A temperature control system, which includes a modified wheatstone bridge with a resistive-capacitive (RC) circuit in one leg of the bridge, is disclosed. The RC circuit includes a resistor which provides an effective resistance as a function of its absolute resistance and the on-time to off-time ratio of pulses supplied to a switch connected across it. A sawtooth voltage is produced across the RC circuit. The voltage is compared with the voltage across a temperature sensor with heat being applied during each pulse period portion when the sawtooth voltage exceeds the voltage across the temperature sensor

Heyser, R. C.

English

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United States Patent ii»i
Low et al.
tin 3,733,463
[45] May 15, 1973
154] TEMPERATURE CONTROL SYSTEM
WITH A PULSE WIDTH MODULATED
BRIDGE
[76] Inventors: George M. Low, Acting Administra-
tor of the National Aeronautics and
Space Administration with respect
to an invention of; Richard C.
Heyser, Tujunga, Calif.
[22] Filed: Dec. 24, 1970
[21] Appl. No.: 101,214
[52] U.S. Cl ! 219/499, 219/50
[51] Int. Cl HOSb 1/02
[58] Field of Search 219/494, 497, 499,
219/501; 317/42; 323/75 H, 75 K
[56] References Cited
UNITED STATES PATENTS
2,819,372 1/1958 Booker et al 219/499
3,526,272 9/1970
2,681,431 6/1954
3,159,796 12/1964
3,546,434 12/1970
Watts etal 219/497 X
Wannamaker, Jr 323/75 K
Van Sandwyk 323/75 K
Apel 219/499 UX
Primary Examiner—Bernard A. Gilheany
Assistant Examiner—F. E. Bell
Attorney- Monte F. Mott, Paul F. McCaul, John R.
Manning
[57] ABSTRACT
A temperature control system is disclosed which in-
cludes a modified wheatstone bridge with a resistive-
capacitive (RC) circuit in one leg of the bridge. The
RC circuit includes a resistor which provides an effec-
tive resistance as a function of its absolute resistance
and the on-time to off-time ratio of pulses supplied to
a switch connected thereacross. A sawtooth voltage is
produced across the RC circuit, the voltage being
compared with the voltage across a temperature sen-
sor, with heat being applied during each pulse period
portion when the sawtooth voltage exceeds the voltage
across the temperature sensor.
8 Claims, 2 Drawing Figures
1
28V
L I
OVEN HEATER
ELEMENT
PATENTEDMAYl 51373 3,733,463
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RICHARD C. HEYSER
INVENTOR.
ATTORNEYS
3,733,463
1 2
TEMPERATURE CONTROL SYSTEM WITH A In accordance with the teachings of the present in-
PULSE WIDTH MODULATED BRIDGE vention, a capacitor is connected across the two resis-
ORIOIN OF THE INVENTION toIS- D"ring each Switchin8 cycle the volta8e ac1055 ^ 6ORIGIN OF THE INVENTION capacitor which is in the form of a sawtooth is com-
The invention described herein was made in the per- 5 pared with the voltage across the temperature sensor in
formance of work under a NASA contract and is sub- the bridge. During each cycle, power is applied to a
ject to the provisions of Section 305 of the National heater only if the capacitor voltage exceeds the temper-
Aeronautics and Space Act of 1958, Public Law a ture sensor in the bridge.
85-568 (72 Stat. 435; 42 USC 2457). The novel features of the invention are set forth with
R APKr.HOl iNin OF THF INVFIMTION 10 Paiticvlaiity "> ^e appended claims. The invention willBACKGROUND OF THE INVENTION ^ ^
 mdeKtood from ^ following description
1 . Field of the Invention when read in conjunction with the accompanying draw-
The present invention generally relates to a tempera- ings.
» **»* ™*™™H OF i. DRAWLS
within an oven or the like. FIG. 1 is a diagram of a basic embodiment of the
2. Description of the Prior Art present invention; and
A typical temperature control system such as that de- FIG. 2 is a multiline diagram useful in explaining the
scribed in U.S. Pat. No. 3,136,877 consists of a conven- embodiment shown in FIG. 1.
tional wheatstone bridge circuit. The bridge circuit in- 20 DESCRIPTION OF THF PRFFFRRFD
eludes a temperature sensitive resistor located in the DESCRIPTIONOF THEPREFERRED
environment, whose temperature is to be controlled. EMBODIMENTS
When the temperature is too low, a first bridge unbal- Attention is now called to FIG. 1 which is a cornbina-
ance condition arises which is detected by a sensor tion block and schematic diagram of the present in ven-
which closes a switch, thereby enabling power to be 25 tion. Therein, numeral 10 represents a wheatstone
supplied to a heater which heats up the environment. bridge circuit, connected across a source of potential,
The environment is heated up until the temperature ex- such as a DC battery 12. The bridge 10 includes a resis-
ceeds a desired nominal temperature. As a result, a sec- tor Rl which is connected in series with a temperature
ond bridge unbalance condition arises, causing the sen- sensitive resistor or simply a temperature sensor Pt
sor to open the switch and thereby terminate the supply 30 across the battery 12. It is sensor Pt which is located in
of power to the heater. , the environment or oven whose temperature is to be
With such a prior art system, the environment tern- controlled, and whose resistance varies as a function of
perature can only be controlled within a finite signifi- temperature. The junction point X between Rl and Pt
cant range, since power supply to the heater occurs is connected to. one input of an operational amplifier
whenever the temperature drops below the nominal 35 15. The bridge also includes another resistor R2 which
temperature and is terminated whenever the tempera- is serially connected with variable resistors PI and P2
ture is above the nominal temperature. Even with sensi- across battery 12. A capacitor C is connected in paral-
tive instruments, the temperature range is quite signifi- lei across PI and P2 and the junction point Y between
cant, and is often greater than that required for very R2 and PI and C is connected to the other input of am-
precise studies or applications, such as in space explo- *" plifier 15.
ration studies. Thus a need exists for an improved sys- The latter is operated as a comparator to provide an
tem for accurate temperature control. output at point Z of a preselected level which closes a
OBJECTS AND SUMMARY OF THE INVENTION SWitCh 1? Only When ** V°ltage * ^^ Y fa greatCrt b b h I t INVtN I ION ^^ ^
 at ^^ x when ^ ^.  1?> which fa
It is a primary object of the present invention to pro- 45 represented in FIG. 1 by a transistor Ql, is closed it
vide a new improved temperature control system. connects an oven heater element 20 across a source of
Another object of the present invention is to provide potential, shown in FIG. 1 at +28V. Thus, only when
an improved temperature control system which incor- switch 17 is closed is heat supplied to the oven by ele-
porates a novel wheatstone bridge circuit. ment 20.
A further object of the present invention is to provide As shown in FIG. 1, connected across the resistor P2
a new system which is programmable to control the rise is a switch 22, in the form of a transistor Q2, which is
in and the maintenance of an environment tempera- activated by pulses such as pulses 26 and 27, from a
ture. source 25, which are supplied at a selected rate. Each
These and other objects of the present invention are pulse, such as pulse 26, has an on-time 260 during
achieved in one embodiment by incorporating in one which the switch is closed so that P2 is shorted out, and
leg of a wheatstone bridge circuit first and second sen- an off-time 266, during which the switch is open so that
ally connected resistors, with the second resistor being P2 is not affected. The ratio of on-time to off-time is as-
connected across a switching transistor. The transistor sumed to be controlled by source 25, which may in turn
is successively switched between on and off states, dur- be controlled by a programmed computer (not shown).
ing which the second resistor is either shorted out or is The control of the on-time to off-time ratio may be
in series with the first resistor. During each switching thought of as controlling or modulating the pulse width
cycle the effective resistance of the second resistor of each pulse. Therefore, source 25 can be thought of
which is in series with the first resistor depends on the as a source of pulse-width-modulated (PWM) pulses.
ratio of the transistors on-time to off-time. By varying
 65 In effect, during the on-time of each pulse P2 is
this ratio, namely by pulse width modulating the shorted out, thus its resistance is zero, neglecting the
switching of the transistor, the effective resistance can collector to emitter resistance of Q2. On the other
be changed. hand, during pulse off-time the full resistance of P2 is
3
3,733,463
in series with PI. Thus, by controlling the on-time to
off-time ratio the effective resistance of P2 during the
full pulse duration is controlled. For example, if the on-
time is only 25 percent of the pulse period, the effective
resistance is one-fourth the resistance of P2. On the
other hand, if the on-time is half the pulse period, the
effective resistance is half the P2 resistance. In practice
PI is selected to equal the resistance of Pt at an ambi-
ent temperature, while the effective resistance of P2 is
chosen to equal the change (increase) in the resistance
of Pt due to a change from the ambient temperature to
a chosen temperature to be controlled.
The voltage at point Y changes due to the change in
the resistance which P2 provides in series with PI. In
the absence of C the voltage at Y would have a square
wave shape as shown in line a of FIG. 2. The upper volt-
age level 31 would be present when Q2 is off and the
lower voltage level 32 would be present when Q2 is on
and P2 is shorted out. Therein, it is assumed that the
off-time to on-time ratio is about 1:3. That is, during
each pulse period the transistor Q2 is on for about 75
percent of the period. However, with C in the circuit
due to the time constant of the circuit formed by C, PI
and P2, the voltage at point Y during the period of each
pulse from source 25 does not have the square wave
shape shown in line a but rather the sawtooth shape as
shown in FIG. 2, line b and designated Vx.
From the foregoing description it should thus be ap-
preciated that in the present invention, during each
pulse period, heat is applied to element 20 only during
that portion of the pulse period when the voltage at
point X designated in FIG. 2 by the dashed line Vx is
less than Vy. Assuming that V* is less than Vy during
an entire pulse period, power is supplied to heater ele-
ment 20 during the entire period, as represented by nu-
meral 35 in line c which is used to diagram the power
profile supplied to heater element 20, and which corre-
sponds to the output of amplifier 15. On the other
hand, if V* is greater than the maximum value of Vy
during an entire pulse period, the heater element is
completely off during the entire period. Thus, the
amount or quantity of heat which is supplied by heater
element 20 to the oven during each pulse period is a
function of the actual oven temperature as represented
by Vjf and the shape of Vy which is controllable by the
time constant and the pulse width modulation.
From the foregoing it should be appreciated that, as-
suming a constant pulse frequency, by controlling the
pulse width modulation, the rate at which the oven
temperature is raised to a desired temperature may be
controlled. Furthermore, once this temperature is
reached it may be accurately maintained by controlling
the pulse width modulation so that during each pulse
period only that amount of heat which is necessary to
maintain the oven at the desired temperature is pro-
vided by heater element 20. Maximum rate of tempera-
ture rise may be achieved by making the on-time per
pulse period equal to zero thereby raising the level of
Vy to a maximum so that power is supplied to the
heater element during the full period of each pulse. The
rate at which temperature is increased from a first tem-
perature such as (1 to (2 may be controlled by the pulse
width modulation of the pulses to produce a desired
rate at which power is supplied to tile heater element
20. Once the desired temperature is reached, the pulse
width modulation is selected to insure the supply of
power per pulse period which is required to maintain
10
1 5
20
25
30
35
45
50
60
,.
the oven at the desired temperature. Experience with
the novel invention has demonstrated the ability to
control an oven temperature to within less than 0.01°C,
e.g., 0.001°C.
Referring again to FIG. 2, let it be assumed that the
final desired temperature is that represented by the line
36 for Vx and that the power or heat per pulse needed
to maintain that temperature is represented by numeral
37. Let it further be assumed that the oven is below the
desired temperature. Thus, more power than is re-
quired to maintain the desired temperature is supplied
to the heater per pulse, as represented by lines 35 and
38. In practice, the oven temperature may briefly oscil-
late above and below the desired temperature until the
temperature is stabilized. In FIG. 2, lines 39 and 40 rep-
resent Vx when the temperature is above the desired
temperature, hi response to the level represented by
line 39, less than the required heat is supplied as repre-
sented by line 41. Since level 40 is above the maximum
value of Vv no power is applied to heater element 20
as represented by line 42. Line 44 represents an oven
temperature below the desired temperature. Conse-
quently, more power than necessary to maintain the de-
sired temperature is applied as represented by line 45.
Lines 48 and 49 respectively, represent Vx at the de-
sired temperature and the power per pulse period
needed to maintain the desired temperature.
Although particular embodiments of the invention
have been described and illustrated herein, it is recog-
nized that modifications and variations may readily
occur to those skilled in the art and, consequently, it is
intended that the claims be interpreted to cover such
modifications and equivalents.
What is claimed is:
1. A temperature control system comprising:
a wheatstone-like bridge having first and second par-
allel legs and including a temperature sensitive ele-
ment, whose resistance varies as a function of tem-
perature, connected in series with a first bridge re-
sistor in said first leg, and a resistor-capacitor cir-
cuit connected in series with a second bridge resis-
tor in said second leg;
a source of power;
heating means for providing heat when connected to
said source;
means for applying:a voltage across said bridge,
whereby the voltages across said element circuit
are functions of the resistances thereacross, respec-
tively;
means coupled to said element and to said circuit for
connecting said heating means to said source when
the voltage across said circuit is greater than the
voltage, across said element; and
control means coupled to said circuit for controlling
the resistance across said circuit to vary in a prese-
lected manner during each of a succession of pulse
periods with the capacitor of said circuit control-
ling the voltage of said circuit to vary in a prese-
lected manner from a first level to a second level
during a first portion of each pulse period and from
said, second level to said first level during the rest
of the pulse period following said first portion.
2. The arrangement as recited in claim 1 wherein said
circuit consists of first and second resistors connected
in series and said capacitor is connected in parallel
across at least one of said resistors and said control
means is coupled to said second resistor and includes
3,733,463
first means for shorting out said second resistor during
said first portion of each pulse period, with said first
portion being selectively variable.
3. The arrangement as recited in claim 2 wherein said
control means include second means for providing a
succession of equal period pulses, each pulse being of
a first level during said first portion of each pulse pe-
riod and of a second level during the rest of the pulse
period, and means for applying said pulses to said first
means to short out said second resistor during the first
portion of each pulse period.
4. The arrangement as recited in claim 2 wherein said
capacitor is connected in parallel across both said first
and second resistors, whereby when said second resis-
tor is shorted out the voltage across said capacitor,
representing the voltage across said circuit, rises gradu-
ally from said first level to said second level and falls
gradually from said second level to said first level dur-
ing the rest of each pulse period when said second resis-
tor is not shorted out, with the voltage across the circuit
during each pulse period having the shape of a saw-
tooth.
5. The arrangement as recited in claim 4 wherein said
control means include second means for providing a
succession of equal period pulses, each pulse being of
a first level during the first portion of each pulse period
and of a second level during the rest of the pulse ..pe-
riod, and means for applying said pulses to said first
means to short out said second resistor during the first
portion of each pulse period.
6. In a temperature control system of the type com-
prising a bridge with a temperature sensitive element
whose resistance varies as a function of temperature in
one leg of said bridge, said system further including
means for comparing the voltage across said element
with the voltage across another leg of said bridge, an
arrangement comprising:
a resistor having first and second terminals in said
other leg;
control means for controlling the resistance across
said terminals to'vary from the full resistance of
said resistor to substantially zero during a selected
portion of each of a succession of periods, whereby
.the effective resistance across said terminals during
each period is less than the resistance of said resis-
tor; and
a capacitor in said other leg connected in parallel
across at least said resistor whereby the voltage
across said other leg increases toward a peak value
when the resistance across said terminals is the full
resistance of said resistor and decreases from said
peak value when the resistance across said termi-
nals is substantially zero.
7. The arrangement as recited in claim 6 wherein said
control means includes a switch which is connected
across said first and second terminals, said switch being
switchable to an on state in which said terminals are ef-
fectively shorted out and an off state during which the
resistance across said terminals is the resistance of said
_5 resistor, and source means coupled to said switch
means for controlling said switch to be in the on state
during a selected portion of each of said periods and in
the off state during the rest of each of said periods.
8. The arrangement as recited in claim 7 wherein said
30 source means is a source of a succession of equal pe-
riod pulses, each pulse being of a first level which
switches said switch to its on state during a variably se-
lected period portion and of a second level which
switches said switch to its off state during the rest of the
35 pulse period.
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Temperature control system with a pulse width modulated bridge

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