Note: Descriptions are shown in the official language in which they were submitted.
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TWO WIRE HEATER REGULATOR CONTROL CIRCUIT
BACKGROUND OF THE INVENTION
Field of the Invention
The invention is directed to a control circuit
which in conjunction with a heating coil provides precise,
automatic temperature regulation. More particularly, both
the power for heating the coil and the power for the
determination of the temperature thereof as reflected by
the resistance of the coil can be simultaneously provided
through the same two wire leads of the coil. Specifically,
the temperature of the heater is established by continuous
excitation and sensing that is independent of the actual
lion of the source of heating power to the coil.
Description of the Prior art
Several approaches to the control of temperature
for a heater have employed a two wire heater regulator
method. Typically the heating coil has a positive temper-
azure coefficient of resistance and the voltage derived
from passing a current through the heating coil is come
pared to a voltage developed across a reference resistor.
The difference, if any, between these two voltages is
determined and either an increase or a decrease in the
level of power delivered to the heating coil is effected.
The power level adjustment is reflected in a corresponding
increase or decrease in the temperature of the heating
coil.
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A convenient method for obtaining the voltage
difference measurement is to configure the heating coil as
one leg of a four leg impedance bridge circuit. Any
variance from the null balance condition at the center of
the bridge indicates that a power adjustment is required.
In a bridge circuit configuration, a first voltage source
excites both the heating coil and the reference resistors
for temperature sensing and resistance balancing functions
and a second voltage source provides power only across the
heating coil for temperature adjustment thereof. Bridge
excitation for sensing and heater coil power for tempera-
lure adjustment are provided across the same two heater
coil leads.
The fundamental drawback experienced in this
arrangement is centered in the practical need to isolate
the low level control voltage for sensing excitation from
the AC line voltage for temperature adjustment, because
both voltages are simultaneously required to implement
their respective functions.
It has been suggested that the heating coil
power source be modified to function as a low power exci-
station source. For example, a heating coil power source
can be modified to provide either low level continuous or
periodically pulsed or synchronously pulsed sensing volt-
age. However, either power stage heat dissipation, closed
loop regulation, or noise generation must be compromised
in order for the heating coil power source to provide the
voltage necessary for both the heating and sensing
functions.
Other proposed solutions to the dual power
requirements of a two wire heating coil include alternate
in, either periodically and/or synchronously, the excite
lion source with the power source, or providing a source
of continuous AC excitation with a DC power source for
heater coil temperature adjustment. In the former propose
Ed solution, the alternate switching of the excitation and
power sources as well as their time separation is manna-
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tory, while in the latter, bridge capacitors and a more
complex power stage are required.
A further drawback inherent in the prior art of
two wire heater control circuits is the impracticability
of adequate fusing for overall circuit protection because
the initial current is often much higher than the final
current at set point. Another problem rests in the fact
that a component failure in the circuitry controlling the
alternation of the excitation source with the power source
will most probably result in the cataclysmic failure of
-the entire heater control circuit.
It is therefore, an object of this invention to
provide a two wire heater regulator control circuit which
overcomes the aforementioned prior art limitations. The
present invention utilizes a continuous sensing voltage
and a heater coil power voltage which can be simultaneously
applied to the heater coil.
It is also an object of this invention to pro-
vise means for the isolation of the sources of sensing
voltage and heater coil voltage.
It is a further object of this invention to
provide a short circuit protection circuit responsive to
heater coil failure.
SUMMARY OF THE INVENTION
Thea invention is a system for controlling the
temperature of a heater apparatus having a heater element
therein, the electrical resistance of which is a function
of the temperature of the apparatus. The system includes
circuitry for providing a continuous sensing excitation
voltage to a bridge circuit having the heater element as
one leg thereof, a heater voltage source and circuitry
isolating the sensing voltage from the heater voltage. A
comparator is responsive to differences between a refer
once voltage and the voltage at the heater element and
control circuitry is responsive to the comparator's out-
put. The heater voltage is applied as required to raise
the temperature of the heater. This invention can provide
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full cycle phase control firing or integral cycle control
through the use of line voltage zero detecting circuitry
and an appropriate logic circuit.
The entire circuitry of this invention is also
protected from an electrical short in the heater element
by identifying a voltage at the heater element in excess
of a predetermined limit by means of a short circuit
protection circuit.
GRIEF DESCRIPTION OF THE DRAWINGS
The above as well as other features and ad van
taxes of this invention will become apparent through
consideration of the detailed description in connection
with the accompanying drawings in which:
Figure l is a block and circuit diagram of a two
wire heater regulator control circuit according to this
invention;
Figure 2 is a block and circuit diagram of an
alternative embodiment of this invention;
Figure 3 is a lock and circuit diagram of
another alternative embodiment of this invention; and
Figure 4 is a schematic representation of the
alternative embodiment of figure 3.
DETAILED DESCRIPTION OF THE INVENTION
Considering Figure l, a four leg resistive
bridge includes resistors R1, R2, R3 which make up three
legs of the bridge and the resistive device RHO to be
controlled by the regulator circuit of this invention,
which makes up the fourth leg of the bridge. While the
particular resistive device RHO illustrated herein is a
heater coil having a positive temperature coefficient of
resistance, it should be understood that the control
circuit of this invention can be utilized in other apply-
cations in which power is applied to, or withheld from a
load according to the resistance of the load. When the
resistance of the heating coil RHO is accurately measured,
changes therein reflect changes in the temperature of the
heating coil itself.
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The voltage necessary for heater coil tempera-
lure adjustment is provided directly from an AC power line
as at 5 and is controlled by switch means 7. A low level
DC excitation voltage as at 9 is continuously applied to
the four legs of the resistive bridge. The DC excitation
voltage is floating with respect to the AC line voltage.
Isolation between the two sources is effected by a DC
blocking means 11 that prevents the continuous DC excite-
lion from flowing into the AC line when the switch means 7
is actuated. The DC blocking means 11 also protects the
heater coil RHO from being shorted by the low source
impedance of the AC line S. Such a short would prevent
the sense and compare function from occurring simultane-
ouzel with the application of AC power for coil heating.
To a lesser extent, circuit isolation is also provided by
an AC blocking device such as diode Do and the high input
impedance of the continuous DC error sense circuit 13.
Because the bridge circuit is purely resistive, it allows
a pure DC signal representing bridge imbalance to be
accurately generated by the error sense circuit 13. A
control circuit 15 is responsive to the DC error imbalance
signal and effects the actuation of the AC switch means 7.
The invention also provides a short circuit
protection circuit 17 responsive to an error imbalance
signal from the DC error sense circuit 13. The protection
circuit 17 continuously monitors the DC error signal for a
predetermined voltage level which is indicative of a short
at the heater coil. If this predetermined voltage level
is sensed, the protection circuit 17 signals the immediate
shutdown of AC power to the heater coil RHO and the switch
means 7 is deactivated. Under normal operating condo-
lions, the switch means 7 would be activated and Dakota-
voted according to the continuous DC error sense circuit
13 output and the control circuit 15.
Turning to F guru 2, an alternative embodiment
of the invention includes additional details which can be
readily incorporated into the embodiment described in
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conjunction with Figure 1. This embodiment of the two
wire heater regulator control circuit provides full cycle
phase control firing. AC line voltage can be applied to
the heater coil RHO, beginning at a point along a single
cycle's development from zero through the positive peak
and back to zero again and lasting for a variable percent-
age of the cycle. This system is particularly useful in
low power applications in which a significant degree of
accurate temperature regulation is desired
lo The floating DC continuous excitation 9 is
blocked from the AC line power source 5 by a blocking
device consisting of an AC line capacitor C1. The contain-
use DC error sense circuit is comprised of the amplifiers
Al and A which together form a high common mode range
instrumentation amplifier. When AC line voltage is being
applied to the heater coil RHO, the voltage at the junk-
lion of R3-RHC is the line voltage centered about a DC
offset created by the DC excitation divided by R3-RHC, and
the voltage at the junction of R1-R2 is the line voltage
divided by Rl-R2 (R3 can be neglected from a practical
standpoint) centered about a DC offset created by the DC
excitation divided by R1-R2. The difference in the two DC
offsets is the required bridge imbalance signal. The
unwanted AC in the error signal generated by the bridge is
rejected by appropriately setting the gains of the amply-
liens Al and A.
The amplifier A is summing the input from the
amplifier Al with the signal from the junction of resistor
R3 and the heater coil RHO, and as such, these two inputs
can be amplified with different gains, GUY and GUY
respectively. With the inputs from the bridge at R1-R2
and R3-RHC as described above and if:
GAY = -K R2
G 21 = -1; and
GUY
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wherein K is a constant which equals less than 1.0 so that
both of the amplifiers Al and A are protected from being
driven into saturation, it follows directly that the
output of the amplifier A is:
K VDC-EXC 1 RHO
If the impedance of resistor R3 is equal to the
impedance of the heater coil RHO at the desired tempera-
lure set point for the heater, the DC error is:
0.5 K VDC-E~C
The conditions for bridge balance are therefore controlled
by calibrating the set point reference circuit I to equal
the DC error voltage at the desired temperature as de-
scribed above. A set point reference modify circuit 21
can be provided to allow adjustments to the set point,
with time, to achieve different operating points for the
temperature of the heater coil. The error comparator 23
evaluate the several inputs and actuates a switch control
means 25 in communication with switch means 7.
The output of amplifier A is also evaluated by
the short circuit protection circuit 17 preferably con-
sitting of an operational amplifier employed as a compare-
ion. The short circuit protection circuit 17 identities
unsatisfactory conditions such as an electrical short of
the heater coil RHO. For example, with the set point
reference lo set as described above and with the switch
means 7 actuated to provide AC power to the heater coil,
if the DC error, as established by the output of amplifier
A, approaches a value equal to K VDc EXC' a short
circuit at the heater coil is indicated. This voltage
level is sensed by the short circuit protection circuit
17. The switch control 25 which is responsive to both the
error comparator 23 and the short circuit protection
circuit 17, is immediately signaled by the short circuit
protection circuit 17 to disable the closure of switch
means 7 so that AC line voltage is removed from the heater
coil.
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In the situation in which the thermally respond
size resistive value of the heater coil RHO is at or above
the desired set point level, A line power is not present
on the bridge and therefore, AC voltage is not being
rejected. With this exception when AC line power is not
present on the bridge, all other circuit operations con-
tinge identically as described above. When the resistive
value of the-heater coil falls below the set point value
but is above the short circuit protection value, the
switch means 7 is closed and AC line power applied to the
heater coil.
Figure 3 illustrates another alternative embody
mint ox this invention in which a two wire heater regular
ion control circuit provides integral cycle control of the
AC switch means. In this embodiment, the integral cycle
control results in low switching noise. Since error
information well beyond the AC line zero crossing point
will not effect the firing decision until the next zero
crossing, sensing is conducted at the zero crossing begin-
nine) of every integral power cycle for approximately microseconds during which closure of the power switch is
prohibited. The delay period allows time for transients
to settle so that a correct error voltage results. As
shown in Figure 3, the DC blocking means 11 is a bridge
rectifier.
This design presents low noise integral cycle
control, because the output of the error comparator 23 is
sampled only at complete cycle zero crossings by grating it
with the line cycle mark pulse circuit 27 output through
NAY gate 29. The switch control circuit 25 is responsive
to both the RAND gate 29 output and the short circuit
protection circuit 17 output in the actuation of switch
means 7. Since the error comparator 23 is sampled only at
the beginning of each line cycle when AC firing has not
yet occurred, at all times when the bridge does have AC
present, the continuous DC error sense circuitry consist
in of amplifiers Al and A need not reject AC line volt-
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age because the sense circuitry output is not required.
In other words, the output of amplifier A is required
only when the DC excitation voltage is present on the
heater coil bridge. Because a bridge rectifier 11 is
providing DC blocking action, any AC on the bridge is
referenced to the AC line voltage rather than being eon-
toned about the DC offsets as is the case when DC blocking
is effected a capacitor. With the error comparator 23
output being grated with a line cycle mark pulse 27, the AC
rejection function of the high common mode range incitory-
mentation amplifier (Al and A) can be dispensed with
entirely. In fact the gain of each amplifier component,
Gal GUY and GUY can all be set to -1, although Saturn-
lion of the instrumentation amplifier should still be
avoided. This step is desirable in this alternative
embodiment since AC rejection by Al and A, although still
possible, would be more complex due to the different AC
reference and is in fact, not required in this embodiment
of the invention.
While the above described function of the elect
ironic circuitry of this invention would enable one skill-
Ed in the art to implement such a function through number-
out combinations of conventional circuit elements, one
such implementation employing commercially available
elements is disclosed in the detailed schematic of Figure
4. Several specific features of the integral cycle,
heater regulator control circuit of Figure 3 are described
herein in detail, particularly, the DC excitation voltage
source, the AC power source, and the heater coil bridge
circuit are reviewed.
The floating DC voltage continuous excitation
source 9 is comprised of four basic elements. An isolated
AC voltage is provided by transformer To and is rectified
by diode bridge rectifier Do. This provides an unrig-
fated input voltage via the filter network consisting of resistor R5 and capacitor C2, for the voltage regulator
VRl. The regulated excitation voltage (15 VDC) is applied
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to the resistive bridge through the AC blocking device,
diode Do. This diode Do provides blocking voltage gape
ability for the protection of the low level excitation
source from the AC line voltage.
The AC power source 5 is provided via the switch
means 7, a trial Al. The AC line voltage is applied in
series to the DC blocking means 11, which is a diode
bridge rectifier Do. The closing of the AC switch 7
permits power to flow through the bridge rectifier Do
directly from the line 5 into the heater coil RHO. The
diode bridge rectifier Do simultaneously prevents the DC
excitation current from flowing into the AC line.
The four leg resistive bridge consists of no-
sisters R1, R2 and R3, and the heater coil RHO. The
signals taken from the bridge at points 31 and 33 provide
the input to the amplifiers Al and A respectively.
What has been described is a two-wire heater
regulator control circuit using a resistive bridge compare
atop circuit of which the heater coil is one leg with
separate and independent excitation and heater voltage
sources applied thereto. Sensing excitation voltage is
continuously applied to the bridge circuit while the
heater voltage source can be applied to the heater coil
element US an integral cycle or a portion of a cycle.
