Temperature controller is described which reduces difference between temperature oscillations about control point. Standard temperature-sensitive resistor element is utilized which is highly stable and reproducible. Temperature sensing circuitry is conventional dc bridge with power supplied by battery or highly regulated supply source

Heyser, R. C.

English

NASA Technical Reports Server

December 1972	 B7240507 
•	 NASA TECH BRIEF 
NASA Pasadena Office 	 W 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Pulse-Width-Modulated Device for Precision 
Temperature, Control 
S
Conventional temperature controllers supply power 
to heating elements in an on-off mode; as a result, the 
temperature of the system under control oscillates 
above and below a nominal temperature, and the dif-
ference between the two extremes is used as a measure 
of the degree of refinement of the controller. Power 
is turned off at one extreme of the temperature oscil-
lation and on at the other extreme; thus, there is a 
"dead band" because the controller is essentially in-
operative between the two temperature extremes. An 
alternative statement is that the controller sampling 
rate is very low and occurs only during the brief inter-
vals when the temperature is nearly at or beyond 
either extreme. 
In order to obtain a closer temperature control, that 
is, to reduce the difference between temperature os-
cillations about the control point, expensive complex 
devices are often incorporated in controllers so that 
the power delivered to heating elements is in propor-
tion to the difference between the selected temper-
ature and the actual temperature of the system under 
control; since dead bands are eliminated and the con-
troller is active at all times, it is evident that the con-
troller sampling rate has been increased. 
The controller shown in the diagram provides an 
inexpensive means of reducing the difference between 
temperature excursions. The temperature sensor, RT, 
is any standard temperature-sensitive resistor element 
of a type which is highly stable and reproducible; as 
a result, temperature settings can be duplicated at 
any time. The temperature-sensing circuitry is a con-
ventional DC bridge consisting of 111, 112, RT, and Pi
+ Pa; power is supplied by a battery or a highly reg-
ulated supply, and ordinarily, 111 = R2. 
The effective resistance of the combination of P1 
and Pa is governed by the conduction state of Qi; the 
A B C D E 
tCtE	 [41 Oven I	 I Heater 
	
R 1 	 R2 
	
X	 A	
Element 
Sensor r 
-1
cl^ 
1^ 'r
	
01
To Clock or 
	
RT:	
Square-WaveL -i
0 Source 
clock or square-wave source supplies a symmetrical 
signal which compels Qi to perform as a switch, that 
is, all on or all off for equal intervals. Consequently, 
P2 is alternately shorted or allowed to remain in the 
network at a rate determined by the clock or square-
wave source, and the effective resistance of the com-
bination of P1, P2, and Qi over a relatively long inter-
val of time can be made to vary between P1 and P1 + 
P2 as a function of the switching rate of Qi. Accord-
ingly, at a given switching rate, Pi and P2 can be 
selected so that the effective resistance of the combi-
nation of Pi, F2, and Qi equals RT, the resistance of 
the temperature sensor at the selected control temper-
ature; when the control temperature is achieved, the 
(continued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
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0
https://ntrs.nasa.gov/search.jsp?R=19720000506 2020-03-17T03:34:21+00:00Z
bridge is balanced and there is no DC potential dif-
ference between points X and Y. The operational 
amplifier shown in the diagram controls power de-
livered to the oven heater element via Q2; when no 
potential difference exists across points X and Y, Q2 is 
biased off by the operational amplifier and no power 
is supplied to the heater. 
The value of capacitor C1 is selected for the partic-
ular clock-rate to produce a sawtooth voltage at point 
Y. The combination of DC voltage produced by bridge 
unbalance at points X and Y and the sawtooth re-
sulting from insertion of Ci give rise to pulse width 
modulation (PWM) at point Z. Referring to thechar-
acteristic sawtooth patterns shown in the diagram, at 
pattern A the bridge is unbalanced because the sensor, 
temperature is too low and the heater "on" time is in-
dicated as tA; at pattern C, the sensor temperature is 
much closer to the control temperature and the heater 
"on" time is shorter (ta). Sawtooth pattern D indicates 
that the resistance of the temperature sensor is identi-
cal to the set point; no power is supplied to the heater, 
but natural cooling will drop the temperature a small 
amount to yield pattern E. The heater "on" time, tE, 
is very close to the power input required to compen-
sate for heat losses from the controlled system and, 
ideally, the system should idle at this point with only 
infinitesimal power level adjustments made at the 
rate of sampling equal to the clock frequency. 
The conduction state of Qi can be controlled by 
programming its switching frequency; thus, the effec-
tive resistance of the combination of P1 and P2 (P2 can 
be omitted) can be controlled by a digital computer 
to give a precise rate of temperature change without 
overshoot for a properly designed system. For ex-
ample, an oven can be brought up to a presoak temper-
ature as fast as possible, held at presoak temperature 
for a given time, then heated at (e.g.) one degree per 
minute to a new temperature and held there for a 
time, and then brought back to presoak temperature 
in exactly the same length of time it took to achieve 
the higher temperature, and finally turned off. 
Notes: 
1. The final temperature in an experimental oven was 
held to ±0.001°C with a thermistor sensor. 
2. Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP72-10507 
Patent status: 
This invention is owned by NASA, and a patent 
application has been filed. Inquiries concerning non-
exclusive or exclusive license for its commercial de-
velopment should be addressed to: 
NASA Patent Counsel 
Mail Code 1 
NASA Pasadena Office 
4800 Oak Grove Diive 
Pasadena, California 91103 
Source: Richard C. Heyser of
Caltech/JPL
under contract to
NASA Pasadena Office
(NPO-11407)
. 
I 
I 
B72-10507	 Category 02


Pulse-width-modulated device for precision temperature control

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