Geomagic Geomagic Phantom Premium 1.56DOF, Device Manual

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D-2 Premium 1.56DOF/1.5HF6DOF Device Guide
A
PPENDIX
: M
AXIMIZING
P
OSITIONING
A
CCURACY
The goal of this procedure is to line up the four
rotary joints of the Phantom 4-bar linkage to
form a rectangle. If the two joints on the short
linkage closest to the base are called Joints 1
and 2, and the joints on the second linkage are
called Joints 3 and 4, the lines between Joints 1
and 3 and Joints 2 and 4 must be horizontal, and
the lines between Joints 1 and 2 and Joints 3 and
4 must be vertical.
To eyeball or not to eyeball
Given that the initialization position dictates future positioning accuracy of the Phantom,
is it sufficient to eyeball the startup position or is it necessary to create a fixture to ensure
accurate and repeatable system initialization?
The answer to that question depends greatly on the application. The following figure
shows typical positioning errors when the initialization position is translated from the
optimal startup position by an “eyeballing” error of 0.2" in the -x and -z directions. The
best-case error is around the initialization point with the errors growing larger near the
edges of the workspace. Furthermore, there is some skewing of the coordinate system.
Due to the error in reported position, the forces presented to the user will also be similarly
skewed.
However, the human is an imprecise position and force measurement device and such
differences are generally not perceivable. For applications in which a single Phantom is
used to present forces in a virtual reality simulation, eyeballing is generally adequate.