Present Shuttle Operations requirements call
for performing pressurization and pressurization decay tests to validate
the Shuttle Orbiter tires and struts for flight. It is necessary to identify
any leak paths during processing before the Orbiter is ready for flight.
To achieve this task, an instrumentation system capable of working at 450
pounds per square inch absolute (psia) and detecting 0.1-psi changes at
that high pressure is required. Currently, the configuration and acquisition
of these measurements is time-consuming, involving the setup of long, pressurized,
flexible tubing and significant warmup time of the measurement system.
Among other concerns, there are safety issues related to having long pressurized
lines. The TPM project goal was to design a handheld system to accurately
measure Orbiter tire and strut pressure and temperature, thus removing
the need for the large and unwieldy system presently in use.
The approach was to place pressure and temperature sensors as close
to the tire or strut measurement location as possible, allowing
the user to
make accurate measurements rapidly, to minimize the length of high-pressure
lines, and to allow reasonable distance between the tire or strut and
the operator. Ideally, the pressure and temperature sensors should
attach directly
to the pressure supply/relief valve on the tire and/or the strut, with
the necessary electronics contained in a handheld enclosure connected
by a 6-foot instrumentation cable. Therefore, the project team
set out to
select small, highly accurate pressure and temperature sensors, to
design the optimum mechanical interface between the tire/strut
valve and the sensors,
and to design a compact, handheld electronics unit that provides rapid,
reliable pressure/temperature data display.
The system was designed to have a sensing unit connected by instrumentation
cable to the data acquisition and display unit. The pressure measurement
portion of the proposed system centers on a small, highly accurate
pressure sensor. The commercially available sensor is capable of
delivering pressure
measurements repeatable to within 0.03 psia. Temperature compensation
and correction are required to maintain this tight tolerance throughout
the
wide operating temperature range. The complete pressure sensor assembly,
housed in a stainless-steel enclosure, contains the pressure sensor,
a temperature sensor and electronics for precise excitation to the
pressure sensor, and amplification of pressure and temperature measurements.
The
sensing unit housing and fittings and unit’s orientation were designed
to allow for easy installation and removal by technicians.
The TPM user interface is a handheld device that can be powered by
12-volt (V) alternating current or by 9-V direct current batteries.
The handheld
device provides voltage to the sensing unit electronics, as well
as low-pass filtering and analog-to-digital conversion of sensors
measurements.
The
system includes a smart software algorithm embedded in a microcontroller,
utilizing complex conversion equations developed from pressure
and temperature sensor calibration data; the system is capable
of optimizing
measurement
accuracy at any given operating temperature. Because of the developed
software algorithm, the system is capable of maintaining the required
accuracy throughout
the required temperature range. The handheld electronics provide
the user with an easily read visual display of pressure/temperature
or
the streaming
of pressure/temperature data via an RS-232 interface.
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Field-Grade Prototype of TPM
Designed to operate
in a temperature environment anywhere from 20 to 120 degrees Fahrenheit,
this new measurement tool will enhance Shuttle Operations tire and
strut pressure measurements. Sensor accuracy, electronics design, and
a simple user interface will allow operators quick, easy access to
required measurements. Coupled with a laptop computer, this new measurement
system can provide users with automated data recording and trending,
eliminating the chance for data hand-recording errors. In addition,
calibration software will allow for calibration data to be automatically
utilized for the generation of new data conversion equations, simplifying
the calibration processes that are so critical to reliable measurements.
Key accomplishments:
- Developed, fabricated,
and preliminarily tested sensing unit and handheld display unit.
- Delivered prototype
unit to United Space Alliance Operations for field testing.
Key milestone:
- Automated calibration
station is being developed to support required accuracies.
Contact: J.M. Perotti (Jose.Perotti-1@ksc.nasa.gov),
YA-D5-E, (321) 867-6746
Participating Organization: Dynacs Inc. (Dr. C.T. Mata, A.J. Eckhoff, R.T.
Deyoe, J.J. Randazzo, B.M. Burns, and N.N. Blalock)
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