Laboratory optimization of the Helium Carrier Gas Flow System of MODULUS Ptolemy

Andrews, Dan; Morse, Andrew; Barber, Simeon; Leese, Mark; Morgan, Geraint; Pillinger, Colin; Sheridan, Simon; Wright, Ian and Zarnecki, John (2009). Laboratory optimization of the Helium Carrier Gas Flow System of MODULUS Ptolemy. Aerotecnica, Missili e Spazio, 88(4) pp. 139–144.

URL: http://www.aidaa.it/4-2009/articolo_mini3.pdf

Abstract

Ptolemy is a miniature laboratory aboard the Philae Lander of the Rosetta mission, comprising a Gas Chromatograph- Isotope Ratio-Mass Spectrometer (GC-IR-MS), as well as a chemical pre-processing suite. Central to the Ptolemy instrument is the gas chromatograph (GC) system for the separation of analytes prior to analysis. The GC system uses helium carrier gas contained in two tanks, each with a volume of 300 cm3 at 50 bar. To operate the GC columns, this gas is _rst admitted to a 30 cm3 plenum chamber and then maintained at a required setpoint pressure between 2.6 and 6 bar (_5%), over a large temperature and tank pressure range. To control the plenum pressure, Ptolemy uses a variable ow thermal expansion valve per tank, driven by feedback from a pressure sensor recording the plenum chamber pressure, and controlled by a Proportional, Integral and Derivative (PID) algorithm. The Ptolemy Flight Model was launched with a default set of generic PID parameters. The present study documents the optimisation of these parameters for the range of conditions in which the ight instrument will operate. A test rig was produced to simulate the operation of the ight carrier gas system, and to produce the required updated PID coe_cients. These coe_cients were then successfully tested on the Flight Spare instrument under a variety of conditions to simulate ight operations, with the result that these updated coe_cients will be used during future ight operations of the Ptolemy instrument.

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