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9.2
General Terms and Definitions
Motion Control Coordinate System
: In free space, an object is considered to have six degrees of
freedom: three linear, along the x, y, and z-axes and three rotational around those (roll, pitch, and yaw
respectively). All motions described here follow the right-hand coordinate system convention. The
cross-product of +X and +Y axes (pointer and middle fingers) is the +Z axis (thumb). Also, if the thumb
of the right-hand points in the positive direction of an axis, the fingers will wrap around the axis in the
direction of positive rotation about that axis. All movements are composed of translations along and/or
rotations about the coordinate axes. Generally, the X and Y axes are on the horizontal plane, the
direction of travel of the first or bottom stage is aligned with the X axis, and the Z-axis is vertical. For
parallel kinematic systems, cardan angles are used.
Linear Stage:
A linear stage or translation stage (also axis), is a component of a precise motion system
used to restrict an object to a single axis of motion.
Stack:
A nanopositioner stack refers to linear stages being stacked on top of each to provide a moving
foundation that can be controlled in multiple dimensions, commonly an xyz stack that provides control
over the 3 linear degrees of freedom. The sample foundation at the top of a positioner stack is not to
be confused with the Platform temperature in Cryostation systems.
Open-Loop Operation
: In open-loop control, the control of the positioners is independent of the
actual location of the stage. It does not use feedback to determine if the stage has achieved the desired
movement goal of the input control command. Open-loop control is often used in simple applications
because of its simplicity and lower cost, especially in systems where feedback is not critical.
Closed-Loop Operation:
The term "closed-loop" in nanopositioning indicates that a sensor is used to
monitor the position of a stage in real time. The position readings are fed back to the controller and
compared to the command positioner, which provides error correction for the system. Control
technology is used to reach the optimum target position, for high position resolution, for uniform feed
velocity, and greater dynamic velocity and acceleration ranges.
