High frequency electronic welding system

A high frequency welding system for tubular members applies high frequency current I to the gap for creating an alternating field. According to magnetic flux of the alternating field, an opposed current i is generated in a conductive portion of the welding apparatus. In close proximity, inductance is effected between the welding current I and the response current i. An output circuit is provided for generating high frequency current for establishing a welding heat with suppression of ripple current at the output. The welding processing is observed by a CCD camera which is in communication with an image processing portion for analyzing image data for determining welding conditions. A signal from the image processing portion is output to a monitoring portion which continuously monitors welding operation and activates an alarm if welding conditions exceed predetermined values. A correction processing portion also receives the signal from the image processing portion and effects adjustment of the power circuit for maintaining a welding heat at a desired level.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
The present invention relates generally to a high frequency welding system. 
Particularly the present invention relates to a high frequency seam 
welding system which can control a manufacturing process for providing 
sealing by welding between opposite sides of a material being formed into 
a cylindrical shape such as piping, for example. 
2. Description of The Prior Art 
Production systems for piping and other tubular members are known in which 
a workpiece is fed from a roll of metal strip in a tubular formation such 
that opposite sides thereof are positioned adjacently. An upset pressure 
is supplied to butt the sides of the workpiece together at a jointing 
point and supplying a high frequency electrical power to the workpiece to 
weld the opposite side surfaces at a welding point. It is conventional 
practice to adjust the intensity of the welding heat generated at, and 
near the jointing point by controlling the high frequency power to the 
workpiece based upon various conditions which are monitored by sensors 
during the production process. However, it is very difficult to monitor 
each of the many conditions which may affect welding heat during such a 
production process. 
FIG. 7 shows an electromagnetic inductance type conductive portion for a 
welding system, FIG. 8 shows contact type conductive portion of 
conventional seam welding systems for forming cylindrical members. 
According to the drawings, a material 1 for forming a pipe undergoes a 
multistage process for rolling the material 1. When the material 1 is 
first rolled to approach a cylindrical shape, a V-shaped gap, or seam, 2 
is formed along one side of the rolled material 1 as the material 1 is 
rolled in the direction of the arrow A of FIGS. 7 and 8. The V-shaped gap 
is known as a V throat. According to the electromagnetic inductance method 
of FIG. 7, a heating coil 3a is powered from a high frequency power source 
through a power circuit. The welding heat under which the workpiece, or 
material 1 is welded, at a welding point 1a, is determined by the level of 
power applied to the heating coil 3a. According to the contact type system 
of FIG. 8, a high frequency current I is applied from an electrical 
source, or work coil 3 which is connected to opposed sides 2a and 2b of 
the V-shaped gap 2 via electrodes 4a and 4b respectively. 
After either of the above described steps, the pipe material 1 is put 
between squeeze rollers 5a and 5b which apply an upset pressure in the 
directions of arrows B and C of FIGS. 7 or 8 for joining the opposed sides 
2a and 2b for continuously forming a welded line seam 10. 
FIG. 14 shows a cross section of end pieces 2a and 2b of a seam to be 
joined by welding. Heated portions of the seam are shown in the drawing by 
hatching. Referring to FIG. 14(A), the flat ends of each side 2a, 2b of 
the seam 10 to be joined are heated. According to this arrangement wherein 
a welding current I is applied to sides of the seam 10, a proximity effect 
is conspicuous between the opposed ends 10a, 10b of the seam 10. FIG. 
14(B) is a close-up view of a thickness portion of the end pieces 10a and 
10b of the seam 10, as can be seen from the drawing, according to this 
effect, a current I is stronger at a corner portion of the ends 10a and 
10b, thus heating is stronger at each corner of each of the ends to be 
joined. Thus, as seen in FIG. 14(C), when pressure is applied by the 
squeeze rollers 5a and 5b for joining the ends 10a and 10b of the seam 
10, a center portion thereof is heated less than the corner portions which 
can lead to spattering of heated metal when the ends 10a, 10b are joined 
under pressure and may further lead to formation of `pinholes` along the 
seam thus degrading the quality of welded seam. 
In order to deal with the problem outlined above, Japanese Patent 
Application 2-139244 discloses an alternative type of conventional seam 
welding system as shown in FIG. 9. According to this arrangement, before 
the seam 10 proceeds to the seam welding portion 6 of the apparatus, it is 
preheated at a preheating portion 7. The preheating portion includes a 
guide means 8 and a second electrical source 9 for supplying mid and low 
frequency current to the seam 10. The guide means is interposed between an 
inner and outer surface of the material 1 for supplying relatively low 
frequency heating to a core, or center portion of the ends 10a and 10b of 
the seam 10 allowing substantially even heating of the core and corner 
portions of ends 10a, 10b to be achieved at the welding stage for forming 
the seam 10. 
According to the above arrangement, a relatively high cost is incurred due 
to the more complex apparatus and, according to the application of high 
and lower frequency currents for heating, a high output electrical source 
is required. Such high output sources are subject to current variation at 
high frequencies. 
FIG. 19 shows a induction heating circuit for such conventional welding 
systems. The circuit includes a hot cathode electron tube 40, and an 
oscillator circuit 50 therefor, a direct current voltage Edc is required 
for causing oscillation of the electron tube 40. A three phase voltage 
e.sub.1 is introduced through a stepdown transformer TR1 to be limited to 
a withstand threshold of a thyristor 100, the thyristor 100 regulates the 
output which is supplied to an amplifying transformer TR2 and is then 
supplied to a three phase rectifier circuit 20 and a filter 30 is provided 
for smoothing. 
Further shown in FIG. 19 is a filament circuit 70 for the electron tube 40. 
A single phase source voltage e.sub.2 is supplied to the filament circuit 
70 through an AVR (Automatic Voltage Regulator). The stabilized output 
from the AVR is supplied to a filament transformer TR3 and the output of 
the transformer TR3 is supplied to the filament 40a of the electron tube 
40 for heating thereof. Also associated with the electron tube 40 is a 
grid bias circuit 80, capacitors Ct.sub.1, Ct.sub.2 and feedback 
capacitors Cg.sub.1 and Cg.sub.2. 
The above described type of circuit is subject to ripple current which 
requires provision of a filter. However, for effectively smoothing such 
ripple current, a large capacity choke coil and a condenser must be added, 
increasing the size, weight and complexity of such a circuit. 
Further, for low frequency ripple a filter for higher harmonic frequencies 
is needed, and the size and cost of the circuit is increased. In addition, 
the thyristor 100 provided for voltage regulation has too slow a response 
to effectively deal with such ripple current. 
When such as circuit as the above-described is used as a heating circuit 
for induction welding, for example, ripple current present in the circuit 
creates fluctuation in the high frequency output voltage in the emissions 
of the electron tube 40 causing unevenness in the resulting welds. 
For monitoring such a welding system, one of the following three methods 
are conventionally employed; 1) visual monitoring by a system operator, 2) 
measuring irradiated temperature of the welding operation, 3) 
electronically detecting oscillation frequency variation for 
discriminating excess applied heating 4) monitoring the shape and 
projection of a welding bead; 
SUMMARY OF THE INVENTION 
It is therefore a principal object of the present invention to overcome the 
drawbacks of the prior art. 
It is a specific object of the invention to provide a welding system in 
which sufficient heating is supplied with suppressing ripple current and 
in which welding operation is continuously monitored for warning a system 
operator when welding conditions fail outside of optimum values. 
There is provided a welding system, comprising: 
a work piece fed to a welding point at which a V throat present in said 
work piece is fuzed into a welded seam; 
a CCD euipped camera for continuously scanning a welding operation and 
outputting a first signal indicative thereof; 
masking means, interposed between said camera and said welding point for 
providing a visual reference for dividing a camera image into zones; 
conversion means for receiving an output from said camera and converting 
said output to a digital for an outputting a second signal indicative 
thereof; 
first memory means for storing digital welding image data based on said 
digital signal; 
second momory means for storing reference image data; 
processing means for accessing said first and second memory means and 
comparing said reference image data with said welding image data and 
producing a third signal indicative of said comparison; 
monitoring means, receiving said third signal and monitoring a welding 
condition based thereon, said monitoring means outputting sequentially 
updated image data based on said third signal for showing a current 
welding condition and outputting an alarm signal indicative of undesirable 
welding conditions including upper and lower heat values when said welding 
condition is excessive of said predetermined conditions; 
display means receiving said sequenstially updated image data; 
alarm means receiving said alarm signal; 
correction adjustment means, recieving said third signal and calculating a 
degree of adjustment of an output power of said welding system based on 
said third signal and outputting a fourth signal indicative of said degree 
of adjustment; 
second conversion means, receiving said fourth signal and converting said 
signal for outputting a fifth, analog signal corresponding to said degree 
of adjustment; 
signal regulating means, receiving said fifth signal and further receiving 
a sixth signal indicative of a reference power level, said signal 
regulating means comparing said fifth and sizth signals and outputting a 
seventh signal indicative of a power variation value; 
power output means, recieving said seventh signal and adjusting a power 
level of a heating portion of said welding system; 
a conductive member, positioned in a Vthroat of a tubular member being 
welded for establishing an inductive current at said V throat, 
sufficiently heating a welding point of said tubular member for effecting 
continuous welding of a seam along said tubular member in accordance with 
control effected by saidimage processing portion, said monitoring portion 
and said power control portion.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Although the welding system of the invention will be described in 
connection with a high-frequency welding unit for production of tubular 
members, it will be understood that the invention is also applicable to 
other types of production processes. 
Generally, a welding system comprises three components, these being a 
production monitoring portion, a conductive, or heating, portion and an 
output portion for supplying high frequency voltage to the conductive 
portion. 
FIG. 1 shows a production monitoring portion according to the invention, 
including an image processing unit. The image processing unit monitors a 
welding arrangement 210. FIG. 2 shows a plan view of the welding unit 210 
as seen by a CCD equipped camera 55 (see FIG. 1) of the image processing 
unit. The camera 55 monitors the workpiece 1 at a welding point 1a. An 
analog/digital (A/D) converter 65 receives an analog signal S1 from the 
camera 55 and converts the analog signal S1 to a digital signal S2. An 
image memory 77 stores image data received via the digital signal S2 and 
outputs an image signal S3. A reference memory 85 is included, which 
contains image pattern information, stored in a ROM memory for example, 
which may be used for comparison with the image data stored in the image 
memory 77. The reference memory 85 image data is conveyed via a signal S4 
output by the reference memory 85. A CPU 95 receives the image signal S3 
and the reference image signal S4 for carrying out discrimination 
processing of the image data for detecting a present welding condition. 
Collectively, the A/D converter 6, the image memory 77, the reference 
memory 85 and the CPU 95 form an image processing portion 101 of the 
welding system of the invention. 
The image processing portion 101 transmits an analysis signal S5 from the 
CPU 95 to a correction portion 117. The correction portion 117 comprises a 
correction amount calculating circuit 122, a signal conversion circuit 
130, and a signal regulating circuit 140. 
As stated above, the correction portion 117 receives an analysis signal S5 
from the image processing portion 101. In addition, the correction amount 
calculating circuit receives a welding condition signal S9 for calculating 
a correction amount and the signal regulating circuit receives a welding 
condition reference signal S0. The welding condition signal may contain 
parameters indicative of high frequency electrical power level, high 
frequency impedance, welding speed, material (workpiece) width, material 
thickness, material resistance, V throat edge positional variation, 
squeeze roller rotational deviation etc., or any other desired processing 
information by which defects in the welding process may be detected. The 
reference signal S0 represents an optimal, or standard, welding condition 
for serving as a basis of comparison with the welding condition signal S9. 
The signal conversion circuit 130 converts the signal S6 from the 
correction amount calculating circuit 122 into an conversion signal S7. 
The conversion signal S7 is output to the signal regulating circuit 140. 
The signal regulating circuit 140 outputs a regulating signal S8, based on 
the conversion signal S7 to a power control portion 150. The power control 
portion 150 regulates power to a conductive portion 10, based on the 
regulating signal S8 from the signal regulating circuit 140. 
Also, analysis signal S5 data from the image processing portion 101 is 
input to an inference portion 160. Depending on the content of the 
analysis signal S5 data, the inference portion is active to infer, or 
determine a cause of undesirable welding conditions which may be present 
in the vicinity of the welding point 1a. In conjunction with the inference 
portion, a welding condition monitoring portion 170 is further provided to 
monitor welding condition. 
Referring to FIG. 3, the inference portion receives data from the image 
processing portion 101 for inferring axial length, contour length, and 
inclination of a molten metal portion 10 of the welding in progress. The 
monitoring portion 170, based on the result of the inference portion 160 
may be active to activate an alarm circuit 180 for warning of undesirable 
welding conditions inferred by the inference portion 160. Data output from 
the inference portion is further output to a display circuit 190 for 
forming a display of the welding point 1a essentially corresponding to the 
image shown in FIG. 3. 
Furthermore, when a noise level is low, edge position fluctuation of less 
than 100 .mu.m may be detected since the inference portion 160 provides 
image data to the display portion of substantially high resolution. 
The operation of the system of FIG. 1 will be explained hereinbelow with 
reference to the drawings. 
First, The camera 55 monitors an image of the vicinity of the welding point 
1a, as shown in FIG. 6. Each picture element (n.times.m), of the image 
monitored by the CCD of the camera 55 is arranged on a X, Y axis referring 
to width x length of the image, and the luminance of each picture element 
is detected for providing an overall luminance distribution pattern. 
The scanning image signal S1 of the camera 55 is output to the A/D 
converter 65 for conversion to a digital signal. The A/D converter outputs 
a digital signal S2 having the luminance distribution of the image data 
and the X, Y coordinates of the picture elements of the digital signal S2. 
The digital signal S2 contains a digital luminance value (i.e. 0-128) for 
each picture element. The digital signal S2 is then stored in the image 
memory portion 77. The image data from the image memory portion 77 is then 
input to the CPU 9 via a digital image memory signal S3. 
The CPU 95 further receives reference image data from the reference memory 
85 and makes a determination as to whether the welding condition is 
appropriate by comparing the luminance patterns from the image memory 77 
with those of the reference memory 85 via a reference image data signal 
S4. For this purpose the image data may be divided into scanning zones. 
Thus, the CPU 95 analyzes the V throat of the welding in progress and the 
edges 10a and 10b of the seam 10 for determining whether a welding heat is 
excessive, insufficient or appropriate and an analysis signal S5 is 
generated in the CPU and output to the correction portion 117 and to the 
inference portion 160. 
Referring to FIG. 1, a mask 210 is mounted below the CCD camera 55. The 
mask 21 is of a transparent material and has a window 21a. The window 21a 
has scanning standard lines F1-F2, E1-E2, V1-V2, V3-V4 and V5-V6 
corresponding to scanning lines of the CCD of the camera 55. Referring to 
FIG. 3, the CCD may, for example, scan across, in the direction of the 
line E1-E2, and sequentially downward in the direction of the line F1-F2. 
In FIG. 3, an E zone is defined between the lines E1-E2 and V3-V4. The 
lines V3-V4 and V5-V6 define a V zone and lines V5-V6 and F1-F2 define an 
F zone. Line E1-E2 is a squeeze roll side of the material 1 being welded 
and line F1-F2 is a forming roll, or material supply side of the material 
1 being welded. A line C1-C2 defines an imaginary center line 
substantially corresponding to a position of the seam 10 after welding is 
accomplished. 
The image processing portion 101 receives the scanning image data from the 
camera and processes same as mentioned above for generating the analysis 
signal S5. After the CPU 95 outputs the analysis signal S5 to the 
inference portion, calculation is carried out as described hereinafter. 
1) Zones (F+V+E) correspond to the area within the points E1, E2, F1, and 
F2 which is the vicinity of the welding point 1a from which high heat 
energy is radiated. The width, center, circumference and highest degree of 
luminance for this area is calculated according to the data received from 
the image processing portion 101. 
2) The F zone, defined between points F1, F2, V5, and V6 is differentiated 
for discriminating lines B1-C1 and B2-C1 and the angles .theta.1 and 
.theta.2 thereof in relation to the axial line C1-C2 of the tubular member 
being formed. A width A1-A2 of the highly heated portion in the vicinity 
of the welding point 1a is also determined. 
3) The V zone, define between the points V3, V4, V5, and V6 represents a 
center of gravity of the image, or a center area of the highly heated 
portion is determined. 
4) The E zone defined by points E1, E2, V3 and V4 representing a welded 
portion is discriminated. 
Referring to the above, 1) width, luminance, circumference, represent 
proportionally rising input heat temperature; 2) The angles .theta.1 and 
.theta.2 based on the inclination of the edges 10a and 10b of the V throat 
2 represent the balance of the workpiece (material 1) and whether an entry 
angle of the V throat 2 is large or small; 3) the X axis position 
corresponding to the line of the welded seam 10 is representative of a 
longitudinal center whether a welding upset condition is large or small; 
4) the result of discrimination of the seam 10 determines whether or not 
an output frequency for heating is suitable. 
FIG. 5 shows a simplified example of the operation of the monitoring 
portion 170 representing monitoring of a center line or X axis movement of 
the monitored welding operation. The broken line C0 represents permissible 
variation of the monitored parameters. The line C1 represents actual 
variation occurring in a welding operation. C2 represents an alarm signal 
for lower limit monitoring and C3 is a signal for upper limit monitoring. 
Line L1 represents a lower picture element luminance value of 80.0, for 
example, and L2 is an upper picture element luminance value of 100.0, for 
example. The lines L1 and L2 define a lower hysteresis region. Line L3 is 
lower luminance value of an upper hysteresis region, representing a value 
of 350.0, for example, and line L4 is an upper luminance value of the 
upper hysteresis region and represents a level of 400.0, according to the 
present embodiment. Values below the line L1 and above the line L2 
represent undesirable welding conditions. 
Further, the time increments between a time t0 and a time t3 represent a 
image processing cycle Ts. As seen in the drawing, when the monitored 
center of gravity reaches the lower monitoring limit at a time t2, the 
inference portion 160 is active to send a lower limit alarm signal to the 
alarm circuit 180. Similarly, as the center of gravity reaches beyond the 
upper hysteresis region at a time t5, the inference portion is active to 
send an upper limit alarm signal to the alarm circuit 180. The monitoring 
portion has a display means, associated with a display circuit 190 which 
displays an image such as shown in FIG. 4 the image is updated 
sequentially to show a current status, or welding condition. According to 
this, determination of the welding condition may be assessed by a human 
operator by monitoring the image. 
Further to say, the ranges of the upper and lower hysteresis regions may be 
determined optionally, by experiment, etc., or no hysteresis region may be 
provided. Further, the hysteresis regions may be associated with an alarm 
or an ON/OFF signal for providing warning of undesirable welding 
conditions. Although, in the method for determining whether a welding 
condition is good or bad according to the above described embodiment, X 
axis movement of the workpiece is monitored, a Y axis position, overall 
area of the highly heated portion, axial length of the highly heated area, 
axial width of the highly heated area, circumference, or other parameters 
may be used in image processing according to the invention. 
The present invention is effective in analyzing welding conditions wherein 
a molten metal portion occurs around the edges defining the V throat. 
Specifically, observation by CCD scanning is made to divide the upstream 
and downstream regions which contain the point where both edges of the V 
throat are merged. The CCD scanning lines are used to divide the high 
temperature portion in to scanning zones. In digital image processing of 
the illuminated state of the image from the camera, each picture element 
has a luminance value which is measured. The luminance value is digitized 
and converted into a monochrome image and a characteristic amount of the 
monochrome image is determined. In this case the image consists of a V 
throat with divergent side edges which merge into a single image, or 
welded seam. Masking is accomplished from the upstream side of the merging 
point and digitizing of the image is accomplished and the image is divided 
into zones and the luminance distribution of each of the edges of the V 
throat may be determined. Characteristic amounts of each of the images is 
determined and calculation is made to give the average over E zone and F 
zone. Subtraction is made from the characteristic amounts of the area of 
the F zone and the remainder represents the balance of the heated state of 
the edges defining the V throat. 
Thus, the correction portion 122 receives the analysis signal S5 form the 
image processing portion 101 and the welding condition signal S9 for 
calculating a correction amount. The correction amount signal S6 is then 
input to the signal conversion circuit 130. The conversion signal S7 is 
then output to the signal regulating circuit 140. The signal regulating 
circuit 140 then compares the level of the conversion signal S7 (i.e. an 
analog signal) to the power setting signal S0 for producing a power 
adjusting signal S8 which is output to the power control portion 150. The 
power control portion 150 then sets a power level to the work coil for 
adjusting welding heat. 
Since, more than 100 picture elements are utilized at each side of the 
image, observation of positional variation of 100 .mu.m may be 
accomplished. For optimal performance of the system, it will be noted that 
the monitored area should be shielded from external light. 
Further, the image processing portion 101 uses the reference memory 85 as a 
standard for analyzing image data from the CCD camera 55, thus, according 
to the above described arrangement, highly accurate adjustment of welding 
heat can be accomplished. Alternatively to providing the reference memory 
values, a linearizer may be utilized. 
Thus, the production monitoring portion of the welding system of the 
invention can appropriately monitor various welding conditions, such as 
temperature, shape, operating level, etc., for establishing optimum 
conditions for welding operation and further, can provide visual 
information for a system operator in a continuous fashion with the 
capability of sounding an alarm if monitored welding conditions fall 
outside of a predetermined range. 
Referring now to FIG. 10, the electrical characteristics of the high 
frequency welding system according to the invention will be described in 
detail in connection with a conductive portion 10 of the welding system. 
In FIG. 10, a material 1 which is a metallic, plate material being formed 
into a pipe, for example, is shown. Opposing longitudinal edges of the 
material 1 are contacted with each other at one end of the material 
forming a cylindrical member. Contacting of the sides of one end of the 
material 1 forms a V throat 2 having a first side 2a and a second side 2b. 
As seen in FIG. 11, a high frequency welding current I is applied to the V 
throat 2 for forming an alternating field. Magnetic flux from the 
alternating field crosses over to a wedge-shaped conductive portion 10 
which is arranged in the V throat 2. The causing a cyclic inductive 
current i. When the inductive current i is adjacent the welding current I, 
the inductive current is present at outer edges 10a and 10b of the 
conductive portion 10 and distribution of the adjacent welding current I 
fluctuates. Current distribution is high at a center region of the opposed 
edges 10a and 10b of the conductive portion 10 and 2a and 2b of the V 
throat 2 and low at corner portions of the opposed edges 10a, 10b and 2a, 
2b. Current distribution is essentially even in the thickness direction of 
the material 1 along the edges 2a and 2b of the V throat 2, providing 
substantially identical heating characteristics of the edges 2a and 2b. 
Thus, along the edges 2a and 2b of the V throat, a comparatively low 
frequency heating action is established which is optimal for a welding 
apparatus. 
Since the conductive portion 10 is formed of a metal such as copper, for 
example, with low electrical resistance, gradual heating of the conductive 
portion is avoided and cooling means is therefore desirable to prevent 
damage by melting etc. The cooling means may comprise, for example, means 
for circulating a cooling medium through the conductive portion 10 
including a supply/discharge tube 11 communicating with the interior of 
the conductive portion 10. As a cooling medium, either gas or liquid state 
cooling means may be employed. Further, in order to prevent corrosion of 
the conductive portion 10, an inert, reducing gas should be utilized for 
cooling. 
FIG. 12 shows an alternative construction of a conductive portion of a high 
frequency welding system according to the invention. According to this 
arrangement, an outlet nozzle 12 for emitting an ionized gas is arranged 
in the V throat 2. For this purpose, either a combustible gas or a plasma 
gas may be utilized. In welding operation, the ionized gas is emitted from 
the nozzle 12 into the V throat 2 with substantially the same results as 
in the above-described first embodiment. That is to say, the welding 
current I forms an alternating field and magnetic flux, causing generation 
of the induction current i in the welding gas as shown in FIGS. 13(A) and 
(B). According to this, a Lorentz force is generated between the ionized 
gas and the welding current I at the edges 2a and 2b of the V throat 2 for 
effectively sealing the edges 2a and 2b. 
When a combustible gas or plasma gas is used as the ionized gas, it is 
preferable that the gas temperature be substantially high for enhancing a 
heating effect of the edges 2a, 2b of the V-shaped opening 2. Namely, for 
plasma gas, a temperature several times the combustion temperature 
(2300.degree. K.) is preferable. Further regarding plasma gas, in order 
not to encourage oxidization of a metal being welded, an inert and/or 
reducing gas should be employed. 
Thus, according to the present invention, ionized gas, being either a 
combustible gas a reducing gas, may be utilized according to the 
invention. and, at high gas temperatures, optimal sealing of edge portions 
2a and 2b of the V throat 2 can be achieved. 
For providing a suitable high frequency current for effecting a welding 
system according to the invention, a high frequency oscillator is further 
provided. FIG. 15 shows a schematic diagram of a high frequency oscillator 
according to the invention. Description which corresponds to that given in 
relation to the previously described prior art circuit of FIG. 19 will be 
omitted for brevity. 
A higher harmonic frequency generator circuit 111 includes an all wave 
rectifier circuit 110 for connection with a primary voltage and a filament 
40a of the electron tube 40 via a first transformer TR3 connected at a 
first side of the rectifier circuit 110. A second transformer TR5 is 
connected at a second side of the all wave rectifier circuit 110 for 
providing a secondary voltage via a grid resistor Rg to a grid bias 
circuit 80 for the electron tube 40. A dc filter 120 acts to cut a dc 
(direct current) voltage component to the second transformer TR5. 
The functioning of the above-described circuit will be explained herein 
below with reference to FIGS. 16 and 17. 
Referring to FIG. 16, the all wave rectifier circuit 110 receives the 
primary voltage e.sub.f, via the transformer TR3 (FIG. 16(A)). The primary 
voltage generates an output voltage e.sub.fd in the rectifier circuit 110 
(FIG. 16(B)). The second transformer TR5 receives the voltage e.sub.fd via 
the dc filter 120 with a direct current component removed and the 
secondary voltage e.sub.ff (FIG. 16(C)) is generated having a higher 
harmonic frequency than the primary voltage e.sub.f. 
When the secondary voltage e.sub.ff is supplied to the filament 4a via the 
grid resistor Rg of the grid bias circuit 8, if large fluctuation in the 
applied voltage occurs, the grid bias voltage oscillates on the minus side 
and is superimposed on the applied voltage for suppressing he fluctuation. 
When heating of the electron tube by alternating current occurs, on 
average, a filament charge time for achieving a given heat value may be 
given as (J/sec=W). Theoretically, a thermal energy Q according to the 
following equation is applied to the filament: 
EQU Q=Q.sub.1900 +.intg.Wf sin (.omega.t-.psi.)dt-.intg..omega.rad sin 
(.omega.t-.psi.)dw (1) 
wherein: 
Wf=Quantity of heat per unit time (Joule/sec) 
Q.sub.1900 =quantity of heat required 
.psi.=phase shift 
by this, a filament temperature of 1900.degree. K. may be achieved. 
If a charge heat is R.sub.if.sup.2 (R=filament resistance, .sub.if 
=filament current), is added to the filament single phase voltage e.sub.f, 
the frequency of the filament temperature T is doubled. 
Therefore, referring to FIG. 17, fluctuation of the filament temperature 
based on high frequency output voltage pulsation of the single phase 
alternating current can be minimized, since the voltage e.sub.ff is 
superimposed with the bias voltage based on a canceling, or compensating, 
high frequency output voltage pulsation. Also, ripple in the rectifier of 
the direct current electrical source circuit based on high frequency 
output voltage upper harmonic pulsation can also be minimized. Further, 
from the voltage of the transformer TR5, a variable potential resistor, or 
the like, may be added for adjusting the grid resistance. 
Hereinbelow, an alternative construction of a high frequency oscillator 
according to the invention will be described with reference to FIG. 18. 
Elements which are identical with those of the above-described oscillator 
circuit will be omitted. 
As seen in FIG. 18, a voltage regulator 130 supplying the primary voltage 
e.sub.f for the filament 40a is provided. A second side of the voltage 
regulator 130 is connected to an all wave rectifier 110. Grid resistors 
Rg.sub.1, Rg.sub.2 provide the grid resistance Rg. The grid resistor 
Rg.sub.2 is applied a current e.sub.fd' FIG. rom the single phase all wave 
rectifier 110 at both terminals thereof. 
According to this arrangement, since the grid bias voltage adjusts the 
frequency doubled applied voltage to the filament 40a, high frequency 
fluctuation in an output voltage can be effectively minimized. 
While the present invention has been disclosed in terms of the preferred 
embodiment in order to facilitate better understanding thereof, it should 
be appreciated that the invention can be embodied in various ways without 
departing from the principle of the invention. Therefore, the invention 
should be understood to include all possible embodiments and modification 
to the shown embodiments which can be embodied without departing from the 
principle of the invention as set forth in the appended claims.