capable, lower cost devices and circuits are applicable to an ever increasing number of requirements. As this trend continues, more ways are needed for electronic circuits to interface with non-electronic devices and systems. Generally this interface is accomplished through a combination of sensors and actuators. Position sensing is commonly used to allow an electrical circuit to gain information about an event or a continuously varying condition. For example, when a sewing machine operator depresses a pedal, a pedal position sensor is used to signal a demand for activation of the drive motor. In addition, the sensor may be used to establish an amount of demand, or a desired speed at which a motor will operate. Position sensors must endure many millions or even billions of small motions referred to as dithers. These dithers are often the result of mechanical motion and vibration carried into the sensor. During the life of a sensor there may also be a million or more full cycles required. There are many applications for position sensors, and a wide variety of technologies to fill these needs. In each of these technologies the position sensor offers a unique set of advantages and limitations. Of these technologies, magnetic sensing is known to have a unique combination of long life components and excellent resistance to contaminants. However, in the prior art, these devices were only applied where little precision was required, such as in proximity detection. Thus, magnetic sensors have been limited in application. These limitations are generally derived from the need for linearity and precise output. In the case of a pedal position sensing application, an operator gently depressing the pedal will expect to see a measurable change in the output of a motor or engine. In fact, the first few degrees of rotation may be the most consequential in terms of percentage change in motor output. Sensitivity and precision are most important close to the zero or no demand position. Deviations in linearity of less than one per cent may have very adverse effect on performance and even on motor control functions. Sensing a demand for motor output when the operator is not depressing the pedal will obviously have adverse consequence. Therefore, at the motor zero set point, sensors are typically required to possess extremely tight and reproducible tolerances through all extremes of climate, contamination, and other external factors. Magnetic circuits offer very good performance upon exposure to the usual contaminants. However, linearity and tight tolerances are another problem. Position sensors are subjected to forces that change the alignment of the moving portion of the sensor with respect to the stationary portion. Somewhere in the system must be provided at least one bearing, and this bearing will have a finite amount of play, or motion, which can result in false movement between the fixed and moving components of the sensor. Unfortunately, magnetic circuits known from the prior art tend to be very sensitive to the type of mechanical motion occurring in a sensor bearing. The problem is accentuated with poor or worn bearings. Typical magnetic circuits use one or a combination of magnets to generate a field across an air gap. The magnetic field sensor, whether this is a Hall effect device or a magnetoresistive material or some other magnetic field sensor, is then located in the gap and aligned centrally within the cross-section thereof. Magnetic field lines are not constrained anywhere within the gap, but tend to be most dense and of consistent strength centrally within the gap. Various means may be provided to vary the strength of the field monitored by the sensor. Regardless of the arrangement and method for changing the field about the sensor, the magnetic circuit has several associated problems which have heretofore not been overcome. Movement of the sensor relative to the gap as a result of bearing play leads to a variation in field strength measured by the sensor. This effect is particularly pronounced in Hall effect, magnetoresistive and other similar sensors, where the sensor is sensitive about a single axis and insensitive to perpendicular magnetic fields. The familiar bulging of field lines jumping a gap illustrates this, where a Hall effect sensor not accurately positioned in the gap measures the vector fraction of the field strength directly parallel to the gap. In the centre of the gap, this will be equal to the full field strength. The vector fraction perpendicular thereto is ignored by the sensor, even though the sum of the vectors is the actual field strength at that point. As the sensor is moved from the centre of the gap, the field begins to diverge, or bulge, resulting in a greater fraction of the field vector being perpendicular to the gap. As this is not detected by the sensor, the sensor will provide a reading of insufficient magnitude. In addition to the aforesaid limitations with regard to position and field strength, another set of considerations arises. A position sensor must be precise in spite of fluctuating temperatures. In order to gain useful output, the magnet must initially be completely saturated, as failure to do so results in unpredictable performance. However, operating at complete saturation leads to another problem referred to in the trade as irreversible loss. Temperature cycling, particularly to elevated temperatures, permanently decreases the magnetic output. A magnet also undergoes aging processes not unlike those of other materials, including oxidation and other forms of corrosion. This is commonly referred to as structural loss. Structural and irreversible loss must also be considered and dealt with in order to provide a reliable device with precision output. Another significant problem in the design of magnetic circuits is the sensitivity of the circuit to surrounding ferromagnetic objects. For some applications a large amount of iron or steel may be placed in very close proximity to the sensor. The sensor must not respond to this external influence. The prior art is exemplified by U.S. Patent No. 4 570 118, wherein a number of different embodiments are illustrated for forming the magnetic circuit of a Hall effect position sensor. The patent discloses in one embodiment the use of a sintered samarium cobalt magnet material formed into two shaped magnets of opposite polarity across an air gap of varying length. No information is provided in the patent as to the manner in which each magnet is magnetically coupled to the other, though from the disclosure it appears to be through the use of an air gap formed by a plastic moulded carrier. Furthermore, no information is provided as to how this magnetic material is shaped and how the irreversible and structural losses will be managed. Sintered samarium cobalt is difficult to shape with any degree of precision, and the material is typically ground after sintering. The grinding process is difficult, expensive and imprecise. The known device may be designed and ground, for a substantial price, to be linear and precise at a given temperature and a given level of magnetic saturation, presumably fully saturated. However, such a device would not be capable of performing in a linear and precise manner, nor be reliable, through the production processes, temperature cycling and vibration experienced by sensors. Furthermore, devices made to this known design are highly susceptible to adjacent ferromagnetic objects. The variation in adjacent ferromagnetic material will serve to distort the field and adversely affect both linearity and precision. The open magnetic circuit not only adversely affects sensitivity to foreign objects, but also sensitivity to radiated energies, commonly referred to as Electro-Magnetic Interference (EMI or EMC). Moreover, all the embodiments described in U.S. Patent No. 4 570 118 are very sensitive to bearing play. The combination of an open magnetic circuit and radially narrow permanent magnet structure provides no tolerance for motion in the bearing system. This motion will be translated into a changing magnetic field, since the area within the gap in which the field is parallel and of consistent magnetic induction is very small. In U.S. Patent No. 3 112 464 several embodiments of a brushless Hall effect potentiometer are disclosed. In the first embodiment, a shaped, radially magnetized structure varies an air gap between the magnetic structure and a casing. However, there is no allowance for radial or axial motion of the magnet carried upon the rotor. Furthermore, the large magnetic structure, like those of U.S. Patent No. 4 570 118, is difficult to manufacture and relatively expensive. U.S. Patent No. 5 159 268 discloses a similar shaped magnet structure, which suffers from the same disadvantages. Additionally, the device of this last-mentioned U.S. patent offers no protection against influence from extraneous ferromagnetic objects. U.S. Patent No. 5 164 668 discloses a sensor which is less sensitive to radial and axial play. This device, however, requires a large shaped magnet for precision and linearity. The size of the magnet structure places additional demand upon the bearing system. No information in the patent deals with the problems of magnet materials, methods for compensating for irreversible and structural losses, or shielding from extraneous ferromagnetic objects. The combination of large magnet, enhanced bearing structure, and added shielding combine to make a more expensive package. Summarising, all the prior art devices suffer the disadvantages of high field strengths at the zero set point. In the devices of U.S. Patents No. 5 159 268 and No. 5 164 668, a bipolar field is utlized. A strong magnetic field is encountered at or near the zero point, progressively diminishing in strength to a zero magnetic field at or near a mid-point, and then returning to a strong magnetic field. In the first embodiment of U.S. Patent No. 4 570 118, a bipolar field is also utilized, in that near the mid-point of rotation the two opposing fields are designed to cancel each other. As the sensor is rotated out from the mid-point however, the magnetic induction no longer cancels and the sensed field becomes progressively stronger. In the last embodiment of U.S. Patent No. 4 570 118 a unipolar field is disclosed. However, the use of sintered samarium cobalt magnet materials together with the necessary grinding operations will limit the minimum thickness of the magnet. Attempting to grind too thin a section will lead to large manufacturing fall-out due to chipping and breakage. Yet grinding is essential for a sintered magnet in order to produce a linear output. In view of the large magnetic induction generated by samarium cobalt, the field will be substantially even at the thinnest point in the magnet. The present invention aims to overcome the aforementioned limitations of the prior art and problems restricting the use of a precision magnetic position sensor by the use of magnetic structure of special geometry. According to one aspect of the invention, there is provided a rotary position sensor, comprising: a housing, including a stationary magnetic field sensing means and a closed magnetic assembly comprising a first magnet and an air gap, said assembly rotatably mounted in the housing about an axis, the magnetic field sensing means being positioned in said air gap, the magnetic assembly being shaped to generate a magnetic field over the air gap, parallel to said axis, and the strength of said magnetic field being dependent on the rotary position of said assembly, the magnetic field sensing means sensing the strength of said magnetic field, characterised in that the closed magnetic assembly includes a second magnet, the first and second magnets being opposite said air gap, and a magnetically permeable pole piece interconnecting the first and second magnets, the first magnet is provided with a first permanent magnet lip portion, and the second magnet is provided with a second permanent magnet lip portion. According to another aspect of the invention, there is provided a rotary position sensor comprising: a housing; an assembly for providing a closed magnetic circuit including: a first magnet; a second magnet; a magnetically permeable pole piece interconnecting the first and second magnets; and a varying dimension air gap defined between the first magnet and the second magnet; the first and second magnets being structured and dimensioned to provide the varying dimension air gap and to form a variable magnetic field coupled therebetween; the assembly being rotatably mounted in the housing about an axis, the axis being generally parallel to the magnetic field coupled between the magnets; wherein a magnetic field sensing means is positioned in the air gap for sensing the variable magnetic field coupled between the first and second magnets; and a first permanent magnet lip portion is provided on the first magnet and a second permanent magnet lip portion is provided on the second magnet; the first lip portion and the second lip portion being structured and dimensioned to expand the variable magnetic field coupled between the magnets. The position sensor in accordance with the invention is further described with reference to the accompanying drawings, in which:- Figure 1 shows one practical embodiment of the invention in a plan view, with the cover removed for clarity; Figure 2 shows the embodiment of Figure 1 in cross-sectional view taken along line 2' of Figure 1; Figure 3 is a schematic view of the magnet and Hall effect device structure; and Figure 4 shows the pole piece in accordance with the present invention. In Figures 1 and 2, a practical embodiment of rotary sensor in accordance with the present invention is designated generally by the numeral 100. The sensor includes a housing 300 and a magnetic structure or assembly 200 of arcuate periphery and generally "C"-shaped cross section, mounted to the housing . The magnet assembly 200 includes therein a magnetically permeable pole piece 210, shaped magnets 212 and 214, and air gap 216, and is supported by a base in the form of a moulded rotor cup 220. The pole piece 210 is bonded to the magnets 212 and 214 such that the air gap is formed between and is bordered by magnets. This use of two magnets substantially reduces loss through the air gap which otherwise occurs with only a single magnet. The closed magnetic circuit which is completed by the pole piece 210 improves performance in that it is less sensitive to bearing play and less sensitive to external ferromagnetic objects. A closed magnetic circuit exists, for the purposes of this invention, when the external flux path of a permanent magnet is confined with high permeability material. Air is understood to be low permeability material. The pole piece 210, the magnets 212, 214 and the air gap 216 form the closed magnetic circuit. The pole piece 210 further reduces the size of the magnets 212 and 214 required, and may be manufactured from moulded or sintered metals. Most preferably, pole piece 210 is formed from sheet steels such as ANSI 430 stainless steel. As shown in Figures 1 and 2, the pole piece has a generally pie shape configuration in plan view and has a radius from an axis 250 to outer edges 261, 263. The shaped magnets 212 and 214 are preferably formed by moulding magnetic materials such as bonded ferrite. Bonded ferrite offers both a substantial cost advantage and also a significant advantage over other similar magnetic materials in structural loss due to corrosion and other environmental degradation. Additionally, bonded ferrite may be produced having a very thin, very low field strength region close to the zero set point. The advantage of this low field at the zero set point is discussed further hereinbelow with reference to Figure 4. Other magnetic materials may be suitable, as may be determined by one skilled in the art. The magnets 212 and 214 should desirably extend substantially from the outer pole piece 210 to a region very close to, or, design permitting, in line with the axis of rotation 250. This large extension of magnets 212 and 214 in the radial direction greatly reduces the effects of radial motion of magnetic structure 200. Additionally, the magnets 212 and 214 are formed with lip structures 474 and 472, as best illustrated in Figure 2. These formations extend out beyond and partially around pole piece 210. The lips 472 and 474 serve to expand the "sweet zone" of operation of the Hall effect device 510, by forcing a larger area of linear magnetic field lines to pass through the air gap and coupled between magnets 212 and 214. This larger area of linear field lines directly corresponds to greater tolerance for both radial and axial play, and yet minimizes the radial extension of the magnets. The moulded rotor cup 220 includes a surface designed to engage with a shaft extending from a device whose position is to be measured. In use, the moulded rotor cup 220 rotates about an axis identified from end view as 250 in Figure 1 and carries therewith the remainder of magnetic structure 200. The moulded rotor cup 220 is retained by housing 300, seal 350, helical spring 360 and cover 310. The cover 310 engages with the housing 300 and may, for example, be ultrasonically welded in place. The cover 310 is strengthened against warpage and deflection through the formation of ribs 312. Within the gap formed by the magnets 212 and 214 is a hybrid circuit substrate 500 carrying thereon a Hall effect device 510. The Hall effect device 510 is positioned somewhere between the edges 265, 267 of magnets 212 and 214, respectively, and the ends 511, 513 of the magnets near the axis 250, but not particularly close to either the edges or the ends, so as to avoid the field bulging effect mentioned earlier. The hybrid substrate 500 is attached by heat staking or another similar method to the housing 300, and it additionally carries thereon electrical circuitry within a tray 520. This tray 520 acts as a container into which appropriate potting compounds may be placed to provide all necessary environmental protection to the associated circuitry. The tray 520 should be electrically grounded for protection against radiated fields (EMI and EMC). The hybrid substrate 500 is electrically interconnected to electrical terminals 410 through wire bonds 530, though it will be understood that any of large number of electrical interconnection techniques would be suitable. Electrical connector terminals 410 emerge from housing 300 at a connector body 400, for interconnection to standard mating connectors. In use, the magnetic structure 200 rotates about the generally central axis 250 relative to the housing 300, thereby rotating the magnets 212 and 214 together with the pole piece 210. The Hall effect device 510 is stationarily fixed relative to the housing 300. As best illustrated in Figure 3, the magnets 212 and 214 are shaped generally helically so as to have a relatively thicker end and a relatively thinner end. At the thicker ends 211 and 215, which is at the same angle of rotation of magnetic structure 200 for both the magnets 212 and 214, there is a narrow air gap 217. At the thinner ends 213 and 216, there is a correspondingly wider air gap 218. The result is the generation of less magnetic induction across the gap 218, with more magnetic induction across the gap 217. Rotation of the pole piece 210 about the axis 250 results in a changing field magnetic induction which is directly measured by the Hall effect device 510. Proper shaping of the gap produces a linear output from the Hall effect device 510. However, such a system will not perform linearly and with precision and resistance to bearing play over its life without further design considerations. In order to stabilize a magnet against irreversible losses, it is necessary first to saturate magnets 212 and 214 and then to demagnetize the magnets by a small amount. The magnetic structure 200 does not demagnetize evenly from magnet ends 211 and 215 to magnet ends 213 and 216, without special consideration. Absent the appropriate features for demagnetization, the resulting device will either lose precision as a result of temperature excursions or will lose linearity as a result of stabilizing demagnetization. Figure 4 illustrates the pole piece design 210 having two small extensions or dams 482 and 484. These dams serve to attract magnetic flux at the low field end of rotation, within gap 218 of Figure 3, thereby further to reduce the strength of the vector portion of the magnetic field that is parallel to the axis of rotation 250. This deflection of the magnetic field reduces the measured field strength, thereby lowering the output at the low end of rotation. The lower the output at this low end, the less the impact of non-linearities and variances, such as the effects of temperature and irreversible and structural losses. These non-linearities and variances are most important, as noted hereinabove, at or near the zero set point. The use of a zero Gauss field at the zero set point offers substantial advantage in maintaining very tight tolerances by eliminating gain errors caused by the magnets, the magnetic sensor 510 and the conditioning electronic circuit located on the substrate 500. The apparatus for measuring angular or rotary position described herein as the preferred embodiment is a low cost structure due to the minimal weight and reduced demands upon magnetic components. In addition, there are many performance advantages not heretofore obtainable, including reduced sensitivity to bearing play, resistance to contamination and environment reduced sensitivity to externally located fields, energies and objects, durability for motion and dithers, precision, linearity, compactness, and reduced complexity. Various modifications of the above-described arrangement are possible within the scope of the invention defined by the appended claims. Thus, while a Hall effect device is illustrated, magnetoresistive and other magnet sensors may be similarly adapted for use in the sensor of this invention.

Claims: A rotary position sensor comprising: a housing (300), including a stationary magnetic field sensing means (510) and a closed magnetic assembly comprising a first magnet (212) and an air gap, said assembly rotatably mounted in the housing about an axis, the magnetic field sensing means being positioned in said air gap, the magnetic assembly being shaped to generate a magnetic field over the air gap, parallel to said axis, and the strength of said magnetic field heing dependent on the rotary position of said assembly, the magnetic field sensing means sensing the strength of said magnetic field, characterised in that the closed magnetic assembly includes a second magnet (214), the first and second magnets (212, 214) being opposite said air gap, and a magnetically permeable pole piece (210) interconnecting the first and second magnets, the first magnet (212) is provided with a first permanent magnet lip portion (474), and the second magnet (214) is provided with a second permanent magnet lip portion (472). The rotary position sensor of claim 1, wherein the first and second lip portions (474, 472) extend out beyond and partially around the pole pice. The rotary position sensor of claim 1 or claim 2, characterised in that the first magnet (212), the second magnet (214) and the magnetically permeable pole piece (210) are arranged in a generally C-shaped cross-sectional configuration with said first and second magnets aligned in parallel planes extending from said magnetically permeable pole piece. The rotary position sensor of claim 1 or claim 2 or claim 3, characterised in that the sensing means (510) is stationarily mounted in the housing. The rotary position sensor of any of claims 1 to 4, characterised in that said sensing means (510) comprises a Hall effect device. The rotary position sensor of any of claims 1 to 5, characterised in that said first and second magnets (212, 214) are shaped generally helically so that each of the magnets has a relatively thicker end and a relatively thinner end. The rotary position sensor of claim 6, characterised in that said first and second magnets (212, 214) are aligned in a manner such that the relatively thicker ends of the first and second magnets face each other across the air gap (216) and the relatively thinner ends of the first and second magnets face each other across the air gap. The rotary position sensor of any of claims 1 to 7, characterised in that the magnetically permeable pole piece (210) is generally C-shaped in cross-sectional profile and includes a first arm and a second arm with the first magnet (212) affixed to the first arm and the second magnet (214) affixed to the second arm. The rotary position sensor of claim 8, characterised in that the generally C-shaped magnetically permeable pole piece (210) includes at least one dam thereon for attracting magnetic flux. A rotary position sensor comprising: a housing (300); an assembly (200) for providing a closed magnet circuit including: a first magnet (212); a second magnet (214); a magnetically permeable pole piece (210) interconnecting the first and second magnets (212, 214); and a varying dimension air gap defined between the first magnet and the second magnet; the first and second magnets (212, 214) being structured and dimensioned to provide the varying dimension air gap and to form a variable magnetic field coupled therebetween; the assembly (200) being rotatably mounted in the housing about an axis, the axis being generally parallel to the magnetic field coupled between the magnets; wherein a magnetic field sensing means is positioned in the air gap for sensing the variable magnetic field coupled between the first and second magnets; and a first permanent magnet lip portion (474) is provided on the first magnet (212) and a second permanent magnet lip portion (472) is provided on the second magnet (214), the first lip portion (474) and the second lip portion (472) being structured and dimensioned to expand the variable magnetic field coupled between the magnets (212, 214). The rotary position sensor of claim 10, wherein the first magnet (212), the second magnet (214) and the magnetically permeable pole piece (210) are arranged in a generally C-shaped cross-sectional configuration with said first and second magnets aligned in parallel planes extending from said magnetically permeable pole piece. The rotary position sensor of claim 10 or claim 11, wherein the sensing means (510) is stationarily mounted in the housing. The rotary position sensor of any of claims 10 to 12, wherein said sensing means comprises a Hall effect device (510). The rotary position sensor of any of claims 10 to 13, wherein said first and second magnets (212, 214) are shaped generally helically so that each of the magnets has a relatively thicker end and a relatively thinner end. The rotary position sensor of claim 14, wherein said first and second magnets (212, 214) are aligned in a manner such that the relatively thicker ends of the first and second magnets face each other across the air gap and the relatively thinner ends of the first and second magnets face each other across the air gap. The rotary position sensor of any of claims 11 to 15, wherein the magnetically permeable pole piece (210) is generally C-shaped in cross-sectional profile and includes a first arm and a second arm with the first magnet (212) affixed to the first arm and the second magnet (214) affixed to the second arm. The rotary position sensor of claim 16, wherein the generally C-shaped magnetically permeable pole piece (210) includes at least one dam thereon for attracting magnetic flux. The rotary position sensor of claim 16 or claim 17, wherein the lips (474, 472) on the magnets extend beyond and partially around the arms of the C-shaped magnetically permeable pole piece. The rotary position sensor of any of claims 16 to 18, wherein said first and second magnets (212, 214) are shaped generally helically so that each of the magnets has a relatively thicker end and a relatively thinner end. The rotary position sensor of claim 19, wherein said first and second magnets (212, 214) are aligned in a manner such that the relatively thicker ends of the first and second magnets face each other across the air gap and the relatively thinner ends of the first and second magnets face each other across the air gap.

Inventors: GUTHRIE MICHAEL S, MCCURLEY JEFFREY L, WHITE JAMES E, GUTHRIE, MICHAEL S., MCCURLEY, JEFFREY L., WHITE, JAMES E.
Applicants: CTS CORP, STACKPOLE CORP, CTS CORPORATION, THE STACKPOLE CORPORATION
Application year: 1995
Main classification: G01D5
IPC codes: G01B7, G01D5