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The pressure modulator is a mechanical device which performs the function of varying the amount of gas in an optical cell by varying the pressure. It achieves this by the use of a piston and cylinder arrangement to compress and rarefy the gas, forcing it in and out of the optical cell.
The pressure of the gas in the system is controlled and stabilised by the use of a molecular sieve material.
The pressure modulator is a fully qualified unit, requiring only modest tailoring to the requirements of the MOPITT program. It has been used extensively in space applications in the past.
The pressure modulator sub-system consists of the mechanical compressor unit, the optical cell, the molecular sieve assembly and the electronic drive system.
The use of pressure modulators was pioneered at Oxford University in the 1970's. Many units have been flown on many space missions. Several forms of pressure modulator for space applications have been developed: The original modulators used in the PMR instrument of the NIMBUS 5 spacecraft used a single piston, an internal magnet and external drive coils. The next generation, used on the SAMS instrument on NIMBUS 6 used a single piston and an internal "voice-coil" drive. This drive has subsequently been developed for use in Stirling Cycle Coolers for space applications such as those marketed by British Aerospace and Lucas-Lockheed. The current generation of modulators which is described in more detail below is being used on the ISAMS instrument and the PMIRR instruments. This current generation of modulators uses opposing pistons for momentum cancellation, internal magnets and external drive coils.
Optically, a Pressure Modulator Cell consists of a sealed cell of gas with two plane parallel windows for passage of the optical beam. The mechanical system for varying the gas pressure is attached to the optical cell via a hollow tube. This is illustrated in Figure B.1
As the gas pressure is varied, the absorption profiles of the spectral lines of the gas in the cell also vary thus modulating radiation at those same optical frequencies. By converting the radiation to an electrical signal and then electronically extracting the component of that signal which varies in frequency and phase with the pressure cycling, a measure of the amount of radiation passing through the cell at optical frequencies close to the gas absorption lines is obtained. Optical frequencies far from gaseous absorptions lines pass through unattenuated and are thus ignored by the electronic selection system.
The performance of the system varies with the extent of the pressure cycling. In general the greater the compression ratio achieved in the cell, the better the performance. The mean cell pressure also varies the performance, but in a more complex manner and the science requirements dictate this value. As a broad generalisation, sounding at lower altitudes requires higher mean cell pressures.
The pressure variation of the gas in the optical cell is complex. Simple theories which postulate a sinusoidal variation in gas pressure are generally adequate for a qualitative to semi-quantitative evaluation of cell performance. The actual variation in optical transmission is complicated by the form of the volume changes, the flow in the various parts of the system, leakage past the piston and temperature cycling of the gas itself. Various models of modulator operation have been used to investigate the details of the cycling and this work is still being pursued.
The optical cell of the modulator is manufactured from pure aluminum. The optical windows are calcium fluoride. The windows are soldered to the cell body. The optical cell is 10mm in length and is shaped to provide an optimum beam clearance which minimises the cell volume. Since cell volume affects the compression ratio, the affect of the cell shaping is to enhance the performance. Similar considerations are applied to the rest of the mechanism to maximise the performance, in particular the length of the connecting tube from the compression volume to the optical cell should be minimised. Compression ratios of 2:1 are believed to be achievable at 7.5kPa mean cell pressures with a cell of dimensions commensurate with the MOPITT instrument.
The pressure cycling is supplied by a mechanical system of a pair of pistons moving in opposition to each other in a common bore. The pistons are suspended on beryllium-copper spiral springs clear of the bore itself and thus the moving system has no rubbing parts. In addition, the pistons are coupled by a third spring as shown in Figure B.1. This gives the moving system two principal modes of vibration. In the undesired mode, the two pistons move in the same direction at the same time and the volume between them is constant. In the desired mode the pistons run in opposition, the volume between the pistons varies and the momentum sum of the pistons is zero (to first order). A tube from the volume between the pistons is used to connect to the optical cell. The relative orientation of the optical cell to the modulator body is immaterial but the connecting tube is rigid and therefore the orientation must be specified in the design. It is also possible to use a bent connecting tube if the radius of curvature is reasonable.
The system is driven at its resonant frequency by a separate moving magnet drive for each piston activated by coils attached to the outside of the modulator body. The positions of the pistons are sensed using twin inductive pick-offs at the extrema of the system. The frequency of operation is about 40Hz and this frequency varies depending upon the pressure of the gas within the modulator. Monitoring this frequency gives information regarding the pressure through a pre-flight calibration curve.
The modulator is constructed almost entirely of titanium. The springs are of beryllium copper. The vacuum seals are gold o-rings. Particular care is given to the minimisation (and hopefully the elimination) of any organic material from the internal chambers of the modulator.
The electronic drive circuitry for the modulators has been designed by the manufacturers and the design will be supplied for incorporation into the MOPITT electronics. A block diagram is shown in Figure B.2.
The drive system is required to drive the mechanical system at its resonant frequency, to control the amplitude to a preset value and to provide phase references to the signal processing electronics. These functions are to be performed with maximum reliability and minimum power consumption.
The positions of the pistons are sensed via the variable reluctance position sensors. These utilise an inductance bridge which is unbalanced by a ferrite bead attached to the end of the piston axial shaft. The bead is within the vacuum system and the coils which comprise the inductance bridge outside. The bridge is excited at about 10kHz and the imbalance signal is detected and filtered to produce the piston position.
The amplitude of the summed output from the two position pick-offs is compared to the desired amplitude of the modulator and the error signal is integrated and used as a control signal on the drive amplitude.
The phase of the summed output from the two position pick-offs is compared to the phase of the drive current and the result is compared to the desired phase difference.
The required amplitude and phase of the drive are then combined to produce a drive signal for both drive coils.
It is emphasised that it is not intended that the electronics will be supplied as GFE, but the design will be supplied for incorporation into the rest of the MOPITT electronics.
The resonant frequency of a modulator is dependent upon the gas pressure. For low pressure modulators this dependence is a square-root function (see Taylor, 1983). For higher pressure modulators, this relationship is not maintained and a more linear relationship is found. An approximate (and easy to remember) formula for an ISAMS modulator has been found to be:
For a more precise discussion of the pressure vs frequency characteristic see Gibson (1993).
The pressure of gas in a modulator is maintained by the use of a "molecular sieve" system which consists of a capsule of silicalite material attached by a tube to the modulator system. The attachment of the sieve to the gas system is such that the compression ratio is not compromised. The equilibrium pressure of the gas in the system is a function of the temperature of the silicalite capsule in the range 20-100oC. The actual relationship between the temperature and the pressure is determined by a pre-flight calibration. The molecular sieve capsule is maintained at a constant temperature by a thermostat.
The mean pressure in the modulator may be adjusted by changing the sieve temperature.
At a constant temperature the large volume of gas stored on the sieve material effectively buffers the volume of the modulator stabilising the mean gas pressure. In the event of small modulator leaks this gas reservoir allows the leak to be compensated within the limits of the reservoir mass. It should be noted that this leak compensation is only effective for very small leaks due to the small reservoir mass and the long time period of a space mission.
The modulators are specifically manufactured and filled for each application. Minor variations in the mounting configuration can be accommodated if they do not compromise the modulator operation. The distance of the optical cell from the compressor unit should be minimised to maximise compression ratio in the cell. The distance of the molecular sieve capsule from the compressor is immaterial within reasonable limits (1m).
The internal pressure of a modulator is set according to the channel required for the instrument. The maximum pressure in a modulator is limited by the ability of the drive system to compress the gas and is generally less than a peak pressure of 10kPa. The filling pressures for the MOPITT instrument will be in the range 2.5-7.5kPa.
Since the modulators are supplied integrated with the molecular sieve unit, it is required that the assembly procedure be able to accept the completed unit without stressing the tube linking the modulator to the molecular sieve unit.
There is a slight preference for mounting a modulator with the piston shaft horizontal in a 1g environment. Other mountings are not considered detrimental to operation or longevity but the pressure vs frequency characteristic can only be verified (and therefore internal pressures verified) with a horizontal orientation in a 1g field.
The electronics requires approximately 200cm² with conventional layout techniques. Conventional layout assumes the use of appropriate dual-inline packages and double-sided printed circuit boards. It is probable that this area could be reduced by the use of more compact technologies.
Schematics of the modulator interface and the molecular sieve interface are shown in Figure B.3 and Figure B.4 (not available on-line). These are preliminary drawing only, produced in advance of specific specification of the interface requirements.
The performance of the PMCs supplied will be individually verified at the sub-system level using an agreed test procedure. The purpose of the test will be to verify that the "as delivered" units have the required performance to enable the contractor to meet the performance specification of the MOPITT instrument.
As noted above, the electronics will not be GFE.
The power consumption of a modulator is 5W. Power consumption of the molecular sieve is 2W(TBC).
The mass of the modulator is 1200g. The mass of the sieve is 200g(TBC).
The residual momentum of the modulator system is specified as <3% of the momentum of a single piston. A single piston system weighs 30g and oscillates through 3mm pk-pk at 40Hz. Measurements conducted on a modulator show residual axial forces at the oscillation frequency of 0.3N or less, depending upon the conditions (Gibson, 1993).