LIMITED ANGLE TORQUE MOTOR WITH HIGH STIFFNESS AND NATURAL FREQUENCY

Abstract: Limited angle torque motor (1) includes a motor power section (2) comprising a rotor assembly (3) having one or more permanent rotor magnets (5) which are magnetized in a diametrical direction and a stator assembly (10) surround­ ing the rotor assembly. The magnetic flux in the motor power section (2) produces an inherent magnetic centering spring rate tending to keep the rotor assembly (3) at the midpoint of its rotational angle range. However, when a direct or pulse width modulated current is applied to the stator windings (14), a cross flux is produced through the stator and rotor assemblies of the motor power section which produces the desired limited rotational angle range for the rotor assembly. The motor also includes a separate magnetic centering spring section (30) which provides another centering spring function in addition to the basic centering spring function inherent in the motor power section (2). The magnetic centering spring section (30) is located immediately adjacent to the motor power section (2), and the permanent rotor magnet (5) of the rotor assembly (3) extends into the centering spring section (30), such centering spring section additionally including two or more permanent magnets (32, 33) with polarities in alignment with respect to the rotor magnet (5).

Description: This invention relates generally as indicated to a limited angle torque motor with high stiffness and natural frequency. The motor of the present invention is particularly suited for use in certain types of high pressure fluid proportional servo control systems including, but not limited to, aircraft controls to drive a proportional control valve of relatively short stroke. The fluid pressure is normally on the order of 1,000 psi or more. In such a motor, the magnetic reluctance torque of the motor can be controlled within certain limits to obtain a particular desired inherent magnetic centering spring rate at a particular torque or power level. However, such a motor has a somewhat limited magnetic spring capability inherent in the design, and there are some applications in which it is desirable for the motor to have a higher centering spring gradient to increase the dynamic stiffness of a servo system, e.g. where the motor is being used to drive a load having substantial and varying force levels. Such a motor also has a torsional undamped natural frequency which is generally related to the magnetic spring rate through the center position and rotational inertia of the rotor. Typically such natural frequencies have been limited to about 60 Hertz (Hz) for motors of practical design. However, for certain high response servo system applications, it is desirable to have a substantially higher motor natural frequency, on the order of 75 to 100 Hz or more. According to the present invention, there is provided a limited angle torque motor comprising a motor power section including a rotor assembly having a permanent rotor magnet magnetized in the diametrical direction and a stator assembly surrounding said rotor assembly, characterized by a separate magnetic centering spring section axially adjacent said motor power section, said centering spring section comprising an outer magnetic ring at one end of said stator assembly, said rotor assembly including said rotor magnet extending coaxially into said outer ring, and additional permanent magnets carried by said outer ring on diametrically opposite sides of the extended portion of said rotor assembly, the polarities of said additional permanent magnets being aligned with respect to said rotor magnet. Thus, by using the present invention, there may be constructed a limited angle torque motor with a relatively high spring gradient to increase the undamped natural frequency of the motor and/or provide high system stiffness when coupled to a servo system having significant and varying load forces. The magnetic flux in the motor power section produces an inherent centering spring rate tending to keep the rotor assembly at the midpoint of its rotational angle range. However, when a direct or pulse width modulated current is applied to the stator windings, a cross flux is produced through the stator and rotor assemblies of the motor power section which produces the desired limited rotational angle range for the rotor assembly. The magnetic centering spring section, which is separate from the motor power section, provides another centering spring function in addition to the basic centering spring function inherent in the motor power section. Non-limiting embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a fragmentary longitudinal section through one form of limited angle torque motor in accordance with the present invention; Figure 2 is a fragmentary transverse section through the motor power section of the motor of Figure 1 taken substantially along the plane of the line 2-2 thereof; Figure 3 is a transverse section through the magnetic centering spring section of the motor of Figure 1 taken substantially along the plane of the line 3-3 thereof; Figure 4 is a schematic diagram showing the rotor magnetic pull both of the magnetic centering spring section by itself and when combined with the inherent magnetic spring capability of the motor power section to increase the net centering gradient of the motor rotor; Figure 5 is a graphic illustration showing the motor shaft position for different levels of applied direct or pulse width modulated current for a representative motor constructed in accordance with this invention; and Figures 6 and 7 are transverse sections similar to Figure 3 but showing alternate forms of magnetic centering spring sections for use in place of the centering spring section of Figure 1. Referring now in detail to the drawings, and initially to Figures 1 and 2, there is shown a preferred form of limited angle torque motor 1 in accordance with this invention. Such motor is primarily intended for use in high pressure fluid systems for directly driving a valve in proportional servo control systems including, but not limited to, aircraft controls. However, it should be understood that such motor could also be used for driving secondary valves as well as other mechanical devices. The motor is a non-commutated two-pole stationary coil rotary magnet motor having a motor power section 2 including a rotor assembly 3 having a rotor shaft 4 with one or more permanent magnets 5 supported for rotation therewith. The rotor magnet or magnets 5 are rare earth magnets made, for example, of samarium cobalt, and are magnetized in the diametrical direction, that is, the North and South poles N and S of the rotor magnets are diametrically oriented as shown in Figure 2. The rotor shaft 4 is journal mounted in suitable bearings within a rotor casing 6. At the outer end of the rotor shaft is an eccentric or other suitable form of rotary to linear gearing 7 that may be used to translate the rotary movement of the rotor shaft to linear movement of a valve member or other external load through a suitable drive linkage, not shown. Surrounding the rotor assembly is a stator assembly 10 which in turn is contained within an exterior housing 11 having a mounting flange 12 at one end for use in attaching the motor to a valve housing or other suitable support structure. The stator assembly includes one or more high density layer wound stator coil windings 14 which extend around a pair of circumferentially spaced stator magnetic pole structures 15, 16 with the axis of the stator coils being generally parallel to the rotor axis. At the opposite end of each of the stator pole structures is a stator pole flange 17 which extends radially outwardly from the respective stator pole blade 18, whereby when the stator coil windings 14 are excited by a direct or pulse width modulated current, the stator pole flanges will cause a cross or transverse flux through the stator and rotor assemblies which produces a turning torque in the motor for turning the rotor shaft through a limited rotational angle range. A magnetic housing 20 made of a suitable magnetic material such as soft iron surrounds the stator assembly, and the stator magnetic pole flanges 17 extend radially outwardly into contact with the inner surface of the outer magnetic housing which provides a magnetic return path for the magnetic field when the stator coil is excited. Such magnetic return path is partially interrupted to strengthen the centering spring tendency by providing one or more air gaps xx, xx in the outer magnetic housing shown in Figure 2. The stator magnetic pole structures 15, 16 are internally supported by a suitable coil support sleeve comprising the rotor casing 6. Also, at the ends of the stator pole structures opposite the stator pole flanges 17 suitable end plates 21 are provided. Such a motor power section 2 produces an interaction of magnetic fields torque when the stator coil 10 is excited which is proportional to the amount of direct or pulse width modulated current passing through the stator coil. When a positive current is applied, a positive torque tending to rotate the rotor shaft in a clockwise direction as viewed in Figure 2 is produced over a relatively large excursion of the rotor shaft which varies, for example, from a maximum torque output at the null or centered position of the rotor shaft (ϑ = 0°) to approximately 50% of the maximum torque output at ± 60° of rotation of the rotor shaft from such centered or null position. Conversely, when a negative current is applied, a negative torque tending to rotate the rotor shaft in a counterclockwise direction is produced over the same excursion of the rotor shaft. The magnetic flux of the rotor magnet 5 of the motor power section 2 also produces a magnetic reluctance torque providing an inherent magnetic centering spring rate tending to keep the rotor assembly at the midpoint of its rotational angle range. As will be apparent, this inherent magnetic centering spring capability of the motor power section may be effectively utilized to provide a centering function for returning the rotor shaft to the centered or null position whenever the current to the stator coil is interrupted. However, some applications require a higher centering spring gradient than is obtainable with the inherent magnetic centering spring capability of a typical motor power section of the type described, as when it is desired to increase the dynamic stiffness of a servo system so that the torque motor can be used to drive a load having substantial and varying force levels. Also, such a motor power section as described has a torsional undamped natural frequency generally related to the magnetic spring rate through the center position and rotational inertia of the rotor which may not be high enough for certain high response servo system applications. By using the present invention, both the undamped natural frequency and centering spring gradient of the motor are substantially increased by providing the motor with a separate centering spring section 30 adjacent to the power motor section 2. In the motor construction shown in Figures 1 and 3, the centering spring section 30 consists of an outer ring 31 structure of highly permeable soft magnetic iron adjacent one end of the motor power section 2, with the rotor assembly 3 including its rotor magnet 5 extending coaxially into the outer ring 31, and a pair of rare earth magnets 32, 33 attached to the outer ring 31 on diametrically opposite sides thereof with their polarities aligned with respect to the rotor magnet 5 as shown in Figure 3. As the curve A of Figure 4 graphically illustrates, such a magnetic centering spring section 30 provides a strong centering tendency for the rotor shaft 4. Also, as curve B further illustrates, such magnetic spring capability of the magnetic centering spring section 30 combines with the inherent magnetic spring capability of the motor power section 2 to increase the net centering gradient of the motor. The typical rotational characteristic of such a motor with combined power and centering spring sections is further graphically illustrated in Figure 5 in terms of shaft rotation versus applied current. An alternate arrangement of the permanent magnets in the centering spring section and rotor assembly is shown in Figure 6. There the rotor assembly 3' is shown as including a pair of permanent magnets 35, 36 in diametrical alignment with each other on opposite sides of a soft magnetic iron core 37 interposed therebetween. Also, the outer ring 31' of the magnetic centering spring section 30' is shown as having a pair of diametrically opposed extensions or poles 39, 40 extending radially inwardly therefrom to permit the use of shorter magnets 41, 42 in the centering spring section than would otherwise be required. The polarities of all of the magnets 35, 36 and 41, 42 are of course still aligned in substantially the same manner as shown in Figure 3. Figure 7 shows still another form of magnetic centering spring section 45 in which the outer ring 46 is provided with a plurality of circumferentially spaced permanent magnets 47-49 and 50-52 on opposite sides of the rotor magnet 53 with the polarities of the respective permanent magnets aligned as shown to increase the magnetic field intensity at the rotor surface 54, and thus increase the spring gradient of the centering spring section 45. As shown in Figure 7, the permanent magnets in the centering spring section may be supported in closely spaced relation to the inner surface of the outer ring 46 by a pair of magnetic blocks or holders 55, 56 attached to such inner surface on opposite sides thereof. The holders may have notches or slots therein for receipt of the respective magnets. Also, the radial inner ends of the holders may be semi-cylindrical in shape to correspond substantially to the cylindrical outer surface of the rotor assembly and are in closely spaced relation to the outer diameter of the rotor assembly. If desired, a non-magnetic spacer 60 may be inserted between the motor power section 2 and centering spring section 30 as further shown in Figures 1 and 3 to help isolate the sections from magnetic interactions that might otherwise degrade performance.

Claims: A limited angle torque motor (1) comprising a motor power section (2) including a rotor assembly (3) having a permanent rotor magnet (5) magnetized in the diametrical direction and a stator assembly (10) surrounding said rotor assembly, characterized by a separate magnetic centering spring section (30) axially adjacent said motor power section (2), said centering spring section comprising an outer magnetic ring (31) at one end of said stator assembly (10), said rotor assembly (3) including said rotor magnet (5) extending coaxially into said outer ring (31), and additional permanent magnets (32, 33) carried by said outer ring (31) on diametrically opposite sides of the extended portion of said rotor assembly (3), the polarities of said additional permanent magnets (32, 33) being aligned with respect to said rotor magnet (5). The motor of claim 1, wherein said rotor magnet (5) produces a magnetic flux in said motor power section (2) that provides an inherent magnetic centering spring rate tending to keep said rotor assembly (3) at the midpoint of its rotational angle range, and said centering spring section (30) produces an additional centering spring function assisting the inherent magnetic centering spring rate of said motor power section (2) in keeping said rotor assembly (3) at such midpoint of its rotational angle range. The motor of claim 1 or claim 2, wherein said centering spring section (30) includes a plurality of said additional permanent magnets (47, 48, 49 and 50, 51, 52) on each of said diametrically opposite sides of said rotor magnet (5). The motor of any one of claims 1 to 3, wherein said outer ring (31') has a pair of diametrically opposed projections (39, 40) extending radially inwardly from the opposite sides of said outer ring, said projections providing an attachment support for said additional permanent magnets (41, 42). The motor of claims 3 and 4, wherein said projections (55, 56) comprise holders for said additional permanent magnets (47-49 and 50-52). The motor of claim 5, wherein said holders (55, 56) have slots therein for receipt of said additional permanent magnets (47-49 and 50-52). The motor of claim 6, wherein said additional permanent magnets (47-49 and 50-52) are supported by said holders (55, 56) in closely spaced relation to the inner peripheral surface of said outer ring (46), and said holders extend radially inwardly into close proximity to the outer surface of the extended portion of said rotor assembly (3). The motor of claim 7, wherein the radial inner ends of said holders (55, 56) are cylindrically concavely curved to correspond substantially to the cylindrical outer surface of the extended portion of said rotor assembly (3). The motor of any one of claims 1 to 8, further comprising a non-magnetic spacer (60) between said motor power section (2) and centering spring section (30) to help isolate said sections from magnetic interactions that might otherwise degrade performance.

Inventors: KELEMEN, JOSEPH A., VANDERLAAN, ROBERT D.
Applicants: PNEUMO ABEX CORPORATION
Application year: 1985
Main classification: H02K26
IPC codes: H01F7, H02K33, H02K26