LRS Micro-g LaCoste FG5, Manual, Page: 85 / 149

Adjustment and Maintenance
4
4.5.2.1.Cart-Position
The cart position is given by an optical shaft encoder that is mounted on the
motor shaft. The base resolution is 500 counts per revolution. The outputs are
two quadrature signals which give information about the amount and direction
of shaft rotation.
The shaft encoder quadrature outputs are preconditioned by a custom
programmed gate array logic (GAL) chip called the AXQD2X. The outputs of the
AXQD2X are glitch-free clock pulses and an up/down bit. The resolution of the
shaft encoder is multiplied by two, giving 1000 counts per revolution. The
AXQD2X also has logic that helps clear the counters and keeps them from an
overflow or underflow condition.
The sixteen-bit counters feed a twelve-bit digital to analog chip (DAC) for use in
the analog servo. The top seven bits are also fed to a comparator that is used for
level settings that trigger different phases of the servo. The output of the DAC is
available on the front panel BNC called CART position.
The DAC output has a programmable offset corresponding to either (1) the
manual position controlled by the front panel knob, or (2) the hold position
trimpot on the controller board. The servo adds in some derivative or damping
that is set by the CART DAMP trimpot on the PC-board. The overall gain is set
by the HOLD GAIN trimpot on the PC-board.
4.5.2.2.Cart Velocity
The servo takes the cart position derivative. A velocity lead trimpot on the PC-
board adds phase margin which tends to speed up or damp the servo in the
velocity mode. Servos that measure position and control velocity tend to have a
slow exponential response without this precaution.
A programmable reference voltage is added to the velocity signal. The servo
makes the actual cart velocity track these reference voltages. The four references
are called: throw, lift, soft throw, and catch.
The throw and lift are constant offsets set by trim pots on the circuit board.
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