APPARATUS FOR DETECTING SPEED OF ELECTRIC MOTOR

Abstract: An electric motor-speed detecting apparatus which includes: a first gear (3) with one notched groove which is secured to a rotating shaft (2) of an electric motor (1) driven by an electric-motor driving unit (7); a second gear (4) with a plurality of notched grooves; a first magnetic sensor (5) which is provided in close proximity to the first gear (3) and adapted to output two signals with a phase difference; a second magnetic sensor (6) which is provided in close proximity to the second gear and adapted to output two signals with a phase difference and inverted signals thereof; and a detection circuit (8) which receives the respective outputs of the first and second magnetic sensors. The detection circuit (8) differentially amplifies the two output from the first magnetic sensor (5) and compares the same with a reference voltage and then outputs one pulse per one full turn of the rotating shaft (2). The detection circuit (8) further subjects the four outputs from the second magnet sensor (6) to differential amplification; inverting amplifica­ tion; comparison; and vector comparison composition, whereby pulses are generated at intervals of 22.5°, deter­ mined on the basis of the fact that the phase of the output waveform of the magnetic sensor corresponding to the width of the notched groove is 180°. Consequently, it is possible to output sine waves which respectively correspond to the notched grooves of the second gear (4) and have a phase difference of 90° in relation to each other and a rectangular wave of frequency four times as great as the number of the notched grooves of the second gear (4) as well as a rectangular wave having a phase difference of a 1/4 period in relation to the former rectangular wave. Employ­ ment of this apparatus makes it possible to detect the speed of even a rotating shaft (2) which is hollow and to obtain detection accuracy four times as high as that obtained by the conventional apparatus.

Description: TECHNICAL FIELD The present invention relates to a motor speed detecting device, and more particularly, to a motor speed detection device utilizing a magnetic sensor and a slotted disc having an undercut notched wedge and mounted on the rotational shaft of the motor. BACKGROUND ART In the prior art, a motor speed detecting device comprises a coded disc composed of a glass plate mounted on the rotational shaft of the motor, and as an opaque portion of the coded disc passes between a light source and a light sensor, an electrical signal is obtained from the light sensor. The electric signal is then amplified to shape the waveform, thus providing a series of pulses in response to the rotational speed of the shaft. Usually, this device outputs two types of pulses, that is to say, one pulse per one revolution of the coded disc and a plurality of pulses per one revolution of the coded disc. The device counts the number per hour of these pulses, and thus detects the rotational speed of the motor. There is an unavoidable problem wherein, when the detected rotational shaft is hollow, the above-described speed detection device cannot be applied. When a motor is used for driving a machine tool such as a lathe and the like, it is necessary to render a drive rotational shaft hollow and to make a cylindrical workpiece pass therethrough, on the basis of the mechanism for delivering the workpiece. In such a case, the apparatus using the coded disc cannot be adapted from the structural point of view, although it is recognized that the interference from other sources of light considerably affects for the worse the apparatus using the coded disc, and there remains a problem in that the precision of the device cannot always be maintained. United States Patent Specification No. US-A-4024458, from which the preamble of the attached single claim has been derived, describes a system for generating a periodic output signal of a predetermined duration and another series of output signal pulses of a predetermined frequency. The described system uses reference pulses and differential amplifiers processing signals coming from magnetic sensors. The system is used in an automotive-type internal combustion ignition system, and is mainly concerned with an improved electrical signal generating and multiplying system which generates two phase displaced alternating current waveforms which are employed by additional separate gating circuitry combinations, a periodic output electrical signal of a predeterminable duration, and a separate series of output electrical signal pulses of a frequency which is a selected integer multiple of the frequency of the generated alternating current waveforms. The system of US-A-4024458 is concerned with the change of the phase relationship between two phase displaced magnetic pick-up generated alternating current waveforms at predetermined intervals. A gating circuit combination responsive thereto initiates and terminates periodic output signals upon alternate changes of phase relationship. Another gating circuit combination, responsive to the generated waveforms and other equally phase displaced auxiliary alternating current waveforms produced from the generated waveforms, produces a series of other output signal pulses of a frequency which is a multiple of the generated waveforms as determined by the total number of generated and auxiliary waveforms. The system does not generate one signal for each rotation, nor does it deliver analog output signals suitable for control of a motor. The present invention is intended to provide an improved motor speed detecting device. According to the present invention, there is provided a motor speed detecting circuit comprising a slotted disc of ferromagnetic material having a plurality of slots and mounted on a rotational shaft of a motor driven by a motor drive unit, a magnetic sensor mounted in close proximity to said disc for outputting two signals having a predetermined phase difference between one another, a detector circuit for receiving and detecting the output of the sensor, the detector circuit comprising a vector synthetic comparator circuit for generating signals having predetermined relative phase relationships from the output signals of the sensor, and a logic circuit for processing the output signals of the vector synthetic comparator circuit, sinusoidal waves being produced to control the motor drive unit and the logic circuit providing a rectangular wave the frequency of which corresponds to four times the number of said plurality of slots, characterized in that first and second slotted ferromagnetic discs and first and second magnetic sensors are provided, the magnetic sensors comprising magnetic resistance effect elements and permanent magnets, the first slotted disc having one slot and the second slotted disc having a plurality of slots, the first magnetic sensor being mounted in close proximity to the said first slotted disc for outputting two signals having a phase difference corresponding to one half of the width of said one slot, the second magnetic sensor being mounted in close proximity to said second slotted disc for outputting, in respect of each of said plurality of slots, a first pair of signals having a phase difference corresponding to the width of one half of one of said plurality of slots, and a second pair of signals each of which is an inversion of a respective one of said first pair of signals, and the detector circuit receiving the output of said first and second magnetic sensors, said detector circuit comprising said vector synthetic comparator circuit, a first differential amplifier circuit for detecting the phase difference between the said two signals from said first magnetic sensor, a first comparator circuit for comparing the output from said first differential amplifier circuit with a reference voltage, a second differential amplifier circuit and a third differential amplifier circuit for receiving the signals output by said second magnetic sensor, the second differential amplifier circuit receiving a first one of said first pair of signals and the inversion of that signal from said second pair of signals and detecting the difference therebetween, and the third differential amplifier circuit receiving the second one of said first pair of signals and the inversion of that signal from said second pair of signals and detecting the difference therebetween, a first inverting amplifier circuit for inverting and amplifying the output of said second differential amplifier circuit, and a second inverting amplifier circuit for inverting and amplifying the output of said third differential amplifier circuit, the said vector synthetic comparator circuit receives the outputs of said second and third differential amplifier circuits, generates signals having respective phase angles of 0, 22.5, 45, 67.5, 90, 112.5, 135 and 157.5 degrees when the phase angle of the signal derived from the output waveform of the first magnetic sensor is 180 degrees, and compares said signals with said reference voltage to generate a series of eight rectangular waves, the logic circuit is responsive to the output of said vector synthetic comparator circuit, the outputs of said first and second inverting amplifier circuits are sinusoidal waves that are used to control said motor through said motor drive unit, one pulse per revolution of said rotational shaft is provided at the output of said first comparator circuit, a rectangular wave having a frequency four times as large as the frequency corresponding to the number of slots of said second disc is provided at the output of said logic circuit, and a further rectangular wave having a 90 degrees phase difference from said rectangular wave is provided at the output of said logic circuit. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of the constitution of a motor speed detecting device according to an embodiment of the present invention; Fig. 2 is a diagram showing a slotted disc and magnetic sensor portion in Fig. 1; Fig. 3 is a diagram showing the first slotted disc and the first magnetic sensor in Fig. 2; Fig. 4 is a diagram showing the second slotted disc and the second magnetic sensor in Fig. 2; Fig. 5 is a waveform diagram showing the output waveforms of the magnetic sensor in Fig. 2; Figs. 6A and 6B are block circuit diagrams of each detector circuit in Fig. 1; Fig. 7 is a circuit diagram of a vector synthetic comparator in Figs. 6A and 6B; and Fig. 8 is a waveform diagram showing the waveforms of signals in the circuits of Figs. 6A and 6B. BEST MODE FOR CARRYING OUT THE INVENTION The constitution of the motor speed detecting device and the slotted disc and magnetic sensor portion thereof are shown respectively in Fig. 1 and Fig. 2 in accordance with an embodiment of the present invention. The device comprises a motor 1 having a rotational shaft 2, a first slotted disc 3, a second slotted disc 4, a first magnetic sensor 5, a second magnetic sensor 6, a detector circuit 8, and a motor drive unit 7. The discs 3 and 4 are composed of ferromagnetic materials and are fixed to the rotational shaft 2. A slot 31 is cut in the disc 3, as shown in Fig. 3. The magnetic sensor 5 is mounted opposite to the disc 3. The magnetic sensor 5 contains a magnetic resistance effect element (denoted by a resistor symbol in the figure) and a permanent magnet (not shown). When the width of the slot 31 is taken as λ, the magnetic resistance effect element is arranged in the peripheral direction of the disc 3 by a spacing of λ/2. The permanent magnet is disposed in relation to the disc 3 to sandwich therebetween the magnetic resistance effect element. When the slot of the disc 3 is opposite the permanent magnet, it is likely to weaken a magnetic field that is provided to the magnetic resistance effect element. The magnetic resistance effect element has the property of varying the value of an electric resistance in response to the strength of the magnetic field. Consequently, when the magnetic resistance effect element is connected its a way such as shown in Fig. 3, and applied to a power source, and the slot of the disc 3 passes by in close proximity to the magnetic sensor 5, both outputs Z and Z of the magnetic sensor 5 have a signal waveform as shown in the waveform (1) of Fig. 5. The disc 4 is provided with a plurality of slots as shown in Fig. 4. To simplify the explanation, in the figure the width of the slot is shown enlarged and the slots arranged uniformally around the periphery are shown by chain lines. The magnetic sensor 6 comprises two magnetic resistance effect elements, in comparison with the magnetic sensor 5 of Fig. 3, to provide the inverted outputs having phase angles different by 180 degrees A and B of the A and B outputs, other than A and B outputs in response to the Z and Z outputs of the magnetic sensor 5. If the width of slot is λ, as shown by the resistance symbol in Fig. 4, in the set of elements providing A and B outputs each element is arranged with a spacing of λ/2, in a set of elements providing A and B outputs each are arranged, for instance, with a spacing of λ/2, and the power source is connected in opposite polarity therewith, to be further arranged with a spacing of 2λ to a set of the elements. In this way, with the output of the magnetic sensor 6, an output is provided such as the waveforms (2) and (3) shown in Fig. 5. The passage of the disc 4 in the neighbourhood of a permanent magnet causes the magnetic field to vary; this is the same operational principle, in which the resistance value of the magnetic resistance effect element is changed to provide an output signal, as that in the case of the disc 3 and the magnetic sensor 5. A block circuit diagram of the detector circuit 8 is shown in Fig. 6A and Fig. 6B. The outputs Z and Z from the magnetic sensor 5 are supplied to a differential amplifier 813 as the first differential amplifier circuit to provide the difference signal. The difference signal is compared with a reference voltage, for instance, 2.5 V, in a comparator 849 as the first comparator circuit, so one rectangular wave per one revolution of the disc is output through a driver 873. The outputs A and A from the magnetic sensor 6 are applied to a differential amplifier 811 as the second differential amplifier circuit to count the difference, and are then supplied to a vector synthetic comparator 846 to 848 forming a vector synthetic comparator circuit, through a comparator 841, vector synthetic comparators 842 to 844 forming a vector synthetic comparator circuit, and an inverter 831. The outputs B and B from the magnetic sensor 6 are applied to a differential amplifier 812 as the third differential amplifier circuit to count the difference, and are then supplied to a comparator 845 and vector synthetic comparators 842 through 844, and 846 through 848 forming the vector synthetic comparator circuit. Each vector synthetic comparator 842, 843, 844, 846, 847, and 848 comprises an operational amplifier 91 and resistors respectively as shown in Fig. 7. The resistance values differ in each vector synthetic comparator. Considering the phase of the output signal S (811) of the differential amplifier 811 as a reference, as the output signal S (812) of the differential amplifier 812 has a phase difference of 90 degrees, these two signals S (811) and S (812) are added with a proper amplitude ratio to provide a signal of a desired phase difference. That is to say, when the signal S (811) of a phase difference of 0 degrees is applied to an input terminal IN1, and the signal S (812) of a phase difference of 90 degrees to an input terminal IN2, as shown in Fig. 7, then if the resistance of resistor R1 and the resistance of resistor R2 are selected to satisfy the equation: R1/R2 = tan 22.5° a signal having a phase difference of 22.5 degrees is applied to the input of the operational amplifier 91 and compared with a reference voltage (V ref ) for instance, 2.5 V, to output the rectangular signal having a phase difference of 22.5 degrees. A vector synthetic comparator 843 selects the resistors R1 and R2 so as to satisfy the equation: R1/R2 = tan 45° = 1 to provide a rectangular wave signal having a phase difference of 45 degrees at the output. On the other hand, a vector synthetic comparator 844 selects a resistor R1 and R2 so as to satisfy the equation: R1/R2 = tan 67.5° to provide a rectangular wave signal having a phase difference of 67.5 degrees at the output. The signal S (812) is applied to the input IN1 of the vector synthetic comparator 846 and an inverted signal of the signal S (811) to the input IN2 through an inverter 831, so the resistors R1 and R2 are selected so as to satisfy the equation: R2/R1 = |tan 112.5°| to provide a rectangular wave signal having a phase difference of 112.5 degrees at the output. Similarly a vector synthetic comparator 847 selects resistors R1 and R2 so as to satisfy the equation: R2/R1 = |tan 135°| = 1 to provide a rectangular wave signal having a difference of 135 degrees. A vector synthetic comparator 848 selects resistors R1 and R2 so as to satisfy the equation: R2/R1 = |tan 157.5°| to provide a rectangular wave having a phase difference of 157.5 degrees at the output. The signal S (811) is applied to the comparator 841, and compared with the reference voltage, to output a rectangular wave having a phase difference of zero degrees, while the signal S (812) is supplied to the comparator 845, to output a rectangular wave having a phase difference of 90 degrees. If the output signals of the above-mentioned vector synthetic comparator 842 to 844 are respectively S (842) to S (844), the output signals of the vector synthetic comparator 846 to 848, respectively S (846) to S (848), and the output signals of the comparator 841 and 845, respectively S (841) and S (845), the waveforms of the signals S (841) to S (848) are as shown in Fig. 8. The signals S (841) and S (843) are applied to the exclusive OR gate 851 to provide an output signal S (851). The signals S (842) and S (844) are applied to the exclusive OR gate 852 to provide an output signal S (852). The signals S (845) and S (847) are applied to the exclusive OR gate 853 to provide an output signal S (853). The signals S (846) and S (848) are applied to the exclusive OR gate 854 to provide an output signal (854). The signals S (851) and S (853) are applied to the OR gate 861 to provide the output signal S (861). The signals (852) and S (854) are applied to the OR gate 862 to provide the output signal S (862). The waveforms of the above-mentioned signals S (851) to S (854), S (861) and S (862) are as shown in Fig. 8. If the signal S (861) is compared with the signal S (841) derived from the signals A and A , the frequency is four times larger and there is a phase difference of 90 degrees between the signal s (861) and the signal S (862). The signals S (861) and S (862) are supplied respectively through the driver 871 and 872 as the output of the detector circuit 8. Additionally, the signal S (811) is output through the inverting amplifier 821 as the first inverting amplifier circuit to provide the sinusoidal wavesignal to the above-mentioned motor drive unit 7, while the signal S (812) is output through the inverting amplifier 822 as the second inverting amplifier circuit to provide the sinusoidal wave signal to the motor drive unit 7 in the same manner. As mentioned above, according to an embodiment of the present invention, there is provided one pulse per one revolution of the rotational shaft of the motor, the sinusoidal wave signal per one revolution being equivalent to the number of slots of the slotted disc having a plurality of slots mounted on the rotational shaft, the sinusoidal wave signal having the phase difference of 90 degrees to the sinusoidal signal and the rectangular wave signals having the phase difference of 90 degrees to each other at four times the frequencies of the sinusoidal signals.

Claims: A motor speed detecting circuit comprising a slotted disc (4) of ferromagnetic material having a plurality of slots and mounted on a rotational shaft of a motor driven by a motor drive unit, a magnetic sensor (6) mounted in close proximity to said disc for outputting two signals having a predetermined phase difference between one another, a detector circuit (8) for receiving and detecting the output of the sensor (6), the detector circuit comprising a vector synthetic comparator circuit (842-844, 846-848) for generating signals having predetermined relative phase relationships from the output signals of the sensor, and a logic circuit for processing the output signals of the vector synthetic comparator circuit, sinusoidal waves being produced to control the motor drive unit and the logic circuit providing a rectangular wave the frequency of which corresponds to four times the number of said plurality of slots, characterized in that first and second slotted ferromagnetic discs (3, 4) and first and second magnetic sensors (5, 6) are provided, the magnetic sensors comprising magnetic resistance effect elements and permanent magnets, the first slotted disc (3) having one slot and the second slotted disc (4) having said plurality of slots, the first magnetic sensor (5) being mounted in close proximity to the said first slotted disc for outputting two signals (Z, Z ) having a phase difference corresponding to one half of the width of said one slot, the second magnetic sensor (6) being mounted in close proximity to said second slotted disc for outputting, in respect of each of said plurality of slots, a first pair of signals (A, B) having a phase difference corresponding to the width of one half of one of said plurality of slots, and a second pair of signals ( A , B ) each of which is an inversion of a respective one of said first pair of signals, and the detector circuit (8) receiving the output of said first and second magnetic sensors, said detector circuit comprising said vector synthetic comparator circuit, a first differential amplifier circuit (813) for detecting the phase difference between the said two signals (Z, Z ) from said first magnetic sensor (5), a first comparator circuit (849) for comparing the output from said first differential amplifier circuit with a reference voltage, a second differential amplifier circuit (811) and a third differential amplifier circuit (812) for receiving the signals output by said second magnetic sensor, the second differential amplifier circuit (811) receiving a first one of said first pair of signals (A,B)and the inversion of that signal from said second pair of signals ( A , B ) detecting the difference therebetween, and the third differential amplifier circuit (812) receiving the second one of said first pair of signals (A,B) and the inversion of that signal from said second pair of signals ( A , B ) detecting the difference therebetween, a first inverting amplifier circuit (821) for inverting and amplifying the output of said second differential amplifier circuit, and a second inverting amplifier circuit (822) for inverting and amplifying the output of said third differential amplifier circuit, the said vector synthetic comparator circuit receives the outputs of said second and third differential amplifier circuits, generates signals having respective phase angles of 0, 22.5, 45, 67.5, 90, 112.5, 135 and 157.5 degrees when the phase angle of the signal derived from the output waveform of the first magnetic sensor is 180 degrees, and compares said signals with said reference voltage to generate a series of eight rectangular waves, the logic circuit (851-854, 861, 862) is responsive to the output of said vector synthetic comparator circuit, the outputs of said first and second inverting amplifier circuits (821, 822) are sinusoidal waves that are used to control said motor through said motor drive unit, one pulse per revolution of said rotational shaft is provided at the output of said first comparator circuit (849), a rectangular wave having a frequency four times as large as the frequency corresponding to the number of slots of said second disc is provided at the output of said logic circuit, and a further rectangular wave having a 90 degrees phase difference from said rectangular wave is provided at the output of said logic circuit.

Inventors: ISHIDA, HIROSHI, TANIGUCHI, MITSUYUKI
Applicants: FANUC LTD.
Application year: 1984
Main classification: G01P3
IPC codes: G01P3, G01D5