US3599069A - Multiphase stepper motor and controller including means to short circuit an inhibited winding - Google Patents

Abstract

A stepper motor is provided with three-phase stator windings for generating magnetic flux fields in directions sequentially spaced 60*/Np where Np is the number of stator poles per phase. A controller for the stepper motor includes three switch means for the three-phase windings respectively, each of the switch means being capable of three states wherein the associated winding is energized in one sense, or an opposite sense, or is inhibited from energization. By causing the switch means to assume various states in accord with a predetermined sequence, multiple-winding excitation is realized in such a way that high efficiency in terms of ampere turns per watt results with the added advantage that a smaller stepping angle and thus higher stepping speed is achieved. A rotor for the stepper motor is preferably of the permanent magnet type and incorporates a number of permanent magnet poles equal to the number of stator poles per phase. The use of the permanent magnet provides desirable damping characteristics.

Description

United States Patent (72] Inventor ElvinCJVeldl Cllvel'Cly,Cali.

(21] AppLNo. 853,321 [22] Filed Ang.27, 1969 [45] Patented Aug. 10, 1971 [73] Assignee AmerkuDflaICoI-poratlou [54} MULMASES'I'EPI'ERMOTORAND CONTROLLER lNCLUDlNG MEANS T0 SHORT CIRCUIT AN INHIBIT) WINDING 9Clahna,l1 Drawing?- [52) 318/696, 318/379 [51] l-LCI. ..ll02k37/00 [50] I'lelrlolSeu-eh 318/138,

156] ReferencesClted UNITED STATES PATENTS 3,127,548 3/1964 VanEmden 318/254 3,229,179 1/1966 Hetzel 318/138 3,297,927 1/1967 Blakesleeetl..... 318/138 3,385,984 5/1968 O'Regan........... 3l8/138X 3,445,741 5/1969 318/138 Primary Examiner-G. R. Simmons Anorney-Pastoriza & Kelly ABSTRACT: A stepper motor is provided with three-phase stator windings for generating magnetic flux fields in directions sequentially spaced 60/Np where Np is the number of stator poles per phase. A controller for the stepper motor includes three switch means for the three-phase windings respectively, each of the switch means being capable of three states wherein the associated winding is energized in one sense, or an opposite sense, or is inhibited from energization. By causing the switch means to assume various states in accord with a predetermined sequence, multiple-winding excitation is realized in such a way that high efficiency in terms of ampere turns per watt results with the added advantage that a smaller stepping angle and thus higher stepping speed is achieved. A rotor for the stepper motor is preferably of the permanent magnet type and incorporates a number of permanent magnet poles equal to the number of stator poles per phase. The use of the permanent magnet provides desirable damping characteristics.

PATENTEUAummsn SHEET 3 OF 3 STEPPING RATE STEPS/SEC l5 STEPS) PRIOR PRESENT MOTOR CONFIGURATION aeumve RELATIVE COILS ENERGIZED FLUX FLUX I COIL ONLY L000 2 COILS SOMULI'. 0.707 L240 3 COILS SIMULT 0.000 .230

FIG. 7

nTEETH FOR EXTERNAL TORQUE OZ.-IN.

FIG. H

l/VVE/VTOR: ELVlN C. WELCH warm/72415;

ATTOR/V Y3.

MULTIPIIASE STEPPER MOTOR AND CONTROLLER INCLUDING MEANS TO SHORT CIRCUIT AN INHIBI'I'ED WINDING This invention relates generally to stepper motors and controllers and more particularly, to an improved stepper motor of the three-phase type and controller therefore, although the principles are applicable to multiphase motors of more than three phases BACKGROUND OF THE INVENTION Stepper motors of the type under consideration are characterized by causing a shaft to rotate through precise step angles in either a clockwise or counterclockwise direction in response to a clockwise input electrical pulse or a counterclockwise input electrical pulse. If exactly l,000 input pulses are provided, the shaft will successively step through 1,000 discrete angular positions in clockwise or counterclockwise direction depending upon the terminal to which the pulses are supplied. Such motors thus find wide use as substitutes for known servomotors for controlling in a digital manner the positioning or indexing of devices, such as machine cutting tools.

It is very desirable in operations of the foregoing type that the available stepper motor be capable of making a large number of steps per unit time in order that a particular position change may be carried out rapidly. After a position change has been completed and the stepper motor must return a machine or other device to an initial position or to another starting point, the time for the return motion is normally wasted and therefore it is desirable to operate the stepper motor at a very high speed during such movement.

The discrete movements effect by the machine may be of any given incremental size by utilizing a suitable gear reduction at the output shaft of the stepper motor. The actual angular step of the stepper motor shaft can thus be made relatively small and the resulting linear or other movement of the machine made of an incremental length large or small depending upon the type of gearing employed. It is thus the number of steps per unit time or rate of stepping of the output shaft of the stepper motor that is important in minimizing the time of any particular machining operation sequence.

It will be evident that in general the smaller the angle of each individual step of a stepper motor rotor the more steps per unit time may be generated. This increased rate follows from the fact that for the smaller step angles, the physical movement of the rotor is smaller and can be executed in less time. Small step angles have been achieved heretofore by utilizing variable reluctance-type rotors for stepper motors. These rotors comprise soft iron with radially projecting teeth, the number of teeth and the stator winding configuration determining the discrete angle through which the rotor is caused to step as a consequence of the stepped magnetic flux field generated by the stator windings. An undesirable characteristic of soft iron or variable-reluctance-type rotors, however, is the fact that damping of the rotor movement is relatively poor with a consequent loss in precision stepping. This poor damping is a result of coupling or resonance of the rotor oscillatory motion with the stepping rate or a submultiple of the stepping rate. lmproved damping is realized by utilizing a rotor containing a permanent magnet. In this case, the rotating flux field of the permanent magnet itself serves to dynamically brake movement of the rotor by interacting with the stator windings. However, most permanent magnet rotors are of the two-pole type; that is, north and south poles diametrically opposite each other so that it is difficult to provide small steps of the rotor in response to a stepped magnetic field without employing an inefficient design which utilizes a large number of stator windings to provide a stepped magnetic field which will result in the desired small steps of the rotor.

[n my 0.8. Pat. No. 3,239,738 issued Mar. 8, I966, and entitled Stepper Motor Circuits," there are described twoand four-phase stepper motors wherein a magnetic flux field may be generated in sequential directions spaced at 90 to cause a permanent magnet rotor to step through discrete angles of 90. By properly energizing the stator winding in accord with a predetermined sequence, the generated magnetic flux fields may be caused to sequentially step at 45 angles thereby causing the rotor to step through discrete angles of 45. For the 0", 90, I", and 270 positions, only 50 percent of the available stator windings are energized and thus, in one sense, ineffi cient use is made of the available stator ampere turns.

In addition to stepper motors of the type described in my above referred to patent, there are presently available threephase stepper motors wherein first, second and third stator windings are arranged to generate magnetic flux fields in successive sequentially spaced directions when the windings are individually energized. In this arrangement, by energizing each individual winding in one sense or an opposite sense, the stator magnetic net flux field can be caused to sequentially step at 60 so that a suitable two-pole permanent magnet rotor will step through angles of 60. By utilizing a variablereluctance-type rotor of sofi iron having four radially extending teeth at to each other, the stepping angle of the rotor can be made 30 and by increasing the number of teeth to eight teeth, the stepping angle of the rotor can be made equal to 15. However, in each instance, only one of the three windings is energized at a time and thus there is an even more serious lack of utilization of available ampere turns in the stator windings. Exciting two windings at a time to provide a vector sum equivalent to that of one winding is possible, but then twice the power input is necessary.

With respect to the foregoing, it should be noted that motor performance is limited by the maximum allowable temperature which in turn places a limit on the maximum power input for a given-sized motor. If higher ampere turns per input watt can be achieved, a higher performance motor in terms of torque and stepping speed results.

Many important commercial applications of stepper motors require that a stepper motor provide incremental steps of i5". In the design of a stepper motor for such purpose, it is desirable to minimize the number of stator windings necessary and yet still achieve the desired small stepping angles of 15. A three-phase stator winding represents the minimum number of stator phase windings to achieve conveniently 15' steps. In the past, variable-reluctance-type rotors employing eight teeth have been used. The effectiveness of these motors, however, has been limited as noted heretofore because of the relatively poor damping characteristics of variable-reluctance-type rotors resulting in stepping inaccuracy and limitations in the torque output of the motor for particular stepping speeds.

BRIEF DESCRIPTION OF THE PRESENT INVENTION With all of the foregoing in mind, the present invention contemplates the provision of an improved multiphase-type stepper motor and controller wherein multiple winding excitation is used in such a way that higher efficiency in terms of stator ampere turns per watt is obtained over prior art stepper motors. By the greater utilization of the available ampere turns in the stator windings, a higher performance motor in terms of torque and stepping speed results.

Briefly, the preferred embodiment of the invention contemplates the provision of a three-phase stepper motor utilizing first, second, and third stator windings which may be connected in a Y-configuration or a delta configuration for generating magnetic flux fields in directions sequentially spaced 60lNp where Np is the number of stator poles per phase. A controller means for the windings includes first, second, and third switch means, each of the switch means being capable of three states. A first state causes the associated winding to be energized in one sense, a second state causes the associated winding to be energized in an opposite sense, and a third state causes the associated winding to be inhibited from energization.

Each of the switch means may thus be caused to assume any one of the states in accordance with a predetermined sequence such that the net magnetic flux field produced by the windings may successively assume step directions space at 30 for a motor with two stator poles per phase. This step angle for the three-phase stator windings is one-half the angle of the stepped magnetic field possible when only one of the windings is energized at a time and resulm by utilizing in each of the stepped positions at least two of the windings simultaneously. As a consequence, more of the available ampere turns of the stator windings are utilized than is the case were only one winding energized at a time. Further, by utilizing a four-pole pennanent-magnet-type rotor wherein north and south poles are circumferentially spaced 90, the rotor will be caused to execute l steps to thereby provide a stepper motor which exhibits far superior performance to known stepper motors of the same capacity insofar as the stepping rate, output torque and stepping accuracy characteristics are concerned.

In accord with a feature of this invention, the inhibit from energization state of the respective switch means may be eliminated by a simple switching in the controller so that the stepping of the rotor may be changed from 15 to 30 without in any way having to modify or change the stator windings or connections thereto.

In addition, by utilizing a variable-reluctance-type rotor having eight teeth, a stepper motor is provided wherein discrete steps of 7% is realized and a unique type of damping of rotor oscillatory motion is provided.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the preferred embodiments of the present invention will be had by now referring to the accompanying drawings, in which:

FIG. 1 is a schematic circuit diagram of a typical prior art variable-reIuctance-type three-phase stepper motor;

FIG. 2 is a block illustrating the controller logic for the motor of FIG. 1;

FIG. 3 is a table defining a predetermined sequence of energization of the winding of the motor of FIG. I to effect successive stepping in a clockwise and counterclockwise direction;

FIG. 4 is a schematic circuitdiagram of the three-phase stepper motor of the present invention together with part of the controller circuit connected thereto;

FIG. 5 is a block representing a subcireuit portion of the controller circuit of FIG. 4;

FIG. 6 is a table illustrating a predetermined sequence of conditions or states applied to the motor of FIG. 4 by the controller to effect 30 steps;

FIG. 7 is a table showing relative vector flux strengths realizable by the present invention over prior art devices;

FIG. 8 illustrates a four-pole permanent magnet rotor within a three-phase four-pole stator-winding configuration to realize l5 rotor steps;

FIG. 9 shows a variable-reluctance-type rotor having eight teeth which could be utilized in place of the rotor in the motor of FIGS. 4 or 8;

FIG. 10 illustrates a delta field-winding configuration which could be substituted for the y-field-winding configuration il- Iustrated in the motor of FIG. 4; and,

FIG. 11 illustrates stepping rate versus output torque characteristics for a typical prior art stepper motor and stepper motor of the present invention for comparison purposes.

DETAILED DBSCRIPT ION OF THE PRESENT INVENTION Referring first to FIG. I, there is schematically shown a stepper motor and controller designated generally by the arrows 10 and II. The stepper motor is of the three-phase variable'reluetance type wherein a variable reluctance rotor 12 having diametrically opposite teeth is shown in the center of a three-phase stator comprising first, second, and third windings 13, I4, and 15 in a Y-configuration. The inner ends of the windings are connected together by a common lead 16 grounded at 17. The outer ends of the windings connect to terminals l8, l9, and 20 respectively.

The controller portion is diagrammatically illustrated as constituting a switch means comprised of relays R1, R2, and R3 for operating switch arms S1, S2, and S3 respectively. Each of the relays is arranged to be energized by a corresponding input terminal such as indicated at 21 22,, and 23. The switches are normally open so that a signal is required on one of the relay input ten'ninals to close the corresponding switch.

The switch arms connect to a common lead 24 in turn provided with positive voltage from a battery 25, the other end of which is grounded at 26.

With the forging arrangement, it will be evident that if any one of the switch means is closed the corresponding winding connnected to the switch means will be energized in a given direction.

FIG. 2 illustrates a block 27 representing a controller logic subcircuit having clockwise and counterclockwise input pulse terminals 28 and 29 33 leads terminating in the terminals 21, 22, and 23 corresponding to the like terminals for the relays of FIG. I designated by the same numbers. The controller of FIG. 2 includes circuitry responsive to clockwise input pulses for sequentially applying signals to the terminals to the relays in accord with a predetermined sequence such that the stator magnetic flux field generated by the windings is caused to step sequentially at angles of 6 0 to thereby cause the asymmetrical rotor 12 to align itself to a position of minimum reluctance and thus also he stepped at the same angles.

FIG. 3 illustrates the predetermined sequence wherein for a 0 step position in which the rotor 12 is vertically aligned the winding 13 of FIG. 1 is not energized, the winding 14 is energized, and the winding 15 is not energized. The resulting magnetic flux field under these conditions is illustrated by the vector 30 in FIG. I. The soft iron variable reluctance rotor will align its teeth with the direction of the magnetic field.

When a next pulse in a clockwise direction is received in the controller logic subcircuit of FIG. 2, the coil I5 is caused to be energized by closing of the relay switch S3 while the coils l3 and 14 are both deenergized. The result is a new magnetic field flux vector indicated by the dashed lines 31 lying in a direction at 60 to the original vector 30. The rotor 12 will thus align itself by stepping through an angle of 60' in a clockwise direction.

lf 0 represents an open-switch condition or deenergized coil and represents a closed-switch condition or energized coil, the states of the various coils are as depicted in FIG. 3 for clockwise pulses and counterclockwise pulses to effect 60 steps of the rotor.

The same magnetic flux vector steps may be generated by exciting two coils at a time rather than one. For example, energizing coil 15 and coil I3 simultaneously will produce a flux vector sum equal in magnitude but exactly opposite in direction to flux vector 30 produced by exciting coil 14 alone. The soft iron variable reluctance rotor I2 will align its teeth with the magnetic field, regardless of the direction; therefore, the stepping action shown in FIG. 3 may be exactly duplicated by a similar switching sequence in which two coils are always excited simultaneously.

Exciting two coils at a time produces a desireable damping action of the variable reluctance rotor; however, the power input to the motor is doubled since two coils must be simultaneously excited by battery 25 rather than one only. If the motor was properly designed for the maximum allowable tempersture rise with only one coil excited, then exciting two coils would cause excessive temperature rise and the windings would burn out. The other altemstive of reducing the voltage of battery 25 such that the power input is brought back to normal, results in a weaker magnetic field and less torque and mechanical power output from the motor.

Three-coil excitation is not feasible in FIG. 1, since the vector sum of the three flux vectors will be zero.

If a permanent magnet rotor were substituted for the soft iron variable reluctance rotor 12 of FIG. 1, the north pole of the magnet would seek a position closest to the south pole of the particular winding through which a current is passing. Therefore, to effect a controlled stepping of a permanent magnet rotor, the switching arrangement should be modified such as to cause current to flow in one direction or the other of each of the windings in order that the polarity or north and south poles of the windings can be switched or changed. In either event, in the known prior art three-phase stepper motors as depicted in FIGS. 1, 2, and 3 either one winding at a time is energized or two are energized simultaneously in such a manner that the motor mechanical output power is significantly reduced.

Referring now to FIG. 4 there is illustrated schematically a preferred embodiment of the three-phase stepper motor wherein for illustrative purposes the three-phase windings are shown in a y-configuration 32 and are understood to define two poles per phase and a controller designated generally by the numeral 33 is shown connected to the windings.

Again, for the sake of illustration, the three-phase two-pole stator windings are depicted as first, second, and third coil A, B, and C. The rotor is illustrated at 34 and is of the permanent magnet type, the north and south poles for the cylindrical configuration being indicated by the arrowhead and arrow tail respectively.

The inner ends of the windings connect to a common lead 35. The outer ends in turn connect to terminals A1, B1, and C1.

The controller portion 33 of the circuit includes first, second, and third switch means connected to the terminals Al, Bl, Cl respectively. For illustrative purposes, these switch means are shown as simple relay-operated switch arms as was the case in the description of FIG. I. It should be understood throughout the description of the invention that these switches are purely schematically shown in this manner for the sake of simplicity and clarity. In actual circuit embodiments of the controller, the switches would be electronic-type switches such as transistors.

The first switch means connected to the terminal Al is capable of three states and these three states are respectively depicted by switch arm SA+ and associated relay coil RA+, switch arm SAG and associated relay coil RAG, and switch arms SA] and 8A1 ganged together for simultaneous operation and associated relay RAI.

The three states of the associated second and third switch means for the terminals B1 and C1 are similarly depicted by switch arms 83+, 550, SB], SB! and associated relay coils RB+, RBT, and RBI respectively and switch arms SC+, SCG, SCI, and SCI and associated relay coils RC+, RCG, and RC].

A voltage source depicted by a battery 36 provides a positive voltage on a common lead 37 connected to the switch arms SA+, 58+, and SC+. The other side of the battery 36 designated G connects through a common line 38 to the switch arms SAG, S86, and SCG. When the corresponding relays for these switch arms are not energized, the switch arms are normally open. Energization of any one or more of the corresponding relay coils as by signals applied to the relay terminals A+, AG, B+, BG, 0+, or CG will cause closing of the corresponding switch arm.

The lower ends of the various relays connect through the switch arms SAl, SBI, and SCI to a common lead 39 grounded at 40. These respective switches are normally closed when the corresponding coils RAI, RBI, and RC] are not energized and are opened when the corresponding coils are energized.

The controller circuit is completed by leads 4], 42, and 43 respectively passing from the terminals Al, Bi, and Cl to the switch arms SAI', SBI', and SCI. These latter switches are closed to a common lead 44 when the corresponding relays RAI, RBI, and RC] are energized to thereby place a voltage value of V12 from the battery 36 on the corresponding terminal A1, B1, or C]. An alternative connection for line 44 would be to the common lead 35 for the stator field windings since this lead is at the same potential V/2. The purpose for this portion of the circuit will become clearer as the description proceeds.

From the description thus far, it will be evident that any one of the states of the first, second, or third switch means associated with the terminals A1, B1, and Cl may be assumed by provision of a signal on one of the three relay coil terminals associated with the switch means. Thus, a first state for the first switch means connected to terminal A1 is assumed when the relay coil RA+ is energized on its terminal A+ by a signal to close the switch arm SA+. A second state for the first switch means is assumed when the relay coil RAG is energized by a signal on its terminal AG to close the switch arm SAG. Finally, the third state is assumed when the relay coil RAI is energized by a signal on its terminal Al to move the switch arms SA] and 5A1 to open and closed positions respectively.

FIG. 5 depicts by the block 45 a controller logic subcircuit having clockwise and counterclockwise input terminals 46 and 47 for receiving clockwise and counterclockwise electrical pulses. For simplicity in the description, there are illustrated output terminals in groups of three designated by the Roman numerals I, II, and II] for connection to the respective relay terminals A+, AG, AI; B+, BG, BI; and Ch CG, and CI in FIG. 4. In response to reception of a clockwise or a counterclockwise pulse on the terminals 46 or 47, the controller logic subcircuit will cause certain ones of these terminals to be either energized or not energized so that the states of the first, second, and third switch means can be defined in accord with a predetermined sequence.

FIG. 6 is a table illustrating such a predetermined sequence for varying the three states of the three switch means respectively to effect clockwise and counterclockwise stepping of the resulting net magnetic flux field of the stator windings in 30 steps. In this table, the symbols G, and I indicate the state of the respective terminals A1, B1, and CI, for the given stepped positions. The symbol G simply means that the G side of the battery 36 is applied to the corresponding terminal, the symbol means that the positive voltage side of the battery 36 is applied to the terminal, and the symbol 1 means that the terminal is inhibited from energization.

OPERATION Assumes in FIG. 4 that the rotor 34 is aligned in a vertical direction with the north pole of the permanent magnet pointing upwardly. Assume also that the various switches constituting the switch means are in the positions shown. Under these conditions, it will be evident that the terminal Bl will be provided with positive voltage from the battery 36 through the lead 37 and the closed switch This means that there is a voltage signal applied to the relay RB+ from the controller logic subcircuit of FIG. 5. Also, there will be applied to the terminals A1 and C1 of FIG. 4 and G side of the battery 36 as through the lead 38 and switches SAG and SCG respectively. The corresponding relays RAG and RCG will thus have signals on their terminals from the controller logic subcircuit of FIG. 5. The outside of the windings of FIG. 4 are accordingly marked for the winding B and G for the windings A and C. This state or condition is depicted in the table of FIG. 6 for the O-step position by the symbols G, and G for the ten'ninals A1, B1, and Cl respectively.

As a consequence of current flow through the winding B which current flow will split and pass through the windings A and C back to the G terminal of the battery, there will be developed a net magnetic field flux vector 2 which corresponds to the sum of the vector developed by the winding B and the vertical components of the vectors developed in the windings A and C. The rotor 34 is thus locked in the position shown since the rotor will magnetically align its flux vector in the same direction as the stator flux vector such that the south pole of the rotor is juxtaposed to the north pole of the stator.

If the logic from the controller of FIG. 5 provides signals on the relay terminals A1, B+, and CG, all other terminals being open, terminal A1 of FIG. 4 for the stator field windings will be inhibited from energization, the B1 terminal will have a voltage applied thereto, and the C1 terminal will have a G applied thereto.

With the foregoing condition, current will flow through the winding B and the winding C back to the G terminal of the battery. There will be no current flow through the winding A. The sum of the flux vectors generated by the windings B and C are depicted by the net flux vector 2' which assumes a stepped position of 30 from the vertical or -position. Therefore, the rotor 34 will step through 30 to align itself with this new vector. It will be noted that this latter state of the terminals Al, BI, and Cl is again depicted in the table of FIG. 6 for the 30' position by the symbols I, and G respectively.

The next stepped position will be 60 and it will be evident from the table which relay terminals could be energized to set the various states for the terminals of the field windings shown in the table.

It will be clear from the foregoing description that at least two of the windings are energized simultaneously in any stepped position of the magnetic flux field. It is significant that in this case the two-coil excitation, or three-coil excitation, is accomplished in a manner which increases motor mechanical power output and efficiency, otherthan causing a decrease as in the prior state of the art use of two-coil excitation. This improvement is accomplished by providing the capability of exciting the motor coils in either sense, rather than in only a single sense, as shown in FIG. 1. The summing of flux vectors may then be accomplished more efficiently.

In the case of the variable reluctance rotor motor, an improvement in damping of rotor motions is effected by the twocoil or three-coil simultaneous excitation, as well as an improvement in motor efficiency. A smaller stepping angle may also be attained which results in higher stepping speeds.

In the case of the permanent magnet rotor motor, good rotor damping and high motor torque and efficiency are achieved with either two-coil or three-coil excitation. In addition, smaller stepping angles with resultant higher stepping speeds may be achieved.

A comparison of stator flux vector strength is shown in FIG. 7 for the various motor configurations with equal power input and resulting maximum motor temperature held constant.

An important feature of the circuit as noted resides the provision of proper damping for the permanent magnet rotor 34. This damping is inherent when the rotor is in the 60, 120, 180, 240, or 300 position; that is, when all three of the windings are simultaneously energized in a proper sense to position the flux field vector in these directions. However, when the half steps between these six steps are generated as at 30, 90, I50, 210, 270, and 330, it will be evident from the table of FIG. 6 that only two of the windings are energized, the third winding being inhibited from energization. The axis of this third winding is perpendicular to the direction of the rotor vector in these particular stepped positions and if this winding were merely left open, its back e.m.f. generated by the rotor motion could not be used to effect the dynamic braking of the rotor. It is for this reason that the additional switch arms SAI S81, and SCI are provided for simultaneous operation with the inhibit switch arms when the inhibit relays RAI, RBI, or RCI are energized. Essentially, these additional switch arms function to effectively short circuit the inhibited winding in order to produce the desired braking or damping action. As mentioned, line 44 could connect to the common connection 35 of the windings thereby directly effecting the short-circuiting action.

As an example of the foregoing, consider the stepped position of the rotor at 30 wherein the net resultant flux vector is indicated by the dashed line 2' in FIG. 4. It will be recalled that in this stepped position, the windings B and C were energized while the Winding A was inhibited. The inhibiting of winding A from energization by either the or G terminals of the battery V is efiected by opening the switch arm SA] upon energization of the relay coil RAI. This movement of the switch arm SAl removes ground on the lead 39 from the bottom ends of the relay coils RA+ and RAG so that these coils are no longer effective to close either the SA+ switch arm or SAG switch arm. In addition, the switch arm SAP is closed to the lead 44 upon energization of the inhibit relay RAI thereby placing a voltage VIZ from the line 44 on the terminal A1 for the winding A. Referring to the winding configuration of FIG. 4, it is assumed that the windings are all identical and have identical resistances. Under these conditions, and in the 30 stepped position, the total voltage drop across the windings B and C will be the voltage of the battery V and the voltage appearing on the common lead 35 between the windings B and C will be VIZ corresponding exactly to the voltage applied to the outside of the winding A. Since these potentials are exactly equal, the winding A is effectively short circuited and can thus function to provide proper damping to the permanent magnet rotor when stepped to the 30 position.

The other additional inhibit switch arms SBI' and SCI function in a similar manner to short circuit the corresponding windings B and C when they are in an inhibit state.

It will be understood that as successive clockwise pulses are received, the successive states depicted by the table in FIG. 6 under the clockwise columns take place so that a clockwise stepping of the rotor 34 will take place through angles of 30. If counterclockwise pulses are received, the stepping will take place in sequence in the reverse direction, the proper states for the three terminals to the windings being shown in the table under the counterclockwise columns.

In order to realize a stepping of the rotor of only 15 rather than 30, it is only necessary to provide a four-pole threephase stator winding configuration in place of the two-pole three-phase windings of FIG. 4 and a four-pole permanent magnet rotor wherein the circumferential distance between a north and south pole is 90 rather than I". Substitution of this type of permanent magnet motor will then result in the stepping of the rotor taking place in sequential steps of 15. A larger number of stator poles per phase, Np, may be provided for even smaller steps with a corresponding number of rotor poles.

FIG. 8 illustrates at 49 such a four-pole permanent magnet rotor wherein the arrows indicate the flux pattern for the north and south poles. The sets of four stator coils making up each of the three-phase windings connecting to terminals A1, B1, and Clare shown at al, a2, a3, a4; bl, b2, b3, b4; and cl, c2, c3, and c4, respectively. Each stator phase produces a flux pattern to match that of the rotor 49.

FIG. 9 depicts a variable-reluctance-type rotor 50 provided with eight teeth which could be used with the three-phase winding configurations of FIGS. 4 or B. It is understood that for an actual motor, the circumferential span of the stator windings must be reduce to coincide with the rotor tooth span. Since the characteristic of the soft iron variable-reluctancetype rotor is simply to have its closest teeth aligned with the direction of the generated magnetic flux field, it will be evident that when the field windings generate a magnetic flux field which steps through angles of 30, the rotor of FIG. 9 will step through an angle of 756. A desired damping of rotor motion is also provided as described heretofore.

FIG. [0 depicts a three-phase stator winding configuration of the delta type rather than the Y-type described in FIG. 4. In this configuration, there are provided three windings designated A, B, and C' connected to define a delta or triangle, the vertices of the triangle constituting the three terminals which are designated by the same letters A1, B1, and Cl as the Y-terminals for the configuration of FIG. 4. Changing the states of the terminals A1, B1, and CI of FIG. 10 in accord with the table of FIG. 6 will result in an identical stepping of the resulting net magnetic flux field generated by the delta windings so that the rotor will be stepped in a like manner. If the motor is a two-pole permanent magnetic type as shown in FIG. 10, it will step through 30 whereas if the rotor is a fourpole permanent magnet type as shown in FIG. 8, it will step through 15. The delta and Y-stator winding configurations are thus equivalent.

In the particular controller logic subcircuit of FIG. 5, it will be noted that the inhibit from energization state may be eliminated from the three terminals A1, B1, and C1 by opening ganged switches SL to their dotted line positions so that only one of two states is applied to the terminals A1, B1, and Cl at any one time.

With the switch SL thus opened, the resultant magnetic flux field in FIG. 4 will be stepped through 60" angles rather than 30 angles. Thus, with reference to the table of FIG. 6, only those states in which no symbol I appears can be provided on the terminals so that for each successive clockwise or counterclockwise pulse received, a stepping of the field through an angle of 60 will result.

If the four-pole permanent magnet motor of FIG. 8 is utilized, it will be evident that by changing the logic switch SL of FIG. from a closed position to an open position, the step angle of the rotor will automatically be changed from l5 to 30 without in any way having to change any connections to the stator field windings.

As mentioned heretofore, a permanent-magnet-type rotor is preferable to a variable reluctance type because of the inferior damping characteristics of the latter. On the other hand, it is easier to manufacture a variable-reluctance-type rotor having many teeth than a permanent magnet rotor with many poles.

FIG. 11 illustrates the vast improvement in operation and performance realizable by the present invention as compared to prior art stepper motors. In FIG. ll, the ordinates represent the stepping rate in steps per second, each of the steps being l 5 steps while the abscissa represents the output torque of the motor in ounce-inches.

The dashed curve 51 is typical of the performance of prior art stepper motors capable of executing 15 steps. The solid line curve 52 illustrates the characteristics of the stepper motor of this invention incorporating the circuit of FIG. 4 with a four-pole permanent-magnet-type rotor and four stator pole coils for each of the three-phase windings as shown in FIG. 8. The four-pole rotor of the permanent magnet type, as described heretofore, will result in 15 steps of the rotor and output shaft with switch SL closed and it will be noted that stepping rates almost twice as high for the equivalent torque output can be realized by the motor of the present invention. Similarly, for equal stepping rates, it will be evident from the curves 51 and 52 that a greater output torque results from the motor of the present invention as compared to prior art motors. Finally, it will be evident that the curve 52 is much smoother than 51 indicating improved damping and stability.

Again, it is to be emphasized that throughout the description of the present invention the reference to relay-type switches, switch arm, relay coils, and the like is merely for purposes of simplicity and it is to be understood that these various portions of the circuit may be replaced by conventional transistor switching circuitry to enable rapid and efficient switching to take place. The term switch means" as used in the specification and claims is thus meant to cover all equivalent means for switching such as by solid state devices including transistors, equivalent and other equivalent semiconductors. In addition, while three separate relay terminals have been depicted to enable visualization of the three states of each of the switch means, it should be understood that the actual controller for the three-phase motor will only have three outputs which connect to the three terminals A1, B1, and C1. The circuitry within the overall controller will effect the change in state of the terminals from G, and l as depicted in the table of FIG. 6 in accord with the prearranged sequencing to provide the stepping desired.

Further, while permanent magnet and variable reluctance motors with three-phase stators with two or four poles for each winding together with two-, fouror eight-pole rotors have been described for illustrative purposes, the same principles are applicable to motors with other numbers of stator phases and rotor poles or teeth.

From all of the foregoing description, it will be evident that the present invention has provided a greatly improved stepper motor wherein smaller steps with higher stepping speeds can be realized, a greater utilization of the available ampere turns is effected, the advantages of a permanent-magnet-type rotor in providing efiicient damping is inherent together with the additional feature of being able to readily change the step angle of switching by a simple switching operation in the logic circuit alone.

I claim:

I. In a multiphase stepper motor defined in part by a number of stator windings for generating magnetic flux fields in sequentially space directions, a controller means including: a number of switch means for connection to said windings respectively, each switch means being capable of three states wherein: the associated winding is energized in one sense, the associated winding is energized in an opposite sense, or the associated winding is inhibited from energization, each of said switch means including means for effectively short circuiting its associated winding when in said inhibited from energization state whereby each of said switch means may be caused to assume any one of said states in accordance with a predetermined sequence such that the net magnetic flux field produced by said windings may successively assume step directions spaced at a given angle.

2. The subject matter of claim 1, in which said stator windings define three phases, said switch means are three in number and said given angle is 60lNp where Np is the number of stator poles per phase.

3. The subject matter of claim 2, in which Np is four.

4. A three-phase stepper motor and controller comprising, in combination:

a. first, second, and third stator windings physically positioned such that the directions of magnetic flux fields generated by said windings, when individually energized in the same sense, are sequentially spaced 120;

b. a rotor means constituting a permanent magnet responsive to magnetic flux fields generated by said windings to align itself in the directions of said fields;

c. first, second, and third terminals connected to said windings respectively;

d. first, second, and third switch means respectively associated with said first, second, and third terminals, each of said switch means being capable of assuming three states, wherein: the associated winding is energized in one sense, the associated winding is energized in an opposite sense, or the associated winding is inhibited from energization each of said switch means including means for effectively short circuiting its associated winding when in said inhibited from energization state;

e. clockwise and counterclockwise input terminals; and

f. a controller logic subcircuit means connected to said clockwise and counterclockwise input terminals and to said first, second, and third switch means and responsive to clockwise and counterclockwise input pulses to cause each of said first, second, and third switch means to as sume any one of said states in accordance with a predetermined sequence such that the direction of the net magnetic flux field produced by said windings successively assumes step directions spaced at 30, at least two of said windings in all stepped positions being energized to thereby utilize more ampere turns than if only one winding were energized at a time, and wherein the stepped direction is clockwise in response to clockwise input pulses and counterclockwise in response to counterclockwise input pulses, the short circuiting of a winding when in said inhibited from energization state resulting in more effective damping of said rotor means than is the case when the winding is not short circuited.

5. The subject matter of claim 4, wherein said rotor means is polarized to define north, south, north, south poles sequentially spaced whereby said rotor means steps at angles of 15.

6. The subject matter of claim 5, including a logic switch means for effectively eliminating said inhibited from energizawith the direction of said magnetic flux field so that the angle of stepping of said rotor is defined by 60ln where n is the total number of teeth.

8. The subject matter of claim 4, in which said first, second, and third windings are connected in a Y-configuration.

9. The subject matter of claim 4, in which said first, second, and third windings are connected in a delta configuration.

Claims (9)

1. In a multiphase stepper motor defined in part by a number of stator windings for generating magnetic flux fields in sequentially space directions, a controller means including: a number of switch means for connection to said windings respectively, each switch means being capable of three states wherein: the associated winding is energized in one sense, the associated winding is energized in an opposite sense, or the associated winding is inhibited from energization, each of said switch means including means for effectively short circuiting its associated winding when in said inhibited from energization state whereby each of said switch means may be caused to assume any one of said states in accordance with a predetermined sequence such that the net magnetic flux field produced by said windings may successively assume step directions spaced at a given angle.

2. The subject matter of claim 1, in which said stator windings define three phases, said switch means are three in number and said given angle is 60*/Np where Np is the number of stator poles per phase.

3. The subject matter of claim 2, in which Np is four.

4. A three-phase stepper motor and controller comprising, in combination: a. first, second, and third stator windings physically positioned such that the directions of magnetic flux fields generated by said windings, when individually energized in the same sense, are sequentially spaced 120*; b. a rotor means constituting a permanent magnet responsive to magnetic flux fields generated by said windings to align itself in the directions of said fields; c. first, second, and third terminals connected to said windings respectively; d. first, second, and third switch means respectively associated with said first, second, and third terminals, each of said switch means being capable of assuming three states, wherein: the associated winding is energized in one sense, the associated winding is energized in an opposite sense, or the associated winding is inhibited from energization each of said switch means including means for effectively short circuiting its associated winding when in said inhibited from energization state; e. clockwise and counterclockwise input terminals; and f. a controller logic subcircuit means connected to said clockwise and counterclockwise input terminals and to said first, second, and third switch means and responsive to clockwise and counterclockwise input pulses to cause each of said first, second, and third switch means to assume any one of said states in accordance with a predetermined sequence such that the direction of the net magnetic flux field produced by said windings successively assumes step directions spaced at 30*, at least two of said windings in all stepped positions being energized to thereby utilize more ampere turns than if only one winding were energized at a time, and wherein the stepped direction is clockwise in response to clockwise input pulses and counterclockwise in response to counterclockwise inpUt pulses, the short circuiting of a winding when in said inhibited from energization state resulting in more effective damping of said rotor means than is the case when the winding is not short circuited.

5. The subject matter of claim 4, wherein said rotor means is polarized to define north, south, north, south poles sequentially spaced 90*, whereby said rotor means steps at angles of 15*.

6. The subject matter of claim 5, including a logic switch means for effectively eliminating said inhibited from energization state of each of said first, second, and third switch means and rearranging said predetermined sequence such that the net magnetic flux field produced by said windings successively assumes step directions spaced at 60* to thereby step said rotor means at angles of 30*.

7. The subject matter of claim 4, in which said rotor means constitutes soft iron having a plurality of radially extending teeth equally circumferentially spaced, a tooth aligning itself with the direction of said magnetic flux field so that the angle of stepping of said rotor is defined by 60*/n where n is the total number of teeth.

8. The subject matter of claim 4, in which said first, second, and third windings are connected in a Y-configuration.

9. The subject matter of claim 4, in which said first, second, and third windings are connected in a delta configuration.

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