JPH09331089A - Nozzle height sensor for laser processing system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the electrostatic capacity to be precisely detected without bring affected by the cable floating capacity by a method in which a sine wave AC voltage in rated voltage and frequency is fed to the space between a sensor electrode and a work so as to convert the electrostatic capacity between the sensor electrode and the work into voltage signals. SOLUTION: A conversion circuit 7 is closely fitted to a sensor electrode 3. The purpose of the conversion circuit is to convert the electrostatic capacity Cx fed to the space between the sensor electrode 3 and a work 2 into electric signals. Besides, the purpose of the a transmission circuit is to feed the sine wave AC voltage in rated voltage and frequency. In such a constitution, the terminal voltage only of the electrostatic capacity Cx is detected by the conversion circuit 7 even if the output current from the oscillation circuit 8 shunts into the floating capacity Cf between a cable 5 and earth thereby enabling the electrostatic capacity Cx to be detected precisely.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a height sensor used as a position detector for keeping a nozzle for emitting laser light at a predetermined distance from a workpiece in a laser processing apparatus.

[0002]

2. Description of the Related Art A laser processing apparatus guides a laser beam output from a laser resonator to a torch through a condenser lens.
The laser beam is emitted from the nozzle attached to the tip of the torch to the workpiece, and the workpiece is processed. At this time, it is necessary to keep the focusing position of the laser beam at a predetermined position with respect to the surface of the workpiece. For this reason, a device has been devised to control the position of the torch so that the distance between the nozzle tip and the workpiece is always kept at a predetermined distance. For this purpose, it is necessary to detect the distance between the nozzle and the surface of the workpiece. Is required. Usually, a mechanism for detecting the distance is called a height sensor, and the distance is detected by utilizing the fact that the distance between the tip of the nozzle and the surface of the workpiece and the capacitance therebetween are inversely proportional.

FIG. 3 shows an example of a conventional height sensor. In FIG. 1, reference numeral 1 denotes a laser processing torch, which collects laser light from a laser resonator (not shown) with a condenser lens 11 and emits it toward a workpiece 2 from a nozzle 12 at the tip. Further, the torch 1 is moved along a machining line by a driving means (not shown), and the position of the torch 1 is vertically controlled with respect to the workpiece 2. 3 is the nozzle 1 at the tip of the torch
2 is a sensor electrode provided in the vicinity of the workpiece 2, and forms a capacitance Cx between the workpiece 2 and the workpiece 2 which is determined by the distance d between the two. Reference numeral 4 denotes a sensor circuit unit, which includes an oscillation circuit 41 that generates a sine wave AC voltage Vs having a constant voltage and a constant frequency, and a conversion circuit 6 that converts a signal from the sensor electrode 3 into a voltage signal. The conversion circuit 6 is composed of, for example, an inverting circuit including an operational amplifier 61 and a resistor 62. Further, Cf in the figure indicates a stray capacitance formed between the cable 5 connecting the sensor circuit portion 4 and the sensor electrode 3 and the workpiece 2 or the ground.

In the apparatus of FIG. 3, the output current I from the oscillation circuit 41 of the sensor circuit section 4 flows through the ground, the stray capacitance Cf and the cable 5 to the operational amplifier 61, and the ground, the workpiece 2, the electrostatic charge. There is a route through the capacitance Cx, the sensor electrode 3 and the cable 5 to the operational amplifier 61. By the way, the following is generally known in the operational amplifier. The potentials of the negative input terminal and the positive input terminal of the operational amplifier are almost the same. Further, the impedance between these two terminals, that is, the input impedance is very large (infinite in the ideal operational amplifier), so the output current I from the oscillation circuit 41 is I = Vs / Z (1). Here, Z is a composite impedance of the electrostatic capacitance Cx and the floating capacitance Cf, and Z = 1 / (jω (Cx + Cf)) (2). (However, j is the imaginary unit, ω is the angular velocity [rad / se
c]) As a result, assuming that the resistance value of the resistor 62 is R, the output voltage Vo of the conversion circuit 6 is Vo = (− R · Vs) / Z (3) from the resistance value R and the equation (1). Becomes From the equations (2) and (3), Vo = (− R (jω (Cx + Cf)) Vs = A (Cx + Cf) Vs (4) (where A = −jωR) and the output voltage Vo of the conversion circuit 6 is obtained. Has a value proportional to the sum of the electrostatic capacitance Cx and the stray capacitance Cf, so the value of (Cx + Cf) can be known from the output voltage Vo.

Here, the change amount Δd of the distance d between the sensor electrode 3 and the surface of the workpiece 2 and the change amount ΔCx of the electrostatic capacitance Cx are in inverse proportion to each other and the change of the output voltage Vo. Since ΔVo = ωΔCx · R · Vs (5) is established between the amounts ΔVo and ΔCx from the equation (4), the change amount ΔCx of the capacitance Cx can be known from the change amount ΔVo of the output voltage Vo. . Therefore, by detecting the output voltage Vo, the change amount Δd of the distance d between the sensor electrode 3 and the surface of the workpiece 2 can be obtained as 1 / ΔCx from the equation (5), and the distance d can also be obtained. Can be obtained from equation (4) as 1 / (Cx + Cf).

[0006]

In the above-mentioned conventional device, the change amount Δd of the distance d becomes a function of ΔCx from the equation (5) and can be directly obtained by the change amount ΔVo of the output voltage Vo of the conversion circuit 6. it can. However, the distance d is a function of (Cx + Cf) from the equation (4). Here, the capacitance Cx between the sensor electrode 3 and the workpiece 2 is as small as a few picofarads [PF], whereas the capacitance between the cable 5 and the workpiece 2 or the ground is floating. Capacity Cf
Is as large as several picofarads [PF] to several tens of picofarads [PF]. Furthermore, this stray capacitance Cf
Varies depending on how to route and install the cable 5, and especially when the torch 6 is attached to the robot and moved along the machining line, the stray capacitance Cf also changes because the positional relationship between the cable 5 and the ground changes momentarily. It changes with this. Therefore, the conventional device cannot detect only the value of the capacitance Cx of the sensor electrode 3. Further, even in the case where the torch 1 is fixed and the workpiece 2 is moved on an XY table or the like, the stray capacitance Cf also changes when the cable 5 is replaced for maintenance and inspection,
In order to detect an accurate distance, it is necessary to readjust each time.

[0007]

SUMMARY OF THE INVENTION The present invention solves the problems of the above-mentioned conventional device and accurately detects the electrostatic capacitance between the sensor electrode and the work piece. And an oscillating circuit for supplying a constant voltage / constant frequency sine wave AC voltage, and the electrostatic capacitance between the sensor electrode mounted close to the sensor electrode and the workpiece is converted into a voltage signal. And a conversion circuit for. Further, this conversion circuit is composed of an operational amplifier that receives the output voltage Vs of the oscillation circuit, and the capacitance Cx is calculated from a circuit in which the output signal Vo is the following formula Vo = ACx · Vs (where A is a constant). A nozzle height sensor for a laser processing apparatus is proposed.

[0008]

FIG. 1 is a diagram showing an embodiment of the present invention. In the figure, reference numeral 7 denotes a conversion circuit attached in the vicinity of the sensor electrode 3, and is a circuit for converting the electrostatic capacitance Cx between the supplied sensor electrode 3 and the workpiece 2 into a voltage signal. Reference numeral 8 is an oscillating circuit for supplying a constant voltage / constant frequency sine wave AC voltage. Others in FIG.
Those having the same functions as those of the conventional device shown in FIG.

In the device shown in FIG. 1, even if the output current from the oscillation circuit 8 is shunted to the stray capacitance Cf between the cable 5 and the ground, the conversion circuit 7 detects only the terminal voltage of the electrostatic capacitance Cx. Therefore, the stray capacitance Cf between the cable 5 and the ground is
Therefore, even if the output of the oscillation circuit 8 leaks, the capacitance Cx between the sensor electrode 3 and the workpiece 2 can be accurately detected without being affected by the leakage.

FIG. 2 is a diagram in which, in FIG. 1, parts such as nozzles that are not directly related to the conversion circuit are omitted in order to explain the operation of the conversion circuit 7. In the figure, the conversion circuit 7 is composed of an operational amplifier 71 and a resistor 72. One input terminal of the operational amplifier 71 is connected to the sensor electrode 3, and the other input terminal is connected to the output terminal of the oscillation circuit 8.

In the device of FIG. 2, the output current I from the oscillation circuit 8 passes through the ground and the stray capacitance Cf, and then the cable 5
To the operational amplifier 71 through the ground, the workpiece 2, the capacitance Cx and the sensor electrode 3. Therefore, only the current flowing through the capacitance Cx flows to the operational amplifier 71. By the way, the following is generally known in the operational amplifier. The potentials of the negative input terminal and the positive input terminal of the operational amplifier are almost the same. Further, the impedance between these two terminals, that is, the input impedance is very large (infinite in the ideal operational amplifier), so that the current I flowing through the electrostatic capacitance Cx flows through the resistor 72 and becomes I = Vs / Z (6) . Here, Z is an impedance consisting of only the capacitance Cx, and Z = 1 / (jω · Cx) (7). (However, j is an imaginary unit, ω is an angular velocity [rad / sec]) As a result, assuming that the resistance value of the resistor 72 is R, the output voltage Vo of the conversion circuit 7 is Vo = from this resistance value R and the equation (6). -R (Vs / Z) (8) From the equations (7) and (8), Vo = (− R (jω · Cx)) Vs = A · Cx · Vs (9) (where A = −jω · R). Therefore, the output voltage V
o is an output proportional to the electrostatic capacitance Cx, and this output Vo
Therefore, the distance d inversely proportional to the capacitance Cx can be directly detected.

Even if the resistor 72 is replaced with a capacitor or a combination of a capacitor and a resistor, the output voltage Vo of the operational amplifier 72 has a capacitance Cx.
It goes without saying that the output is proportional to and the distance d inversely proportional to the capacitance Cx can be directly detected.

Further, in the above description, only the conversion circuit 7 is provided close to the sensor electrode 3, but if the oscillation circuit 8 is also provided close to the sensor electrode 3, noise mixed from the cable 5 can be eliminated. it can.

[0014]

Since the height sensor of the present invention is as described above, accurate detection is possible without any influence of the stray capacitance of the cable. Therefore, when the laser torch is attached to the robot and moved along the two-dimensional or three-dimensional machining line, no error occurs. In addition, even when the cable is replaced in maintenance and inspection, there is no need to make any adjustments, and stable detection accuracy can always be ensured.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a height sensor of the present invention.

FIG. 2 is a diagram showing an example of a conversion circuit used in the height sensor of the present invention with some other parts omitted.

FIG. 3 is a diagram showing an example of a conventional height sensor.

[Explanation of symbols]

 1 Torch 2 Workpiece 3 Sensor electrode 4 Sensor circuit part 5 Cable 6,7 Conversion circuit 8 Oscillation circuit 11 Condenser lens 12 Nozzle 41 Oscillation circuit 61,71 Operational amplifier 62,72 Resistor Cx Sensor electrode and workpiece Capacitance between Cf Stray capacitance between cable and ground (workpiece) Vo Output voltage of conversion circuit Vs Output voltage of oscillation circuit d Distance between sensor electrode and work piece

Claims (2)

[Claims] 1. A nozzle height sensor for a laser processing apparatus for detecting a distance between a tip of a nozzle that emits laser light and a workpiece, and a sensor electrode mounted near the tip of the nozzle, and the sensor. An oscillating circuit for supplying a constant voltage / constant frequency sine wave AC voltage between the electrode and the work piece, and a capacitance between the sensor electrode and the work piece mounted in proximity to the sensor electrode. Height sensor for a laser processing apparatus, which comprises a conversion circuit for converting the voltage into a voltage signal. 2. The conversion circuit is composed of an operational amplifier having an output voltage Vs of the oscillation circuit as an input, and an output signal Vo = ACx · Vs (where A is a constant and Cx is between the sensor electrode and the workpiece). 2. The nozzle height sensor for the laser processing apparatus according to claim 1, wherein the electrostatic capacity of the nozzle is obtained.