GB2283865A - Magnetic stator circuit assembly for a stepping motor - Google Patents

Abstract

A stepping motor having coil bobbins 22, a flux plate assembly 23, end plates 20, 21 and a terminal block 31 has the terminal block received between the ends of the flux ring 30. Pole fingers 33, 41 interdigitate and the rotor bearings are retained by bearing retainers 36 formed integrally with the bobbins 22. Appropriate detent means locate the magnetic parts in fixed relation to each other. <IMAGE>

Description

STEPPER MOTOR This invention relates to stepper motors and in particular to the construction of the housing or stator for a stepper motor.

Previous constructions of stepper motors such as that shown in GB 2210207A have used a drawn metal housing of a two part construction which is welded together to form the return magnetic flux path for the stator. A separate mounting flange is welded or otherwise attached to the housing. This flange does not form part of the flux path. Also, as the housing is produced by drawing, the thickness of the walls of the housing cannot be closely controlled. As the housing parts are one piece, the thickness of the housing parts are governed by the part of the housing requiring the greatest thickness. This part is usually the pole fingers which are pressed from the housing part. The thickness of the pole fingers determines the maximum magnetic flux capacity of the flux path through the housing. As the fingers represent the smallest area of the flux path through the housing, the fingers need to be the thickest part to achieve the required capacity. However, as the housing parts are one piece, the entire housing part including the side walls are formed from the same thickness sheet which means that the walls, being of greater area than the pole fingers and thus could be thinner, are over sized. Hence, material usage is not optimized. Additionally, the walls of the housing parts need a large section removed to accommodate a terminal block, leading to greater material wastage.

It is the purpose of the present invention to mitigate the above problems and, in particular, to provide a stepper motor in which the thickness of the motor housing can be readily matched to the capacity of the motor.

This is achieved by providing a stepper motor with a housing comprising a discontinuous flux ring and two end plates fitted to the flux ring.

Thus, according to the invention, there is provided a stepper motor comprising: a housing; two coil bobbins disposed within the housing, each bobbin having a coil wound thereon and a central recess; a permanent magnet rotor disposed within the central recess of the bobbins with a space there between, a flux plate assembly disposed between the two coil bobbins and having pole fingers extending into the space between the bobbins and the rotor; and a terminal block for making external connections to the coils, wherein the housing comprises a discontinuous flux ring disposed about the coil bobbins and two end plates fitted to the flux ring. The end plates have pole fingers which extend through apertures in the bobbins and into the space between the bobbins and the rotor in fixed spaced relationship with the pole fingers of the flux plate assembly to form stator poles.

The use of a discontinuous flux ring as part of the housing means that the thickness of the flux ring and thus the capacity of the return flux path is not dependent on the thickness of a drawn housing part as in the prior art but on the thickness of the sheet of material from which the ring is formed. Thus the thickness of the flux ring can be more accurately controlled and designed for optimum flux density, thus reducing material wastage. For example, in a design having a pole finger thickness of 0.8 mm, the flux ring thickness may be reduced to 0.7 mm, a saving of over 10%. The end plates can now be stamped instead of being drawn which also speeds up production times as well as saving energy in producing the parts.

Preferably the flux ring is formed from a rectangular sheet with detent means formed near the corners of the sheet and on the bobbins to hold the flux ring in position around the bobbins. This aids assembly of the motor by avoiding the need for separate fastening means and the flux ring holds the bobbins together during assembly.

Preferably the detent means comprises apertures in the flux ring co-operating with spigots formed on the bobbins as this is easier to produce than other means such as hooks on the flux ring co-operating with recesses in the bobbins.

Preferably the coils are terminated directly on terminals mounted on the coil bobbins and which extend through the terminal block to make direct external connections.

This avoids the need for separate terminal members or pins fitted to the terminal block which are required to be connected, usually by soldering, to the bobbin terminal post.

By this arrangement, the normal terminal posts are replaced by the actual terminal pins of the terminal block eliminating the connection between the terminal post and the terminal pins. Also, as the terminals are mounted on the bobbins, the coils can be terminated directly onto the terminals at the time of winding to avoid unravelling of the coils which may occur if termination of the coil directly to the terminal pins is delayed until the bobbins are fitted to the terminal block.

Ideally the terminal block fills the gap between the ends of the discontinuous flux ring and securing means secure the terminal block directly to the end plates. This avoids the need for additional parts of the housing to protect the coils which should not be exposed. Securing the terminal block directly between the end plates ensures that the terminal block is reliably secured.

Preferably the end plates have depending lips which extend over the axial ends of the flux ring to hide the edge of the flux ring and capture the flux ring between the end plates. The lips may be discontinuous with intervening peripheral depressions cooperating with tabs deformed from the flux ring to secure or key the end plates to the flux ring.

To avoid the need for a separate mounting flange, one of the end plates may have apertured ears for mounting the motor.

According to a second aspect of the invention, there is provided a stepper motor comprising a housing; two coil bobbins disposed within the housing; two stator coils wound on respective bobbins and terminated on terminals; a permanent magnet rotor disposed within the bobbins; two bearing retainers integrally formed with the bobbins; two bearings held by the bearing retainers for rotatably supporting the rotor; a flux plate assembly disposed between the two bobbins and having pole fingers which extend axially of and adjacent the rotor; and a terminal block arranged to facilitate external connection of the terminals; wherein the housing comprises two end plates having pole fingers which extend axially inwardly through apertures in the bobbins and adjacent the rotor, in fixed spaced relationship with the flux plate assembly pole fingers to form stator poles, characterised by the housing further comprising a discontinuous flux ring, the flux ring being formed from a rectangular sheet with apertures formed adjacent each corner, which co-operate with spigots formed on the bobbins to hold the flux ring in position around the bobbins, with the ends of the flux ring abutting the terminal block which is fitted to the coil bobbins, and by the end plates, having discontinuous depending lips broken by peripheral depressions, the lips concealing the axial edges of the flux ring with the end plates being secured to the flux ring by tabs deformed from the flux ring and engaging the peripheral depressions of the end plates, and having apertures co-operating with posts formed on the terminal block to secure the terminal block to the housing.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an exploded view of a stepper motor constructed in accordance with the invention; Figure 2 is a plan view ofthe assembled stepper motor of Figure 1; Figure 3 is a sectional elevation viewed along Line III-III of Figure 2; Figure 4 is an end view of the motor of Figure 3; Figure 5 is a plan view of a top end plate, a component of the motor of Figure 1; Figure 6 is a sectional side view of the top end plate of Figure 5 viewed along Line VI VI; Figure 7 is a plan view of a flux plate assembly, another component part of the motor of Figure 1; Figure 8 is a side view of the flux plate assembly of Figure 7; Figure 9 is a plan view of a coil bobbin, another component part of the motor of Figure 1; Figure 10 is an end view of the coil bobbin of Figure 9 viewed in the direction of arrow X; Figure 11 is a view of a flux ring, a component part of the motor of Figure 1, viewed prior to rolling and on a reduced scale; Figure 12 is an end view of a terminal block, a further component part of the motor of Figure 1, viewed on an enlarged scale; and Figures 13 and 14 are detail views of the connection between the end plate and the flux ring cut along lines A-A and B-B, respectively.

As can be seen from Figure 1, the stepper motor has a number of discrete parts. They are: front and rear end plates 20, 21; two coil bobbins 22; a flux plate assembly 23; a permanent magnet rotor 24 including a shaft 25 and a cylindrical magnet 26, bearings 27, a spacer 28 biased by a helical spring 29; a flux ring 30 and a terminal block 31.

The rotor 24 is of standard construction with the magnet fitted to the shaft for rotation therewith and the shaft is journalled on the bearings 27. The spacer 28 and spring 29 are used to limit the play of the rotor between the bearings and to bias the rotor axially in one direction as is well known.

The sectional view of Figure 3 more clearly shows the arrangement of the various component parts when fitted together. The housing of the motor comprises the front and rear end plates 20, 21 and the flux ring 30. The front and rear end plates 20, 21 are located, as illustrated, on the top and bottom of the motor respectively. The front end plate 20 is shown in Figures 5 and 6. The rear end plate 21 is not shown separately as it is similar to the front end plate with the exception of the mounting ears.

The front end plate 20 has ears 32 with holes there through for mounting the motor.

The front end plate thus also acts as the mounting flange for the stepper motor, alleviating the need for a separate flange to be welded to the motor housing. The end plates have a series of tapered spikes forming pole fingers 33 extending vertically from the plates.

The end plates fit snugly over the outer end of an associated coil bobbin 22. The coil bobbins have a winding coil 34 wound on an annular bobbin section 35 and connected to terminals 44. The coils are connected directly to the terminals at the time of winding. The outer end of the bobbin has an integral bearing retainer in which a bearing 27 is seated. Between the bobbin portion and the bearing retainer are a number of holes 37 through which the pole fingers 33 of the end plate extend. The annular bobbin portion 35 defines a central recess in which the rotor is centrally disposed. A gap or space 39 is formed between the rotor 24 and the bobbin portion 35. The pole fingers extend into this space.

The flux plate assembly 23, comprising two flux plates 40 joined together back to back, is located between the bobbins. The flux plates have a corresponding number of pole fingers 41 which extend into the space 39 in fixed spaced relationship with the pole fingers 33 of the end plates to form the stator poles, as is well known. The flux plate assembly has a cut-out portion 42 to accommodate an enlarged portion 43 in the bobbins in which the terminals 44 are mounted. The cut-out portion and the terminal mounting portion 43 provide a key to lock the flux plate assembly in the correct orientation with respect to the bobbins.

Each bobbin 22 has at least two terminals 44 for connecting the coils 34 to an external electrical supply. The terminal mounting portions 43 interlock and the coil bobbins themselves are identical. They are shown more clearly in Figures 9 and 10.

In Figure 10 the terminal mounting portion 43 is shown end on and a locking pin 45 is shown depending from the mounting portion. This pin 45 locates in a hole (not shown) in the other bobbin and the corresponding pin from the other bobbin engages hole 46 to key the two bobbins together.

The bobbins 22 each have two spigots 53 formed on the periphery as shown in Figure 9. These spigots co-operate with holes 54 formed in the flux ring to form detent means to hold or hook the flux ring around the assembled bobbins.

The flux ring, as shown in Figure 11 in the unrolled state and on a reduced scale, is formed from a rectangular strip of sheet steel which is rolled to form a discontinuous ring. The inside diameter of the rolled flux ring would be slightly less than the outside diameter of the bobbins to ensure a tight fit between the flux ring and the bobbins and the flux plate assembly. The strip is formed with holes 54 adjacent each corner. The strip has additional holes 55 along the major edges which are square or rectangular in shape. The material between the end of the strip and the rectangular holes 55 form tabs 56 which are used to secure the flux ring to the end plates as will be described.

The end plates have a plurality of depending lips 57 which hang over and conceal the edge of the flux ring thereby capturing the flux ring between the end plates as shown in Figure 14. In between the lips are peripheral depressions 58 which extend inside and past the edge of the flux ring adjacent the tabs 56 and are aligned with the rectangular holes 55. The tabs 56 are then deformed radially inwardly on to the depressions 58 to trap the end plates as shown in Figure 14, thereby securely fastening the flux ring to the end plates without welding.

The terminals 44 extend outwardly from the motor and pass through the terminal block 31. The terminals thus fulfil the function of terminal posts for the bobbins and terminal contact members or pins for the terminal block. The terminal block has a guard or plug guide 49 to assist the connection of an external plug to the terminals.

The outer wall 47 of the terminal block rests on the outer surface of the bobbins to further hold the bobbins together while a small wall or ridge 48 on the bobbins forms a stop against which the terminal block abuts.

The terminal block has four posts 50 which engage two holes 51 in each end plate.

The end plates have a depressed portion 52 surrounding the holes 51. The depressed portions bear on the wall of the terminal block to capture the wall of the terminal block between the end plate and the bobbins. The posts 50 and holes 51 ensure that the terminal block 31 is securely fastened to the housing. The terminal block fills the gap formed between the ends of the discontinuous flux ring to protect the coils from exposure.

The motor is assembled by fitting the bearings to the bearing retainers and placing the rotor and flux plate assembly between a pair of previously wound coil bobbins. By fitting the terminal block to the bobbins, the bobbins are held together while the flux ring is fitted around the bobbins. The end plates are then fitted over the ends of the bobbins and the terminal block and the tabs 56 of the flux ring deformed to securely fasten the housing and thus the motor together.

Claims (12)

1. A stepper motor comprising: a housing; two coil bobbins disposed within the housing, each bobbin having a coil wound thereon and a central recess; a permanent magnet rotor disposed within the central recess of the bobbins with a space therebetween; a flux plate assembly disposed between the two coil bobbins and having pole fingers extending into the space between the bobbins and the rotor; and a terminal block for externally connecting the coils, wherein the housing comprises: a discontinuous flux ring disposed about the coil bobbins; and two end plates fitted to the flux ring and having pole fingers which extend through apertures in the bobbins and into the space between the bobbins and the rotor in fixed spaced relationship with the pole fingers of the flux plate assembly to form stator poles.

2. A stepper motor according to Claim 1, wherein the flux ring is formed from a rectangular sheet, and detent means are formed adjacent the corners of the sheet and on the bobbins to hold the flux ring in position around the bobbins.

3. A stepper motor according to Claim 2, wherein the detent means comprises apertures in the flux ring co-operating with spigots formed on the bobbins.

4. A stepper motor according to any one of the preceding claims wherein the coils are terminated directly on terminals which are mounted on the coil bobbins and which extend through the terminal block to make direct external connections.

5. A stepper motor according to any one of the preceding claims wherein the terminal block fills the gap between the ends of the discontinuous flux ring.

6. A stepper motor according to any one of the preceding claims wherein securing means are provided between the terminal block and the end plates to secure the terminal block to the housing.

7. A stepper motor according to Claim 6, wherein the securing means comprises apertures in the end plates co-operating with posts formed on the terminal block.

8. A stepper motor according to any one of the preceding claims wherein the end plates have depending lips which extend over the axial edges of the flux ring.

9. A stepper motor according to Claim 8, wherein the lips of the end plates are discontinuous with a plurality of peripheral depressions which co-operate with tabs deformed from the flux ring to secure the end plates to the flux ring.

10. A stepper motor according to any one of the preceding claims wherein at least one end plate has apertured ears for mounting the motor.

11. A stepper motor comprising: a housing; two coil bobbins disposed within the housing; two stator coils wound on respective bobbins and terminated on terminals; a permanent magnet rotor disposed within the bobbins; two bearing retainers integrally formed with the bobbins; two bearings held by the bearing retainers for rotatably supporting the rotor; a flux plate assembly disposed between the two bobbins and having pole fingers which extend axially of and adjacent the rotor; and a terminal block arranged to facilitate external connection of the terminals; wherein the housing comprises two end plates having pole fingers which extend axially inwardly through apertures in the bobbins and adjacent the rotor, in fixed spaced relationship with the flux plate assembly pole fingers to form stator poles, characterised by the housing further comprising a discontinuous flux ring, the flux ring being formed from a rectangular sheet with apertures formed adjacent each corner, which co-operate with spigots formed on the bobbins to hold the flux ring in position around the bobbins, with the ends of the flux ring abutting the terminal block which is fitted to the coil bobbins, and by the end plates, having discontinuous depending lips broken by peripheral depressions, the lips concealing the axial edges of the flux ring with the end plates being secured to the flux ring by tabs deformed from the flux ring and engaging the peripheral depressions of the end plates, and having apertures co-operating with posts formed on the terminal block to secure the terminal block to the housing.

12. A stepper motor substantially as hereinbefore described with reference to the accompanying drawings.