Flame rod structure, and a compensating circuit and control method thereof

A flame rod structure is comprised of a silicon alloy or is coated by a silicon material on a metal flame rod. A compensating circuit applies the A.C. bias to the D.C. bias of the flame rod structure, generates the excitation frequency signal and mixes the excitation frequency with the D.C. bias to produce a reference frequency according to the flame sensing of the flame rod structure, whereby a calorific step is accurately detected to control the optimum heating of a burner or a combustion apparatus.

BACKGROUND OF THE INVENTION 
The invention relates to a flame rod structure provided with a cladding of 
a semiconductor material, and a compensating circuit for promoting 
reliable operation of the flame rod structure and a control method 
thereof. 
PRIOR ART 
Generally, a custom heater or heater and a combustion apparatus have used 
fossil fuels to control a number of calories generated during heating 
based on flame sensing, in which flame sensing requires a metal material 
which can endure relatively high heat. In other words, the flame rod 
structure is made in the form of a metal rod as a sensor for detecting the 
generated amount of calories, oxygen concentration, firing and non-firing 
relative to the flame state of fossil fuels like petroleum. The flame rod 
detects a flame current of a predetermined voltage generated upon the 
combustion of fossil fuels, so that its equivalent circuit can be designed 
in a modelling pattern. 
The flame current on the flame rod is first converted into a small amount 
of ion current according to the combination of carbon and oxygen during 
the combustion of fossil fuels. Therefore, as shown in FIG. 1 the flame 
rod may be considered an ideal diode because its ion current flows in one 
direction or is forward-biased. Then, the difference Ri of the flame 
resistance dependent upon the number of calories generated during heating 
is generated. The drift of charge components (relative to the time 
elapsed) forces the flame rod to have a small amount of electrostatic 
capacity Ci according to the heating state. Also, the flame rod has a 
resistance value RL due to the leakage current generated by its structural 
factors and the combustion condition. 
The characteristic of the voltage to the current of the flame rod is 
illustrated in FIG. 2. The forward characteristic of direct current (D.C.) 
represents the vector value of the leakage current value RL to the flame 
resistance value per hour. The flame resistance Ri is proportional to the 
total calories Ro (related with the temperature and the time), and the 
leakage current resistance RL is proportional to the value of R (the 
structural factor of the flame rod) multiplying the square of flame 
resistance Ri. 
The electrical alternate current (A.C.) characteristic of the flame rod is 
shown in FIG. 3. That is, the relationship of the electrical A.C. to the 
flame current fluctuates with a heating change according to the amount of 
calories absorbed, which is determined by the combustion combination rate 
according to the combustion ratio condition. The capacitance load C is 
inversely proportional by an exponential function to the heating step. It 
represents the equation of Ci.alpha.Co(w), wherein w is the combustion 
ratio condition factor. 
The flame rod is made of metal material which serves as a conductor 
(medium) of the heat generated by the combustion flame, but has a 
conductivity characteristic which degrades according to the time elapsed 
and the temperature rise. It furthermore has problem with reliability the 
exterior disturbance (the petroleum quality and the efficiency reduction 
of the complete combustion under a predetermined combustion ratio 
condition) and the electrical problems of its associated circuitry leading 
to the degradation of conductivity, thereby causing function as a 
conductivity medium as shown in FIG. 4. 
The metal flame rod also increases its flame resistance Ri more and more 
according to the temperature rise as shown in FIG. 5, because its 
conductivity characteristic accelerates the elastic collision of free 
electrons. 
Additionally, the ion current converted from the flame current flows along 
the skin surface of the flame rod. The amount of charge is reduced 
according to the time elapsed, and the smaller the calorie is, the more 
the electrical AC characteristic of the skin current relative to the 
heating step is deteriorates as illustrated in FIG. 6. 
As a result, the metal flame rod is under the relatively large exterior 
influences including the calories absorbed, the time and the temperature, 
so that its electrical characteristic is abruptly changed. It is 
appreciated that the metal flame rod is not ideal as a conductivity medium 
or device with respect to the associated circuitry. 
The, the electric charge quantity Q.sub.F is represented as follows: 
EQU Q.sub.F &gt;.alpha.Q.sub.C ; .alpha..ltoreq.1, 
wherein 
Q.sub.F : a quantity of the electric charge generated by the flame current 
Q.sub.C : a quantity of the Electric Charge supplied to the electrical 
network by the flame rod 
.alpha.: the flame rod conductivity .sigma. (dependent on the temperature 
characteristic and the supplied electric charge quantity Q.sub.F) 
On the other hand, the metal flame rod is resistance abruptly increases in 
the non-conductor area based on the time characteristic curve thereby 
causing the electrical loss shown in FIG. 4. The flame resistance Ri 
relative to the time elapsed of the metal flame rod is dependent upon the 
fuel quality, but the increase of the flame resistance should be 
introduced only within the scope of the conductivity area. At that time, 
assuming that the skin current component has a maximum electric charge, 
the combustion of carbon material occurs adjacent to the skin surface of 
the flame rod to form a carbon cladding thereon, and the carbon cladding 
acts as a resistor to increase the flame resistance Ri, infinitely. 
Accordingly, it is noted that the flame rod may be remarkably improved by 
using materials which are not subject to the exterior influences like the 
calorie, the time and the temperature, etc. 
Considering these points, if the flame rod structure is able to facilitate 
the generation of the flame current, be heat resistant property and reduce 
the skin current, it can be supposed an ideal flame sensor. In other 
words, the material which causes the reduction of the skin current and the 
improvement of the conductivity and temperature characteristics related to 
the flame resistance Ri for overcoming the deficiencies disclosed in FIGS. 
4 and 5 is a semiconductor semiconductors are known as a conductivity 
medium having the excellent characteristics of conductivity and a 
heat-resistance property. 
Therefore, an ideal flame sensor can exist in practice, if the defects of 
the metal flame rod are removed, and the merits of the semiconductor 
material are adapted to the flame rod. In order to realize the ideal flame 
sensor, the flame rod structure can be made of the combination of a metal 
and a semiconductor. 
The semiconductor material is heated to raise its temperature, so that the 
interior electrons become excited from a bound energy level to become free 
electrons. As a result, the flame component is charged due to the negative 
change of negative ions, so that the electrons of the metal flame rod are 
combined with the positive holes of the semiconductor material to serve as 
charge components and promote a current drift corresponding to the heating 
temperature. This effect causes the charge components of the semiconductor 
material to compensate for the skin current reduction of negative ions 
generated in aging by raising the temperature. 
Therefore, the charge flux is shown as follows: 
EQU Ju=n(E.sub.C -.mu.+3/2 K.sub.B T) (-.mu.e)E 
Wherein, 
Ju: charge flux 
N: the number of the semiconductor atoms 
E.sub.C : energy level (conductivity band) 
K.sub.B : Boltzman constant 
T: absolute temperature 
-.mu.e: mobility of electron 
E: applied drift electric field strength 
As shown in FIG. 7, the conductivity of the negative ion represents a state 
when the gradient in the non-conducting region of the high temperature is 
slightly smaller than that in the usage region. 
As FIG. 8 illustrates a flame rod including a cross-sectional portion A and 
a lateral portion B. The flame rod is wrapped at a predetermined thickness 
by a semiconductor material. The electric field strength E at the center 
of the cross-sectional portion A is zero, and the cross-sectional electric 
field strength E is inversely proportional to a length 1 and proportional 
to the difference value obtained by subtracting the final voltage V.sub.2 
from the initial voltage V.sub.1. 
EQU E=(V.sub.1 -V.sub.2)/l 
At this point, the metal electrons can be combined with the positive holes 
of the semiconductor material. The semiconductor material not only 
increases the interior mobility of the semiconductor material relative to 
the negative ions by forming the drift electric field, but also conducts 
the drift current due to the effect (Ju=3/2 KT) obtained by the charge 
flux thereof according to the temperature rise. At that time, the drift 
current components compensate for the reduction of the skin current 
occurred in aging by the combustion flame skin resistance. It is believed 
not to influence the conductivity of the flame rod as a whole. 
Consequently, the semiconductor material contributes to draw out the 
conductance current. 
On the other hand, a fluid (air) vibration of the flame rod caused by a 
combustion flame causes charge fluctuation, so that the conductivity of 
the medium constituting the flame rod is reduced, if a heating step is low 
as shown in FIG. 3. This flame rod during the combustion is represented as 
the equivalent circuit illustrated in FIG. 9. Herein, the D.C. direction 
is the same as that in FIG. 1. However; the flame rod relative to a 
current Ii, as illustrated by the A.C. electrical characteristic in FIG. 
2, has difficulty in processing the electrical signal like a constant 
voltage regulated power source, because its interior impedance is 
relatively large and variable. 
It is represented by the following equation. 
EQU Zi (calorie)=V.sub.FR /Ji 
EQU Zi (W)=V.sub.FR (W)/Ji(W) 
Wherein, 
V.sub.FR : voltage of the flame rod 
Ji: current of the flame rod 
W: change of calorie 
Consequently, the flame resistance Ri acts as an interior impedance at low 
frequencies in shown as FIG. 9. Thus, in order to transmit a larger signal 
the flame rod needs an associated bias/excitation circuit due to the 
variability of the resistance value. The method for processing the low 
frequency signal is illustrated in FIG. 10. It is noted from FIG. 10 that 
the low frequency band (between WE to WB) is added to the reference 
frequency h.sub.W and then removed. 
For example, 
EQU Q'.sub.FR =Q.sub.FR *h.sub.W 
Also, the interior impedance mainly occurs due to the skin resistance. 
Therefore, the impedance value of Zi can be decreased by properly applying 
the A.C. bias to a D.C. bias circuit without reducing the capacity 
component Ci generated by the flow of the skin current illustrated in FIG. 
1. 
For example, 
EQU Zi=1/WCi 
Therefore, the design condition of the bias circuit is as follows. 
As illustrated in FIG. 11, the current loss at the skin surface should be 
prevented. 
For example, 
EQU Z.sub.A =Z.sub.B +Z.sub.C 
EQU Z.sub.A (W)=Z.sub.B (W)+Z.sub.C (W) 
Wherein, 
Z.sub.A : interior impedance of excitation circuit 
Z.sub.B : combustion flame voltage+impedance Z of flame rod medium 
conductivity voltage 
Z.sub.C : impedance Z of circuit C 
Accordingly, it is necessary to apply the maximum A.C. bias to circuit C so 
as to minimize the impedance value of Z.sub.A. 
As described above, if it is based on the flame rod structure and the 
associated circuit, the flame rod, a so called flame sensor, acts as a 
variable signal source during operation, so that its conductivity loss may 
be reduced, and processes the associated with electrical signals 
associated with relative to the fractionized flame states. 
Accordingly, it is an object of the present invention to provide a flame 
rod structure for restraining current from being generated at the skin 
surface to reduce the loss due to the variability of the structural 
interior impedance thereof. 
It is a further object of the present invention to provide a flame rod 
structure including the semiconductor material for improving the 
conductivity and the temperature characteristic influenced by the flame 
resistance to reduce the skin surface current thereof. 
It is another object of the invention to provide a flame rod structure 
acting as a sensor having the interior characteristic of high impedance 
which is minimally affected by the time elapsed and the exterior 
disturbance. 
It is still another object of the invention to provide a compensating 
circuit for improving the function of the flame rod structure by applying 
the A.C. bias to the D.C. bias which is the generation factor of the skin 
current due to the interior impedance of the flame rod. 
It is still another object of the invention to provide a method of 
controlling the compensating circuit for applying the A.C. bias to a flame 
rod structure. 
SUMMARY OF THE INVENTION 
According to the present invention, there are two types of flame rod 
structures. One is comprised of a composition of a semiconductor material 
and a metal component. A semiconductor material may be silicon and 
germanium, etc. in the form of a micro-powder, and the metal powder may be 
iron and nickel, etc. for use as a ferromagnetic substance. Accordingly, 
these powders are simultaneously sintered and ground into the 
micro-particles, melted with a predetermined adhesive agent, cooled and 
pressed/molded at a high pressure, thereby providing the flame rod 
structure. 
The other flame rod structure may be constructed to cover the semiconductor 
material on a metal flame rod. At that time, the junction portion formed 
between the semiconductor material and the metal can be considered as a 
high frequency diode including the resistance rectifying junction portion. 
Also, according to the present invention, there is provided a circuit for 
compensating a skin current by applying an A.C. bias to the D.C. bias of a 
flame rod structure. The circuit comprises a constant voltage regulated 
circuit applying the D.C. bias to the flame rod structure; means for 
generating an excitation frequency signal, which is A.C. biased relative 
to the flame rod structure; means for mixing the signal of the constant 
voltage regulated circuit with the signal of the excitation frequency 
generating means and generating a signal of the predetermined frequency 
band; means for receiving the signal from the flame rod structure to sense 
the flame; means for trapping only the excitation signal among the signals 
from the flame sensing means; means for filtering the trapped signal to 
leave the actual sensing frequency of the flame signal; means for shaping 
the waveform of the filtered signal to output it to the first 
analog/digital converting port of a microprocessor; The microprocessor is 
connected to the flame rod structure for sensing the flame signal at the 
second A/D port, converting only the reference frequency corresponding to 
the excitation frequency into the voltage signal and outputting the D/A 
converting signal of the reference signal at the third analog/digital port 
of the microprocessor. Also included is for converting the analog signal 
from the microprocessor into a voltage to frequency signal and applying it 
to the excitation signal generating means. 
Also, the present invention is provided with a microprocessor constituting 
a compensating circuit including a method of determining whether the 
frequency to voltage signal inputted at the first and second A/D port is 
equal to the previously set frequency to voltage data, outputting the 
inputted voltage to frequency signal if they are equal. If they are not 
equal the flame rod voltage is compared with the corresponding set minimum 
voltage at the sensing time point. If the flame rod voltage is equal to 
the sensed minimum voltage, then outputting the inputted voltage to 
frequency signal is outputted. If they are not equal, then the contents of 
the predetermined RAM data are charged and output as the voltage to 
frequency signal.

DETAILED DESCRIPTION OF THE INVENTION 
According to the present invention, as shown in FIG. 8, a flame rod 
structure 100 generates a drift current to increase the interior electron 
mobility as a conductor, thereby reducing the skin current and increasing 
the quantity of drift current according to a rise in temperature. The 
temperature rise increases the charge flux Ju by about 3/2 KT. 
The flame rod structure of the present invention is produced through one of 
two methods. One method is to prepare a semiconductor composition 
comprised of a micro-particular magnetic substance. For example, an 
iron-semiconductor alloy is prepared by using the micro-powder of silicon, 
germanium as a semiconductor and iron, nickel as a metal powder. For 
example, the typically silicon alloy is formed so that the silicon powder 
of 3-5% by its weight ratio is sintered with the metal powder, crushed 
into the micro-particles, melted with the predetermined adhesive agent, 
such as an elastic adhesive agent, cooled and pressed/molded at a high 
pressure. 
The flame rod structure is electrically adapted to a high frequency 
application like the clad structure of a laminate type on the metal flame 
rod as described below, so that it improves the electrical characteristic 
such as the conductivity in the high temperature condition and especially 
has a low core loss, a high permeability and a low eddy current loss due 
to the increasing of the electrical resistance. 
The other flame rod structure is prepared by coating the semiconductor 
material on the metal flame rod. Herein, metal semiconductor junction 
portion constitutes the low-resistance region of a rectifying junction 
portion. Therefore, this junction portion can be used as a high-frequency 
diode. Meanwhile, this semiconductor(dielectric substance) surface 
provides an electrical conduction path in parallel with the volume portion 
of the metal flame rod, where the electrical conduction is characterized 
by the surface resistance value. 
The dielectric substance of silicon causes the skin electrical conduction 
in a humid environment. At that time, if it is used as a flame sensor, the 
dielectric substance can not generate charge drifting on the skin surface, 
thereby losing the conductivity function. 
Additionally, the current at a high frequency is induced adjacent to the 
skin surface of the conductor or the flame rod structure, in which the 
skin depth is defined to reduce the current density by 1/e on the skin 
surface, and the skin resistance Rs is the D.C. resistance value of the 
conductor having the thickness of the skin depth. 
The surface (skin) resistance is as follows: 
EQU Rs=.rho./.delta.=1/.alpha..delta. 
Wherein, 
.rho.=electrical resistance (.OMEGA.-m), .delta.=thickness (m) 
.sigma.=electrical conductivity (.upsilon./m) 
FIG. 12 is a block diagram of a compensating circuit according to the 
principle of the present invention. 
The compensating circuit is provided with a microprocessor 20 so as to 
generate the excitation frequency relative to the flame rod structure 
which is a flame sensor. In the other words, the flame rod structure 100 
is connected to a mixer 24 at one end thereof, which receives input 
signals from a reference voltage generating circuit 22 and an excitation 
signal generating circuit 26. 
The reference voltage generating circuit 22 is formed as a constant voltage 
regulated circuit for applying the D.C. bias to the flame rod structure 
100, in which the D.C. bias is the signal of a waveform A shown in FIG. 
14. 
The excitation frequency signal generating circuit 26 creates the 
excitation signal of an A.C. component having a predetermined frequency, 
which is adjusted by the microprocessor 20. The excitation signal appears 
as the waveform B of FIG. 14, wherein a voltage Vm or Vex(t) is 
represented as follows: 
EQU Vex(t)=Vmsin (wt+.phi.). 
Thus, the mixer 24 generates the signal of the frequency band for improving 
the electrical characteristic of the flame rod structure 100, in which the 
signal has a waveform C adding the waveform A to the waveform B, which 
represents the A.C. component voltage as follows: 
EQU Vc=Vref+Vmsin (wt+.phi.) 
The flame rod structure 100 senses the flame state in addition to receiving 
the signal from the mixer 24 and then generates the flame sensing voltage 
according to the medium material of the flame rod structure 100. The flame 
sensed signal is inputted to a flame signal detecting circuit 28 and an 
excitation frequency separating circuit 34. 
The flame signal detecting circuit 28 convolutes (raises) the flame 
detected signal to a voltage according to the frequency and the calorific 
step. Herein, the voltage is represented as follows: 
EQU V.sub.D =Vref+Vmsin (wt+.phi.)*V.sub.FR. 
The convoluted flame detecting signal is applied to a low pass filter 30. 
The low pass filter 30 receives only the flame detecting signal V.sub.FR 
by means of an attenuator 38 connected through a voltage-frequency 
converter 36 to the third A/D converting port P.sub.3, because the 
attenuator 38 forces the signal from the flame signal detecting circuit 28 
to be made into a voltage signal of the A.C. component adding the waveform 
A to the waveform B to remove the excitation signal component from the 
flame signal detecting circuit 28. Herein, the voltage signal is 
represented as follows: 
EQU V.sub.E =V.sub.D -V.sub.B 
Thus, the low pass filter 30 permits only the frequency component of the 
actual flame detecting signal to be applied to a waveform shaping circuit 
32. That is, the flame detecting signal is represented as follows: 
EQU V.sub.F =V.sub.FR =V.sub.E 
The waveform shaping circuit 32 applies the predetermined rectangular wave 
signal to the first analog/digital(A/D) converting port P.sub.1 of the 
microprocessor 20. At the same time, the signal from the flame rod 
structure 100 is applied to the excitation frequency separating circuit 
34. The excitation frequency separating circuit 34 removes the excitation 
frequency, converts it into a frequency-voltage signal, and then applies 
this converted signal to the second A/D converting port P.sub.2 of the 
microprocessor 20. 
The microprocessor 20 controls the compensating circuit as shown in FIG. 
13. 
Referring to FIG. 13, at step 40 the microprocessor 20 receives the signals 
from the excitation signal separating circuit 34 and the waveform, shaping 
circuit 32. Step 40 goes onto step 41 to judge whether the input 
frequency-voltage data is the frequency-voltage data previously stored in 
RAM. When they are equal, step 41 moves onto step 44 to convert the 
excitation signal into a voltage-frequency signal and outputs the 
converted signal at the third A/D converting port P.sub.3 to the 
voltage-frequency converter 36. Otherwise, step 41 goes onto step 42 to 
judge whether the flame detecting signal V.sub.FR is equal to the minimum 
voltage previously stored in RAM of the microprocessor. If not, step 42 
moves onto step 44 to converts the flame detecting signal into the 
voltage-frequency signal and output the converted signal at the third A/D 
converting port P.sub.3 to the voltage-frequency converter 36. If the 
flame detecting signal V.sub.FR is equal to the minimum voltage, step 12 
goes onto step 43 to convert the previously set RAM data into the minimum 
voltage and then moves onto step 44. 
Therefore, the microprocessor 20 outputs the voltage-frequency converting 
signal having the predetermined excitation frequency through the D/A 
converting port P.sub.3 to the voltage-frequency converter 36 according to 
the heating step of the flame rod structure, in which the 
voltage-frequency converter 36 converts the signal of the microprocessor 
20 into the voltage-frequency signal and supplies it to the excitation 
frequency signal generating circuit 26. 
As described above, a compensating circuit of the present invention 
supplies the current of the A.C. component to a flame rod structure 100 in 
addition to the signal of the D.C. component, so that it prevents the flow 
of skin current from being reduced.