DEVICE FOR DETECTING OUTPUT VOLTAGE OF A LASER OSCILLATOR

Abstract: A matching circuit (3) is connected between a high-frequency power source (1) and a laser tube (4). The matching circuit (3) has reactance elements (31,33) in parallel with the load. A current transformer (5) for detecting current is provided between ground and the reactance element (33) at the output of the matching circuit, and the current flowing through transformer (5) is utilised to detect the voltage applied to the laser tube (4).

Description: Technical Field The present invention relates to an output voltage detecting device in a laser oscillator, and more pariticularly to an output voltage detecting device in a laser oscillator which is operable at a low voltage and is composable at an inexpensive cost. Background Art A laser tube has an electrical impedance which varies depending upon an interiorly confined gas status, as a consequence, a voltage supplied to the laser tube varies. For example, in a laser oscillator having a current feedback loop with which a current is fedback to the laser tube, the variation in the electrical impedance of the laser tube causes to vary the supply voltage, since the current is being controlled to be at constant. If this condition is left as it stands, semiconductor elements, the laser tube or other elements which constitute an inverter in a high frequency power supply are caused to be damaged. Furthermore, the output voltage needs to be monitored to ascertain if the supply voltage is stable. In view of the foregoing, it is a conventional practice to detect the voltage supplied to the laser tube, and when the resultant voltage is above a reference voltage, an alarm is generated. When the detected voltage is above a danger-level voltage, the driving is stopped. In addition, it has been necessary to detect and monitor the output voltage in order to control the output voltage to be at constant. Accordingly, it can be appreciated that in the laser oscillator, monitoring of the voltage supplied to the laser tube and measurement thereof are important measuring items. Conventionally, such measurements have been carried out using a transformer or voltage-division capacitors. However, the transformer is required to have a considerably high voltage insulation resistivity. The transformer is further required to have a high frequency characteristic attendant to the high frequency power supply unit. For such reasons, the transformers which are extremely expensive in cost have to be employed. A method in which the voltage is divided by capacitors is involved with a problem such that a power supply line led to the laser tube cannot be isolated from a ground connection of a control apparatus, so that noises are liable to be introduced into a control circuit. Disclosure of the Invention It is an object of the invention to resolve the above-mentioned problems and to provide an output voltage detecting device in a laser oscillator, which is operable at a low voltage and composable at an inexpensive cost. In order to resolve the above-mentioned problems, the present invention provides an output voltage detecting device in a laser oscillator, including an impedance matching circuit connected between a high frequency power supply unit and a load comprising a laser tube, the matching circuit having reactance elements connceted in parallel with the load characterised by: a current transformer for detecting a current flowing in a ground terminal of one of the reactance elements connected to an output terminal of the matching circuit, wherein a voltage applied to the laser tube is detected on the basis of the current detected by the current transformer. Since the current transformer is coupled to the ground terminal of the reactance element, the voltage induced in the current transformer is extremely low. In addition, the output voltage is given by the product of the current detected by the current transformer and the impedance of the parallel-connected reactance. Brief Description of the Drawing Fig. 1 is a block diagram showing one embodiment of the present invention. Best Mode for Carrying Out the Invention One embodiment of the invention will be described with reference to a drawing. Fig. 1 is a block diagram showing one embodiment of the invention. In the figure, a high frequency power supply unit 1 supplies a high frequency voltage upon rectifying a current from a commercial power supply to provide a D.C. current and then converting it to the high frequency voltage using a high frequency inverter. A booster transformer 2 boosts the high frequency voltage supplied from the high frequency power supply unit to a required voltage. A matching circuit 3 performs an impedance matching between the transformer 2 and a laser tube, so that power fed from the high frequency power supply unit is effectively supplied to the laser tube. In this embodiment, a π-type matching circuit is employed. Capacitors 31 and 33 are parallel-connected reactances and an inductor 32 is a serially-connected reactance. The laser tube 4 has a tube 41 formed with, for example, a silica glass. In the interior of the laser tube, a laser medium gas 42 is being circulated. Through electrodes 43a and 43b, a high frequency voltage is applied to the tube 41. A discharge is taken place for the laser medium gas 42 confined within the laser tube 41 while applying a high frequency voltage, whereupon the laser light is oscillated and amplified. The direction in which the laser light emits is perpendicular to the sheet of drawing. A current transformer 5 is coupled to the ground side terminal of the parallel-connected capacitor 33 disposed in the output terminal. Accordingly, the voltage induced in the current transformer 5 is extremely low, and thus the current transformers of low insulation voltage are available for use. Since a ground terminal of the current transformer 5 is isolated from the matching circuit 3, there is little possibility that noises are introduced into the current transformer 5. An amplifier 7 is provided in which assuming that its amplification factor K is an impedance of a capacitor, the output of the amplifier is defined by a product of a current fed from the current transformer 5 and the impedance of the parallel-connected capacitor. A voltage of the high frequency power supply unit applied to the laser tube is thereby calculated. Alternatively, dispensing with the amplifier 7, the voltage supplied to the laser tube can be calculated in a control circuit 6 in such a manner that the current fed from the current transformer 5 is converted to a digital value with the use of a DA converter and a product of the resultant value and the impedance of the capacitor 33 is obtained, where the impedance of the capacitor 33 has been stored in advance. The voltage thus obtained is compared with a predetermined reference voltage, and when it is above the reference voltage by a predetermined value, an alarm is generated. When the voltage obtained has reached or is above a danger-level voltage, the driving is stopped. Furthermore, while monitoring the voltage obtained, it may automatically be controlled so as to be at constant. The control circuit 6 may be arranged to carry out a positional control of a table of a laser processing machine, serving as a computerized numerical controller (CNC). In this case, an arithmetic processing of the impedance can be executed by a microprocessor provided within the computerized numerical controller (CNC). Although the π-type matching circuit is exemplified for the matching circuit, other types of matching circuits are available if they have a parallel-connected impedance in their output terminals, where the current transformer can be used in the same fashion. As described above, according to the present invention, the voltage detection is performed on the basis of a current detected by the current transformer coupled to the ground side terminal of the parallel-connected reactance at the output terminal of the matching circuit. Therefore, the insulating voltage of the current transformer may be low, thereby allowing to use an inexpensive current transformer, and further the voltage supplied to the laser tube can be measured with a simplified arrangement.

Claims: 1. An output voltage detecting device in a laser oscillator, including an impedance matching circuit (3) connected between a high frequency power supply unit (1) and a load comprising a laser tube (4), said matching circuit having reactance elements (31, 33) connected in parallel with the load, characterised by: a current transformer (5) for detecting a current flowing in a ground terminal of one of said reactance elements connected to an output terminal of said matching circuit, wherein a voltage applied to said laser tube is detected on the basis of said current detected by said current transformer. 2. An output voltage detecting device in a laser oscillator as recited in claim 1, wherein said reactance elements are capacitors. 3. An output voltage detecting device in a laser oscillator as recited in claim 1, wherein while converting said current detected by said current transformer to a digital value, said voltage is computed in accordance with a product of said digital value and an impedance of said one of said reactance elements.

Inventors: EGAWA, AKIRA FANUC MANSION HARIMOMI 3-303
Applicants: FANUC LTD.
Application year: 1988
Main classification: H01S3
IPC codes: H01S3