Pre-Phase A Report - Lisa - Nasa

3.2 The inertial sensor 71
Figure 3.10 Scheme of one channel loop.
to the full scale range of acceleration required for the control of the proof mass. It shall
be changed according to the different operation modes from tens of volts to hundreds of
milli-volts.
A supplementary set of electrodes, called injection electrodes in Figures 3.8, 3.9 and 3.10, is
used to control the proof-mass electrical potential, in particular at the pumping frequency
of the capacitive sensing. This solution has been preferred to a thin gold wire used in
other space accelerometers because of the stiffness and of the damping that the wire may
generate. Experimental investigations have demonstrated that the wire is only compatible
with a resolution of several 10−14 ms−2 / √ Hz at room temperature [86].
3.2.4 Evaluation of performances
The performances of the inertial reference sensor have been evaluated by considering the
geometry and the characteristics of the sensor head, the characteristics of the electronics
configuration and the environment on board the LISA satellite. In particular this last
point leads to a required pressure inside the housing of less than 10−6 Pa.
The evolution of the charge Q of the isolated proof mass is one of the most critical error
sources. On the one hand, the proof mass is subjected to the electrostatic forces appearing
with the image charges on the electrodes in regard to the proof mass; the resultant of
these forces is not null when the configuration is not perfectly symmetric, in particular for
any off-centering of the proof mass with respect to the sensor cage. On the other hand,
this charge Q induces a Lorentz force when it moves in the interplanetary magnetic field.
In fact Q is the sum of the charge acquired when the proof mass separates from the
accelerometer cage at suspension switch-on, and of the charge resulting from the cosmic
particle bombardment, especially from the proton flux over 100 MeV. When considering
achargingrateof10 −17 to 10 −18 C/s [87, 88], the 2×10 −14 C limit required is reached in
less than one hour. Therefore the charge has to be measured exploiting the electrostatic
device itself, and the proof mass has to be discharged, continuously or time by time, for
instance by photo-electric emission induced by ultraviolet light [87].
Other electrostatic-force noises must be considered: they are induced either by the fluc-
Corrected version 2.08 3-3-1999 9:33