JP2018132341A - Highly accurate detector for motor current - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a current detector which reduces a resistance value added exclusively for detecting a current and has a high precision.SOLUTION: The current detector includes a motor 1, a motor driving circuit 2, and conduction path means for attaining conduction to the motor, and calculates a current value by detecting the voltage of a current flowing in the motor 1 by a current detection resistor 4 and operating the current by a detection circuit. The current detection resistor 4 has the conduction path means for attaining conduction to the motor.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a motor control device used in a laser processing apparatus, and more particularly to a galvano motor.

  The present applicant has developed a galvano motor for use in machine tool devices such as a laser drilling machine, a laser trimmer device, and a laser repair device. The galvano motor is a laser beam reflected by a mirror and positioned with high accuracy by attaching a mirror to the shaft of the rotary motor and performing positioning with high accuracy in the rotation direction. In order to perform highly accurate positioning, a capacitance sensor or an optical or magnetic encoder is used as a rotation angle detector. Since machine tool devices are also required to have high speed, a PWM amplifier is often used in a motor drive circuit in consideration of heat generated by a current amplifier.

  As a conventional method for detecting the driving current of a motor, there is widely known a method in which a detection resistor is provided in a DC motor and a potential difference between both ends of the detection resistor is detected by a differential amplifier. However, the PWM detection component is included in the current detection signal due to the performance of the common-mode voltage rejection ratio (CMRR) in the differential amplifier, and the current detection accuracy decreases. One method for improving the accuracy is to increase the voltage of the differential signal component compared to the common-mode voltage component by increasing the detection resistance. As a PWM modulation method, there are bipolar modulation and unipolar modulation (Non-Patent Document 1).

  In bipolar modulation, the gate drive is switched on and off almost simultaneously in the full-bridge drive circuit. In order to improve the detection error due to the in-phase component of the potential at both ends of the detection resistor in the drive circuit using this modulation, a detection resistor is provided on both sides of the DC motor, and the potential difference between both ends of each detection resistor is detected by a differential amplifier Is also known (Patent Document 1).

JP 2008-064506 (paragraph 0027, FIG. 3)

Atsuo Kawamura, Tomoki Yokoyama, Hiroto Funato, Shinichi Hoshi, Teruo Yoshino: “Introduction to Power Electronics”, Corona (2009)

  However, when the detection resistance is increased, there is a problem that power loss at the detection resistance increases.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a highly accurate current detection device that reduces the resistance value added exclusively for current detection applications.

  In order to achieve the object, a current detection device according to one aspect of the present invention includes a motor, a motor drive unit, and an energization path unit to the motor, and detects a current flowing through the motor by detecting a voltage using a detection resistor. In the current detection device that calculates a current value by calculating with a circuit, the current detection resistor includes a current-carrying means for the motor.

  It is possible to provide a highly accurate current detection device while reducing the resistance value added exclusively for current detection applications.

It is a block diagram which shows the structure of the galvano scanner including the electric current detection apparatus of 1st Embodiment of this invention. It is the figure which showed the motor drive circuit and current detection circuit of 1st Embodiment of this invention. It is the figure which showed the current detector of 1st Embodiment of this invention. It is the figure which showed the current detector of 2nd Embodiment of this invention. It is the schematic which shows the structure of the laser processing apparatus of 1st Embodiment of this invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[First embodiment]
FIG. 1 is a block diagram showing the configuration of the galvano scanner according to the first embodiment of the present invention. In the galvano scanner, a mirror is attached to the shaft of the motor 1 and positioning is performed with high accuracy in the rotation direction, whereby laser light is reflected by the mirror and positioning is performed with high accuracy.

  The galvano scanner of this embodiment detects the angular position of a galvanometer mirror by an encoder as shown in FIG. The position feedback controller outputs a current command so that the position command to the galvano scanner matches the position detection signal from the encoder. Further, the current flowing through the motor 1 is detected by the current detector 3, and the current feedback controller compensates for feedback to operate the motor drive circuit 3 so that the current command value and the motor drive current become the same.

  FIG. 2 is a detailed view of the motor drive circuit 2 and the current detector 3 shown in FIG. The motor drive circuit 2 is a so-called PWM amplifier. In the motor drive circuit 2, the input signal is first compared with the PWM triangular wave signal. The signal after the comparator is connected to the gate driver directly or through an inverting buffer. Furthermore, the gate driver applies a voltage to the motor by switching the FET on and off. In the current detector 3, the current flowing through the motor 1 is converted into a voltage by the current detection resistor 4, and the potential difference between both ends of the current detection resistor 4 is detected by the differential amplifier 5 as a current detection signal.

  Generally, in the current detector 3, an electric resistor dedicated to current detection mounted on a substrate is used as the current detection resistor 4. For this reason, for example, when using an electric resistor having an electric resistance of 132 mΩ and a current rating of 15 A, a power rating of 30 W is required. Electric resistors with a power rating of 30W are large and expensive. Moreover, the power loss of 30 W increases by using an electric resistor dedicated to current detection. For this reason, it is common to reduce the power rating by setting the electrical resistance to about 10 mΩ. However, current detection accuracy decreases with a reduction in electrical resistance value.

  FIG. 3 is a current detector that well represents the present invention. In a galvano scanner, a motor drive circuit and a current detector are generally mounted on one electric board. Further, in the laser processing machine, the electric board and the motor 1 are often arranged at a distance of about 5 m due to the device configuration. For this reason, a motor cable 6 having a length of about 5 m is used as an energization path means from the motor drive circuit 2 to the motor 1. Also, the thickness of the motor cable 6 is often about AWG 19 in consideration of the cable routing. The electrical resistance of AWG19 is 26.41Ω / km, and it is 132mΩ at a length of 5m.

  Moreover, the current rating of a cable of about AWG 19 is generally allowed to be about 15A. Therefore, as shown in FIG. 3, by using the electric resistance of the motor cable 6 as a current detection resistor, the potential difference between both ends of the motor cable 6 is detected as a current detection signal by the differential amplifier 5. In this way, by using the electric resistance of the motor cable 6 as the current path means to the motor 1 as the current detection resistance, a highly accurate current detection device can be provided without increasing the power loss.

  Hereinafter, a laser processing apparatus 1000 to which the galvano system shown in FIG. 1 is applied will be described with reference to FIG. The laser processing apparatus 1000 is an apparatus that irradiates an object (irradiated object) OB to be processed with a laser beam LL and performs processing such as cutting, drilling, and welding of the object OB.

  The laser processing apparatus 1000 includes an X axis motor 1002 and a Y axis motor 1004. The X-axis motor 1002 rotates the X-axis mirror 1006 attached to the rotation shaft, moves it from the first position, and stops it at the second position. Similarly, the Y-axis motor 1004 rotates the Y-axis mirror 1008 attached to the rotation shaft, moves it from the first position, and stops it at the second position.

  The X-axis detection unit 1010 is composed of an incremental rotary encoder, and detects the rotation angle of the X-axis mirror 1006 (that is, the rotation angle of the rotation axis of the X-axis motor 1002). Similarly, the Y-axis detection unit 1012 is composed of an incremental rotary encoder, and detects the rotation angle of the Y-axis mirror 1008 (that is, the rotation angle of the rotation axis of the Y-axis motor 1004).

  The main control unit 1014 controls the whole (operation) of the laser processing apparatus 1000 and applies an ON signal that instructs the light source 1016 to irradiate (emit) the laser beam LL and non-irradiate (non-emit) the laser beam LL. An instructing OFF signal is output. The main control unit 1014 instructs the X-axis motor position command unit 1018 and the Y-axis motor position command unit 1022 about the X coordinate position and the Y coordinate position on the machining surface.

  The X-axis motor position command unit 1018 and the Y-axis motor position command unit 1022 calculate a command rotation angle to the motor from the given coordinate position, and commands the X-axis motor control unit 1020 and the Y-axis motor control unit 1024. The X-axis motor control unit 1020 detects the rotation angle of the motor by the X-axis detection unit 1010, performs feedback control calculation by the DSP, and drives the X-axis motor.

  Similarly, the Y-axis motor control unit 1024 corresponds to the control controller 200 in FIG. The Y-axis motor control unit 1024 detects the rotation angle of the motor with the Y-axis detection unit 1012, performs feedback control calculation with the DSP, and drives the Y-axis motor.

  In the processing of the object OB, the laser light LL from the light source 1016 is reflected by the Y-axis mirror 1008 and the X-axis mirror 1006 and is irradiated to the object OB. The laser beam LL is scanned in the X-axis direction (positive side and negative side) on the object OB as the X-axis mirror 1006 rotates. Further, the laser beam LL is scanned in the Y-axis direction (positive side and negative side) on the object OB as the Y-axis mirror 1008 rotates. At this time, each of the X-axis mirror 1006 and the Y-axis mirror 1008 is driven by the X-axis motor 1002 and the Y-axis motor 1004 to realize high-speed and high-precision positioning. In such a laser processing machine, it is required to perform laser processing at the correct position of the processing object. By applying the current detection device described above, highly accurate position control can be performed, and laser processing to the correct position can be performed.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

[Second Embodiment]
Next, a galvano scanner according to a second embodiment of the invention will be described. The configuration of the galvano scanner of the second embodiment of the present invention is the same as that of FIG. 1 of the first embodiment. FIG. 4 is a diagram showing a current detector of a galvano scanner according to the second embodiment of the present invention.

  As shown in FIG. 4, the current detector of the galvano scanner of this embodiment detects the potential difference between both ends of the motor cable 6 with a differential amplifier as in the first embodiment. By the way, when a motor current is passed through the motor cable 6, electric power is consumed by the electric resistance of the motor cable 6. Therefore, it is conceivable that the temperature of the motor cable 6 rises due to heat generation. The temperature of the motor cable 6 is affected by the ambient temperature. The motor cable 6 is generally made of copper wire, and its electric resistance value varies depending on the cable temperature. Therefore, in order to realize current detection with higher accuracy, it is necessary to estimate a change in electric resistance value.

  Therefore, as shown in FIG. 4, an electric resistor 7 having a resistance temperature coefficient smaller than that of the motor cable 6 is added, and a current detection compensation signal is generated by a differential amplifier. Here, a signal obtained by differentially amplifying the current detection compensation signal and the potential difference between both ends of the motor cable 6 is input to the motor cable resistance estimator. Here, let the resistance value of the electric resistor 7 be about 1/10 of the resistance value of the motor cable. The current detection compensation signal is 1/10 lower in S / N ratio than the signal measured by the motor cable 6. Therefore, the S / N ratio can be expected to be improved by a factor of 10 from the central limit theorem, where the motor cable resistance estimator measures the current detection compensation signal 100 times and obtains the time average.

  Similarly, a time average is similarly obtained for a signal obtained by differentially amplifying the potential difference between both ends of the motor cable 6. Next, the electric resistance value of the motor cable 6 is estimated so that the time averages of the respective current detection values coincide with each other. Finally, a current detection signal of the motor current can be obtained from the signal obtained by differentially amplifying the potential difference between both ends of the motor cable 6 and the estimated value of the electric resistance of the motor cable 6.

  Now consider the temperature rise of the motor cable. The temperature coefficient of resistance value of copper is α = 4.3 × 10 −3 / ° C. When the motor cable temperature rises by 10 ° C, the motor cable resistance changes by + 4.3%. For example, if you want the current detection accuracy to be 1% or less, at least the resistance variation must be 1% or less. Therefore, the motor cable temperature fluctuation should be 10 ℃ / 4.3 = 2.3 ℃ or less.

  The motor cable temperature can be roughly calculated from the ambient temperature, cable characteristics, and motor current. Therefore, when the AWG19 cable temperature was calculated at a constant motor current of 15 A, a result that fluctuated by about 1.5 ° C / second was obtained. Therefore, if the resistance value of the motor cable can be estimated in 1.5 seconds or less, the detection error due to the temperature fluctuation of the motor cable can be suppressed to 1% or less. If the motor cable resistance estimator is composed of a microcomputer and an AD converter, the above time average calculation can be easily performed in 1.5 seconds or less.

  In this way, by using the electric resistance of the motor cable 6 as the current path means to the motor 1 as the current detection resistance, a highly accurate current detection device can be provided without increasing the power loss. In this way, the current detection resistor has an electric resistor with a resistance temperature coefficient smaller than that of the motor cable, and by including an estimator of the electric resistance value of the motor cable, the detection resistance value dedicated to current detection can be reduced, and high accuracy A current detection device can be provided. Further, although a simple time average is used for the motor cable resistance value estimator, a low pass filter may be used. Further, the motor cable resistance value estimator may be constituted by an analog circuit.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

DESCRIPTION OF SYMBOLS 1 Motor 2 Motor drive circuit 3 Current detector 4 Current detection resistor 5 Differential amplifier 6 Motor cable

Claims (6)

In a current detection device that has a motor and motor drive means, and energization path means to the motor, and calculates a current value by calculating a current flowing through the motor from a potential difference between both ends of the detection resistance by a detection circuit, the current detection resistance An electric current detection device comprising an energization path means to the motor. 2. The electric current detection resistor includes an electric resistor having a resistance temperature coefficient smaller than that of an energization path means to the motor, and includes an estimator of an electric resistance value of the energization path means to the motor. The current detection device described in 1. In a current detection device that has a motor and motor drive means, and energization path means to the motor, and calculates a current value by calculating a current flowing through the motor from a potential difference between both ends of the detection resistance by a detection circuit, the current detection resistance An electric current detection method comprising energization path means to the motor. 4. The electric current detection resistor includes an electric resistor having a resistance temperature coefficient smaller than that of the energizing path means to the motor, and includes an estimator of an electric resistance value of the energizing path means to the motor. The current detection method described in 1.   A motor position control device comprising the current detection device or the current detection method according to claim 1.   A laser processing apparatus comprising the motor position control apparatus according to claim 5.