FORCE MOTOR

Abstract: A dual working airgap force motor has a centrally located stator (10) includes two toroidally shaped electromagnets (26, 28) with the axis of the stator (10) coinciding with the axis of the toroids. Each toroidal coil (26, 28) is separated from the other toroidal coil (28, 26) by an axial distance. Toroidal permanent magnets (18A, 18B) are also mounted preferably inside the toroidal electromagnet coils (26, 28) and also spaced apart axially. The permanent magnets generate a flux flow in opposite axial directions whereas, upon energisation, the toroidal coils generate flux flow in the same axial direction at a given radial position. Two armatures (14A, 14B) are located on an output shaft (16) at either end of the stator (10) and each armature is spaced apart from the stator by inner and outer axial airgaps (22A, 22B, 24A, 24B). Energisation of the coils (26, 28) with current causes a greater flux flow across the inner and outer airgaps at one end than is caused through the inner and outer airgaps at the other end, thus tending to reduce the airgap at the end with the largest flux flow and consequently causing movement of the respective armatures (14A, 14B) and the output shaft (16) upon which they are mounted.

Description: FORCE MOTOR. The invention relates to force motors, for instance of the type which produces a relatively short displacement which is proportional to a driving current. Solenoids are generally characterised by an actuation direction which does not change with regard to the direction of the energising current. In other words, if a direct current supply has its polarity reversed, the solenoid still provides axial movement in the same direction. Force motors are distinguished from solenoids in that they use a permanent magnet field to prebias the airgap of a solenoid such that movement of the armature of the force motor is dictated by the direction of current in the coil. Reversal of the polarity of current flow will reverse the direction of the force motor armature displacement. Force motors are frequently used to drive a valve spool in a high performance aircraft where efficiencies of weight, size, cost and power consumption are of prime consideration. It is therefore advantageous to minimise losses associated with producing high magnetic forces and to minimise the size of the permanent magnets which normally have densities and relative costs higher than the solenoid iron. Figure 1 in the present application illustrates a conventional force motor with a simplified construction for ease of explanation. A stator 10 includes mounting brackets 12 and an iron core which provides a path for flux travel. An armature 14 is mounted on and moves with output shaft 16. Included in the stator is magnet 18 which generates a flux flow through the stator and the armature as indicated by the solid line arrows 20. This flux from magnet 18 travels in opposite directions across airgaps 22 and 24. Coils 26 and 28 are wound so as to provide flux flow paths indicated by broken line arrows 30 which cross airgaps 22 and 24 in the same direction. Obviously if the current flow in the coils 26 and 28 were reversed, the direction of the coil generated flux flow paths shown by dotted line arrows 30 would be reversed for both airgaps 22 and 24. It is noted that the permanent magnet 18 can be mounted in the stator assembly, as shown, or may be part of the armature. Operation of the prior art force motor provides an output movement by the shaft 16 when current in one direction is supplied to the coils 26 and 28 and movement of the output shaft in the opposite direction when the opposite current flow is supplied to the coils 26 and 28. This movement direction is caused by the fact that, as shown in Figure 1, flux flow generated by the permanent magnet 18 (shown by solid line arrows 20) is in the same direction as coil generated flux flow (indicated by dotted line arrows 30) across the airgap 22 but in an opposite direction across the airgap 24. This causes a greater attraction at the airgap 22 than would exist at the airgap 24 and thus the armature is attracted towards the left hand stator portion moving the output shaft to the left. If the coil generated flux flow were reversed (by winding the coil differently or merely reversing the polarity of the direct current supply), the flux flow would be cumulative across the airgap 24 and differential across the airgap 22 resulting in armature movement to the right and consequent output shaft movement to the right. The airgaps 22 and 24 are designated working airgaps in which the flux passes through an airgap and, as a result, generates an attractive force between the stator and the armature which is in the axial direction. The prior art force motors have an additional airgap 32 which may be characterised as a non-working airgap in that flux flow is in the radial direction and thus, even though there is an attraction between the stator and the armature, this does not result in any increase in force in the axial or operational direction of the force motor. In order to maximise flux flow (minimising airgaps) this dimension is made as small as possible (minimising reluctance of the flux flow path) although a sufficient clearance must be maintained to allow for relative movement between the stator and the armature. It will be further recognised by those familiar with the utilisation of permanent magnets in force motors that the magnet will have a preferred optimum energy product point on its de-magnetisation curve about which the magnet should operate for maximum efficiency. The closer the magnet operates to this point, the smaller the magnet can be. Further, the magnet length, cross sectional area and strength are dictated by the level of flux required to drive through the magnetic circuit to achieve the desired performance of the force motor. Thus, force motors having a high force requirement typically have a low reluctance magnetic path due to the cross sectional area of the iron necessary for producing high forces and a relatively large volume of permanent magnets to produce the necessary airgap flux. Of course, attendant with the desired high flux level of a low reluctance magnetic circuit are losses which may be expressed in ampere-turns in the iron and also in the non-working airgap(s) which further detract from the efficiency of the motor. These losses are accounted for by increases in the electrical power source and/or the requirement of a larger permanent magnet than would otherwise be necessary. According to the invention, there is provided a force motor as defined in the appended Claim 1. Preferred embodiments of the invention are defined in the other appended claims. It is thus possible to provide a force motor whose magnetic circuit reduces or minimises energy losses inherent in prior art force motors. It is also possible to reduce the overall mass of a force motor to less than that of prior art force motors for a given force/displacement requirement. It is further possible to reduce the volume and/or mass of permanent magnet material utilised in a force motor and its associated costs. It is also possible to provide a force motor with a magnetic circuit of relatively higher reluctance but having airgaps only in a direction which contributes to force production, i.e. in the axial direction of the force motor, and to eliminate the need for a non-working airgap. A stator is provided with two axially separated coils mounted therein, and wound in the conventional manner for a force motor. Adjacent either end of the stator are two separate armatures separated from the stator by working airgaps both inside of and outside of the coils, the gaps extending in an axial direction. Permanent magnets are provided to generate a flux flow across the respective working airgaps in opposite directions so as to operate in a manner similar to the prior art force motor. However, because there is no radial non-working airgap, there is no attendant increase in reluctance and decrease in flux flow and therefore decrease in operational efficiency due to flux being forced to flow in a radial direction across a non-working airgap. Consequently, a higher force output for a given force motor size can be achieved. The present invention will be further described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a schematic illustration of flux flow in a conventional prior art force motor; Figure 2 is a schematic representation of flux flow in a force motor constituting a preferred embodiment of the present invention; Figure 3A is a side view of a force motor constituting a preferred embodiment of the present invention partially in section; Figure 3B is an end view of the force motor of Figure 3A; Figure 4A is a graph of a demagnetisation curve for a conventional permanent magnet showing flux density vs. magnetic intensity; Figure 4B is a graph comparison of single vs. dual working airgap force motors indicating force for various airgaps lengths; and Figure 4C is a graph of flux density vs. magnetic intensity for a single and double airgap solenoids. Figure 2 illustrates schematically a preferred embodiment of the present invention. A stator 10 includes mounting flanges 12 for fixing the position of the stator with respect to two armatures 14A and 14B. The armatures are fixedly mounted on a shaft 16 and are positioned for axial movement relative to the stator in the operational direction of the force motor. The mounting structure which permits such movement is not shown in Figure 2 for clarity of illustration. Coils 26 and 28 are wound as in the prior art. A single permanent magnet could be used and mounted essentially between the coils as in the prior art although in the preferred embodiment two separate permanent magnets 18A and 18B are used. The flux path generated by the permanent magnets is represented by solid line arrows 20 and the flux generated by the electromagnets 26 and 28 is shown by broken line arrows 30. The flux generated by the permanent magnets and the electromagnets must pass across two axial working gaps 22A and 22B associated with the electromagnet 26 and the permanent magnet 18A and two additional axial working airgaps 24A and 24B associated with the coil 28 and the permanent magnet 18B. There is no radial flux flow across any non-working airgap. Because all airgaps are in the working direction (i.e. all airgap flux travel is in the axial direction), a lower level of flux will be necessary to provide the same force output from the shaft 16. This is a reduction in flux required to be generated by the permanent magnets 18A and 18B and allows them to be even smaller because there is a consequent reduction in iron core losses. The embodiment of Figure 2 operates in a similar manner to the motor of Figure 1. Flux flows from the permanent magnet 18A and the coil 26 add across both airgaps 22A and 22B while at the same time flux flows generated by the permanent magnet 18B and the coil 28 subtract across the airgaps 24A and 24B. Consequently, the armature 14A will be attracted toward the stator with a much greater force than will the armature 14B causing the output shaft 16 to move to the right in Figure 2. One advantage over the prior art force motor can be seen by referring to Figure 4A which is a graph of the demagnetisation curve for the magnets. It shows that the maximum energy product area (the product of H x B) is when the flux density of the magnet is at point P1. It will be noted that an open circuit magnet (no accompanying iron core) will have a large H (low flux density but high ampere-turns per unit length) as represented by point P2 on the curve and a magnet in a low reluctance iron circuit will have a high flux density B and a low H as noted at point P3. Both points P2 and P3 have low energy product areas and are not ideal operating points. For operating point P3 to move toward P1, the magnet size must increase or the reluctance of the iron circuit must increase. In the present embodiment, this is accomplished by replacing the radial non-working airgap whose reluctance is typically made as low as practicable. Thus, because the present circuit has a greater reluctance caused by the presence of two working airgaps for every one working airgap of the prior art, it operates at about point P1 at a reduced flux level which permits a smaller permanent magnet and reduced losses in the iron. A second advantage for the force motor is related to the maximising of the attainable force for a given size of the motor. The utilisation of essentially two working airgaps instead of the single working airgap of the prior art allows the force capability to be doubled. However, due to the large difference in circuit reluctances of the prior art motor and the preferred embodiment, a doubled force improvement is not realised for all conditions and this can be explained by Figures 4B and 4C. In Figure 4B it can be seen that there is a crossover point at a given airgap length where the single airgap, prior art low reluctance motor will pass through a point of maximum iron permeability and be approaching saturation while the higher reluctance motor will be approaching its point of maximum iron permeability. Beyond the point of maximum permeability of the low reluctance motor (the prior art motor) the permeability (B/H) of the high reluctance motor will always be higher assuming equal iron paths, airgap length and coil EMF with its consequent higher force advantage. As shown in Figure 4C, permeability µ is equal to B (the flux density) divided by H and it can be seen that both the single gap solenoid (the prior art solenoid) and the double gap solenoid have operating ranges A to B which are the gap lengths A and B shown in Figure 4B. Therefore, it can be seen that both force motors can operate at the maximum permeability which is the broken line shown in Figure 4C. However, it can also be seen that for a large portion of airgap lengths the dual working airgap is closer to the maximum permeability than the single working airgap as noted in Figure 4B. This is why, when operating in this region (from the crossover point in Figure 4B to the left), the dual working airgap has a dramatically greater force than the prior art force motor even though it might have the same iron paths, airgap length and coil EMF. It can also be seen that, in order to generate the same force, the dual working airgap force motor would have a smaller coil, smaller magnet and smaller iron core thus providing significant cost and weight savings. A preferred practical embodiment of the invention is shown in Figures 3A and 3B where Figure 3A is a partial cross section along section lines 3A-3A of Figure 3B. Structures identified in Figure 3A are all labelled with the same labelling as those in Figure 2. The stator 10 includes the mounting flanges 12 integral therewith. However, the mounting of the armature relative to the stator is shown in Figures 3A and 3B although it was eliminated for purposes of clarity from Figure 2. Four arm springs 40A, 40B, 42A and 42B are shown in Figure 3A. The configuration of each spring is similar to the spring 42B shown in Figure 3B in which there are four separate arms 44 having ends which are connected to the stator through machine screws 46 which pass through small spacers 48 and large spacers 50 and are secured into appropriately threaded apertures in the mounting flange 12 of stator 10. The armature 14B is not only connected to output shaft 16 but is also fixedly connected to the central portion of the four arm springs 42A and 42B. In this configuration, the stator 10 and the armature 14B can move relative to each other only in an axial direction. A similar arrangement is used to secure the armature 14A through the four arm springs 40A and 40B to the mounting flange 12 of the stator 10. Therefore, while the armatures 14A and 14B are fixedly mounted with respect to each other and the output shaft 16, they are free to move in an axial direction with respect to the stator 10. Mounting holes 52 permit the stator 10 to be bolted through another set of spacers and machine screws (not shown) to any flat structure. Alternatively, mounting tabs arranged in a circular mounting hole and extending inwardly could be used in conjunction with short machine screws to mount the stator in its operational position. Because the large spacers 50 and the machine screws connect the four arm springs to both the stator 10 and the armatures 14A and 14B, it is important that the spacers and screws be non-magnetic as they would otherwise permit flux leakage around the outside working airgaps (22B and 24B). For the same reason, the output shaft 16 should be non-magnetic to prevent flux leakage around the inner airgaps 22A and 22A. Many modifications may be made depending upon the particular application desired. For example, in order to obtain a greater amount of force in the axial direction, additional permanent magnets and electromagnets, stators and armatures could be included along the output shaft, making a relatively long but narrow cylindrical force motor. On the other hand, should a very short but wide construction force motor be desired, additional airgaps, permanent magnets and electromagnets could be located radially outwards of the existing airgaps, permanent magnets and electromagnets. Although the present device shows the stator 10 fixedly mounted and the armatures 14A and 14B mounted on the shaft 16 for an output movement, it is possible depending upon a particular application for the armatures 14A and 14B and the output shaft 16 to be fixed and the stator 10 to provide the output movement of the force motor. In this instance, if it was desirable to reduce the inertia of the stator 10, both the permanent magnets 18A and 18B and the electromagnets 26 and 28 could be mounted on the armatures 14A and 14B, respectively. As noted previously, the location of the permanent magnets can be as illustrated in the prior art device and/or as illustrated in Figure 2. The permanent magnets could also be located and fixed relative to the armature so as to move with the armature. There would be a disadvantage in that this would increase the inertia of the armature but this may be desirable in some circumstances. Similarly, the electromagnets themselves, although shown in Figure 2 as being fixed with respect to the stator, could be fixed with respect to the armatures although this would increase the inertia of the armature.

Claims: A force motor comprising first and second members which are movable relative to each other along an axis of operation, characterised in that the first member (10) has first and second ends spaced apart along the axis; and the second member has a first portion (14A) separated from the first end by first and second laterally spaced working airgaps (22A, 22B) and a second portion (14B) separated from the second end by third and fourth laterally spaced working airgaps (24A, 24B), and further comprising: first means (26, 28) for generating a first magnetic flux flow (30) through the first portion (14A) across the first working airgap (22A), through the first member (10), across the third working airgap (24A), through the second portion (14B) across the fourth working airgap (24B), through the first member (10), across the second working airgap (22B), and back to the first portion (14A); and second means (18A, 18B) for generating a second magnetic flux flow (20) in the first portion (14A) and the first member (10) in coincidence with the first flux flow (30) therein and in the second portion (14B) and the first member (10) in opposition to the first flux flow (30) therein. A force motor as claimed in Claim 1, characterised in that the second means comprises at least one permanent magnet (18A, 18B). A force motor as claimed in Claim 2, characterised in that the second means comprises at least two permanent magnets (18A, 18B). A force motor as claimed in Claims 2 or 3, characterised in that the or each permanent magnet (18A, 18B) is mounted on the first member (10). A force motor as claimed in any one of the preceding claims, characterised in that the first means comprises at least two coils (26, 28). A force motor as claimed in Claim 5, characterised in that the coils (26, 28) are mounted on the first member (10). A force motor as claimed in any one of the preceding claims, characterised by means for mounting the first and second portions (14A, 14B) for movement relative to the first member (10). A force motor as claimed in Claim 7, characterised in that the mounting means comprises at least one spring plate (40A, 40B, 42A, 42B) having arms (44) and a central portion and means for connecting the central portion to the second member (14A, 14B) and for connecting the arms (44) to the first member (10). A force motor as claimed in any one of the preceding claims, characterised in that the first member (10) is a stator and the second member (14A, 14B) is an armature. A force motor as claimed in any one of the preceding claims, characterised in that the second means comprises at least two permanent magnets mounted on said stator, each of said magnets having north/south polarity and mounted parallel to said axis of operation, one permanent magnet having its north/south polarity reversed from the north/south polarity of the other of said permanent magnets. A force motor as claimed in Claim 4 or in any one of Claims 5 to 9 when dependent on Claim 4, characterised in that the second means comprises first and second permanent magnets (18A, 18B) whose magnetic axis are parallel to the axis of operation and whose magnetic polarities are opposite each other.

Inventors: MOHLER, DAVID BRIAN
Applicants: LUCAS LEDEX, INC.
Application year: 1989
Main classification: H01F7
IPC codes: H01F7, H02K33