Pre-Phase A Report - Lisa - Nasa

72 Chapter 3 Experiment Description
tuations of the applied voltages, of the contact potential differences and of the patch
effects [89], or by the proof-mass motion; they are reduced by the expected geometrical
accuracy of the ULE cage and of the proof mass (1 µm), by the large gaps and by the
servo-loop controls which maintain the proof mass motionless.
The parasitic effects due to the residual pressure are limited by the good vacuum and
the high thermal stability. The proof-mass magnetic moment is very weak because of the
choice of a material having very low magnetic susceptibility, and the magnetic behaviour of
the satellite should be acceptable. Furthermore, a magnetic shield could be implemented
if necessary.
This preliminary error budget shows that such a sensor concept appears compatible with
the LISA mission requirements. A detailed definition of the sensor, of its operation and of
the envisaged environment on board the satellite is necessary to refine and to confirm the
evaluation of performances. Experimental investigations will be necessary to assess the
behaviour of the sensor and to optimise the configuration. Both activities are being undertaken.
The sensor configuration could be tested in the laboratory under normal gravity if
a lighter proof mass, made in ULE for instance, is used, and if dedicated electronics units
are employed for the one-g proof-mass suspension. Such an ULE proof mass will weight
less than 100 g while the suspension of a 320 g accelerometer proof mass has already been
performed with quite the same 16 cm2 cross-section available for the electrodes used for
the one-g suspension. These investigations will be performed in 1999 after the production
of such a laboratory model. Even with dedicated laboratory facilities, the ground tests are
limited in resolution by the seismic noise and by the coupling with the one-g suspension,
but these tests can be performed over very long periods. They should be complemented
by free fall tests in a drop tower that provides the micro-gravity environment, but for only
very short test periods of 4.7 s. The ASTRE accelerometer, being realised in the frame
of the ESA COLUMBUS program and flown on board the shuttle, has been successfully
tested in the Bremen drop tower [90]. The instrument currently under development, the
configuration of which is very similar and representative to the one proposed for LISA,
could be much better evaluated on board a µ-satellite with a drag compensation system.
3.2.5 Sensor operation modes
The proof mass can be kept motionless in position and attitude by means of six servocontrol
channels acting separately. It can be shown that the operation of the drag-free
control can be represented by two loops. The first loop ensures the electrostatic suspension
which can maintain the proof mass at the centre of the cage with electrostatic forces. The
second one is the satellite drag-free control loop which compensates the satellite external
forces using thrusters.
In the LISA mission, the absolute acceleration of the proof mass should essentially depend
only on the gravitational field. Three sources of disturbances can be considered:
the position-sensor noise, the satellite acceleration depending on the drag-free control
performances, and the parasitic forces acting directly on the proof mass that cannot be
minimised by any servo loop. The first two sources can sufficiently be reduced at low
frequencies, when the electrostatic loop stiffness is adequately lower than the satellite
control-loop stiffness. On the contrary, the stiffness of the electrostatic loop must be
3-3-1999 9:33 Corrected version 2.08