Various contamination sensors are available for
incorporation into a contamination monitor such as ICA (Mir). This
uses QCMs, AO detectors, and impact detectors. All instruments are
flight proven and fabricated to customer specifications in terms
of class A, B, C protocols depending on budget and environment constraints.
Five types of sensors can be used to monitor the environment in
- Temperature-controlled Quartz Crystal Microbalance (TQCM)
- Quartz Crystal Particle Microbalance (QCPM)
- Ionization Pressure Gauge
- Thermal Coating Calorimeter
- Optical Scattering Sensor
Temperature-controlled Quartz Crystal Microbalance.
Temperature-controlled Quartz Crystal Microbalances
(TQCM) are used to measure the deposition of molecular contamination.
The TQCMs measure contamination by means of a frequency shift of
a quartz crystal oscillator. This occurs when the crystal mass increases
as a result of contamination accretion. The device is extremely
sensitive, 1 Hz corresponding to 1.56 E-9 g/cm2. This sensitivity
is achieved by using a specially cut crystal that produces an extremely
small temperature dependence and by using a reference quartz crystal.
The signal from the reference quartz crystal, when mixed with the
signal from the sensing crystal, gives a beat frequency totally
independent of temperature and power supply fluctuations. The TQCMs
should be held at temperatures that are representative of typical
instrument surfaces (optical and thermal) that are to be monitored.
In addition, the TQCM can periodically volatilize collected contaminants
Quartz Crystal Particle Microbalance.
The Quartz Crystal Particle Microbalance (QCPM)
is essentially the same as a TQCM. The only differences being the
use of lower frequency quartz crystals (each resonates at approximately
10 MHz to aid in the accommodation of particulates) and the use
of a vacuum deposited coating with an extremely low vapor pressure
on the crystals. QCPMs have flown on 5 missions including STS, Mir
Ionization Pressure Gauge.
An Ionization Pressure Gauge can be used to measure
the total pressure. The vacuum measurement system consists of three
assemblies; the cold cathode gauge, the signal conditioner, and
the interconnecting cables. The cold cathode ionization gauge is
used to convert the pressure input into an electrical signal. The
gauge is based on the principle that the discharge current in a
transverse magnetic field is dependent on the pressure of the gas.
The cold cathode gauge and the high voltage supply are packaged
into a common housing. Combining the cold cathode gauge and the
high voltage supply in one housing eliminates high voltage connectors
and cables and the possibility of arcing and corona. The output
current of the cold cathode gauge is proportional to the input pressure.
The signal amplifier changes the linear input pressure to current
(voltage) relationship into an approximately logarithmic function.
Thermal Coating Calorimeters.
Thermal Coating Calorimeters (TCCs) have successfully
flown on numerous missions including the IOCM, NOAA-C, ICA, APEX,
OPM and NOVA contamination monitor projects. A sample disk rests
on a 0.254mm thick Kapton ring within an inner cup. The thin ring
support provides conduction isolation for the sample. The inner
cup is flanged and has an exterior surface equal in area to the
sample. This serves to maintain the inner cup surface close in temperature
to the sample inner surface to minimize radiative and conductive
heat transfer. The inner cup is supported by a Kapton ring 0.508mm
thick, once again for conductive isolation. This resides within
the outer cup or frame, which is mounted to the instrument. The
sample mount and inner and outer cups are machined from aluminum
and have very smooth surfaces to minimize their emittance. Several
layers of Mylar aluminized on both sides and crimped and put between
the sample and inner cup and between the inner and outer cup. This
serves as a multilayer insulation (MLI) barrier to minimize radiation
heat transfer within the calorimeter and thus minimize the heat
leak out the back of the calorimeter. Various thermal coatings or
surfaces are available. (E.g. replacing the sample disk with an
arrangement of stacked razor blades (chemically blackened), can
give an indication of the radiative heat flux input to the Contamination
Monitor during on-orbit operations, and secondly, as a calibration
surface for the contamination effects portion of the experiment).
Another example would be to cover calorimeters with a second surface
type mirror (SiO2/Ag). These mirrors are also known as Optical Solar
Reflectors (OSR's). This surface will act as the primary contamination
effects monitor for thermal control surfaces. This surface has been
used for similar purposes to determine contamination effects on
several shuttle missions.
Optical Scattering Sensor.
Contamination by spacecraft effluents can cause
degradation to sensitive optical surfaces. An accepted figure of
merit in the measurement of optical degradation is BRDF that quantifies
the amount of incident radiation that is scattered due to non-uniform
surface characteristics. Measurement of deposited mass is an indirect,
non-linear measurement of the optical performance of degradation
in terms of BRDF and not amenable to analytical solution without
an extensive empirical database which considers the morphology of
the contaminant deposit. For example, a given amount of mass deposited
as a homogeneous film will have vastly different optical properties
compared with the same amount of contaminant deposited as small
droplets, the latter producing a high degree of scattering over
a wide band of wavelengths. While TQCMs are useful in measuring
a threshold amount of mass deposition, which has been determined
by prior experiments to significantly degrade optical performance,
the information provided has limited usefulness. A more direct measurement
of real time changes in BRDF as a function of contaminant deposition
would provide the user with greater flexibility in controlling and
mitigating contaminant effects. The thickness of a layer of removable
contaminant can be inferred from a measurement of changes in surface
resistivity. The BRDF measurement includes degradation caused by
a removable layer of contaminants and any permanent surface damage
that may have occurred. Although direct measurement of the change
in optical properties is important, the optical and resistivity
measurements used in concert provide the amount of removable contaminant
deposition with accuracy, which is not available to each instrument
individually. The device consists of optical quality quartz glass
on which several sub-sensors are placed. Surface resistivity is
measured by several ring electrodes. The optical degradation measurement
device consists of a laser diode and two (2) receiver diodes. The
light beam emitted by the laser diode is partially reflected and
partially scattered from the sensor surface. Light that is scattered
into the receiver diodes is dependent upon the degradation of the
surface and the thickness of deposits that accumulate on the surface.