  The InfraRed Imaging Spectrograph (IRIS) will be a first-light client
instrument for the Narrow Field Infrared Adaptive Optics System (NFIRAOS) on
the Thirty Meter Telescope. IRIS includes three configurable tip/tilt (TT) or
tip/tilt/focus (TTF) On-Instrument Wavefront Sensors (OIWFS). These sensors are
positioned over natural guide star (NGS) asterisms using movable
polar-coordinate pick-off arms (POA) that patrol an approximately 2-arcminute
circular field-of-view (FOV). The POAs are capable of colliding with one
another, so an algorithm for coordinated motion that avoids contact is
required. We have adopted an approach in which arm motion is evaluated using
the gradient descent of a scalar potential field that includes an attractive
component towards the goal configuration (locations of target stars), and
repulsive components to avoid obstacles (proximity to adjacent arms). The
resulting vector field is further modified by adding a component transverse to
the repulsive gradient to avoid problematic local minima in the potential. We
present path planning simulations using this computationally inexpensive
technique, which exhibit smooth and efficient trajectories.
