1. Field of the Invention
The present invention relates to a galvano motor and a galvano motor system which scan or deflect an optical axis of light.
2. Description of the Related Art
Recently, there has been a processing machine such as a laser drilling machine, a laser marker, or an optical forming machine, which continuously scans or positions to irradiate a laser light beam, heats to burn an object to be processed by the laser energy, and sublimates or hardens a light curing resin to process it to be a desired shape. A galvano motor has been used for scanning or positioning the laser light beam in the processing machine.
The characteristics required for the galvano motor is a response and an accuracy of the scanning or the positioning. In order to improve the response of the galvano motor, it has been designed in considering lowering inertia of the motor, heightening the torque, and lowering impedance. Furthermore, when a galvano motor system is constituted by the combination of the galvano motor and a motor controller, frequency response characteristics of a motor which are capable of heightening a control gain have been required.
In order to improve the accuracy of the galvano motor, for example, a capacitance type or an optical type angle position sensor is getting larger, the sensitivity is improved by a multiple division, or the compensation is performed by a differential circuit. Furthermore, recently, a galvano motor in which an encoder is used as an angle position sensor has also been realized.
FIG. 6 is a cross-sectional view showing a schematic configuration of a conventional galvano motor. In FIG. 6, reference numeral 101 denotes a motor shaft, and reference numeral 102 denotes a magnet which is fixed on a substantially central part of the motor shaft 101. Reference numeral 103 denotes a motor case, and reference numeral 104 denotes a fixed yoke which is fixed on the motor case 103 and constitutes a magnetic circuit in cooperation with the magnet 102. Reference numeral 105 denotes a coil which is fixed on the fixed yoke 104 and rotationally drives the magnet 102 and the motor shaft 101.
Reference numeral 106 denotes an encoder scale which is fixed on one end of the motor shaft 101 and has a grid for obtaining angle information (position information). Reference numeral 107 denotes a sensor which reads grid information of the encoder scale 106, and reference numeral 108 denotes a board on which the sensor 107 is mounted. The board 108 converts the information read by the sensor 107 to an electric signal. Reference numeral 109 denotes a mirror and is directly joined to the other end of the motor shaft 101.
As a conventional art shown in FIG. 6, generally, a straight shaft is often used as a motor shaft 101. Inertia of the mirror 109 to be driven is often greater than that of the encoder scale 106. Furthermore, a distance Lmirror between the magnet 102 and the mirror 109 is often greater than a distance Lscale between the magnet 102 and the encoder scale 106.
Specific numerical values in this conventional art are as follows: Gmirror=206 [GPa], Ipmirror=Dmirror4/32=0.0044/32 [m4], Imirror=0.8−7 [Kgm2], Lmirror=0.015 [m], Gscale=206 [GPa], Ipscale=Dscale4/32=0.0044/32 [m4], Iscale=0.6−7 [Kgm2], and Lscale=0.012 [m].
Here, Gmirror is modulus of transverse elasticity of a material of the motor shaft 101 between the magnet 102 and the mirror 109. Ipmirror is polar moment of inertia of area of the motor shaft 101 between the magnet 102 and the mirror 109. Dmirror is a diameter of the motor shaft 101 between the magnet 102 and the mirror 109. Imirror is inertia of the mirror 109. Lmirror is a length between the magnet 102 and the mirror 109.
Gscale is modulus of transverse elasticity of a material of the motor shaft 101 between the magnet 102 and the encoder scale 106. Ipscale is polar moment of inertia of area of the motor shaft 101 between the magnet 102 and the encoder scale 106. Dscale is a diameter of the motor shaft 101 between the magnet 102 and the encoder scale 106. Iscale is inertia of the encoder scale 106. Lscale is a length between the magnet 102 and the encoder scale 106.
When using the above numerical values, a linear torsional resonance frequency fmirror between the magnet 102 and the mirror 109 and a linear torsional resonance frequency fscale between the magnet 102 and the encoder scale 106 are calculated. In this conventional art, the torsional resonance frequencies fmirror and fscale are 5898 Hz and 7614 Hz, respectively.
Since a motor controller of the galvano motor using an encoder as an angle position detector utilizes position information obtained from a digital signal without reducing the accuracy, typically, a digital control circuit is used.
FIGS. 7A and 7B are diagrams of frequency characteristics of a conventional galvano motor. FIG. 7A is gain characteristics and FIG. 7B is phase characteristics. A Bode diagram shown in FIGS. 7A and 7B shows open-loop frequency characteristics when the conventional galvano motor shown in FIG. 6 is driven at a sampling period of around 40 kHz.
As shown in FIG. 7A, the torsional resonance frequency fscale is higher than the torsional resonance frequency fmirror. Furthermore, as shown in FIG. 7B, because of a phase delay caused by a sampling period of a motor controller, the phase crossover frequency, i.e. the frequency whose phase is −180 degrees, is around 3 kHz.
In the phase characteristics of the conventional art, the phase proceeds by 180 degrees by the antiresonance just before the torsional resonance frequency fmirror and delays again by 180 degrees at the torsional resonance frequency fmirror. Further, it delays by 180 degrees at the torsional resonance frequency fscale. Thus, when the resonance is generated at a side of the encoder that is a sensor rather than the motor controller, the phase delays by 180 degrees. In contrast, when the resonance is generated at a side of the mirror opposite to the encoder rather than the motor controller, the phase delays by 180 degrees after it returns by 180 degrees.
According to Bode theorem, if the gain does not exceed 0 dB at the phase crossover point in a Bode diagram, the control system is stable. If the phase does not cross −180 degrees even when the gain exceeds 0 dB, the control system is stable. Therefore, as the conventional art, when the resonance is generated at the mirror side at a frequency slightly higher than the phase crossover frequency, the phase crosses −180 degrees at the torsional resonance frequency fmirror and the gain exceeds 0 dB. Therefore, the control system is unstable and the oscillation state is caused. In order to avoid this, typically, a notch filter is applied to the torsional resonance frequency fmirror.
However, when the notch filter is applied, a phase delay is generated at an area of a frequency lower than the frequency, and the phase crossover frequency is lowered. Therefore, it is difficult to heighten the control gain and the response of the motor is deteriorated.
Japanese Patent Laid-Open No. 2003-84224 discloses a control method of a galvano scanner for scanning an irradiation position of a laser beam with a mirror driven.
As described above, since a discrete processing is performed in a digital control, a dead time element is generated. As a result, when viewed in what is called frequency characteristics in a Bode diagram, a phase delay is generated.
The resonance frequency in a mechanical system is close to the phase crossover frequency (a frequency whose phase is −180 degrees) in the realistic range although they depend upon the size of the motor or the sensor and the ability of an arithmetic element used for the control. Therefore, if a various kind of filters such as a low-pass filter and a notch filter are not used, it is difficult to improve the control gain which directly influences the response of the motor.
In the galvano motor using such a digital control, there are problems as follows, and the improvement of the performance has been difficult.
First, in the digital control system, because a phase delay is generated by a dead time element caused by the sampling, the phase crossover point is lowered. The control band can not exceed the phase crossover frequency. Therefore, the control gain also needs to be lowered and the response of the motor or the force to suppress a disturbance is reduced.
Second, in order to handle the resonance of the motor, typically, a control filter such as a notch filter is also constituted by a digital calculation. However, if the variety or the order of filters increases, the calculation load increases and the sampling period of the control needs to be longer. Furthermore, since the phase crossover frequency is lowered by the influences of the filter itself, it is difficult to improve the control gain.