Hybrid rotary actuator

A rotary actuator (16) includes a rotor (48) which is disposed in a housing (34) between first and second pole pieces (42 and 44) of a stator (40). The rotor (48) is rotatable relative to the stator (48) between an unactuated position (FIG. 4) and an actuated position (FIG. 5). During rotation of the rotor (48), the axial extent of a first working air gap (66) between the rotor and a first pole piece (44) of the stator (40) remains constant. However, the axial extent of the working air gap (64) between the rotor (48) and the second pole piece (42) of the stator (40) decreases as the rotor moves from the unactuated position to the actuated position. In a preferred embodiment, the rotor lobes are made so that the net axial force of all of the rotor lobes is substantially zero thereby reducing stress on the rotor shaft support bearings.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rotary actuator having a rotor which is rotatable relative to a stator.

2. Discussion of Prior Art

A rotary actuator has been utilized in association with a diverter gate in the sorting of mail or other items traveling by a conveyor. The rotary actuator is effective to rotate the diverter gate from one position to another position within a matter of a few milliseconds, typically within about 0.020 seconds, so as to permit a rapid sorting process. The angle of rotation of the diverter gate is typically about 15° to 20° to move the item of mail from one conveyor path to another conveyor path.

The angle of rotation through which the diverter gate is moved is limited by rubber stop bumpers. The rubber stop bumpers are mounted external to the rotary actuator so as to allow precise adjustment and to minimize impact noise by the diverter gate. At the end of its operating stroke, the diverter gate may tend to rebound as it impacts against one of the rubber bumpers.

If the diverter gate can rebound back into the previous conveyor flow path, a missortment or jam may occur. To prevent a missortment or jam from occurring, the flow rate of mail or other items must be decreased to give time for the diverter gate to return to its fully actuated position. Alternatively, the rate of operation of the rotary actuator must be decreased to reduce the kinetic energy of the rotary actuator and diverter gate at an end of stroke position. Of course, both of these solutions to the problem of diverter gate rebound are counter to rapid sorting requirements.

The rotary actuator for the diverter gate must provide for both rapid movement of the diverter gate from an unactuated position to an actuated position and holding of the diverter gate at its actuated position upon impact of the diverter gate against a rubber bumper. In order to provide both functions adequately, the starting torque of the rotary actuator must be high to provide a high diverter gate acceleration rate. The ending torque of the rotary actuator must be high to counteract the rebound energy imparted by the rubber bumper to the diverter gate.

Known rotary actuators have previously utilized either one of two basic design approaches. The first basic design approach utilizes a pole configuration termed as “constant air gap” for the rotor and stator pole pieces. The second basic design approach utilizes a pole configuration termed as “diminishing air gap” for the rotor and stator pole pieces. The air gaps are the working air gaps across which magnetic flux is conducted between the rotor and stator pole pieces.

The “constant air gap” rotary actuator design is characterized by a high starting torque that decreases to a lower torque as the rotary actuator operates through its operating stroke (it being assumed that a constant current is applied to the coil of the rotary actuator). The high starting torque occurs when lobes of the rotor are only partially overlapping, or aligned with, corresponding stator lobes. Typically, there is a 3° overlap of the rotor lobes and stator lobes at the initial starting position of the rotor.

The maximum torque for the “constant air gap” rotary actuator design occurs between the initial position and an overlap position of about 10°. The torque then steadily drops off for the remainder of the stroke. For a rapid response, a high starting torque is essential to overcome inertia of components of the rotary actuator and diverter gate. However, a rotary actuator of the “constant air gap” design has a relatively low torque at the end of its operating stroke. This relatively low torque is insufficient to prevent rebound of a diverter gate upon impacting of the diverter gate against a rubber bumper.

The “diminishing air gap” rotary actuator design is characterized by a relatively low starting torque due to large initial air gaps between the rotor and stator pole pieces at the beginning of the operating stroke of the rotary actuator. As the rotor rotates, the air gaps decrease and the torque steadily rises toward a high ending torque. Therefore, for a given power level and loading conditions, the rotary actuators having a “constant air gap” design will produce a higher starting torque than the rotary actuators having a “diminishing air gap” design. However, the “diminishing air gap” rotary actuator design will have a higher end of stroke torque. Although the “diminishing air gap” rotary actuator design has potential to have a relatively high ending torque, small variations in the final position of the diminishing air gaps, being in a series magnetic circuit arrangement, can result in a large variation in the end of stroke torque of the “diminishing air gap” rotary actuator design.

In the foregoing discussion of the background of the present invention, the rotary actuators have been considered in association with a diverter for mail or other items that are traveling along a conveyor. It should be understood that rotary actuators have and, in all probability, will be used in many different environments. For example, rotary actuators have previously been utilized to actuate valves which control fluid flow.

SUMMARY OF THE INVENTION

The present invention provides a new and improved rotary: actuator having a larger beginning of operating stroke torque than is achieved with a corresponding “diminishing air gap” rotary actuator design and a larger ending of operating stroke torque than is achieved with a corresponding “constant air gap” rotary actuator design. This is accomplished by utilizing features of both the “constant air gap” rotary actuator design and the “diminishing air gap” rotary actuator design. Although it is preferred to utilize the improved rotary actuator of the present invention in association with a diverter for mail or other items, it is contemplated that the improved rotary actuator may be utilized in many different environments in association with many different types of devices.

A rotary actuator constructed in accordance with the present invention includes a rotor which is disposed between pole pieces of a stator. The rotor is rotatable relative to the stator between an unactuated position and an actuated position.

A first stator surface on a first pole piece of the stator faces toward and is spaced from a first rotor surface on the rotor by a first working air gap. The first stator surface and the first rotor surface are spaced apart by the same distance when the rotor is in the unactuated position as when the rotor is in the actuated position. Therefore, the axial extent of the working air gap between the first stator surface and the first rotor surface remains constant during rotation of the rotor between the unactuated and actuated positions.

A second stator surface on a second pole piece of the stator faces toward and is spaced from a second rotor surface on the rotor by a second working air gap. The second stator surface and the second rotor surface are spaced apart by a smaller distance when the rotor is in the actuated position than when the rotor is in the unactuated position. Therefore, the axial extent of the second working air gap decreases during rotation of the rotor between the unactuated and actuated position.

In a further embodiment of the present invention, the configuration of the stator lobes are such that they do not generate any substantial axial force on the rotor shaft. Thus, in one force-balanced embodiment, on a first lobe, the upper surface has a variable spacing configuration and the lower surface has a fixed spacing configuration, and a rotationally adjacent lobe has the lower surface with a variable spacing configuration and the upper surface with a fixed spacing configuration, with the lob configuration alternating around the rotor. In a preferred force balanced embodiment, each lobe is symmetrical about its plane of rotation between the actuated and unactuated positions although lobe configuration may differ between different lobes. At least one lobe has a variable spacing configuration on its upper and lower surfaces and at least one lobe has a constant spacing configuration on its upper and lower surfaces.

DETAILED DESCRIPTION OF THE INVENTION

Diverter Assembly

A diverter assembly10is illustrated inFIGS. 1 and 2. The diverter assembly10is adapted to be utilized in conjunction with a conveyor which moves articles, such as mail. The diverter assembly10includes a gate12which is rotatable relative to a base by an improved rotary actuator16constructed in accordance with the present invention.

The rotary actuator16is operable to pivot the gate12in a clockwise direction, as viewed inFIG. 1, about a central axis18(FIG. 2) of the rotary actuator. The rotary actuator16has an output shaft20which extends in opposite directions from the rotary actuator16and is fixedly connected with the gate12. A helical coil biasing spring24is effective to urge the gate12and the rotary actuator output shaft20to an unactuated position when the rotary actuator16is in a de-energized condition. Electrical energy is conducted to the rotary actuator16through electrical conductor28(FIGS.1and2).

When the rotary actuator16is in a de-energized condition, the biasing spring24is effective to firmly press the gate12against a rubber stop bumper (not shown). By adjusting the position of the rubber stop bumper, the unactuated position of the gate12can be accurately adjusted relative to a conveyor conducting mail or other articles at a high speed.

When the diverter assembly10is to be operated to divert one or more articles, such as mail, from one conveyor path to another conveyor path, the rotary actuator16is operated from the unactuated condition to the actuated condition under the influence of electrical energy conducted through the conductor28. As this occurs, the gate12pivots through approximately twenty degrees in a clockwise direction (as viewed inFIG. 1) about the axis18(FIG. 2A). The biasing spring24is resiliently extended as the gate12is pivoted by the rotary actuator.

When the gate12is moved to its operated or divert position, the gate engages a second rubber bumper to limit movement of the gate12relative to the base14. As long as the rotary actuator16remains energized, the output shaft20of the rotary actuator16remains stationary and the gate12remains in its divert position against the second rubber stop.

When the rotary actuator16is de-energized, the biasing spring24immediately pulls the rotary gate from its divert position back to the initial position illustrated in FIG.1. As this occurs, the gate24moves out of engagement with the second rubber bumper and moves into engagement with the first rubber bumper. The biasing spring24is effective to hold the gate12in engagement with the first rubber bumper as long as the rotary actuator16is in a de-energized condition. Although the illustrated biasing spring24is a helical coil biasing spring, a different type of biasing support could be utilized if desired. For example, a spiral spring could be utilized. Alternatively, a leaf spring could be utilized.

Mail or other articles being conducted by the conveyor are moving at a relatively high speed. Therefore, the gate12must be quickly pivoted from its initial position to its divert position by operation of the rotary actuator16. When the gate12reaches its divert position, the torque transmitted from the rotary actuator16to the gate12must be sufficient to prevent rebound of the gate12from the rubber bumper back toward its initial position.

The torque output of the rotary actuator16to the gate12must be relatively high when the rotary actuator is initially energized. This relatively high initial torque is required in order to overcome the inertia of components of the rotary actuator16and the gate12. The gate12must be moved quickly to its actuated or divert position in order to properly sort the mail and to prevent jamming of the flow of mail.

When the gate12reaches its actuated or divert position, the torque output from the rotary actuator16must be sufficient to prevent rebounding of the gate12. To do this, the end of operating stroke output torque from the rotary actuator16must be sufficient to offset the kinetic energy absorbed and stored as potential energy by the rubber bumper. Thus, the rotary actuator16must have both a relatively high initial output torque and a relatively high end of stroke output torque in order to effect the desired movement of the gate12.

In the foregoing description, the rotary actuator16has been described as being utilized in association with a gate12which rotates through approximately twenty degrees to divert articles being moved by a conveyor. It is contemplated that the rotary actuator16will be utilized in many different environments in association with many different devices other than diverter assemblies. It is contemplated that the rotary actuator16may be operated through an operating stroke which is either greater than or less than twenty degrees.

In the illustrated embodiments of the invention, it is preferred to utilize the biasing spring24to move the gate12from its divert position back to its initial position. However, the gate12could be moved back to its initial position in other ways if desired. For example, a second rotary actuator17could be connected with the gate12as shown inFIG. 2B.

The rotary actuator16(FIG. 3) includes a cylindrical housing34which is formed of a magnetically conductive material. A cylindrical coil36is disposed within the housing34. A magnetizable stator40is fixedly connected with the housing34. The stator40includes an upper pole piece42and a lower pole piece44. A magnetizable rotor48is disposed between the upper and lower pole pieces42and44.

Upon electrical energization of the coil36, the stator40and rotor48are both magnetized by magnetic flux emanating from the coil. The magnetic flux from the coil36is effective to cause the rotor48and output shaft20to be rotated in the direction of the arrows52relative to the stator40and housing34. As this occurs, the rotor48rotates through an operating stroke of approximately twenty degrees relative to the stator40. Rotation of the rotor48causes the output shaft20to rotate in a clockwise direction (as viewed in FIG.3). This rotates the gate12(FIG. 1) through approximately twenty degrees, from its initial position to its divert position.

In the specific embodiment of the rotary actuator16illustrated inFIG. 3, the rotor48rotates through an operating stroke of approximately twenty degrees. It should be understood that the rotary actuator16may be constructed so as to have the rotor48rotate through an operating stroke which is either greater than or less than twenty degrees. The configurations of the stator40and rotor48could be reversed from the illustrated configurations to reverse the direction of actuation of the rotary actuator16.

The housing34is formed of a magnetizable material, such as iron. This enables the housing to conduct magnetic flux emanating from the coil36. The housing34includes a cylindrical side wall56(FIG.3). Circular upper and lower end plates58and60are fixedly connected with the side wall56.

The side wall56and end plates58and60are formed of a magnetic flux conducting material, such as iron. The coil36is disposed within the housing34and has a cylindrical configuration. The cylindrical coil36is disposed in engagement with the side wall56and the end walls58and60of the housing. The cylindrical coil36has a central axis which is coincident with the central axis18of the output shaft20. The coil36extends around and encloses the stator40and the rotor48.

Upon energization of the coil36, the coil generates an elongated toroidal electromagnetic flux field which extends upward through the side wall56to the upper end plate58of the housing34. The magnetic flux flows downward from the upper end plate58of the housing to the upper pole piece42. The magnetic flux then flows through an upper working air gap64to the rotor48. The magnetic flux then flows from the rotor48through a lower working air gap66to the lower pole piece44. The magnetic flux flows from the lower pole piece44through the lower end plate60to the side wall56of the housing34to complete the circuitous flux flow path.

The magnetic flux from the coil36magnetizes the upper pole piece44. A south pole of the upper pole piece42is adjacent to the upper end plate58of the housing34and a north pole of the upper pole piece is adjacent to the upper working air gap64. The magnetic flux from the coil36magnetizes the rotor48. A south pole of the rotor48is adjacent to the upper working air gap64and a north pole of the rotor is adjacent to the lower working air gap66. The magnetic flux from the coil36magnetizes the lower pole piece44. A south pole of the lower pole piece44is adjacent to the lower working air gap66and a north pole of the lower pole piece44is adjacent to the lower end plate60. Of course, the foregoing polarities would be reversed if the direction of flow of current through the coil was reversed.

The output shaft20and rotor48are held against movement along the central axis18of the output shaft. However, the rotor48and output shaft20are freely rotatable about the axis18. The rotor48is held against axial movement by means of retaining rings that bear against the inner races of shaft bearings.

The retaining rings maintain a preset clearance for the air gaps described previously. Alternative methods for axial retainment include interference fitting of the shaft to the bearings, bonding the shaft to the inner race of the bearings, or other additional components internal to the actuator. However, the rotor48is rotatable under the influence of the magnetic flux conducted across the upper and lower working air gaps64and66.

The output shaft20and rotor48are rotated together in the direction of the arrows52inFIG. 3under the influence of the magnetic flux conducted across the upper and lower working air gaps64and66between the rotor and stator pole pieces42and44. The rotor48and stator pole pieces42and44are formed of magnetizable material, such as iron. The output shaft20is formed of a nonmagnetic material, such as aluminum or300series stainless steel, to minimize magnetic flux losses.

In the illustrated embodiment of the invention, the rotor48is formed with three identical lobes or arms70which are fixedly connected with and extend radially outwardly from the output shaft20. Each of the lobes70of the rotor48has a south pole adjacent to the upper (as viewed inFIG. 3) working air gap64and a north pole adjacent to the lower working air gap66when the coil36is energized. Of course, the polarities of the lobes70of the rotor48would be reversed if the direction of flow of the current through the coil36was reverse.

The lobes or arms70of the rotor48are integrally formed as one piece. The lobes or arms70are equally spaced from each other in a circular array about the output shaft20. The lobes70of the rotor48are interconnected by a hub which is integrally formed as one piece with the lobes and is fixedly connected to the output shaft20.

The upper pole piece42is integrally formed as one piece and includes three identical lobes or sections74which extend axially downward (as viewed inFIG. 3) from the upper end plate58toward the rotor48. The lobes or sections74of the upper pole piece42are integrally formed as one piece with a cylindrical base76of the upper pole piece. The base76of the upper pole piece has a cylindrical opening through which the output shaft20extends. The base76of the upper pole piece42has a central axis which is coincident with the axis18of the output shaft20. The three identical lobes or sections74of the upper pole piece42are equally spaced apart in a circular array about the axis18.

The lower pole piece44of the stator40has three identical lobes or sections80which are integrally formed as one piece with a cylindrical base82of the lower pole piece44. The base82of the lower pole piece44has a cylindrical configuration and is disposed in a coaxial relationship with the output shaft20. The lower pole piece44has a cylindrical central opening through which the output shaft20extends. The lobes or sections80of the lower pole piece44are spaced equal distances apart in a circular array about the central axis18. The lobes or sections80on the lower pole piece44are axially aligned with the lobes or sections74on the upper pole piece42.

The rotor48and output shaft20are held against movement along the axis18by suitable bearings (not shown) connected with the end plates58and60of the housing34. However, the rotor48and the output shaft20are freely rotatable about the axis18. Therefore, when the coil36is energized and the upper and lower pole pieces42and44of the stator40are magnetized, the rotor48can rotate relative to the stator40. Regardless of the direction in which current is conducted through the coil36, the resulting magnetic field effects rotation of the rotor48in the direction of the arrows52in FIG.3.

The rotor48and output shaft are rotated in the direction opposite to the arrows52under the influence of the biasing spring42(FIGS. 1 and 2) when the coil36is de-energized. The biasing spring24may be enclosed within the housing34. Whether the biasing spring24is inside or outside of the housing34, the biasing spring may have a construction other than the illustrated helical construction. In order to minimize cost, it is preferred to utilize just the biasing spring24to reverse the rotation of the rotor48.

Working Air Gaps

In accordance with one of the features of the present invention, the axial extent of the upper working air gap64decreases in size and the axial extent of the lower working air gap66remains constant in size during rotation of the rotor48in the direction indicated by arrows52in FIG.5. Thus, when the rotor48is in the initial or unactuated position ofFIG. 4, the axial extent of the upper working air gap64is relatively large. Upon rotation of the rotor48to the actuated position ofFIG. 5, the axial extent of the upper working air gap64is relatively small. During rotation of the rotor48from the unactuated position ofFIG. 4to the actuated position ofFIG. 5, the axial extent of the lower working air gap66remains constant. The combination of the diminishing upper working air gap64and constant lower working air gap66results in the rotary actuator16having a relatively high starting or initial torque, compared to a “diminishing air gap” type of rotary actuator, and a relatively large ending torque, compared to a “constant air gap” type rotary actuator.

The upper working air gap64is the space across which magnetic flux is conducted from the upper pole piece42to the rotor48to effect rotation of the rotor in the direction of the arrow52in FIG.4. Similarly, the lower working air gap66is the space across which magnetic flux is conducted between the rotor48and lower pole piece44to effect rotation of the rotor in the direction of the arrow52in FIG.4.

In order to optimize the operating characteristics of the rotary actuator16, the axial extent of the upper working air gap64diminishes in size as the rotor48moves from the unactuated position illustrated inFIG. 4to the actuated position illustrated in FIG.5. This results in the rotary actuator16having a relatively high end of the stroke torque to hold the rotor48against rebound when the rotary actuator16is operated to its actuated condition. The axial extent of the lower working air gap66remains constant as the rotor48moves from the unactuated position ofFIG. 4to the actuated position of FIG.5. This results in the rotary actuator16having a relatively large initial torque to overcome inertia of components of the rotary actuator and any devices connected with the rotary actuator. The upper working air gap64(FIG. 4) is formed between downwardly facing side surface90on the lobe or section74of the upper pole piece42and an upwardly facing side surface92on the lob or arm70of the rotor48. It should be understood that working air gaps, corresponding to working air gap64, are formed between each of the lobes or arms70on the rotor48and each of the lobes or sections74on the upper pole piece42.

The downwardly facing side surface90on the lobe or section74of the upper pole piece42is skewed at an acute angle to a plane extending perpendicular to the coincident central axes of the upper pole piece42and rotor48. Similarly, the upwardly facing side surface92on the lobe or arm70of the rotor48is skewed relative to the plane extending perpendicular to the coincident central axes of the upper pole piece42and rotor48. The downwardly facing stator side surface90and upwardly facing rotor side surface92extend parallel to each other when the rotor48is in the actuated position of FIG.5.

The upper working air gap64diminishes from the relatively large axial extent illustrated inFIG. 4to the relatively small axial extent illustrated in FIG.5. Although the upper working air gap64diminishes in size, the surfaces90and92do not engage each other when the rotor48is in the actuated position ofFIG. 5due to the external rubber bumpers. Thus, there is a relatively large distance between the upwardly facing side surface92on the rotor48and the downwardly facing rotor side surface90, as measured perpendicular to the parallel side surfaces, when the rotor is in the unactuated position of FIG.4. When the rotor48is in the actuated position ofFIG. 5, there is a relatively small distance between the rotor side surface92and stator side surface90, as measured perpendicular to the side surfaces. The diminishing size of the upper working air gap64as the rotor48moves from the unactuated position ofFIG. 4to the actuated position ofFIG. 5increases the magnitude of the magnetic flux force urging the rotor48to rotate in the direction of the arrows52as the upper working air gap64diminishes.

The downwardly facing side surface90(FIG. 4) on the lobe or section74of the upper pole piece42has an arcuate configuration. In the illustrated embodiment of the invention, the surface90is formed as a portion of a helix which extends around the central axis18(FIG. 3) of the rotary actuator16. Similarly, the upwardly facing side surface92(FIG. 4) on the lobe or arm70of the rotor48has an arcuate configuration. In the illustrated embodiment of the invention, the surface92is formed as a portion of a helix which extends around the central axis18(FIG. 3) of the rotary actuator16. The surfaces90and92(FIG. 4) have the same configuration and are parallel to each other when the rotor48is in the actuated position of FIG.5.

The downwardly facing side surface90on the lobe or section74of the upper pole piece42and the upwardly facing side surface92on the lobe or arm70of the rotor48may have a configuration which is different than the configuration illustrated in FIG.4. For example, the surfaces90and92may be flat parallel surfaces.

The lower working air gap66(FIG. 4) is formed between a downwardly facing side surface96on the rotor48and an upwardly facing side surface98on the lobe or section80of the lower pole piece44. The downwardly facing rotor side surface96and upwardly facing stator side surface98are disposed in parallel planes which extend perpendicular to the central axis18of the rotor48.

When the rotor48is in the unsaturated position of FIG.4and when the rotor48is in the actuated position ofFIG. 5, the distance between the rotor side surface96and the stator side surface98, as measured perpendicular to the rotor and stator side surfaces, is the same. Therefore, as the rotor48moves from the unactuated position to the actuated position, the size of the lower working gap66remains constant, as measured in a direction perpendicular to the rotor side surface96and stator side surface98.

When the rotor48is in the unsaturated position ofFIG. 4, the distance between the downwardly facing side surface90on the stator lobe or section74and the upwardly facing side surface92on the rotor48, as measured perpendicular to these surfaces, is greater than the distance between the downwardly facing side surface96on the rotor and the upwardly facing side surface98on the stator lobe or section80. As the rotor48moves from the unactuated position ofFIG. 4to the actuated position ofFIG. 5, the distance between the stator side surface90and rotor side surface92across the upper working air gap64decreases. However, the distance between the downwardly facing rotor side surface96and the upwardly facing stator side surface98across the lower working air gap66remains constant. This results in the rotary actuator16having operating characteristics which are a combination of the operating characteristics of a “constant air gap” design rotary actuator and a “diminishing air gap” design rotary actuator.

The lobe or arm70of the rotor48has an arcuate outer side surface104. The arcuate outer side surface104is formed as a portion of a cylinder having a central axis coincident with the central axis18of the rotary actuator16. Similarly, the upper and lower pole pieces42and44of the stator40have arcuate outer side surfaces106and108. The stator pole piece side surfaces106and108are formed as a portion of a cylinder. The diameter of the stator pole piece surfaces106and108may be slightly greater than the diameter of the rotor side surface104to provide clearance between the rotor48and the coil36.

The upwardly facing side surface92on the lobe or arm70of the rotor48slopes upward (as viewed inFIG. 4) toward the upper pole piece42. This results in the lobe or arm70of the rotor48having a wedge or ramp-shaped configuration. The lobe or arm70of the rotor has a rectangular leading end surface100which is smaller than a rectangular trailing end surface102.

The downwardly facing side surface96on the lobe or arm70extends perpendicular to the leading and trailing end surfaces100and102. The leading and trailing end surfaces100and102are skewed relative to each other in a direction toward the central axis of the rotor48.

The manner in which the output torque of the rotary actuator16varies with movement of the rotor48through an operating stroke from the unactuated position ofFIG. 4to the actuated position ofFIG. 5is illustrated by a solid line curve110in FIG.6. The manner in which the output torque of a “constant air gap” design rotary actuator varies with an operating stroke of the rotor is indicated by a large dash curve designated112in FIG.6. The manner in which the output torque of a “diminishing air gap” design rotary actuator changes during an operating stroke is indicated by a small dash curve114in FIG.6.

It is desirable to have a relatively large output torque from the rotary actuator16at the beginning of its operating stroke and at the end of its operating stroke. FromFIG. 6, it is apparent that the initial output torque of the rotary actuator16, as indicated by the curve110, is greater than the initial output torque of the “diminishing air gap” type rotary actuator, as indicated by the curve114. However, the initial torque of the rotary actuator16is less than the initial torque of the “constant air gap” type rotary actuator, as indicated by the curve112in FIG.6. This will result in the rotary actuator16being better able to overcome the inertia of components of the rotary actuator and devices connected with the rotary actuator16at the beginning of an operating stroke than with the “diminishing air gap” type rotary actuator.

The torque of the rotary actuator16at the end of its operating stroke, indicated by the curve110inFIG. 6, is substantially grater than the end of the operating stroke torque of the “constant air gap” rotary actuator having the characteristics indicated by the curve112in FIG.6. The rotary actuator16may also have an end of operating stroke torque which is less than the end of stroke torque of the “diminishing air gap” rotary actuator. However, the end of operating stroke torque of the “diminishing air gap” design may not exceed the end of operating stroke torque of the actuator16by as much as is indicated by the curves110and114in FIG.6. This is because the end of stroke position of the rotary actuator16can be more accurately adjusted than the end of stroke position of known “diminishing air gap” rotary actuators.

The known “diminishing air gap” rotary actuators have rotors with sloping or ramped-shaped surfaces on both sides of the rotor. This makes adjustment of the end of stroke position more difficult than with the rotary actuator16. This is because there are two air gaps, corresponding to the air gaps64inFIG. 4, which diminish in size as the rotor rotates through its operating stroke. By having one of the air gaps of constant size, this is, the lower working air gap66, the rotary actuator16can be more easily adjusted than a rotary actuator having two diminishing size air gaps.

In the embodiment of the invention illustrated inFIGS. 1-5, the rotor has three lobes or arms70which extend radially outward from the rotor. In the embodiment of the rotor illustrated inFIG. 7, the rotor has four arms or lobes. Since the embodiment of the rotor illustrated inFIG. 7is generally similar to the embodiment of the rotor illustrated inFIGS. 1-5, similar numerals will be utilized to designate similar components, the suffix letter “a” being associated with the numerals ofFIG. 7to avoid confusion.

The rotor48a is provided with four lobes or arms70a. Each of the lobes or arms70a has the same construction and is connected with a central hub130. The four lobes or arms70a and hub130are integrally formed as one piece of magnetizable material.

The rotor arm70a has a leading end surface100a with a flat rectangular configuration and a trailing end surface102a with a flat rectangular configuration. In addition, the rotor arm70a has an upwardly facing side surface92a with an arcuate configuration. The rotor arm70a has a flat downwardly facing side surface96a.

The rotor48a cooperates with a stator, corresponding to the stator40of FIG.4. However, the stator with which the rotor48a cooperates has upper and lower pole pieces with four lobes or sections rather than three lobes or sections. Thus, the rotor48a cooperates with an upper pole piece having four lobes with downwardly facing arcuate side surfaces, corresponding to the downwardly facing side surfaces90on the lobes or sections74of the upper pole piece42of FIG.4. Similarly, the stator which cooperates with the rotor48a has a lower pole piece, corresponding to the pole piece44ofFIG. 4, with four lobes or sections with flat upwardly facing side surfaces, corresponding to the side surface98of FIG.4.

In addition to changing the number of lobes of the rotor and stator combination, there are further variations on the hybrid rotary actuator that are significant. It will be appreciated that the hybrid rotary actuators shown inFIGS. 3 & 7have a horizontal pole and rotor surfaces which are closely spaced and therefore provide a low reluctance airgap as compared to the angled or helical pole and rotor surfaces which have a high reluctance airgap, at least at the beginning of the stroke. This variation in reluctance at the airgaps provides a variation in force (not only in amplitude of force but the application direction of the force) which in turn is applied to the shaft of the rotor. To the extent that the axial components of the forces applied to the shaft are not equal, they will generate a net axial component.

The shaft, as noted above, is constrained against any axial movement but free for rotational movement between the actuated and unactuated positions. However, the net force difference would tend to try to move the shaft axially as well, providing an additional load on the actuator bearings and possibly resulting in undue wear on the actuator bearings.

FIG. 8illustrates a force-balanced hybrid rotary actuator which does not present force differences on the rotor shaft. Each of the lobes of the rotor generates a symmetrical rotational force (while the rotational force components add, the axial components cancel each other) having little or no axial component. As a result of all rotor lobes being essentially force balanced, there is primarily only a rotational force applied to the shaft which eliminates or at least substantially reduces axial load wear on the actuator shaft bearings.

FIGS. 9a through 9ccompare the rotor/stator configuration of non-force balanced hybrid rotary actuator ofFIGS. 3 and 7(shown inFIG. 9a) to force-balanced hybrid rotary actuators (FIG. 9bcorresponds to the rotor/stator configuration of FIG.8andFIG. 9cis a further embodiment). If the force-balanced hybrid shown inFIGS. 9a through 9chave a similar number of angled or helical pole surfaces, they will have similar torque/stroke curves (regardless of whether they are force balanced or unbalanced. It can be seen that all three embodiments have the same number of angled or helical surfaces and the same number of flat surfaces. Thus the curve110for a hybrid would be applicable for either the force balanced (FIGS. 9b & 9c) or force unbalanced (FIG. 9a) hybrids.

As will be apparent, to obtain the benefit of the force-balanced rotor concept, one must only insure that, after the force components for all of the rotor lobes are summed, the net resultant force in the axial direction is as low as possible. The desired zero net axial force components is achieved in theFIG. 8embodiment by having rotor lobes (and adjacent stator pole pieces) which alternate between those having surfaces parallel to the direction of rotation and those having surfaces inclined to the direction of rotation.

However, if the desired stroke/torque characteristics of the force balanced hybrid are desired to be closer to those of the “constant airgap” rotary actuator (as shown inFIG. 6, i.e., with higher starting torque with lower ending torque), more parallel surface lobes and less inclined surface lobes could be employed. Similarly, more inclined surface lobes and less parallel surface lobes could be used if a lower starting torque and higher ending torque similar to the “diminished airgap” rotary actuator were desired.

In order to substantially balance the axial forces, as long as the same number of inclined surfaces are used on the top and bottom of the rotor, the axial force components will effectively cancel each other resulting in a force-balanced rotor. It will be seen that each of the configurations shown inFIGS. 9b and 9cmeet these requirement. Additionally, combinations of the stator and rotor components shown inFIGS. 9b and 9ccould be combined to provide force balanced rotors with different starting and ending torque effects.

Conclusion

In view of the foregoing description, it is clear that a: rotary actuator constructed in accordance with the present invention has a larger beginning of operating stroke torque than is achieved with a corresponding “diminishing air gap” rotary actuator design and a larger ending of operating stroke torque than is achieved with a corresponding “constant air gap” rotary actuator design. This is accomplished by utilizing features of both the “constant air gap” rotary actuator design and the “diminishing air gap” rotary actuator design. Although it is preferred to utilize the improved rotary actuator16of the present invention in association with a diverter12for mail or other items, it is contemplated that the improved rotary actuator may be utilized in many different environments in association with many different types of devices.

A rotary actuator16constructed in accordance with the present invention includes a rotor48which is disposed between pole pieces42and44of a stator40. The rotor48is rotatable relative to the stator40between an unactuated position (FIG. 4) and an actuated position (FIG.5).

A first stator surface98on a first pole piece44of the stator40faces toward and is spaced from a first rotor surface96on the rotor48by a first working air gap66. The first stator surface98and the first rotor surface96are spaced apart by the same distance when the rotor is in the unactuated position (FIG. 4) as when the rotor is in the actuated position (FIG.5). Therefore, the axial extent of the working air gap66between the first stator surface98and the first rotor surface96remains constant during rotation of the rotor48between the unactuated and actuated positions.

A second stator surface90on a second pole piece42of the stator40faces toward and is spaced from a second rotor surface92on the rotor48by a second working air gap64. The second stator surface90and the second rotor surface92are spaced apart by a smaller distance when the rotor48is in the actuated position (FIG. 5) than when the rotor is in the unactuated position (FIG.4). Therefore, the axial extent of the second working air gap64decreases during rotation of the rotor48between the unactuated and actuated positions.

In order to reduce wear on the actuator rotor mounting bearings, various rotor designs (symmetric or asymmetric) resulting in reduced or eliminated axial force components could be used, such as the force balanced rotors described above.

In view of the above description of the invention, those having ordinary skill in the art will appreciate that many improvements, changes and modifications to the hybrid rotary actuator are possible. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims and thus, the present invention is limited only by the appended claims.