Speed control of a motor

[0002] This invention relates generally to the speed control of electric motors, and specifically to improvements in a speed control system for motors, particularly those suitable for use in a disk drive, such for example as a CD-ROM drive, for moving the transducer or pickup assembly across the track turns of the rotating data storage disk, among other applications.

[0003] Brushless d.c. motors have found widespread use for rotating a data storage disk or for moving a transducer across the data tracks on the disk. Fine control of motor speed is an absolute requirement in such applications. U.S. Pat. No. 5,371,635 to Sakaguchi et al., assigned to the assignee of the instant application, is hereby cited as teaching a system for generating pulses indicative of the angular position and rotational speed of a data storage disk. This prior art system is unsatisfactory for its complexity and expensiveness of construction as an inevitable result of the use of a motor speed sensor aside from the control electronics.

[0004] Japanese Unexamined Patent Publication No. 3-16066 suggests, for speed control of a brushless d.c. motor, to utilize the Hall-effect devices, the magnetoelectric converters, that have been customarily built into this type of motor for detection of the angular position of its rotor relative to the stator. The rotor position signals produced by the magnetoelectric converters are differentiated and further processed into a motor speed signal indicative of the actual rotational speed of the motor preparatory to motor speed control.

[0005] This second prior art system gains a definite advantage over the first recited one in that it does without any dedicated motor speed sensor. It is still objectionable, however, first because the control electronics, particularly those for production of the motor speed signal from the rotor position signals supplied by the magnetoelectric converters, is not as simple and inexpensive as can be desired. Secondly, the motor speed signal so produced is inconveniently high in ripple percentage, running counter to the objective of fine motor speed control.

[0006] The present invention has it as an object, in producing the motor speed signal without use of a motor speed sensor as above, to minimize the ripple of the motor speed signal for controlling the motor speed with a minimum of errors.

[0007] Another object of the invention is, in speed control of a brushless d.c. motor without use of a motor speed sensor of any kind, to materially simplify the circuitry for production, from the rotor position signals supplied by the standard magnetoelectric converts, of a motor speed signal which indicates the actual rotational speed of the motor and which is needed for its speed control.

[0008] Briefly, the present invention may be summarized as a speed control system for a motor of the type having magnetoelectric converter means for providing a first, a second and a third rotor position signal with phase differences of 120 degrees therebetween, each rotor position signal being indicative of the angular position of a rotor relative to a stator. The motor speed control system comprises differentiating circuit means for differentiating the first, the second and the third rotor position signal and providing a first, a second and a third output signal in a prescribed phase relationship with the first, the second and the third rotor position signal, respectively. A first, a second and a third switch are provided for selectively passing the first, the second and the third output from the differentiating circuit means under the control of a first, a second and a third binary switch control signal, respectively. The first, the second and the third switch control signals are derived from the first, the second and the third rotor position signal in a prescribed phase relationship therewith. Outputs from the three switches are added together into a motor speed signal representative of the actual rotational speed of the motor. The motor speed signal is compared with a target speed signal representative of a desired rotational speed of the motor, thereby providing a motor speed control signal indicative of the difference between the actual and the desired motor speeds. A motor excitation circuit responds both to the first, the second and the third rotor position signal and to the speed control signal for causing the motor to rotate at the desired speed.

[0009] The invention is applicable to brushless d.c. motors, d.c. motors with brushes, and alternating current motors. The motor speed signal is obtained from the rotor position signals supplied from the Hall-effect devices or like magnetoelectric converters. For production of the motor speed signal from the rotor position signals, these signals must be differentiated, and parts of the resulting signals must be extracted, or the resulting signals put to what is essentially equivalent to half- or full-wave rectification, prior to being added together into the motor speed signal.

[0010] Such extraction or rectification is accomplished according to the invention as the first to third switches selectively pass the first to third outputs from the differentiating circuits under the control of the first to third binary switch control signals. The present invention particularly features the creation of the switch control signals by shaping the rotor position signals, which are in the form of sine waves, into binary signals. With the motor speed signal produced in this manner, the complete speed control system is greatly simplified in construction compared to the closest prior art set forth above.

[0011] According to another feature of the invention, the rotor position signals are 30 degrees delayed in phase before being turned into binary signals for switch control. Desired segments of the rotor position signals can be ideally extracted by controlling the switches by these binary switch control signals, resulting in the provision of a motor speed signal, or speed control signal, that is less in ripple percentage.

[0012] A further embodiment is disclosed which employs an operational amplifier in place of a simple adder for combining the outputs from the switches. The operational amplifier performs the dual function of differentiation and addition of the outputs from the switches, or even the multiple function of the differentiation and addition of the switch outputs and the comparison of the resulting motor speed signal with the target speed signal, thereby contributing to greater simplicity in circuit arrangement.

[0013] There is also disclosed herein a yet further embodiment that employs another set of switches which coact with the first recited set of switches for what is equivalent to full-wave rectification of the outputs from the differentiating circuit means. The resulting motor speed signal, or speed control signal, is still more reduced in ripple percentage.

[0014] The above and other objects, features and advantages of this invention and the manner of achieving them will become more apparent, and the invention itself will best be understood, from a study of the following description and attached claims, with reference had to the accompanying drawings showing the preferred embodiments of the invention.

[0015] FIG. 1 is a block diagram of the first preferred form of speed control system for a brushless d.c. motor according to the present invention;

[0016] FIG. 2 is a sectional view of the motor controlled by the FIG. 1 system, the motor being shown together with the three magnetoelectric converters for providing as many rotor position signals with predetermined phase differences;

[0017] FIG. 3 is a schematic electrical diagram, partly in block form, of the motor excitation circuit of the FIG. 1 motor control system, shown together with the stator windings of the motor;

[0018] FIG. 4 is a block diagram of the speed detector circuit of the FIG. 1 motor control system, the speed detector circuit forming the gist of this invention;

[0019] FIG. 5 is a diagram of waveforms useful in explaining the operation of the FIG. 4 speed detector circuit;

[0020] FIG. 6 is similar to FIG. 2 but shows the motor together with two magnetoelectric converters and associated circuit means for production of three rotor position signals;

[0021] FIG. 7 is a block diagram of another preferred embodiment of the invention, featuring a modified speed detector circuit for use in the FIG. 1 speed control system in place of the FIG. 4 speed detector circuit;

[0022] FIG. 8 is a schematic electrical diagram showing in more detail one of the binary signal forming circuits of the FIG. 7 speed detector circuit;

[0023] FIG. 9 is a vector diagram explanatory of how the rotor position signal is delayed by the FIG. 8 circuit;

[0024] FIG. 10 is a diagram of waveforms useful in explaining the operation of the FIG. 7 speed detector circuit;

[0025] FIG. 11 is a block diagram of still another preferred embodiment of the invention, featuring a speed control signal forming circuit for use in the FIG. 1 speed control system in place of the FIG. 4 speed detector circuit and the differential amplifier;

[0026] FIG. 12 is a block diagram of yet another preferred embodiment of the invention, featuring another modified speed detector circuit for use in the FIG. 1 speed control system in place of the FIG. 4 speed detector circuit;

[0027] FIG. 13 is a diagram of waveforms useful in explaining the operation of the FIG. 12 speed detector circuit;

[0028] FIG. 14 is a block diagram of a further preferred embodiment of the invention, featuring a modified speed control signal forming circuit for use in the FIG. 1 speed control system in place of the FIG. 4 speed detector circuit and the differential amplifier;

[0029] FIG. 15 is a block diagram of a further yet preferred embodiment of the invention, featuring still another modified speed detector circuit for use in the FIG. 1 speed control system in place of the FIG. 4 speed detector circuit;

[0030] FIG. 16 is a block diagram of another preferred form of speed control system for a d.c. motor according to the present invention; and

[0031] FIG. 17 is a schematic electrical diagram, partly in block form, of the motor excitation circuit of the FIG. 16 motor control system, shown together with the motor.

[0032] The general configuration of the brushless d.c. motor speed control system according to the invention will be understood from a brief study of FIG. 1. The brushless d.c. motor 1 is shown in this figure only insofar as is necessary for an understanding of its speed control system, and is to be detailed with reference to FIG. 2. Suffice it to say for the moment that the motor 1 comprises a stator 9 including three phase windings 9u, 9vand 9w, and a rotor 10.

[0033] Broadly, the motor speed control system comprises: (a) a motor excitation circuit 2 of largely conventional make for controllably exciting the stator windings 9u, 9vand 9wof the motor 1; (b) three magnetoelectric converters 3, 4 and 5 such as those built on the familiar Hall effect for providing rotor position signals indicative of the angular position of the rotor 10 of the motor relative to the stator 9; (c) a speed detector circuit 6 for detecting the actual rotational speed of the motor 1 from the rotor position signals and putting out a motor speed signal Vt; (d) a target speed signal generator 7 for providing a target speed signal Vr representative of a desired motor speed; and (e) a differential amplifier 8 having inputs connected to the speed detector circuit and the target speed signal generator 7 for producing a motor speed control signal Vcon representative of the difference between the actual motor speed signal Vt and the target speed signal Vr, for application to the excitation circuit 2 in order to cause the same to excite the motor windings 9u, 9vand 9wfor motor rotation at the desired speed.

[0034] The brushless c.d. motor 1 with the magnetoelectric converters 3-5, motor excitation circuit 2, and speed detector circuit 6 listed above will be hereinafter discussed in more detail, in that order and under separate headings. Various modifications of the speed detector circuit 6 in particular will also be disclosed.

[0035] As illustrated in FIG. 2, the brushless d.c. motor 1 is of the known rotor-outside-stator configuration, with the stator 9 enclosed by the rotor 10, which is thus exposed. The stator 9 comprises the first 9u, second 9vand third 9wphase windings and a magnetic core 11. The core 11 has twelve slots 11acut radially therein at constant angular spacings and so provide as many teeth 11b. Each divided into three interconnected parts, the three-phase windings 9u, 9vand 9ware wound on nine neighboring ones of the twelve core teeth 11b. It will be seen that the three-phase windings 9u,9vand 9ware arranged cyclically and in that order in a clockwise direction as seen here. The electrical angle between any two slots 11a, or any two teeth 11b, is therefore 120 degrees, or 2 Ti/3.

[0036] Three of the twelve stator teeth 11bare left unoccupied by the windings in the illustrated embodiment. The three magnetoelectric converters 3-5 are disposed in radial centerline alignment with the three stator slots 11alying next to the three unoccupied teeth. The electrical angles between these magnetoelectric converters are therefore also 120 degrees.

[0037] The above noted arrangement of the magnetoelectric converters 3-5 is intended to mitigate the influence of the magnetic fields due to the windings 9u, 9vand 9won these converters. In cases this is no appreciable advantage, however, windings may be provided on all the twelve stator teeth 11b. Either way, the three magnetoelectric converters 3-5 of the FIG. 2 arrangement will provide three rotor position signals with phase differences of 120 degrees.

[0038] The rotor 10 is tubular in shape, concentrically surrounding the stator core 11 and itself divided into eight north- and eight south-seeking magnetic pole sections N and S alternating in its circumferential direction. Immovably supported outside the rotor 10 in prescribed positional relationship to the three-phase windings 9u, 9vand 9w, the three magnetoelectric converters 3-5 are so arranged as to detect magnetic fluxes due to the rotor 10. Therefore, with the rotation of the rotor 10 relative to the stator 9, the first to third magnetoelectric converts 3-5 will provide first to third rotor position signals, Vh₁, Vh₂and Vh₃, respectively in the form of alternating voltages of sinusoidal waveform with a phase lag of 120 degrees from one to the next (FIG. 5).

[0039] As illustrated in detail in FIG. 3, the motor excitation circuit 2 comprises a d.c. power supply 16, a speed control circuit 17 for controlling the rotational speed of the motor 1, a plurality of, six in this embodiment, switching transistors 18, 19, 20, 21, 22 and 23 for causing the three-phase stator windings 9u, 9vand 9wof the motor to be energized selectively, three amplifiers 24, 25 and 26 connected respectively to the magnetoelectric converts 3-5 by way of lines 12, 13 and 14 for amplifying the rotor position signals Vh₁, Vh₂and Vh₃, and a switch control circuit 27 for making on-off control of the switching transistors 18-23 according to the amplified rotor position signals.

[0040] It will be noted from this figure that the three-phase stator windings 9u, 9vand 9wof the motor 1 are interconnected on one hand and, on the other, connected to one terminal of the power supply 16 via the first, third and fifth transistors 18, 20 and 22 and the speed control circuit 17 and also to the other terminal of that power supply via the second, fourth and sixth transistors 19, 21 and 23. The transistors 18-23 are all shown to be of npn type. Of these 18, 20 and 22 have their emitters connected to the motor windings 9u,9vand 9w, and their collectors to the power supply 16 via the speed control circuit 17. The other transistors 19, 21, and 23 have their collectors connected to the motor windings 9u, 9vand 9w, and their emitters connected directly to the power supply 16. The bases of all these transistors 18-23 are connected to the switch control circuit 27.

[0041] Per se of known construction, the switch control circuit 27 makes on-off control of the transistors 18-23 by the familiar two-phase excitation method in response to the three rotor position signals Vh₁, Vh₂and Vh₃. Thus the switch control circuit 27 will cause conduction through the transistors 18 and 21 for simultaneous excitation of the motor windings 9uand 9v, through the transistors 20 and 23 for simultaneous excitation of the motor windings 9vand 9w, and through the transistors 19 and 22 for simultaneous excitation of the motor windings 9uand 9w.

[0042] The speed control circuit 17 functions to control the power fed from power supply 16 to motor windings 9u, 9vand 9waccording to the speed control signal supplied from the differential amplifier 8, FIG. 1, over the line 15. The power may of course be augmented for higher motor speed and lessened for lower motor speed. Alternatively, for lower motor speed, the switch control circuit 27 may be caused to brake the motor 1 by energizing its windings in a direction of rotor rotation opposite to that in which it is currently rotating.

[0043] Notwithstanding the showing of FIG. 3, however, the provision of the speed control circuit 17 is not an absolute necessity; instead, the switch control circuit 27 may be modified to perform the additional function of motor speed control, in both increasing and decreasing direction, through on-off control of the transistors 18-23. The speed control signal line 15 may be connected directly to such a modified switch control circuit, as indicated at 15a. For such motor speed control the modified switch control circuit may be equipped to produce speed control pulses such that their durations are modulated according to the incoming speed control signal. The repetition rate of such duration-modulated speed control pulses should be sufficiently less than that of the pulses for causing motor rotation, and each such motor rotation pulse may be constituted of a series of speed control pulses.

[0044] Still another option is available for motor speed control: The resistance value of each of the transistors 18-23 may be controlled during their periods of conduction.

[0045] The speed detector circuit 6, constituting the gist of the instant invention, does not relay on any external speed sensor other than the standard magnetoelectric converters 3-5 for motor speed detection, as will be noted from FIG. 1. Inputting the rotor position signals Vh₁, Vh₂and Vh₃over lines 31, 32 and 33, the speed detector circuit 6 puts out the motor speed signal Vt for delivery to the differential amplifier 8 over a line 34.

[0046] FIG. 4 is a detailed illustration of the speed detector circuit 6. The following discussion of this circuit 6 will be better understood by referring also to FIG. 5 which indicates the waveforms of the signals appearing in various parts of the circuit.

[0047] The speed detector circuit 6 comprises: (a) input amplifiers 35, 36 and 37 connected respectively to the input lines 31, 32 and 33 for amplifying the rotor position signals Vh₁, Vh₂and Vh₃; (b) differentiating circuits 38, 39 and 40 connected respectively to the amplifiers 35, 36 and 38 for differentiating the amplified rotor position signals; (c) switching transistors 41, 42 and 43 connected respectively to the differentiating circuits 38, 39 and 40 for passing parts of their output signals Vd₁, Vd₂and Vd₃as partial motor speed signals Vs₁, Vs₂and Vs₃; (d) a binary signal forming circuit 44 for on-off control of the transistors 41, 42 and 43; and (e) an adder 45 for adding the partial motor speed signals Vs₁, Vs₂and Vs₃into a desired motor speed signal Vt representative of the running speed of the motor.

[0048] Connected to the rotor position signal lines 31-33 via the amplifiers 35-37, the differentiating circuits 38-40 put out the voltage signals Vd₁, Vd₂and Vd₃which, as shown in FIG. 5, are 90 degrees advanced in phase with respect to the rotor position signals Vh₁, Vh₂and Vh₃. Although these voltage signals Vd₁, Vd₂and Vd₃contain information on motor speed, a simple addition of these alternating voltages would not provide a unidirectional motor speed signal because then unnecessary mutual cancellation would occur among them. It might be contemplated to add the signals Vd₁, Vd₂and Vd₃together after their full-wave rectification by diodes. Full-wave rectification by diodes would not suit very low voltages produced by the magnetoelectric converts 3-5, FIG. 1, when the motor is in low speed rotation. Full-wave rectification by switches or amplifiers, instead, might be possible but is definitely impractical because of the unjustifiable complexity and expensiveness of the circuitry required.

[0049] The solution proposed by the instant invention is the extraction, for subsequent addition, of the positive half-cycles of the sinusoidal output signals Vd₁, Vd₂and Vd₃of the differentiating circuits 38-40 by means of the switching transistors 41, 42 and 43. These transistors are, moreover, turned on and off by the binary switch control signals Vc₁, Vc₂and Vc₃derived from the amplified rotor position signals Vh₁, Vh₂and Vh₃by the binary signal forming circuit 44.

[0050] The binary signal forming circuit 44 is comprised of only three wave-shaping circuits 46a, 46band 46cin this particular embodiment, which are connected respectively between the outputs of the amplifiers 35, 36 and 37 and the bases of the transistors 42, 43 and 41. In practice the wave-shaping circuits 46a, 46band 46cmay take the form of comparators, producing the binary switch control signals Vc₁, Vc₂and Vc₃by translating the positive half-cycles of the rotor position signals Vh₁, Vh₂and Vh₃into rectangular wave trains. The binary switch control signals Vc₁, Vc₂and Vc₃are of course obtainable by connecting one input of each comparator to one of the amplifiers 35, 36 and 37 and grounding the other input.

[0051] A comparison of the output voltage Vd₁of the first differentiating circuit 38 and the binary switch control signal Vc₃from the third wave-shaping circuit 46cin FIG. 5 will indicate that the signal Vc₃is only 30 degrees advanced in phase from the signal Vd₁. The third switch control signal Vc₃can therefore be used for on-off control of the first transistor 41, so that the output of the third wave-shaping circuit 36cis shown connected to the base of the first transistor 41. There is thus obtained on the output side of the first transistor 41 the first partial motor speed signal Vs₁which is equivalent to the −30° to +150° segment of each cycle of the first differentiating circuit output Vd₁. The negative segments included in this first partial motor speed signal Vs₁, as from −30° to 0°, are negligible compared to its positive segments, as from 0° to 150°, in terms of both duration and amplitude.

[0052] An inspection of FIG. 5 will further reveal that a phase relationship similar to that discussed above between first differentiating circuit output Vd₁and third binary switch control signal Vc₃also exists between second differentiating circuit output Vd₂and first binary switch control signal Vc₁, and between third differentiating circuit output Vd₃and second binary switch control signal Vc₂. The first wave-shaping circuit 46ais therefore connected to the base of the second transistor 42, and the second wave-shaping circuit 46bto the base of the third transistor 43. The second and third partial motor speed signals Vs₂and Vs₃are thus obtained on the output side of the second and third transistors 42 and 43.

[0053] The first to third partial motor speed signals Vs₁, Vs₂and Vs₃are all directed into the adder 45 thereby to be combined into the unidirectional motor speed signal Vt. The partial motor speed signals Vs₁, Vs₂and Vs₃have phase differences of 120 degrees, and each goes negative only during 30 degrees of each cycle. Experiment has proved that the maximum percent ripple ΔV of the motor speed signal thus obtained according to the invention is 50%.

[0054] With reference back to FIG. 1 the motor speed Signal Vt thus produced by the speed detector circuit 6 is directed to one input of the differential amplifier 8, to the other input of which is supplied a voltage signal representative of a desired motor speed from the target speed signal generator 7. The resulting output from the differential amplifier 8 is a motor speed control signal in the form of a voltage difference between motor speed signal Vt and target speed signal. As has been explained with reference to FIG. 3, the motor excitation circuit 2 will energize the three-phase windings 9u, 9vand 9wof the motor 1 for rotation at the desired speed in response to the motor speed control signal.

[0055] As has been set forth hereinbefore with reference to FIGS. 1-5, the motor speed control system according to the invention utilizes the rotor position signals Vh₁, Vh₂and Vh₃from the standard magnetoelectric converters 3-5 for production of the motor speed signal Vt indicative of actual motor speed. Such converters have been customarily built into this type of motor in disk drive applications for excitation control purposes and are used for the same purposes, beside being used for motor speed detection, in this and all the other embodiments of the invention to be disclosed subsequently, too. The speed control system according to the invention is therefore much simpler and far less expensive in construction compared to prior art speed control systems employing speed sensors for motor speed detection.

[0056] Perhaps a most pronounced feature of the FIGS. 1-5 embodiment resides in the method of making on-off control of the switching transistors 41-43, FIG. 4, of the speed detector circuit 6. The rotor position signals Vh₁, Vh₂and Vh₃from the magnetoelectric converters 3-5 are turned into the binary signals Vc₁, Vc₂and Vc₃by the wave-shaping circuit 46a-46c. The first binary signal Vc₁is used for on-off control of the second transistor 42, the second binary signal Vc₂for that of the third transistor 43, and the third binary signal Vc₃for that of the first transistor 41. The three partial motor speed signals Vs₁, Vs₂and Vs₃thus obtained from the transistors are then combined into the unidirectional motor speed signal Vt, such that its ripple percentage is reduced to a level that has proved to present no problem at all in practice.

[0057] The present invention is applicable to the speed control of brushless d.c. motors furnished with but two magnetoelectric converters, instead of three as in the foregoing embodiment. FIG. 6 shows such a motor 1 with two magnetoelectric converters 3 and 4, which are disposed in the same positions with respect to the motor as their FIG. 2 counterparts 3 and 4. The absence of the third magnetoelectric converter is overcome by creating a third rotor position signal from the rotor position signals produced by the two magnetoelectric converters 3 and 4.

[0058] Employed to that end, in place of the third magnetoelectric converter 5 of the preceding embodiment, is a circuit designated 49 in FIG. 6. This third rotor position signal forming circuit, so to say, comprises two amplifiers 49aand 49b, an adder 49c, and a phase inverter 49d. Connected to the two magnetoelectric converters 3 and 4 via the amplifiers 49aand 49b, the adder 49cvectorially adds together the two rotor position signals Vh₁and Vh₂having a phase difference of 120 degrees. The resulting output from the adder 49cis then inverted in phase by the inverter 49c, for delivery as the third rotor position signal Vh₃to both motor excitation circuit 2, FIG. 1, and speed detector circuit 6. This third rotor position signal Vh₃has substantially the same voltage waveform as indicated by the same indicia in FIG. 5.

[0059] An advantage gained by this second embodiment is the greater simplicity in construction arising from use of but two magnetoelectric converters 3 and 4. Since three rotor position signals are nevertheless obtained from the two magnetoelectric converters, a speed detector circuit of the same construction as shown in FIG. 4 can be employed for processing them into a motor speed signal.

[0060] FIG. 7 shows a modified speed detector circuit 6afor use in the FIG. 1 motor speed control system in place of the speed detector circuit 6. The modified speed detector circuit 6aemploys binary signal forming circuits 44a, 44band 44c, in lieu of the FIG. 4 binary signal forming circuit 44, for on-off control of the switching transistors 41-43. Each of the binary signal forming circuits 44a, 44band 44cis a serial connection of a 30-degree delay circuit 47a, 47bor 47cand a wave-shaping circuit 46a, 46bor 46c. The delay circuits 47a, 47band 47cimpart a 30-degree phase delay to the three rotor position signals Vh₁, VH₂and Vh₃, respectively, and the wave-shaping circuits 46a, 46band 46ctranslate the delayed rotor position signals Vh₁′, Vh₂′ and Vh₃′ into binary switch control signals Vc₁, Vc₂and Vc₃, respectively.

[0061] More specifically, the delay circuit 47aof the first binary signal forming circuit 44ahas inputs connected to both amplifiers 35 and 36 for processing the amplified first and second rotor position signals Vh₁and Vh₂into the first delayed rotor position signal Vh₁′. The wave-shaping circuit 46aand the first binary signal forming circuit 44ahas an output connected to the base of the second switching transistor 42 for on-off control thereof by the first binary switch control signal Vc₁. The delay circuit 47bof the second binary signal forming circuit 44bhas inputs connected to both amplifiers 36 and 37 for processing the amplified second and third rotor position signals vh₂and Vh₃into the second delayed rotor position signal Vh₂′. The wave-shaping circuit 46bof the second binary signal forming circuit 44bhas an output connected to the base of the third switching transistor 43 for on-off control thereof by the second binary switch control signal Vc₂. The delay circuit 47cof the third binary signal forming circuit 44chas inputs connected to both amplifiers 35 and 37 for processing the amplified first and third rotor position signals Vh₁and Vh₃into the third delayed rotor position signal Vh₃′. The wave-shaping circuit 46cof the third binary signal forming circuit 44chas an output connected to the base of the first switching transistor 41 for on-off control thereof by the third binary switch control signal Vc₃.

[0062] The delay circuit 47a, 47bor 47cand wave-shaping circuits 46a, 46band 46cof each binary signal forming circuit 44a, 44bor 44cmay not necessarily be discrete entities but may be parts of an essentially single circuit. FIG. 8 illustrates by way of example such an integrated first binary signal forming circuit 44a, it being understood that the second and third circuits 44band 44care each of like construction except for connections of their inputs and outputs to other parts, which have been specified above. The exemplified first binary signal forming circuit 44acomprises two resistors R₁and R₂functioning as parts of a 30° delay circuit, an operational amplifier 48 functioning as wave-shaping circuit, and a reference voltage source Er.

[0063] The operational amplifier 48 has one input connected to both amplifiers 35 and 36 via the respective resistors R₁and R₂, and another input connected to the reference voltage source Er. There are therefore impressed to the resistors R₁and R₂the first and the second amplified rotor position signals Vh₁and Vh₂from the magnetoelectric converters 3 and 4, FIG. 1. The first resistor R₁is understood to have a resistance value that is half that of the second resistor R₂. These resistors R₁and R₂have their output sides interconnected. In consequence, as indicated in FIG. 9, the rotor position signals Vh₁and Vh₂with a phase difference of 120 degrees therebetween are added together at a rate of two to one, thereby providing the first delayed rotor position signal Vh₁′, which lags 30 degrees behind the first rotor position signal Vh₁, for delivery to the operational amplifier 48.

[0064] It is understood that the reference voltage from the source Er is equal to the mean value (e.g. 2.5 volts) of the rotor position signals Vh₁, Vh₂and Vh₃. The operational amplifier 48 goes high when the fist delayed rotor position signal Vh₁′ is higher than the reference voltage Er, and low when the first delayed rotor position signal is less than the reference voltage. Thus the positive half-cycles of the delayed rotor position signal Vh₁′ from the operational amplifier 48 are converted into the first binary switch control signal Vc₁in FIG. 10.

[0065] It is self-evident that, as will be understood by referring back to FIG. 7, the second binary signal forming circuit 44bcombines the second and third rotor position signals Vh₂and Vh₃at a rate of two to one into the second delayed rotor position signal Vh₂′ and reshapes this signal into the second binary switch control signal Vc₂shown also in FIG. 10. The third binary signal forming circuit 44clikewise combines the third and first rotor position signals Vh₃and Vh₁at a rate of two to one into the third delayed rotor portion signal Vh₃′ and reshapes this signal into the third binary switch control signal Vc₃shown also in FIG. 10.

[0066] The three delayed rotor position signals Vh₁′, Vh₂′ and Vh₃′ lag 30 degrees in phase behind the FIG. 10 rotor position signals Vh₁, VH₂and Vh₃. The three binary switch control signals Vc₁, Vc₂and Vc₃derived from these delayed rotor position signals are therefore as indicated also in FIG. 10. A comparison of FIGS. 5 and 10 will show that the FIG. 10 switch control signals Vc₁, Vc₂and VC₃lag 30 degrees behind their FIG. 5 counterparts. The FIG. 10 switch control signals Vc₁, Vc₂and Vc₃agree in phase with the second, third and first differentiating circuit outputs Vd₂, Vd₃and Vd₁, respectively.

[0067] Thus, as the transistors 41, 42 and 43 of the FIG. 7 speed detector circuit 6aare turned on and off by the binary switch control signals Vc₃, Vc₁and Vc₂, respectively, the differentiating circuit outputs Vd₁, Vd₂and Vd₃will be broken up into the three partial motor speed signals Vs₁, Vs₂and Vs₃, FIG. 10, which are all positive half waves, from zero to 180 degrees, of the differentiating circuit outputs.

[0068] The adder 45 of the FIG. 7 speed detector circuit 6awill combine the three-phase half-wave signals Vs₁, Vs₂and Vs₃into the unidirectional motor speed signal Vt shown in FIG. 10. The ripple percentage ΔV of this motor speed signal Vt is as low as 13 percent, compared to that of 50 percent in the first disclosed embodiment.

[0069] The speed detector circuit 6 and the differential amplifier 8 of the FIG. 1 motor speed control system can be combined into that may be termed a speed control circuit, capable of producing a speed control signal for direct application to the motor excitation circuit 2. FIG. 11 shows an example of such speed control circuit 50. Essentially, this circuit 50 is a streamlined combination of the FIG. 7 speed detector circuit 6aand FIG. 1 differential amplifier 8, to be coupled directly to the motor excitation circuit 2.

[0070] The speed control circuit 50 comprises: (a) capacitor-resistor circuits 38a, 38band 38cconnected to the rotor position signal input lines 31, 32 and 33 via the buffer amplifiers 35, 36 and 37; (b) the noted switches 41, 42 and 43 connected to the CR circuits 38a, 38band 38c, (c) a second set of switches 51, 52 and 53 also connected to the CR circuits 38a, 38band 38cfor discharging the capacitors C₁, C₂and C₃; (d) the binary signal forming circuits 44a, 44band 44cfor producing the binary switch control signals Vc₁, Vc₂and Vc₃for direct on-off control of the first set of switches 42, 43 and 41 and, via inverter circuits 55, 56 and 54, of the second set of switches 52, 53 and 51; and (e) an operational amplifier 57 forming a part of a differentiating and an adding circuit.

[0071] The serial circuits 38a, 39band 39cof capacitor C₁and resistor R₁, capacitor C₂and resistor R₂, and capacitor C₃and resistor R₃function somewhat like the FIG. 7 differentiating circuits 38, 39 and 40. Comprised of the delay circuits 47a, 47band 47cand the wave-shaping circuits 46a, 46band 46c, the binary signal forming circuits 44a, 44band 44coperate just like their FIG. 7 counterparts bearing the same reference characters, producing the switch control signals Vc₁, Vc₂and Vc₃from the amplified rotor position signals Vh₁, Vh₂and Vh₃.

[0072] The operational amplifier 57 has one input connected to the CR circuits 38a, 39band 40avia the first set of switches 41, 42 and 43 and to the noted target speed signal line 7avia a resistor R₄and a polarity inverter circuit 59, beside being connected to its own output via a feedback resistor R₅. The other input of the operational amplifier is grounded via a source 58 of a reference voltage Er, which is 2.5 volts in this particular embodiment. This reference voltage source 58 is further connected to the CR circuits 38a, 39aand 40avia the second set of switches 51, 52 and 53. Controlled by the phase inversions, made by the inverter circuits 54, 55 and 56, of the switch control signal Vc₃, Vc₁and Vc₂from the binary signal forming circuits 44c, 44aand 44b, the second set of switches 51, 52 and 53 turn on and off in opposite relationship to the first set of switches 41, 42 and 43. Thus, closed while the first set of switches 41, 42 and 43 are open, the second set of switches 51, 52 and 53 function to discharge and reset the capacitors C₁, C₂and C₃.

[0073] Functionally, the FIG. 11 speed control circuit 50 is equivalent to the FIG. 7 speed control circuit 6aplus the FIG. 1 differential amplifier 8. More specifically, the binary signal forming circuits 44a, 44band 44cderive the binary switch control signals Vc₂, Vc₂and Vc₃from the amplified rotor position signals Vh₁, Vh₂and Vh₃. The switch control signals Vc₁, Vc₂and Vc₃are akin to those drawn in FIG. 10. The first set of switches 41, 42 and 43 are closed when the switch control signals Vc₃, Vc₁and Vc₂are high, and open when they are low. The second set of switches 51, 52 and 53 are closed when the switch control signals Vc₃, Vc₁and Vc₂are low, and open when they are high.

[0074] Let us assume that only the first rotor position signal Vh₁is being input to the FIG. 11 speed control circuit 50. The switch 41 will be closed, as from t₉to t₁₅in FIG. 10, under the control of the switch control signal Vc₃from the third binary signal forming circuit 44c, thereby passing the positive half waves of the output (similar to the output Vd₁of the FIG. 7 differentiating circuit 38) from the CR circuit 38a. The capacitor C₁of this CR circuit 38awill be discharged when the switch 41 is open, that is, when the switch 51 is closed, as from t₃to t₉in FIG. 10. There will thus be obtained on the output side of the switch 41 a signal analogous with the first partial motor speed signal Vs₁, FIG. 10, of the FIG. 7 speed detector circuit 6a.

[0075] Similarly, if only the second rotor position signal Vh₂is being input to the FIG. 11 speed control circuit 50, the switches 42 and 52 will be controlled by the switch control signal Vc₁from the first binary signal forming circuit 44a. Thus a signal similar to the second partial motor speed signal Vs₂, FIG. 10, of the FIG. 7 speed detector circuit 6awill be obtained.

[0076] If only the third rotor position signal Vh₃is being input to the FIG. 11 speed control circuit 50, the switches 43 and 53 will be likewise controlled by the switch control signal Vc₂from the second binary signal forming circuit 44b. A signal similar to the third partial motor speed signal Vs₃, FIG. 10, of the FIG. 7 speed detector circuit 6awill then be obtained.

[0077] Unlike their FIG. 7 counterparts bearing the same reference numerals, the first set of switches 41, 42 and 43 of the FIG. 11 speed control circuit 50 have their output sides interconnected for connection to one input of the operational amplifier 57. The second set of switches 51, 52 and 53 are likewise jointly connected to the reference voltage source 58. Consequently, if the target speed signal Vr were to be neglected, the operational amplifier 57 would put out an equivalence of the FIG. 10 motor speed signal Vt, which is produced by the adder 45 of the FIG. 7 speed detector circuit 6ain response to the three incoming partial motor speed signals Vs₁, Vs₂and Vs₃.

[0078] Actually, as the target speed signal Vr is supplied from the target speed signal generator 7, FIG. 1, the operational amplifier 57 will put out the motor speed control signal Vcon by subtracting the target speed signal Vr from the actual motor speed signal Vt. The speed control signal Vcon is to be applied to the FIG. 1 motor excitation circuit 2 for causing the same to excite the three-phase motor windings 9u, 9vand 9wso as to cause motor rotation at the desired speed.

[0079] The operational amplifier 57 functions not merely as differentiator and adder but also as differential amplifier for providing the speed control signal Vcon. The target speed signal Vr on the input line 7ais inverted in polarity into −Vr by the inverter circuit 59 preparatory to delivery to the operational amplifier 57 via the resistor R₄. Therefore, during the conducting periods of the first set of switches 41, 42 and 43, currents will flow from these switches through the feedback resistor R₄. The current that will flow through this feedback resistor R₄due to the negative polarity target speed signal −Vr, on the other hand, is oriented toward the inverter circuit 59. Thus a current subtraction, so to say, will take place at the feedback resistor R₅, resulting in the provision of the speed control signal Vcon, indicative of a difference between actual motor speed signal Vt and target motor speed signal Vr, on the output side of the operational amplifier 57.

[0080] The operational amplifier 57 of the FIG. 11 speed control circuit 50, capable of the triple functions explained above, constitutes the most pronounced feature of this fourth embodiment of the invention. It immensely contributes to the simplicity of this speed control circuit.

[0081] Another advantage of the FIG. 11 embodiment is the removal of d.c. components from the incoming rotor position signals Vh₁, VhH₂and Vh₃by the capacitors C₁, C₂and C₃. Offset-free motor speed control will be possible as only the alternating components are allowed through the capacitors. It is also an advantage that the capacitors C₁, C₂and C₃can be positively reset by the second set of switches 51, 52 and 53 controlled by the phase inversions of the switch control signals Vc₃, Vc₁and Vc₂from the binary signal forming circuits 44c, 44aand 44b.

[0082] Another modified motor speed detector circuit 6bshown in FIG. 12 is somewhat closer in construction to the FIG. 11 speed control circuit 50 than to the FIG. 4 speed detector circuit 6 or to its first modification 6aof FIG. 7. However, the speed detector circuit 6bis so named because it does not input the target speed signal Vr and does put out the actual motor speed signal Vt, rather than the motor control signal Vcon, for delivery to the differential amplifier 8, FIG. 12, over the line 34.

[0083] Thus the inverter circuit 59 and resistor R₄of the FIG. 11 speed control circuit 50 are absent from this speed detector circuit 6b. Further, for providing the motor speed signal Vt as a full-wave rectification of the differentiations of the three rotor position signals Vh₁, Vh₂and Vh₃, the speed detector circuit 6bcomprises a second operational amplifier 60, a second feedback resistor R₆, and another resistor R₇. Other details of construction are the same as in FIG. 11.

[0084] The second operational amplifier 60 has a first input connected to the second set of switches 51, 52 and 53, a second input connected to the reference voltage source 58, and an output connected to its own first input via the feedback resistor R₆and also, via the resistor R₇, to that input of the first operational amplifier 57 to which are connected the first set of switches 41, 42 and 43.

[0085] FIG. 13 shows the waveforms appearing in various parts of the FIG. 12 motor speed detector circuit 6b. Inputting the amplified rotor position signals Vh₁, Vh₂and Vh₃, the binary signal forming circuits 44a, 44band 44cput out the binary switch control signals Vc₁, Vc₂and Vc₃just like its FIG. 7 counterparts with the same reference characters. The first set of switches 41, 42 and 43 will be closed when the switch control signals Vc₁, Vc₂and Vc₃are high, and open when these signals are low. The second set of switches 51, 52 and 53 will be open when the switch control signals Vc₁, Vc₂and VC₃are high, and closed when the signals are low.

[0086] Let us assume that only the first rotor position signal Vh₁is being input to the FIG. 12 motor speed detector circuit 6b. The switch 41 will be closed when the switch control signal Vc₃from the third binary signal forming circuit 44cis high, as from t₉to t₁₅in FIG. 13, thereby passing the positive half waves of the output Vd₁(similar to the output from the FIG. 7 differentiating circuit 38) from the first CR circuit 38a. The switch 51 will be closed when the third switch control signal Vc₃is low, as from T₃to t₉to FIG. 13, thereby passing the negative half waves of the output Vd₁from the first CR circuit 38a. There will thus be obtained the first motor speed signal Vs₁of FIG. 13, which is equivalent to a full-wave rectification of the output Vd₁from the first CR circuit 38a.

[0087] Similarly, if only the second rotor position signal Vh₂is being input to the FIG. 12 speed detector circuit 6b, the switches 42 and 52 will be controlled by the switch control signal Vc₁from the first binary signal forming circuit 44a. Thus the second motor speed signal Vs₂will be obtained as shown in FIG. 13.

[0088] If only the third rotor position signal Vh₃is being input to the FIG. 12 speed detector circuit 6b, the switches 43 and 53 will be likewise controlled by the switch control signal Vc₂from the second binary signal forming circuit 44b. The third motor speed signal Vs₃will then be obtained as shown in FIG. 13.

[0089] Like their FIG. 11 counterparts the first set of switches 41, 42 and 43 are all connected to one input of the first operational amplifier 57. The second set of switches 51, 52 and 53, on the other hand, are all connected to one input of the second operational amplifier 60. The output of the second operational amplifier 60 is connected to the same input of the first operational amplifier as are the first set of switches 41, 42 and 43. Thus the second operational amplifier 60 combines the rectifications of the negative half waves of the voltages Vd₁, Vd₂and Vd₃. The first operational amplifier 57 combines the rectifications of the positive half waves of the voltages Vd₁, Vd₂and Vd₃and, further, the output from the second operational amplifier 60.

[0090] It is thus seen that when the three rotor position signals Vh₁, Vh₂and Vh₃are all being input to the FIG. 12 speed detector circuit 6b, the first operational amplifier 57 puts out the unidirectional motor speed signal Vt of FIG. 13 by adding the three motor speed signals, Vs₁, Vs₂and Vs₃. The thus obtained motor speed signal Vt is notable for its low ripple percentage. It is understood that this motor speed signal Vt is directed into the differential amplifier 8, FIG. 1, thereby to be used for production of the motor speed control signal Vcon to be applied to the motor excitation circuit 2.

[0091] The FIG. 12 speed detector circuit 6bis readily modifiable into a motor speed control circuit shown in FIG. 14 and therein generally designated 50a, by incorporating an inverter circuit 59 and a resistor R₄. As in the FIG. 11 speed control circuit 50, the target speed signal Vr on the input line 7ais directed into the first operational amplifier 57 via the inverter circuit 59 and resistor R₄. The motor speed control circuit 50ais similar to the FIG. 12 speed detector circuit 6bin the other details of construction.

[0092] The first operational amplifier 57 functions not only as differentiator and adder but as differential amplifier for production of the motor speed control signal Vcon. That is because the target speed signal Vr on the input line 7ais inverted in polarity before being fed into the first operational amplifier 57 via the resistor R₄. A current will flow through the feedback resistor R₅of the first operational amplifier 57 toward the inverter circuit 59 because of the negative polarity of the target speed signal. A current subtraction will thus take place at the feedback resistor R₅, with the result that the first operational amplifier 57 provides the speed control signal Vcon as a difference between actual motor speed signal Vt and target speed signal Vr.

[0093] In FIG. 15 is shown a modification 6cof the FIG. 12 speed detector circuit 6b. Instead of being connected to one of the inputs of the first operational amplifier 57 as in the FIG. 12 circuit 6b, the output of the second operational amplifier 60 of this modified circuit 6cis connected via an inverter circuit 70 to an adder 71, to which is also connected the output of the first operational amplifier 57. The other details of construction are as set forth above in connection with FIG. 12.

[0094] The output from the first operational amplifier 57 and the inversion of the output from the second operational amplifier 60 are added together by the adder 71 to provide the motor speed signal Vt for delivery to the differential amplifier 8, FIG. 1, over the line 34. The thus obtained motor speed signal Vt is similar to that produced by the FIG. 12 circuit 6b.

[0095] The present invention is applicable to the speed control of motors with brushes. In FIG. 16 is shown a modification of the FIG. 1 speed control system. The modified speed control system for a d.c. motor 1awith a pair of brushes 81 and 82 has a disk magnet 84. The d.c. motor 1acomprises a stator 9a, a rotor 10a, a brush assembly 80 with the brushes 81 and 82, and a shaft 83. As illustrated in FIG. 17, the stator 9acomprises a pair of magnets 86 and 87. The rotor 10acomprises a winding 85 and a core (not shown). The winding 85 is connected to a motor excitation circuit 2a. The motor excitation circuit 2acomprises a d.c. power supply 16 and a speed control circuit 17. The d.c. power supply 16 and the speed control circuit 17 in FIG. 17 have composition which is same as FIG. 3 counterparts 16 and 17. The speed control circuit 17 functions to control the power fed from the power supply 16 to the winding 85.

[0096] The shaft 83 of the rotor 10ais combined in the center of the disk magnet 84. Therefore, the disk magnet 84 rotates in accordance with the rotor 10a. The disk magnet 84 is divided into eight north and eight south seeking magnetic pole sections N and S alternating in its circumferential direction. The three magnetoelectric converts 3, 4 and 5, the speed detector circuit 6, the target speed signal generator 7, and the differential amplifier 8 in FIG. 16 have the composition which is the same as FIG. 1 counterparts 3-8.

[0097] The first second and third magnetoelectric converters 3, 4 and 5, are furnished on the stator 9aand are so arranged as to detect magnetic fluxes due to the magnet 84. The electrical angles between the magnetoelectric converters 3-5 in FIG. 16 disposed along the disk magnet 8 are 120 degrees. Also, the relationship between the disk magnet 84 and the magnetoelectric converts 3-5 in FIG. 16 correspond with the relationship between the rotor 10 with the magnetic pole sections N and S and the magneto electric converter 3-5 in FIG. 2. Therefore, with the rotation of the rotor 10aand the disk magnet 84 relative to the stator 9, the first to third magnetoelectric converts 3-5 will provide first to third rotor position signals Vh₁, Vh₂and Vh₃, respectively, in the form of alternating voltages of sinusoidal waveform with a phase lag of 120 degrees from one to the next (FIG. 5).

[0098] The teachings of FIG. 16 using the disk magnet 84 are applicable not only to the FIG. 1 embodiment but to the FIGS. 6, 7, 11, 12, 14 and 15 embodiments as well.

[0099] Although the motor speed control system according to the present invention has been shown and described in several preferable forms, it is not desired that the invention be limited by the exact showing of the drawings or the description thereof. The following, then, is a brief list of possible modifications or alterations of the illustrated embodiments:

[0100] 1. The teachings of FIG. 6, using but two magnetoelectric converters for providing three rotor position signals Vh₁, Vh₂and Vh₃, are applicable not only in the FIG. 1 embodiment but to the FIGS. 7, 11, 12, 14 and 15 embodiments as well.

[0101] 2. Contrary to the showing of the waveform diagrams of FIGS. 5, 10 and 13, the motor speed signals Vs₁, Vs₂and Vs₃could be of negative polarity.

[0102] 3. Also with respect to the waveform diagrams of FIGS. 5, 10 and 13, the negative half waves of the outputs Vd₁, Vd₂and Vd₃from the differentiating circuits could be extracted by the switches 41, 42 and 43 of the pertinent embodiments.

[0103] 4. The delay circuits 47a, 47band 47cof the FIGS. 11, 12, 14 and 15 embodiments could be omitted, and the wave-shaping circuit 46a, 46band 46ccould be connected directly to the amplifiers 35, 36 and 37.

[0104] 5. The input line 7, the inverter circuit 59 and the resistor R₄of the FIG. 11 embodiment could be omitted, and the output of the operational amplifier 57 could be connected to the differential amplifier 8 of the FIG. 1.

[0105] 6. The invention is applicable to d.c. motors with brushes, and alternating current motors.

[0106] These and other changes within the usual knowledge of the specialists are intended in the foregoing disclosure. It is therefore appropriate that the invention be construed broadly and in a manner consistent with the fair meaning or proper scope of the claims which follow.