A multi-phase hybrid stepper motor

Abstract: To minimize fluctuation of torque, improve torque stiffness, and reduce resonance vibration during rotation in the hybrid stepper motor. The magnetic poles have two or more teeth placed with the same spaces as with toothpitch of the rotor. The total number (NS) of the teeth of the nonsymmetrical stators is determined by NS = 5 × (n0 + n1), where n0 is the number of teeth of each of five poles and n1 is the number of teeth of each of remaining five poles. The nonsymmetrical stator, of which the teeth are n1, is placed between the nonsymmetrical stators, of which the teeth are n0, so that the nonsymmetrical stator, of which the teeth are n0, is placed on the opposite side of the nonsymmetrical stator, of which the teeth are n1, when the core of the rotation axis is the center. The total number (NS) of the teeth (NR) of the rotor and the teeth of the stators corresponds to the relationship of NS ≧ 0.8 NR (NS is equal to or greater than 0.8 NR), and the difference between NR and NS is defined by the equation NR - NS = K(S0 - S1) + 10(S1 - 1 + B), where K is an integer between 2 and 5, S0 and S1 are integers as small as possible in such a way that S1 can also be equal to S0, B is a fraction of 1, 10B is an integer. The ten windings produce as many north-and south-poles as necessary by using a power supply method to supply applied power to achieve full step angles = electrical angle 90 ° = α F, α F = 360 ° /10NR and half step angle = electrical angle 45° = α H, α H = 360° /20NR.

Description: This invention relates to improvements for a multi-phase hybrid stepper motor. Hybrid stepper motors have been known to be excellent actuators whenever high precision positioning has been required, for more than twenty-five years. Areas of application include any kind of handling--equipment manufacturing, up to full process automation, as well as in the computer peripherals industry (i.e.printers, plotters, facsimiles, and disc-drives). A. The Four-Phase Hybrid Stepper Motor The most common motor is a so called four-phase motor with 8 mainpoles. Figs.9 (a) and (b) illustrate cross-sectional views A-A and B-B of Fig.10, the four-phase hybird stepper motor. Each of these statorpoles has a winding 11 and 5 teeth 12. As illustrated in Fig.10, teeth 12 face the innerrotating teeth 15 of rotorcaps 14 which are mounted one on each side of an axially magnetized permanent magnet 13. In this case the rotorcaps 14 have 50 teeth 15 each, one rotorcap creating north-poles, and the other one creating south-poles with a tooth pitch of 360/50 degrees = 7.2 degrees. Two rotorcaps 14 are offset to each other in rotational direction by one half tooth pitch. These rotorcaps 14, mostly made out of silicon lamination material or solid magnetically soft iron, together with the magnet mounted on a shaft 16, represent a rotor system. Currently there are hybrid stepper motors on the market with more than one rotor system. Most commonly, the tooth pitch of the statorpole teeth is the same as the rotor tooth pitch, Tp. In Fig.9 adjacent poles in this case are 45° away from each other (centre of middle tooth of the adjacent pole). The physical shiftangle between any adjacent pole is now 45° divided by 7.2° (tooth pitch) minus integer numbers of tooth pitch, and equals 1/4 of the tooth pitch. This is equivalent to 90° electrical, since one toothpich represents 360° electrical. This stator and its lamination can be considered to be a symmetrical stator-lamination, since every pole has the same number of teeth with the same toothpitch and the same winding-slot-opening between adjacent outer teeth of adjacent poles. In this case 2.25 Tp minus the tooth width. Theoretically these motors could be built with 48 stator teeth (6 teeth per pole), if the slot openings in the case of 1.25 Tp minus one toothwidth allow the insertion of the winding and create a phase-inductance as required. These motors can have a symmetrical lamination for any number of rotor teeth which fulfills the following requirement of: Number of rotor teeth divided by number of stator mainpoles equal to K + 1/4, K being an integer. As an example, 50/8 = 6.25 will create a symmetrical lamination. 48/8 = 6, does not work because there is no shiftangle. For instance, any motor with a number of rotor teeth NR = 8(K + .25) can have a symmetrical stator. So NR = 10; 18; 26; 34; 42; 50; and 58 will allow a symmetrical lamination with 8 stator-poles. These motors create a sinusoidal detent-torque which is periodical with one fourth of the tooth pitch and keeps the rotor in position without any external statorpole excitation. When considering that every pole when energized, together with the rotor system develops a nearly sinusoidal holding torque (Fig.11), which is periodical with the tooth pitch Tp, and since the poles are 90° electrical away from each other, one can design the torque-vector-diagram shown in Fig.12. It is easy to see that phase I is given by poles 1 and 5, phase II by poles 2 and 6, phase III by poles 3 and 7,and phase IV by poles 4 and 8. It can also be seen that maximum torque is obtained when all of the 8 poles are energized. A fullstep angle of αF = 90° electrical or αF =1/4 Tp will be achived when the polarization of four phases (i.e. 1; 5; 3; 7) gets changed at the same time. A halfstep angle of αH =45° or 1/8 Tp will be reached when those aforementioned poles are switched off instead of being changed in polarization. In this case the torque varies in halfstep mode by √2 to 1. Since two poles are always 180° away from each other (i.e.1 to 3 or 2 to 4), and also since the maximum torque will develop from step to step when 4 poles get changed in polarity at the same time, phase I and III can be conected with each other as well as phases II and IV. This allows a relatively simple, so called H-Bridge driver with a total of 8 transistors to achieve fullstep and halfstep modes. When energized in these two modes, the 8 poles have a polarization as shown in Fig.19. Here step 1 represents a fullstep position and step 2 a halfstep position. As it can be seen in Fig.19, this motor has as many north-poles as south-poles at any time. This means that the radial forces on the north-rotorcap are the same as they are on the south-rotorcap. This is important in order to avoid major bearing damage over lifetime, as well as to ensure a better reaction against mechanical inaccuracy to keep small step angle tolerances. From fig. 13 it is also apparent that whenever all 8 poles are energized, we have two north-poles adjacent, followed by two adjacent south-poles. This gives good stability at fullstep positions, since fig. 11 shows the detent (fourth harmonic) will not support the stiffness of an 8-stiffness of an 8-pole energized holding torque when inphase with a single pole (fig. 11 shows the detent offphase). All of the discriptions above can be summarized by the following general laws for symmetrical hybrid stepper motors: 1. Number of rotor teeth given by NR = mp[(n-1) + (K + 1/m)] where: m = number of phases p = number of poles per phase K = positive integer equal to or greater than one n = number of teeth per pole 2. Highest possible number of stator teeth NS max = NR - p and NS = mpn therefore NS always has to be smaller than NR 3. The smallest possible number of stator poles is equal to the number of phases. This means that the smallest possible number of poles per phase is one. |mp|min = m 4. Stator toothpitch Tp can be the same as rotor toothpitch TpS = TpR with TpR = 360°/NR But for any symmetrical motor lamination the stator toothpitch can also be Tps = 360°/NR - p 5. The basic detent-torque as well as the torque variation at speed (cogging) is periodical with the m th harmonic of the rotor toothpitch. 6. Number of steps per revolution for: fullstep mode: NREVF = mNR halfstep mode: NREVH = 2mNR Consequently the stepangles are α F = 360°/mNR and αH = 180°/mNR B. Disadvantages of Hybrid-Stepper Motors with Four (or less) Phases 1. Large variation in dynamic torque during switching from step to step and also in static positions when in halfstep mode. The variation factor is 1/√2. 2. Relatively high detent-torque which can destroy the stepangle accuracy as well as the movement during micro-stepping. 3. For micro-stepping mode a sinewave-current variation requires very small changes at the top of the sinewave. With small motors the rotor may not move because the torque change is too small. 4. These motors develop relativery strong resonances at step frequencies below 1kHz due to the torque variations. Sometimes those motors have speed zones in which the rotor will not rotate without a certain load or additional damper. 5. Start-and stop-stop frequencies are relatively low. 6. Due to the limited number of possible rotor teeth, most of the achievable step angles and the number of steps per revolution are not very practical for many industrial applications. C. By using a Five-Phase Hybrid Stepper Motor, some of the above mentioned disadvantages can be eliminated by employing a higher number of phases. US-A-3,866,104 and US-A-4,000,452 describe a five-phase hybrid motor which follows the same basic laws (equations) as layed out above. But in this case, the number of phases divided by two (2) is not an integer and in a PM-motor a change in current-direction shifts the polariry by 180° electrical (Fig.13), which is two times higher than normal. Therefore: NREVF = 2mN R and αF =180°/mNR NREVH = 4mN R and αH = 90°/mNR US-A-3,866,104 describes a five-phase motor with a shiftangle between adjacent poles of 3/5 Tp. Therefore an electrical angle of 72° (1/5 Tp) will be achieved between poles 2 and 4 (Fig.14). In this case we will achieve a torque-vector-diagram as shown in fig.15, and a pole polarization table as shown in Fig. 20. These two figures show that one can achieve a fullstep mode with 5 phases on as well as with 4 phases on. A halfstep mode will be achieved when alternating between 5 phases on and 4 phases on. The torque difference in halfstep mode is just 5% (percent). The cogging-torque as well as the detent-torque are the 5th harmonics(fig.16). This is a great improvement compared to the earlier described motor. The resonance-frequencies are in a much higher range and no "none-running" speed zone can be realized. The possible number of rotor teeth for symmetrical laminations follows the equation NR = 5p[(n-1) + (K + 3/5)] Therefore one can achieve rotor teeth numbers of 16; 26; 36; 46; 56; etc. Only the number 36 is of industrial interest or importance. Numbers like 50 or 100, which are very important for many applications, cannot be obtained in a symmetrical design as such. As US-A-4,095,161 describes however, a symmetrical five-phase lamination with a shiftangle of 3/5 Tp can be used together with 4 different numbers of rotor teeth. As an example, the lamination shown in Fig. 14 is designed for NR = 36, but it can be used also with NR = 32,38 or 42. Numbers such as NR = 20,30,40,50, etc. are only possible if we go to a nonsymmetrical lamination design, as described in US-A-3,866,104. This means that the maximum number of stator teeth will be less since there are always 4 rotor toothpitches (T P ), which cannot be used in conjunction with stator teeth. A lamination design of such a motor is shown in Fig. 17(NR = 50). The basic equation for such a five-phase nonsymmetrical motor is: NR = 5p[(n-1)+(K+3/5)]+4 The smallest possible difference between NR and NS is 5p. Therefore for a 10-pole motor the smallest difference is 10 which means in a motor for 500 steps per revolution, with an angle of α F = .72°, NR will be 50, and NS will be 40. This results in a decrease of the maximum possible torque and has to be considered as a disadvantage. Other disadvantages are the following items: 1. Even though the detent-torque is small, it can reduce the stepangle accuracy, since it can change the holding torque equilibrium point → fig. 16. 2. When in halfstep mode with 5 or 3 phases on, the number of stator north-poles will never be the same as the number of stator south-poles → Fig. 20. This creates an imbalance in the radial force( → Fig. 18) on the two rotorcaps, which puts more force on the bearings. It is also more sensitive to manufacturing tolerances and reduces the stepangle accuracy. Since the number of north-and south-poles will change every second step in halfstep mode, the radial force on the rotorcaps will change every second step. This, together with the 4(or 2) largerwinding-slot openings,creates larger vibrations at some speed frequencies. 3. The difference in the number of north-and south-poles, and the variation at every second step, creates higher hysteresis-values in halfstep mode, since the tooth-and pole-induction will be higher over the rotorcap with the smaller number of north-or south-poles. 4. A five-phase motor with a shiftangle between adjacent poles of 3/5 Tp, such as this, will not allow the same polarization on two adjacent poles, as described for the motor with four phases. This reduces the stability in the equilibrium positions. As mentioned above, conventional four or five-phase stepper motors have many faults. The present invention is formed in order to solve the above mentioned faults, and it is an object of the present invention to provide a hybrid stepper motor improving the torque-stiffness whilst reducing the torque variations to a minimum, and which is capable of reducing the resonant vibrations during rotation. US-A-3866104 discloses a multi-phase hybrid stepper motor which comprises a rotor system with at least one axially magnetized permanent magnet, having one polecap on each side, each polecap being made of soft magnetic iron and having on its periphery a number of evenly spaced, radially outward directed teeth; the rotor polecaps on each side being offset in rotational direction to each other by one half tooth pitch, where Tp = 360 /NR; a non-symmetrical stator surrounding the rotor and having ten radially inwardly directed statorpoles, each of the statorpoles having one untapped winding; the statorpoles having two or more teeth spaced with the tooth pitch Tp, a total number of stator teeth NS corresponding to the relation NS = 5(nO + nl), with nO being the number of teeth on each of five poles and nl the number of teeth of each of the remaining five poles, each one of the poles in between the five poles with nO teeth placed in such a way that opposite each of the poles with nO teeth is one pole with nl teeth; a relationship between the number of rotor teeth NR and the total number of stator teeth NS being NS ≥ 0.8NR and with a difference between NR and NS which is defined by the equation NR - NS = K(SO - S1) + lO(S1 - 1 + B), where K is any integer varying from 2 to 5, SO and S1 are integers as small as possible in such a way that S1 can also be equal to SO and where B is a fraction of 1 in such a way that 10B is always an integer; and a winding scheme such that the ten windings can each be individually energized through a voltage source or current source so that the ten windings form as many north and south-poles as necessary to achieve full step angles = electrical angle 90 = αF, where αF = 360 /lONR and half step angles = electrical angle 45 = αH, where αH = 360 /20NR and, according to the present invention, such a stepper motor is characterised by every second of the ten poles having one more tooth than the preceding one in such a way that five poles have a number of teeth nO, and their adjacent poles have a number of teeth nl; [nO = nl + 1; NS = 5(nO + nl)]. In the accompanying drawings : Fig.1 is a front view of an embodiment of the invention. Fig.2 is a front view of another embodiment of the invention. Fig.3 is a front view of another embodiment of the invention. Fig.4 is a vector diagram for a rotor of the invention. Fig.5 is a torque-vector diagram of the invention. Fig.6 is a torque wave form diagram of the invention. Fig.7 is a diagram indicating the varying pole of each of the statorpoles. Fig.8 is another diagram indicating the varying pole of each of the statorpoles. Fig.9 is a front view of a motor of the prior art. Fig.10 is a sectional view of a stepper motor of the prior art. Fig.11 is a torque wave form diagram of a stepper motor of the prior art. Fig.12 is a vector diagram indicating the holding-torque of a stepper motor of the prior art. Fig.13 is a torque-vector diagram of a 5 phase motor of the prior art. Fig.14 is a front view of a symmetrical 5 phase step motor of the prior art. Fig.15 is a torque-vector diagram of 5 phase motor of the prior art. Fig.16 is a torque wave form of the prior art. Fig-17 is a front view of nonsymmetrical statorpoles of the prior art. Fig. 18 is a diagram indicating the varying polarity of each of the statorpoles of a prior art device. Fig.19 is a diagram indicating the varying polarity of each of the statorpoles of a prior art device. Fig.20 is a diagram indicating the varying polarity of each of the statorpoles of a prior art device. Embodiments of the present invention will now be described in detail with reference to the drawings. This invention will improve at least three of the five aforementioned disadvantages of the 10 statorpoles motor with nonsymmetrical stator. This invention can even improve some stator designs under all four areas. The first design would lead to improvements in torque-stiffness, stepangle accuracy, efficiency for a five-phase motor with 10 statorpoles, and NR = 50, if we implement a shiftangle between adjacent statorpoles of 1/5. Instead of having 4 teeth on each of the 10stator-poles, we add one tooth on every second statorpoles in such a way that opposite of a statorpole with 4 teeth, there is always a statorpole with 5 teeth (fig. 1). This creates a difference of five (50 minus 5 times 4, minus 5 times 5) between the number of rotor teeth and stator teeth, while the standard design as described in US-A-3,866,104 and its general equation create a difference of 10 teeth. Therefore the ratio of stator teeth to rotor teeth in the improved design is equal to 45/50 = .9, while the standard design allows only a ratio of .8. This design also improves the torque equilibrium position stability since at least two adjacent stator poles have north-polarization, while two other adjacent stator poles have south-polarization (fig.7). An additional improvement is to achieve a better speed behavior,since this design has three larger winding slot-openings compared with two or four slot-openings in the standard design. This triangular distribution of large slots together with the earlier described rectangular distribution in radial forces reduces the vibration. To achieve major improvements in all four areas for a non-symmetrical stator-lamination with 10 statorpoles, for a number of rotor teeth such as 40,50,80,90 and 100, the next embodiment of the invention uses a shiftangle between adjacent statorpoles of 3/10 Tp or 7/10 Tp. Every second statorpole has one more tooth in the same way as described above. Now all motors with very important stepangles can be designed with differences between the number of rotor teeth and stator teeth, as small as 5 and even some as small as 3. Additionally these motors need only two larger winding-slot openings with only one toothpitch more than the other 8 openings. Important motors, which have 10 equivalent slot openings, can now also be designed. Fig. 2 and Fig. 3 show two typical designs of this invention. The radial forces on the rotorcaps, as shown in Fig. 4, now have a triangular distribution and they are more even on both rotorcaps. These triangular forms are much less sensitive to manufacturing tolerances, and improve the stepangle accuracy as well as leading to a reduction in vibration, due to only two larger slot openings. Fig. 7 shows the statorpole polarization for three consecutive steps, and it is evidient that in any position this motor has the same number of north-and south-poles. This redues the hysteresis. Also, at least two adjacent statorpoles are north, while at least two other adjacent poles are south. The torque improvement is shown in the vector diagram of Fig. 5. Because the shiftangle is selected at 3/10 Tp or 7/10 Tp, these motors with 10 statorpoles can be considered as 10-phase motors with p = 1. In accordance to the earlier developed general laws, these motors will have a detent-torque that is equivalent to the 10th harmonic of the holding-torque-curve. As shown in Fig. 6, this will improve the balance in torque stiffness between 10 statorpoles energized and 8 statorpoles energized. It also improves the step accuracy in halfstep mode, since the holding-torque curves cross the detent-curve at the zero positions. This is the basic equation for the motors of this invention: NR - NS = K(So - S1) + 10(S1 - 1 + B) with NS = 5(n 0 + n 1 ) n 0 = number of teeth on 5 poles n 1 = number of teeth on the other 5 poles k = any integer from 2 to 5 s 0 & s 1 =integers as small as possible s 0 = s 1 is possible B = a fraction of 1 in such a way that 10B is an integer Although these motors have 10 phases, always two opposite phases can be combined while switching to another step position. This was already described for the 8-statorpole motor with 4 phases. Therefore, in principle these 10-phase motors can be energized through a Pentagon-Drive as described in US-A-3,842,332. Also, a bipolar star-drive, the number of switching transistors is reduced by the factor of two, as compared with a so called full H-bridge driver. As mentioned above, the claimed invention has features as follow: accurate stepangle can be obtained, together with the holding-torque equilibrium position stability, and in addition, the mentioned hybrid stepper motor does not bring about an irregular vibration during the rotation at either fullstep mode or halfstep mode due to the same number of north-and south-poles.

Claims: A multi-phase hybrid stepper motor comprising a rotor system (13,14) with at least one axially magnetized permanent magnet (13), having one polecap (14) on each side, each polecap (14) being made of soft magnetic iron and having on its periphery a number (NR) of evenly spaced, radially outward directed teeth (15); the rotor polecaps (14) on each side being offset in rotational direction to each other by one half tooth pitch (½Tp), where Tp = 360 /NR; a non-symmetrical stator surrounding the rotor (13,14) and having ten radially inwardly directed statorpoles, each of the statorpoles having one untapped winding; the statorpoles having two or more teeth spaced with the tooth pitch Tp, a total number of stator teeth NS corresponding to the relation NS = 5(nO + nl), with nO being the number of teeth on each of five poles and nl the number of teeth of each of the remaining five poles, each one of the poles in between the five poles with nO teeth placed in such a way that opposite each of the poles with nO teeth is one pole with nl teeth; a relationship between the number of rotor teeth NR and the total number of stator teeth NS being Ns ≥ 0.8NR and with a difference between NR and NS which is defined by the equation NR - NS = K(SO - S1) + lO(S1 - 1 + B), where K is any integer varying from 2 to 5, SO and S1 are integers as small as possible in such a way that S1 can also be equal to SO and where B is a fraction of 1 in such a way that 10B is always an integer; and a winding scheme such that the ten windings can each be individually energized through a voltage source or current source so that the ten windings form as many north and south-poles as necessary to achieve full step angles = electrical angle 90 = αF, where αF = 360 /10NR and half step angles = electrical angle 45 = αH, where αH = 360 /20NR characterised by every second of the ten poles having one more tooth than the preceding one in such a way that five poles have a number of teeth nO, and their adjacent poles have a number of teeth nl; [nO = nl + 1; NS = 5(nO + nl)]. A stepper motor according to claim 1, wherein the ten pole windings are connected to form a decagon with ten connecting points of which every point can be individually connected through electronic switches with a voltage source, or current source such that at least two adjacent poles are south. A stepper motor according to claim 1 or claim 2, wherein B has a value of 0.3 and an electrical shift angle of 36 is achieved between poles 1 and 8, between poles 2 and 9, between poles 3 and 10, and so on, counted in one direction. A stepper motor according to claim 1 or claim 2, wherein B has a value of 0.7, and an electrical shift angle of 36 is achieved between every fourth pole, for example between poles 1 and 4, between poles 2 and 5, and so on, counted in one direction. A stepper motor according to claim 3 or claim 4, wherein the pole windings of two opposite poles get energized in such a way that the poles have opposite polarity (north and south). A stepper motor according to any one of claims 3 to 5, wherein the constants K, SO and S1 are chosen in such a way that the stator has ten teeth less than the number of rotor teeth NR; (NS = NR - 10). A stepper according to claim 1 or claim 2, wherein B has a value of 0.2 and an electrical shift angle of 72 is achieved between every two adjacent poles. A stepper motor according to claim 7, wherein the pole windings of two opposite poles get energized in such a way that the poles have the same polarity (north or south). A stepper motor according to claim 1 or claim 2, wherein B has a value of 0.3, and an electrical shift angle of 36 is achieved between poles 1 and 8, between poles 2 and 9, between poles 3 and 10, and so on, counted in one direction, and the constants K and SO are chosen in such a way that the stator has 3 teeth less than the number of rotor teeth NR; (NS = NR - 3). A stepper motor according to claim 1 or claim 2, wherein B has a value of 0.3, and an electrical shift angle of 36 is achieved between poles 1 and 8, between poles 2 and 9, between poles 3 and 10, and so on, counted in one direction, and the constants K, SO and S1 are chosen in such a way that the stator has five teeth less than the number of rotor teeth NR; (NS = NR - 5). A stepper motor according to claim 1 or claim 2, wherein B has a value of 0.3, and an electrical shift angle of 36 is achieved between poles 1 and 8, between poles 2 and 9, between poles 3 and 1O, and so on, counted in one direction, and the constants K, SO and S1 are chosen in such a way that the stator has 15 teeth less than the number of rotor teeth NR; (NS = NR - 15). A stepper motor according to claim 1 or claim 2, wherein B has a value of 0.7 and an electrical shift angle of 36 is achieved between every fourth pole, as an example between poles 1 and 4, between poles 2 and 5 and so on, counted in one direction and the constants K, SO and S1 are chosen in such a way that the stator has 7 teeth less than the number of rotor teeth NR; (NS=NR-7). A stepper motor according to claim 1 or claim 2, wherein B has a value of 0.2, and an electrical shift angle of 72 is achieved between every two adjacent poles, and the constants K, S0 and S1 are chosen in such a way that the stator has 5 teeth less than the number of rotor teeth NR; (NS = NR - 5).

Inventors: YOSHIDA KENJI, YOSHIMURA NORIYUKI, YOSHIDA, KENJI, YOSHIMURA, NORIYUKI, YOSHIDA, KENJI, NO. 272, NO. 4 BUILDING
Applicants: MINEBEA KK, MINEBEA CO.,LTD.
Application year: 1993
Main classification: H02K37
IPC codes: H02K37