Inside the 3TB
The signal voltages output by the 3TB are proportional to ground velocity, and are
transmitted from the instrument on balanced differential lines. In addition, mass
position signals are sent as single-ended circuits referred to analogue ground on the
output plug. The 3TB also receives control signals, which are used to clamp and un-
clamp the masses, and to run the motors which level and centre the instrument once
in position. Finally, a line is provided for you to apply a calibrating voltage to the
force transducers, thereby measuring the deflection sensitivity.
6.2 The control system
The internal operations of the 3TB are supervised by a control microprocessor, which
drives the mass clamping and centring adjustment motors. It responds to
60
Issue G - November 2019
