Note: Descriptions are shown in the official language in which they were submitted.
- 2046851
1 61211-1018
BACKGROUND OF THE INVENTION
Field of the invention:
The present inventlon relates generally to the
technology of lnductlon heatlng and more partlcularly to the use
of induction heating devices for case-hardening of rnachine
components such as gears.
Machine components such as gears, splined shaves and
sprockets are frequently sub~ected to high torque loads,
frictional wear and impact loading. Gears of this type are
typlcally used ln power transmisslon drive trains. An apparatus
and method for induction-hardening of such machine components is
disclosed in the U.S. Patent No. 4,845,328 to Storm et al.. The
Storm et al. patent and this application are both owned by the
same assignee, Contour Hardening Inc., of Indlanapolls, Indlana.
As is well known in the art, a known device for gear
teeth hardening includes a dual-frequency arrangement for
induction heating wherein a low frequency current is used for
preheating the gear teeth and then a hlgh frequency (Radio
Frequency) current is then used for final heating prior to quench
hardening of the gear teeth. The dual frequency induction
hardening concept is described in the article
~ B
20~6851
--2--
"Induction Gear Hardening by the Dual-Frequency Method" which
appeared in Heat Treating Magazine, Vol. 19, No. 6, published
in June, 1987.
As explained in the article, dual-frequency heating
employs both high and low frequency heat sources. The gear
is first induction heated with a relatively low frequency
source (3-10 kHz), providing the energy required to preheat
the mass of the gear teeth. This step is followed
immediately by induction heating with a high-frequency source
10 which typically ranges from 100-300 kHz depending on the gear
size and diametral pitch of the gear teeth. The
high-frequency source will rapidly final heat the entire
tooth contour surface to a case hardening temperature. The
gears are then quenched to a desired hardness and tempered.
Induction heating is the fastest known way of heating an
iron alloy gear. In some applications a pre-heat low
frequency heat process precedes the final heat RF heating.
Heating times for the high-frequency RF heating step
typically range from 0.10 to 2.0 seconds. In induction
heating, the gear is mounted on a spindle and spun while
positioned within the induction heating coil. A quick pulse
of power is supplied to the induction heating coil which
achieves an optimum final heat of the gear teeth. Next, the
piece is manually or automatically moved into a water-based
quench. Because induction hardening puts only the necessary
amount of heat into the part, case depth requirements and
distortion specifications are met with great accuracy.
Within the induction heating process, whether dual- or
single frequency, and regardless of the type of part and its
material, the part characteristics dictate the optimum design
of both the induction heating coil or coils and the most
appropriate machine settings. In particular, the amount of
time that the high-frequency power signal is supplied to the
induction heating coil to generate the final heat is a most
critical parameter. The exact amount of heat required to
_3_ 2046851
harden the gear is directly related to the precise amount of
time that the power signal is supplied to the induction
heater coil.
Traditionally, there are two systems well-known in the
art for supplying power to an induction heater coil as
described above. The first system utilizes what is known in
the art as a "solid state" generator approach wherein high
power amplification devices such as transistors, be they
bipolar or CMOS, are used in the high-frequency RF generator
to supply a high-frequency oscillator signal to the induction
heater coil. An alternate approach is to use a vacuum tube
RF generator and utilize thyristor type devices to switch
power on and off to the high-frequency, high power vacuum
tube oscillator circuit. The output of either oscillator
circuit is coupled to the induction heater coil by way of a
transformer. Some experts in the art of induction heating
coil machines designed for case hardening metallic structures
have hefetofore preferred the solid state high-frequency RF
generators for their exact timed control of power delivery to
the induction heater coil. A vacuum tube RF generator
typically receives its input power subject to the on/off
timing characteristics of thyristor devices such as silicon
controlled rectifiers (SCR's) which are also known in their
JEDEC description as reverse blocking triode thyristors. The
power delivery timing variance created by the SCR is
intrinsic in the operation of such devices. Specifically,
once an SCR is "turned on" for a partial cycle, even though
the on/off signal supplied to the gate is removed or
deactivated, the SCR will continue to conduct current so long
as the anode to cathode terminals are biased with a positive
voltage. In the worst case of a 60-cycle power signal being
transferred by the SCR, this results in over an 8 millisecond
additional power signal transmitted by the SCR, since half of
a 60-cycle waveform is 8.33 milliseconds in duration.
It is recognized that the vacuum tube RF generator is
- 2046851
-4-
preferred by some in the induction heating art for its
characteristic power delivery curve in supplying power to an
induction heater coil. Additionally, since SCR's are the
device of choice for repeated high power switching circuits,
a technique for accurately controlling SCR's to deliver
specific quantities of power to a high-power vacuum tube RF
generator is needed.
A method and apparatus for more accurately controlling
the timed power output of a silicon controlled rectifier
power supply is needed for accurately controlling the power
signal supplied to induction heater coils used in case
hardening devices.
20~6851
--5--
SUMMARY OF THE INVENTION
An apparatus for induction hardening machine components
with precise control of power output, according to the
present invention, comprises an AC power source for producing
an AC power signal, zero-crossing detector means connected to
the AC power source for detecting zero crossings of the AC
power signal and producing a zero-crossing signal
corresponding thereto, a high-frequency generator having a
power input and an output for producing a high-frequency,
high-power signal in response to a signal supplied to the
power input, a high-frequency induction heater coil sized to
fit the gear and connected to the output of the generator,
the coil generating a high-frequency electrical signal
through the gear, thyristor power switching means having an
activation input, a power input connected to the AC power
source, and a power output, the power switching means
producing an AC power signal at the power output in response
to a signal supplied to the activation input, and processor
means, connected to the zero-crossing detector and the
thyristor power switching means activation input, for
computing activation times and supplying a corresponding
activation signal to the activation input, the processor
means including:
l) means for entering a desired activation time, Z) means for
computing a delay time so that the sum of the activation time
and the delay time corresponds to a minimum whole number
multiple of the period of the AC power signal, and 3) input
means for receiving a user supplied manual cycle start input
signal, the processor responding to a cycle start input
signal by detecting a zero crossing signal and delaying a
period of time equal to the delay time before supplying an
activation signal to the activation input so that the
activation signal is extinguished substantially
simultaneously with a subsequent zero crossing of said AC
- 204~851
6 61211-1018
power signal.
An induction-hardening apparatus according to another
aspect of the present inventlon include an AC power source for
producing an AC power signal, phase detector means for detecting a
predetermined phase angle of the AC power slgnal, the detector
means producing a detector signal when the predetermined phase
angle is detected, a high-frequency generator means having a power
input and a power output for producing a high-frequency high-
power signal at the power output in response to a power signal
supplied to the power input, a high-frequency induction heater
coil connected to the power output, the heater coil ernitting a
high-frequency electromagnetic signal in response to the high-
frequency high-power signal, power switching means connected to
the AC power signal, the power switching rneans including an
activation input, the power switchlng means supplylng the AC power
signal to the power input ln response to receiving a signal at the
activation input, and timer circuit means responsive to the
detector signal for supplying an activation signal of a
predetermined duration to the activation input.
According to another aspect of the present invention
there is provided, a method for precisely controlling power
supplied to an induction-hardenlng apparatus which includes a line
frequency AC power source which produces a line frequency power
signal at an output, a high-frequency generator having a power
input and a power output, and a high-frequency induction heater
coil connected to the power output of said high-frequency
generator, said method comprising the steps of:
detectlng the predetermined phase angle of said line
"~,
~,s,
., ,~,
6a 20~C851 61211-1018
f requency AC power s lgnal;
connect ing the AC power source output to the power input of
the high-frequency generator for a single predetermined continuous
period of t irne in response to detect ing said predeterrnined phase
angle .
An induct ion-hardening apparatus according to yet
another aspect of the present invention for precisely controlllng
power delivery of an high-frequency induction heater coil,
comprises an AC power source for producing an AC power signal,
lO f irst circuit means for producing a f irst signal in
'-'''B
7 2046851
reponse to detecting a predetermined phase angle of the AC
power signal, switch means for producing a start signal when
the switch means is activated, second circuit means
responsive to simultaneous occurrence of the first signal and
the start signal for producing a predetermined duration
activation signal in response thereto, high-frequency
generator means having a power input for producing a
high-frequency high-power signal in response to a signal
supplied to the power input, and power switching means
connected to the AC power signal and supplying the AC power
signal to the high-frequency generator in response to the
predetermined duration activation signal.
One object of the present invention is to provide an
improved induction hardening machine.
Another object of the present invention is to provide a
method for more accurately controlling the power signal
supplied to induction heater coils of an induction hardening
machine to precisely control the power supplied and thus the
heating of a gear during case hardening.
~nother object of the present invention is to provide a
more accurate high power switching circuit so that the total
power output signal can be controlled with greater precision.
These and other objects of the present invention will
become more apparent from the following description of the
preferred embodiment.
2Q~6851
--8--
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a typical embodiment of an
induction-hardening system according to the present invention.
FIG. 2 is a timing diagram showing variations in the
active or "on" state of an SCR with respect to certain input
conditions applied to the gate of the SCR.
FIG. 3 is a graph depicting a deviation in power output
signals produced by power switching SCR circuits of the
present invention as compared with prior art devices.
FIG. 4 is a block diagram of another embodiment of an
induction-hardening system according to the present invention.
2046851
_9_
DESCRIPTION OF THE PREFERRED EMBODIMENT
For the purposes of promoting an understanding of the
principles of the invention, reference will now be made to
the embodiment illustrated in the drawings and specific
language will be used to describe the same. It will
nevertheless be understood that no limitation of the scope of ;
the invention is thereby intended, such alterations and
further modifications in the illustrated device, and such
further applications of the principles of the invention as
illustrated therein being contemplated as would normally
occur to one skilled in the art to which the invention
relates.
Referring now to FIG. 1, an induction-hardening system 10
according to the present invention is shown. Switch SWl
provides an activation signal to the system processor 12 for
invoking or initiating the case hardening of a gear. System
processor 12 is programmed by the user with timing parameters
for controlling the power signal supplied to the induc~ion
heater coil. Processor 12 supplies an on/off power switching
signal to power switching SCR circuit 14. System processor
12 receives a zero crossing indicator input signal from zero
crossing detector 16. One phase bl from 3b high voltage
power source 18 is supplied to an input of zero crossing
detector 16. The 3b high-voltage power source 18 supplies
three phases of high voltage power to the power switching SCR
circuits 14. Power switching SCR circuits 14, when
activated, supply either half-wave or full-wave AC power
signals to the primary windings of step-up transformer 22.
Transformer 22 steps up the AC power signals bl, b2
and b3, typically 480 volts three-phase signals, to a
voltage level sufficiently high that rectifier and filter 24
produces a 24,000 volts DC signal at its output.
The 24,000 volts DC signal at the output of rectifier
filter 24 is the power source for a vacuum tube type
20~6851
-10-
high-energy RF oscillator 26. The output of the high-energy
oscillator 26 is AC coupled to the induction heater coil 28
via windings 29. Induction heater coil 28 supplies a
case-hardening heating signal to the gear teeth of gear 30
when an RF signal is supplied to its input.
The components 22, 24 and 26 of the system 10 are part of
RF generator 20 which is a high-frequency, high-power RF
generator. The RF generator 20 is an off-the-shelf system
supplied by Pillar Industries, Inc., N92 W15800 Megal Drive,
Menomonee Falls, Wisconsin 53051. The RF generator 20 is
referred to as a "450/600 kilowatt RF Generator".
The particular geometry and physical attributes of gear
30 dictate the precise amount of time that power switching
SCR circuits 14 are "turned on" by system processor 12 in
order to produce the appropriate case hardening result. In
some instances, the amount of time that the SCR circuits 14
are turned on is as small a time period as 0.10 seconds to
accomplish the desired heating and case hardening of gear
30. With this condition in mind, it is easy to see why the
prior art devices which did not include zero crossing
detector 16, were unable to accurately control the amount of
power signal or total power supplied to the induction heater
coil 28.
The system processor 12 of the present invention
typically includes a computer having adequate memory and
computing capability, and a programming input device such as
a CRT/keyboard device. Additionally the processor 12 has
mass storage devices such as floppy or hard disk drives for
use in storing and recalling control programs. Operationally
speaking, an operator programs the system processor 12
through a keyboard for a particular "on-time" or heat time
which is the exact time that the power switching SCR circuits
14 shall be turned on to supply a fixed quantity of
high-frequency power signal to the induction heater coil 28.
In response to the programmed "on time" information, the
2046851
--11--
system processor 12 will compute a complement value for the
specific "on time" which is equal to the difference between
the "on time" divided by 8 . 33 milliseconds (the period of a
60 Hz waveform). The remainder from this calculation is
subtracted from 8.33 milliseconds to produce a time value
which is the delay time that the processor 12 should delay
after detecting a zero crossing of the 60 Hz signal present
at the input of detector 16 prior to activating the SCR
circuits 14 to supply power to the RF generator. The time
10 delay calculation is designed so that the end of the on or
conducting period for the SCR devices corresponds exactly
with or just prior to a zero crossing of the power signal
bl supplied to the input of zero crossing detector 16.
Thus, the SCR's, which remain in the conducting state so long
as the anode to cathode terminals are forward biased, will
not remain on a substantial period of time after the system
processor 12 signals the SCR circuits 14 to turn off by
deactivating the input to the circuits 14.
It is well known in the art that SCR circuits 14 may
supply a half-wave or full-wave 3b output signal to the
transformer 22. If the signal is half-wave in nature, the
divide-by factor described above (8.33 milliseconds) becomes
16.67 milliseconds and the remainder is subtracted from 16.67
milliseconds. Additionally, negative-slope zero crossovers
must be detected to determine the appropriate timing
reference points for activating a half-wave output SCR
circuit. Thus, the "on time" desired is divided by 16.67,
and any remainder therefrom is subtracted from 16.67. The
result of the subtraction process is the delay period
required after a negative-slope zero crossover of the power
signal prior to activating the SCR circuits 14 for half-wave
outputs therefrom. Although the other phases (b2 and
b3) of the SCR circuits 14 may remain "on" after the input
to circuits 14 is deactivated, the above technique
20~851 ~
-12-
produces an accurate and repeatable power output from SCR
circuits 14.
Referring now to FIG. 2, a timing diagram showing
variations in active or "on" state of an SCR with respect to
certain gate signal conditions is shown. Curve 40 is a
standard sine wave power signal representing the bl signal
at the input of detector 16. Curve 40 is a 60 Hz signal
plotted with respect to time. Curves 42 and 46 represent the
signal produced by the system processor 12 and supplied to
the gate input of the SCR circuits 14. Curves 42 and 46 are
the "on time" desired to produce a predetermined amount of
heat in a particular gear 30 to be induction hardened.
The circuits 14 are activated or caused to supply a power
signal to generator 20 at the point in time which is the
off-on transition of the curve 42. At the end of the "on
time" of curve 42, or time TD, the signal changes from the
"on" state to the "off" state. The precise timing of the
on-off transition does not occur near a zero crossing of
curve 40. Since the activation signal represented by curve
42 does not return to the "off" state until after the zero
crossing at time Tc, the power signal which is supplied to
the RF generator 20, represented by curve 44, is continuously
"on" until time Te, which may be as much as 8.33
milliseconds after the on-off transition of curve 42. Thus,
if the on signal produced by system processor 12 begins at
time TB and continues until time TD, the total power
signal supplied to the RF generator will last from time TB
until time TE on the graph, for a total time period of
T2 .
In order to precisely control the power supplied to the
induction heater coil, and thus achieve more accurate control
of the induction hardening process, the system according to
the present invention computes a time delay beyond a zero
crossing (here the zero crossing at To) for turning on the
20~6851
-13~
SCR circuits 14 so that the SCR activation signal,
represented by curve 46, will change from the "on" state to
the "off" state at or just prior to a zero crossing of curve
40. For example, in order to eliminate the additional "on
time" of the power signal 44 as compared to the gate on-time
input signal represented by curve 42 which switches the SCR
circuits, the system processor 12 will compute a time T3
which corresponds to the desired "on time" Tl divided by
8.33 milliseconds and subtract the remainder from 8.33
10 milliseconds to produce time T3. Then, the system
processor delays activating SCR circuits 14 a pe~iod of time
T3 after a zero crossing so that the activation curve 46,
which coincidentally is exactly equal in "on time" duration
to curve 42, changes from the "on" to the "off" state at time
Tc, which corresponds with a zero crossing of the power
signal curve 40.
Since the curve 46 is so closely related at time TC to
a zero crossing, an accurate amount of "on time" of the SCR
circuits 14 is achieved, thereby accurately controlling the
amount of time that power is supplied to RF generator 20 with
precision not heretofore known with SCR circuits. In so
doing, the amount of power which is supplied to induction
heater coil 28 is accurately controlled. Thus, a tube type
RF generator, which is preferred by some skilled in the art
over the solid state semiconductor type high-frequency RF
generators, may be used to produce an accurate quantity of
power signal and a correspondingly precise quantity of power
supplied to the induction heater coil 28.
Although only one phase ~bl) of the power source 18 is
shown in FIG. 2, it should be apparent to one skilled in the
art that in a 3b system all three phases are related by 120
degrees. Thus, a fixed amount of additional power signal
will be supplied by the other phases (b2 and b3) of
the power source 18 beyond the time TC with the activation
signal represented by curve 46. Nevertheless, the additional
- 2046851
-14-
power supplied by the other two phases will be a constant
quantity since the deactivation signal occurs at a
predetermined time and phase relative to the other power
phases. Therefore, the amount of power delivered to the gear
5 30 by the system 10 is repeatable by establishing a fixed
timing reference (with respect to one phase) for switching on
and off a 3b power source.
Referring now to FIG. 3, a graph of the power output of
the RF generator 20 is shown. The maximum power output of
the generator 20, represented by curve 50, can be adjusted
vertically to achieve higher or lower total instantaneous
power output. The variance in "on time", represented by
times Tl and T2, as a result of the intrinsic
functionality of SCR circuits is shown at the bottom of the
graph. If the SCR circuits remain on for a length of time
T2 as opposed to Tl, which is the desired "on time", the
additional power represented by the shaded portion 52
underneath the curve 50 is supplied to the heater coil 28 in
addition to the actual desired power, represented by the
unshaded portion underneath the curve 50 and extending up to
the end of time Tl. The additional amount of power supplied
to the induction heater coil 28 causes excessive heating of
the gear 30.
As is seen in the graph of FIG. 3, timing variations make
for greater variations in the case hardening process,
particularly when the "on time" Tl is approximately 0.10
seconds. The maximum difference between times T2 and Tl can
be as much as 8.33 milliseconds, and thus the power
represented by area 52 can represent as much as 8-10%
difference in power supplied to the induction heater coil 28
when a 0.10 second power signal is desired for heater coil
28. Another recognized fact is that once the gear 30 has
been heated, the additional heating time represented by the
area 52 can seriously increase the heat of the gear, as the
heat transfer properties of the gear are non-linear and cause
~Q46851
-15-
heat to transfer deeper into the gear face once the gear is
heated around the perimeter. Thus, it is highly desirable to
control the power supplied to the induction heater coil 28
via the technique shown and described above.
Referring now to FIG. 4, another embodiment of an
induction-hardening system 110 according to the present
invention is shown. Switch SW2 provides a reset/start signal
to single pulse timer circuit 116. AC power source 118
supplies an AC signal to phase angle detector 112 and power
switching devices 114. Phase angle detector 112 provides a
series of pulses to an input of single pulse timer circuit
116. Each pulse from detector 112 corresponds to the
detection of a predetermined phase angle of the AC power
signal from power source 118. Upon receiving a reset/start
signal from switch SW2, single pulse timer circuit 116 is
triggered or activated by the next pulse from detector 112 to
produce a pulse or signal having a predetermined duration.
The predetermined duration pulse enables the power switching
devices 114. Thus, the initiation of the heating cycle as a
result of the closure of switch SW2 is delayed until a
predetermined phase angle is detected by phase angle detector
112. Phase angle detector 112 provides a phase detector
means for detecting a predetermined phase angle in the power
signal from AC power source 118.
As in the previous embodiment, the RF generator 120
receives a power signal from the power switching devices 114
and in response thereto supplies a high frequency, high power
signal to the induction heater coil 128 via windings 129.
Windings 129 provide impedance matching between the output of
the RF generator 120 and the induction heater coil 128.
Single-phase and multi-phase power supplies are
contemplated.
The phase angle detector 112 is implemented using a triac
phase angle controller Part No. TDA1185A manufactured by
Motorola Incorporated of Phoenix, Arizona. The TDA1185A
- 2û468~1 ~
-16-
device is programmable to produce an output signal
corresponding to detection of a predetermined phase angle of
the AC signal. This predetermined phase angle is variable
with the TDA1185A device in accordance with an external set
5 voltage representing the conduction angle desired. (See
discussion of control signals, infra.) Since the TDA1185A
device detects firing angles only on the positive half of the
AC signal, should a firing angle on the negative half of the
AC signal be desired, an inverting operational amplifier may
be inserted between the AC power source and the phase angle
detector 112 to invert the AC signal, and thus provide an
input signal to the phase angle detector 112 such that
activation in the negative half of the AC signal may occur.
Signal pulse timer circuit 116 is implemented using a
retriggerable monostable multivibrator integrated circuit,
part No. 74LS123 manufactured by Texas Instruments. The
74LS123 is a rising-edge triggered device and thus the pulses
produced by the phase angle detector 112 can be used to
trigger the production of an output pulse from the timer
circuit 116. The signal produced by switch SW2 provides a
retrigger, enable or rearming signal to the timer circuit
116. Since the 74LS123 device can be configured to produce
an output pulse from less than 1 millisecond to a very large
time duration, such as hours, the combination of the phase
angle detector 112 and the timer circuit 116 provides
infinitely variable control of the timing functions necessary
to activate power switching devices 114 in accordance with
the previously described conditions calling for a supply of a
specific duration power signal to the RF generator 120.
Optional control signals, represented by broken lines 132
and 134, provide phase angle selection and pulse width
duration signals to detector 112 and circuit 116,
respectively. Specifically, the phase angle control signal,
present on signal path 134 and supplied to an input of
detector 112, provides phase angle selection information to
2046851
-17-
detector 112. In response to the signal on signal path 134,
detector 112 produces an output pulse corresponding in time
to the occurrence of the desired phase angle established by
the signal on signal path 134. Likewise, the duration
control signal present on signal path 132 controls the time
duration of the pulse produced by circuit 116. The signal on
signal path 132 is typically implemented via a
potentiometer/capacitor combination establishing a decaying
signal well known with such circuits.
The device 110 of FIG. 4 includes several components
which are identical with components of the device 10 of FIG.
1. In particular, the AC power source 118 corresponds with
the three-phase high voltage power source 18, power switching
devices 114 correspond with power switching SCR circuits 14,
RF generator 120 corresponds with RF generator 20, induction
heater coil 128 corresponds with induction heater coil 28,
and gear 130 corresponds with gear 30. Triacs or Silicon
Controlled Rectifiers (SCR's) are contemplated as the power
switching devices in block 114.
Operationally speaking, the pulses produced at the output
of phase angle detector 112 correspond in time with a
predetermined phase angle of the AC signal indicated by time
line TB in FIG. 2. Likewise, the output pulse produced by
timer circuit 116 will correspond with time T2. Thus, the
difficulties of energizing an AC power source with precise
timing and power output control are overcome by the
embodiment of FIG. 1 wherein a time delay after a zero
crossing is used to determine turn on time of the power
signal, or as in the embodiment of FIG. 4, a particular phase
angle is detected to determine the point in time when an
activation signal is desired for activating the power
switching devices. With both embodiments of the invention, a
predetermined timing reference point relative to the AC power
signal is located or detected prior to the activation of the
power switching devices to produce an activation signal which
20~6851
-18-
will subside before or simultaneously with a subsequent zero
crossing of the power signal so that the power switching
devices will be turned off or switched off at a precise
predetermined time, typically a zero crossing as is the case
with most thyristors .
Alternately, it is also contemplated that the phase angle
detector 112 and timer circuit 116 are portions of a
microcomputer based controller (not shown) wherein an A/D
converter (not shown) is used to monitor the amplitude (which
corresponds with the phase angle) of the signal from source
118. Further, user-changeable software enables control of
the desired phase angle detected and the width of the control
pulse supplied to the power switching devices 114.
While the invention has been illustrated and described in
detail in the drawings and foregoing description, the same is
to be considered as illustrative and not restrictive in
character, it being understood that only the preferred
embodiment has been shown and described and that all changes
and modifications that come within the spirit of the
invention are desired to be protected.
