October 4, 2007

NASA Electronic Parts and Packaging program

**Use of COTS (Commercial-Off-The-Shelf) Parts for**

**Wide-Range Temperature Sensing from Hot Jet Engine**

**Distributed Control Systems to Cryogenic Space Missions**

Richard Patterson, NASA Glenn Research Center

Ahmad Hammoud, ASRC Aerospace / NASA GRC

**Scope:**

The implementation of distributed control architecture in the NASA
Subsonic Fixed Wing/Jet Engine Distributed Control applications requires
that sensors and the electronic interface circuitry to be located with
monitoring and control transducers for engines and actuators in hot
environments where temperatures easily exceed 150 °C. The sensor
elements and associated circuits gather and transmit information
pertinent to the engine's temperature, speed, and pressure In addition
to meeting the operational requirements, placement of the electronics in
the harsh environment allows simpler signal multiplexing, improves
system performance, and avoids or minimizes signal degradation.

Similar situations arise in cold temperature environments. Examples
would be the NASA James Webb Space Telescope (JWST) and space-based
infrared (SBIR) satellite systems. In both of these applications,
electronics must be capable of operation at cryogenic temperature due to
operational requirements and for low-noise capture and processing of
very weak signals.

**Temperature Sensing Circuit:**

A temperature sensing circuit was assembled using COTS
(commercial-off-the-shelf) electronic parts to determine feasibility for
use in the harsh environment of a jet engine. Besides operation at
extreme temperatures, the sensor is required to sense temperature and to
produce an output consisting of a stream of rectangular pulses whose
frequency is a function of the sensed temperature. The output frequency
would be fed into a controller or a computer, and 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.
In order to develop this sensor circuitry, a literature survey was
performed to determine the simplest and most efficient circuit
configuration that can be utilized as a temperature-to-frequency
conversion sensor in extreme temperature applications. Also an industry
search was performed for commercial-off-the-shelf (COTS) electronic
parts that can be utilized at extreme temperatures. A
relaxation-oscillator topology was selected to build the
temperature-to-frequency circuit using an RTD (Resistance Temperature
Detector) as the temperature-sensing element. A schematic of the
temperature-to-frequency relaxation oscillator circuit is shown in
Figure 1, and COTS parts used are listed in Table I. The HT1104 is a
monolithic quad operational amplifier manufactured by Honeywell and is
rated for temperature operation between -55 °C and +225 °C. The device
is fabricated using silicon-on-insulation (SOI) technology, and it can
handle single or split supply application \[1\]. The RTD was a high
precision, thin film platinum device in a ceramic package, and the
resistors consisted of high-temperature precision-film elements.
Finally, the capacitor comprised of multi-layer stacked units made of
NPO/COG ceramic for optimum thermal stability.

![](media/image1.png){width="2.640277777777778in"
height="2.129166666666667in"}

Figure 1. Schematic of the temperature-to-frequency relaxation
oscillator circuit.

Table I. COTS parts used in construction of the temperature sensor
circuit \[1-4\].

  ------------- ----------------- ------------------ --------- ----------------------
  **Part**      **Part \#**       **Manufacturer**   **Temp.   **Features**
                                                     (°C)**    

  Operational   HT1104            Honeywell          -55 to    Silicon-on-insulator
  amplifier                                          +225      (SOI) technology,
                                                               hermetic 14-lead
                                                               ceramic DIP package

  RTD           PPG102A1          U.S. Sensor        -50 to    Thin film platinum,
                                                     +500      ceramic package

  Capacitor     SM041A164KAN240   AVX                -55 to    MLC COG ceramic
                                                     +125      

  Resistor      MM177-10K-1%      Caddock            +275      High temperature
                                                               precision film
                                                               resistor
  ------------- ----------------- ------------------ --------- ----------------------

The temperature-to-frequency relaxation oscillator circuit, which was
assembled on a high temperature polyimide board with Teflon-coated wire
interconnects and with high temperature solder. The circuit was
evaluated at selected test temperatures between -195 C and +200 C. A
temperature rate of change of 10 C/min and a dwell time of 20 minutes at
test temperature were used in these investigations. Circuit evaluation
was obtained in terms of its output frequency response with temperature,
variation in the output signal duty cycle and rise time, and the circuit
supply current. A photograph of the actual circuit board is shown in
Figure 2.

![](media/image2.png){width="3.7888888888888888in"
height="3.595833333333333in"}

Figure 2. Temperature-to-frequency relaxation oscillator circuit board.

**Test Results**

A typical output response of the temperature-to-frequency conversion
circuit is shown in Figure 3 at 25 C. Those obtained at the high
temperature of +200 C and at the cryogenic temperature of -195 C are
shown in Figures 4 and 5, respectively.

![](media/image3.png){width="4.65in" height="3.0097222222222224in"}

Figure 3. Output waveform of the temperature-to-frequency circuit at 25
C.

![](media/image4.png){width="4.641666666666667in" height="3.0in"}

Figure 4. Output waveform of the temperature-to-frequency circuit at
+200 C.

![](media/image5.png){width="4.670833333333333in"
height="3.0097222222222224in"}

Figure 5. Output waveform of the temperature-to-frequency circuit at
-195 C.

It can be clearly seen that the circuit performed very well throughout
the temperature range between +200 °C and -195 °C. As expected, the
frequency of the output signal fluctuated with variation in the sensed
temperature; reaching a peak of about 9.2 kHz at test temperature of
-195 °C and a minimum of 2.3 kHz at +200 °C. This frequency response
with temperature is depicted in Figure 6. No major change was
experienced by the duty cycle of the output signal as shown in Figure 7.

![](media/image6.wmf)

Figure 6. Output frequency versus temperature.

![](media/image7.wmf)

Figure 7. Duty cycle of output signal as a function of temperature.

While the rise time of the output signal held steady value between 25 °C
and +200 °C, it did, however, undergo slight increase as the temperature
was decreased below room temperature, as shown in Figure 8. The rise
time changed from 1.3 µs to about 4.3 µs when temperature was lowered
from 25 °C to -195 °C. The supply current of the circuit showed minor
dependence on test temperature as it remained within 4 to 8 mA level
throughout these tests. Figure 9 shows this slight change in the circuit
current with temperature.

![](media/image8.wmf)

Figure 8. Rise time of output signal versus temperature.

![](media/image9.wmf)

Figure 9. Circuit supply current as a function of temperature.

**Conclusions:**

A temperature-to-frequency relaxation oscillator circuit was assembled
using COTS parts for application under extreme temperatures in jet
engine hot environments. The circuit, which can also be applied in
cryogenic applications, employed a high temperature SOI operational
amplifier, NPO multi-layer ceramic capacitors, and precision film
resistors. The circuit was evaluated at selected test temperatures
between -195 C and +200 C. Circuit evaluation was obtained in terms of
its frequency response with temperature, variation in the output signal
duty cycle and rise time, and the circuit supply current. The prototype
circuit performed well throughout the temperature range between -195 C
and +200 C by producing a frequency output that was a smooth function of
the sensed temperature. No major changes were observed in the
characteristics, i.e. duty cycle and rise time, of the output signal
waveform as a result of change in test temperature. It can be concluded,
therefore, that all the COTS parts used in building the circuit
exhibited good performance under wide temperature swing, and the circuit
may have good potential for use in extreme temperature environments from
jet engines to cryogenic. Further testing under long-term exposure to
extreme temperatures, and additional characterization under vibration
conditions, which are typical of a jet engine, are required, however, to
establish suitability of the circuit and to determine its reliability.

**References:**

\[1\] Honeywell Company, "High Temperature Quad Operational Amplifier
HT1104", Datasheet 900134 Rev. B, 9-03.

\[2\]. U.S. Sensor, "Thin Film Platinum RTD's", Data Sheet.

\[3\]. AVX Corp., "SMPS Stacked MLC Capacitors", Data Sheet.

\[4\]. Caddock Electronics, Inc. "Type MM and ML Precision Film
Resistors", Data Sheet 28-IL124.1004.

**Acknowledgments:**

This work was performed under the NASA Glenn Research Center, GESS-2
Contract \# NNC06BA07B. Funding was provided from the NASA Electronic
Parts and Packaging (NEPP) Program Task "Reliability of SiGe, SOI, and
Advanced Mixed Signal Devices for Cryogenic Power Electronics", and the
NASA Fundamental Aeronautics -- Subsonic Fixed Wing Project -
Distributed Engine Control Task (Code AMRD) at NASA Glenn Research
Center.