January 14, 2007

**A Temperature Sensor Using a Silicon-On-Insulator (SOI)**

**Timer for Very Wide Temperature Measurement**

Richard Patterson, NASA Glenn Research Center

Ahmad Hammoud, ASRC Aerospace / NASA GRC

Malik Elbuluk, University of Akron

Dennis Culley, NASA Glenn Research Center

**Scope**

A temperature sensor based on a commercial-off-the-shelf (COTS)
Silicon-On-Insulator (SOI) Timer was designed for extreme temperature
applications. The sensor can operate under a wide temperature range from
hot jet engine compartments to cryogenic space exploration missions. For
example, in Jet Engine Distributed Control Architecture, the sensor must
be able to operate at temperatures exceeding 150 °C. For space missions,
extremely low cryogenic temperatures need to be measured. The output of
the sensor, which consisted of a stream of digitized pulses whose period
was proportional to the sensed temperature, can be interfaced with a
controller or a computer. The data acquisition system would then give a
direct readout of the temperature through the use of a look-up table, a
built-in algorithm, or a mathematical model. Because of the wide range
of temperature measurement and because the sensor is made of carefully
selected COTS parts, this work is directly applicable to the NASA
Fundamental Aeronautics / Subsonic Fixed Wing Program -- Jet Engine
Distributed Engine Control Task and to the NASA Electronic Parts and
Packaging (NEPP) Program. In the past, a temperature sensor was designed
and built using an SOI operational amplifier, and a report was issued
\[1\]. This work used an SOI 555 timer as its core and is completely new
work.

**Temperature Sensor**

A temperature-to-frequency relaxation oscillator circuit was constructed
using a high temperature polyimide circuit board, Teflon wire
interconnects, and high temperature lead-free solder. The circuit
employed a high temperature Silicon-On-Insulator (SOI) precision timer
and other specially selected, temperature-stable
commercial-off-the-shelf (COTS) parts. The CHT-555-DIL14
ceramic-packaged chip was recently introduced by CISSOID Company and is
specified for -30 °C to +225 °C operation \[2\]. The other components of
the circuit included a high temperature precision, thin film platinum
RTD (Resistance Temperature Detector) as the temperature-sensing
element, a solid tantalum input filter capacitor, and two NPO ceramic
capacitors. A schematic of the circuit is shown in Figure 1, and a
photograph of the circuit board is shown in Figure 2. The circuit was
evaluated at selected test temperatures between -190 °C and +210 °C. The
effect of thermal cycling was also investigated by subjecting it to a
total of ten cycles between +200 °C and -190 °C. A temperature rate of
change of 10 °C/min and a dwell time of ten minutes at test temperature
were used in these investigations. The circuit was evaluated in terms of
its frequency-to-temperature conversion, period-to-temperature
conversion, and variation in the output signal duty cycle and rise time
with test temperature.

![](media/image1.png){width="4.653472222222222in"
height="2.6118055555555557in"}

Figure 1. Schematic of the relaxation oscillator circuit.

![](media/image2.jpeg){width="4.272916666666666in"
height="3.004861111111111in"}

Figure 2. Relaxation oscillator circuit populated with CHT-555 timer.

**Results**

A typical output response of the temperature-to-frequency conversion
circuit, which comprised of a rectangular pulse train (yellow signal),
is shown in Figure 3 at 25 °C. The signal at the threshold pin
(magenta), which governs the charge/discharge cycle of the timing
capacitor C1, is also depicted in this figure as a triangular waveform.
Those obtained at the high temperature of +210 °C and at the cryogenic
temperature of -195 °C are shown in Figures 4 and 5, respectively.
(Scale: Yellow 2V/div, Magenta 1V/div; Horizontal 0.1ms/div).

![](media/image3.png){width="3.98125in" height="2.588888888888889in"}

Figure 3.Output and threshold signal waveforms at 25 °C.

![](media/image4.png){width="3.9006944444444445in"
height="2.5277777777777777in"}

Figure 4.Output and threshold signal waveforms at +210 °C.

![](media/image5.png){width="3.9118055555555555in"
height="2.5493055555555557in"}

Figure 5.Output and threshold signal waveforms at -190 °C.

It can be clearly seen that the relaxation oscillator circuit performed
very well throughout the temperature range between +210 °C and -190 °C
as the frequency of the output signal fluctuated with variation in the
sensed temperature. While the frequency of the output signal had a value
of about 3.393 kHz at room temperature, it decreased to about 2.072 kHz
at +210 °C and attained a frequency of 15.048 kHz when the temperature
approached -190 °C. This frequency response, which took on a hyperbolic
trend with temperature, is depicted in Figure 6. Plotting the period of
the output signal versus temperature reveals, as expected, a linear
response as shown in Figure 7. No change was experienced by either the
duty cycle or the rise time of the output signal throughout the test
temperature range as shown in Figures 8 and 9, respectively.

![](media/image6.wmf)

Figure 6. Output frequency versus temperature.

![](media/image7.wmf)

Figure 7. Period of the output signal versus temperature.

![](media/image8.wmf)

Figure 8. Duty cycle of output signal as a function of temperature.

![](media/image9.wmf)

Figure 9. Rise time of output signal versus temperature.

The supply current of the circuit varied slightly with temperature as
depicted in Figure 10. While the current hovered around 0.68 mA between
+20 C and +210 C, it exhibited a slight increase with decreasing
temperature reaching about 2.5 mA at the extreme cryogenic temperature
of - 190 C. This change in the circuit current with temperature is most
likely attributed to the variation in the resistance value of the RTD
element in the circuit as test temperature changed.

![](media/image10.wmf)

Figure 10. Circuit supply current as a function of temperature.

As was mentioned earlier, the performance of the relaxation oscillator
circuit was also investigated after subjecting it to ten thermal cycles
between +200 °C and -190 °C. Post-cycling measurements performed on the
investigated parameters, i.e. frequency output, duty cycle, rise time,
and supply current, revealed no major deviation from those obtained
prior to cycling at any given test temperature. Therefore, this limited
thermal cycling has had no impact on the operation of the oscillator
circuit. In addition, the circuit demonstrated successful start-up
operation while at the extreme temperatures, i.e. +210 °C and -190 °C.

It should be noted that this circuit design is tailored to operate into
a fixed load impedance because the timing components are connected
across the output. Variation in the load impedance would result in a
change in the charge/discharge time constant of the circuit, and thereby
the frequency of the output would change.

**Conclusions**

A temperature-to-frequency relaxation oscillator circuit was constructed
using COTS (Commercial-Off-The-Shelf) parts for application under
extreme temperatures. The circuit employed a recently-developed high
temperature silicon-on-insulator CHT-555 timer, thin-film platinum RTD,
and solid tantalum and ceramic capacitors. The circuit was designed
mainly for hot jet engine environment but it was evaluated also for
potential use under cryogenic conditions. Performance of the oscillator
circuit was investigated in terms of its temperature-sensing response,
output signal duty cycle and rise time, and supply current under a wide
temperature range between -190 C and +210 C and after thermal cycling.
The prototype circuit performed well throughout this temperature range
in producing a pulsed output whose period was a linear function of the
sensed temperature, and no major changes were observed in its
characteristics, i.e. duty cycle and rise time of the output signal, as
a result of change in test temperature. In addition, all of the
individual parts exhibited no physical or packaging damage due to the
extreme temperature exposure or cycling. It can be concluded, therefore,
that all the COTS parts used in designing the circuit exhibited good
performance under wide temperature swing, and these preliminary results
suggest that the circuit has good potential for use in both hot and cold
temperature environments. Issues such as long-term exposure to extreme
temperatures, thermal cycling, and mechanical vibrations (that are
typically encountered in jet engine and spacecraft environs) need to be
addressed to establish reliability of the circuit and to better
determine its suitability for use in hostile aerospace and space
applications.

**References**

\[1\] Patterson, Richard; Hammoud, Ahmad; "Use of COTS
(Commercial-Off-The-Shelf) Parts for Wide-Range Temperature Sensing from
Hot Jet Engine Distributed Control Systems to Cryogenic Space Missions,"
NASA Electronic Parts and Packaging Website, October 4, 2007.

\[2\] CISSOID Company, "CHT 555 High Temperature Precision Timer,"
Datasheet Doc. DS-07006 Rev. A, March 28, 2007.

**Acknowledgments**

This work was performed under the NASA Glenn Research Center, GESS-2
Contract

\# NNC06BA07B. Support was provided by the NASA GRC Fundamental
Aeronautics / Subsonic Fixed Wing Program -- Distributed Engine Control
Task and by the NASA Electronic Parts and Packaging (NEPP) Program.