Absolute magnetic encoder

The encoder rotor of an absolute magnetic encoder mounted on a servomotor shaft has a first drum of a bipolar magnet, and a second drum with a Q-bit multipolar magnetic pole track and a reference track. A signal processor generates absolute signals on the basis of detected signals, which differ in phase by 90° and in which a single rotation represents a single period, from X-phase and Y-phase magnetic sensors disposed facing the first drum, and on the basis of A- and B-phase signals, which differ in phase by 90°, and a reference signal obtained from A-, B-, and Z-phase magnetic sensors disposed facing the second drum. Even if the number of bits Q of the multipolar magnetic pole track is increased in order to enhance the resolution, the number of magnetic pole tracks does not need to be increased, and higher resolution can therefore be obtained without increasing the axial length.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an absolute magnetic encoder for generating absolute signals that indicate an absolute rotational position of a servomotor shaft or other rotational body, and more particularly relates to an absolute magnetic encoder capable of generating high-resolution absolute signals without increasing the number of magnetic pole tracks formed on an external peripheral surface of a magnetic drum and increasing the axial length.

2. Description of the Related Art

FIGS. 9A and 9Bare a front view and side view showing a configuration of a detection portion of a commonly known absolute magnetic encoder. As shown in the drawings, N tracks, or four magnetic pole tracks102to105ranging from 20to 23are aligned in the direction of a drum axis101aon the external peripheral surface of a magnetic drum101fixed to the motor shaft of the measurement object or another rotational body. Hall sensors or other magnetic sensors202to205are disposed facing the magnetic pole tracks102to105, respectively. Shown inFIG. 10are bit signals for each digit obtained from the magnetic sensors202to205for these digits.

In an absolute magnetic encoder with this configuration, when the number of bits is increased in order to enhance the resolution, the number of tracks increases proportionately and the axial length of the magnetic drum101increases as well. This becomes an impediment to reducing the size and weight of a high-resolution absolute magnetic encoder.

SUMMARY OF THE INVENTION

A main object of the present invention is to provide an absolute magnetic encoder that can produce high resolution without increasing the number of magnetic pole tracks formed on the external peripheral surface of a magnetic drum.

To solve the above and other problems, an absolute magnetic encoder of the present invention has a rotor provided with first and second drums fixed coaxially to a rotational member to be measured; a bipolar magnet formed on an external peripheral surface of the first drum; a multipolar track provided with Q-bit (where Q is a positive integer) magnetic poles formed at equiangular intervals on an external peripheral surface of the second drum; a reference track provided with Z (where Z is a positive integer) reference magnetic poles formed at equiangular intervals on the external peripheral surface of the second drum; an X-phase magnetic sensor and a Y-phase magnetic sensor which are disposed facing the external peripheral surface of the first drum and which output an X-phase signal and Y-phase signal that differ in phase by 90°; an A-phase magnetic sensor and a B-phase magnetic sensor which are disposed facing the external peripheral surface of the multipolar track and which output an A-phase signal and B-phase signal that differ in phase by 90°; a Z-phase magnetic sensor which is disposed facing an external peripheral surface of the reference track and which outputs a reference signal for indicating a position of the reference magnetic poles; a first rotor angle computing circuit for computing with P bits (precision α) a rotational angle of the rotor on the basis of the X-phase signal and Y-phase signal; a second rotor angle computing circuit for computing with Q bits (precision β(>α)) the rotational angle of the rotor on the basis of the computational results of the first rotor angle computing circuit and on the basis of the A-phase signal, B-phase signal, and Z-phase signal; and an absolute signal generating circuit for generating an absolute signal that indicates an absolute rotational position of the rotor on the basis of the computational results of the first and second rotor angle computing circuits.

Here, the absolute signal generating circuit may be configured so as to generate the absolute signal on the basis of the computational results of the first rotor angle computing circuit and on the basis of the A- and B-phase signals in an interval of time beginning from the start of rotation of the rotor until the time when the first reference signal is output, to correct the absolute rotational angle on the basis of the reference signal at the time when the reference signal is output, and to generate the absolute signal on the basis of the computational results of the second rotor angle computing circuit after the reference signal has been output.

Also, the first drum and the second drum may be structured as separate members or formed as a single magnetic drum.

In the present invention, X-phase and Y-phase signals which differ in phase by 90° and in which a single rotation represents a single period are generated using a bipolar magnet, and the absolute position of the rotor during initial operation can be detected using these signals and the A- and B-phase signals until the reference signal is generated. After the reference signal has been generated, the absolute position of the rotor can be detected using the X- and Y-phase signals together with the A- and B-phase signals and the reference signal.

Therefore, even if the number of bits of a multipolar track for obtaining A- and B-phase signals is increased to enhance the resolution, the number of magnetic pole tracks does not need to be proportionally increased, as is the case with the prior art. A high-resolution absolute magnetic encoder can thereby be obtained without an increase in the axial length.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Described below with reference to the drawings is an example of a servomotor provided with an absolute magnetic sensor to which the present invention has been applied.

FIG. 1is a schematic diagram showing a servomotor in accordance the present invention. The basic configuration of a servomotor1is the same as that of a common servomotor. Both ends of a cylindrical motor housing2are sealed with end brackets3and4, and a motor shaft5is disposed so as to completely pass through a center portion of the end brackets3and4. The motor shaft5is rotatably supported by the end brackets3and4via bearings6and7. A motor rotor8is integrally formed in a coaxial state in a portion of the motor shaft5inside the motor housing2, and a motor stator9is fixed to an internal peripheral surface of the motor housing2with a fixed gap around an outside of the motor rotor8. A motor winding10is mounted on the motor stator9.

A front end portion5aof the motor shaft5protrudes forward from the front end bracket3, and is connected to a load side via a reduction gear or the like. A rear end portion5bof the motor shaft5protrudes rearward from the rear end bracket4, and an absolute magnetic encoder20is disposed on the rear end portion5b.The absolute magnetic encoder20is covered with an encoder cover20amounted on a rear end portion of the motor housing2.

The absolute magnetic encoder20has an encoder rotor23provided with a first drum21and a second drum22that are fixed coaxially on the rear end portion5bof the motor shaft5, a first magnetic sensor group24disposed facing an external periphery of the first drum21, a second magnetic sensor group25disposed facing an external peripheral surface of the second drum22, and a signal processor27mounted on an encoder circuit board26. A servo driver40of the servomotor1controls to drive the servomotor1in accordance with commands from a host system50on the basis of absolute signals or other signals output from the absolute magnetic encoder20. The servo driver40has a receiver circuit unit41for receiving signals from the absolute magnetic encoder20, and a control circuit unit42that includes a control computational circuit, a power drive circuit, and other components.

FIG. 2is a diagram showing the first drum21and first magnetic sensor group24.FIG. 3is a signal waveform diagram showing detection signals of the first sensor group24. The external peripheral surface of the first drum21is formed by a bipolar magnet21a,and the first magnetic sensor group24disposed facing this external peripheral surface includes an X-phase sensor24X and a Y-phase sensor24Y in positions that differ in phase by 90°. Therefore, as shown inFIG. 3, X- and Y-phase signals can be obtained in the form of sinusoidal waves that are offset in phase by 90° as the first drum21rotates.

FIGS. 4A and 4Bare front and side diagrams showing the second drum22and second magnetic sensor group25. A multipolar track31and a reference track32are formed on an external peripheral surface22aof the second drum22so as to be aligned in a direction of a drum axis22b.The multipolar track31includes Q-bit (where Q is a positive integer) magnetic poles formed at equiangular spacing, and the reference track32includes Z (where Z is a positive integer) reference magnetic poles formed at equiangular intervals. The second magnetic sensor group25disposed facing the outside peripheral surface of the second drum22includes an A-phase magnetic sensor33and a B-phase magnetic sensor34disposed facing the multipolar track31, and a Z-phase magnetic sensor35disposed facing the reference track32. A-phase and B-phase magnetic signals that differ in phase by 90° are output from the A-phase magnetic sensor33and B-phase magnetic sensor34, and Z-phase signals (reference signals) that indicate a position of the reference magnetic poles are output from the Z-phase magnetic sensor35.FIG. 5shows signal waveforms of these A-phase, B-phase, and Z-phase signals.

FIG. 6is a schematic block diagram of the signal processor27of the absolute magnetic encoder20. The signal processor27has waveform-shaping circuits61and62for the X-phase and Y-phase signals, and a first rotor angle computing circuit63for computing with precision α a rotational angle of an encoder rotor23on the basis of the X-phase and Y-phase signals after waveform shaping. The signal processor27has waveform-shaping circuits64,65, and66for shaping the waveforms of the A-phase, B-phase, and Z-phase signals (reference signal), respectively; a second rotor angle computing circuit67for computing with precision β(>α) the rotational angle of the encoder rotor23on the basis of the waveform-shaped signals and on the basis of the computational results of the first rotor angle computing circuit63; and an absolute signal generating circuit68for generating an absolute signal that indicates an absolute position of the encoder rotor23on the basis of the computational results of the first and second rotor angle computing circuits63and67. The absolute signal generated by the absolute signal generating circuit68is output to the servo driver40via an output circuit69.

The detection operation of the absolute magnetic encoder20with this configuration is described with reference toFIG. 7. The absolute signal is generated at the initial startup of the servomotor1, in other words, in the interval of time beginning from a time t0when the rotation of the encoder rotor23rotating integrally with the motor shaft starts until a time t1when the first reference signal Z is output, on the basis of the computational results of the first rotor angle computing circuit63and on the basis of the A- and B-phase signals. A detection precision at this time is α(=360/(2^P×2)), and the resolution is P bits. That is to say, the absolute magnetic sensor20operates as an absolute encoder with precision α and a resolution of P bits.

When the initial reference signal Z is output, the second rotor angle computing circuit67corrects an absolute position on the basis of the reference signal Z, and from a time t2after correction the absolute signal generating circuit68generates an absolute signal on the basis of the computational result of the second rotor angle computing circuit67. Therefore, the absolute magnetic encoder20operates from the time t2as an absolute encoder with precision β(=360/(2^Q×2)) and resolution Q. It should be noted that the number of reference signals Z per rotation is a positive integer that is 1 or greater, and the relationship between the bits is 2^Q>2^P>Z

Thus, in the absolute magnetic encoder20of the present example, X-phase and Y-phase signals which differ in phase by 90° and in which a single rotation represents a single period are generated using the bipolar magnet, and the absolute position of the rotor during initial operation until the reference signal Z is generated is detected using these signals and the A- and B-phase signals. After the reference signal Z has been generated, the absolute position of the encoder rotor23is detected with high precision using the X- and Y-phase signals together with the A- and B-phase signals and the reference signal Z. Therefore, even if the number of bits Q of the multipolar track31is increased in order to enhance the resolution, the number of magnetic pole tracks does not need to be proportionally increased. A resulting advantage is that a high-resolution encoder can be obtained without increasing the axial length, and motors and other rotational machines in which the encoder is mounted can be made smaller and more compact.

OTHER EMBODIMENTS

The encoder rotor23in the above-described example is configured with the first drum31and the second drum32. These may be formed instead as a single magnetic drum.FIGS. 8A and 8Bare front and side diagrams showing an encoder rotor23A comprising a single magnetic drum71. In these diagrams, the same reference numerals are assigned to the portions that correspond to those inFIGS. 2 and 4.