LRS Micro-g LaCoste FG5, Manual, Page: 86 / 149

Adjustment and Maintenance
4
The soft throw and catch are both linear voltage ramps whose slopes are
controlled with trim pots labeled SOFT THROW RAMP and CATCH RAMP.
Each reference voltage also allows an offset to be added using the trim pots
labeled THROW OFFSET and CATCH OFFSET. These ramps are used to
accelerate the cart during the throw phase and decelerate the cart during the
catch phase.
The gain for the velocity servo is set by the VELOCITY GAIN trimpot on the PC-
board.
4.5.2.3.Sphere Servo
The sensor for this servo is an LED focused spot-sensor. The operational
principle is a simple optical lever arrangement. An LED and linear detector are
mounted on opposite sides of the cart. A spherical lens mounted on the test
mass focuses the LED onto the linear detector, giving relative position between
the cart and test mass. The signal is preconditioned by a preamplifier mounted
on the cart. The position can be monitored on a front panel BNC labeled
SPHERE.
The servo consists of an active feedback servo and a passive feedforward servo.
The feedforward provides the approximate correct motor voltage during the
drop.. This reduces the demands on the feedback servo.
4.5.2.4.Active Sphere Servo
The sphere signal is added with an offset called height and another offset called
hover height. The height offset is always added to provide a tracking difference
between the cart and the test mass during free-fall. The hover height is switched
in by the EPROM and gives an overall reduced offset which makes the cart track
the test mass very close to the rest position. This is useful for catching the object
near the bottom of the drop.
The sphere signal (sphere and offsets) servo uses a proportional and a first
derivative term. The proportional term is controlled with a trimpot called
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