Terminology 1: Linear Positioning Stages

If you are new to working with precision mechanical positioning systems the terminology used in specification sheets can be confusing.  The terms below are the most common 

• Resolution - should be defined as either theoretical or actual. 
• The theoretical, or design resolution is the smallest incremental step a positioner can make without taking into account mechanical stiction.  In a closed loop positioner it will be the smallest incremental step that can be measured by the encoder.
• The actual resolution, or minimum incremental step, is the smallest step the positioner can reliably make taking into account all mechanical effects.
• Repeatability - un-idirectional or bi-directional.
• Uni-directional repeatability is how precisely a positioner returns to a given location when always approaching it from the same direction.
• Bi-directional repeatability is how precisely a positioner returns to a given location when approaching it from any direction.
• Backlash - the mechanical play due to slack in a positioner when the motor changes direction. Sometimes this is used to define the bi-directional repeatability.
• Hysteresis - like backlash this error occurs when the positioner changes direction.  Typically hysteresis is much smaller than backlash because it is due to the relaxation of elastic forces that build up in the mechanics.
• Accuracy or Linearity - defines how precisely the positioner follows the programmed trajectory.  Many factors will contribute to accuracy including hysteresis and encoder non linearity.  Positioners with the highest accuracy will have a linear encoder directly measuring the location of the moving platform.
• Runout - how the positioner (moving in X) deviates from a straight path, includes:
• Flatness - deviation in the Z plane
• Straightness - deviation in the Y plane
• Pitch - rotation about Y
• Yaw - rotation about Z
• Roll - rotation about X
• Orthogonality - angular deviation caused by mounting error between two or more axes.
• MTBF - mean time between failure, highly application dependent.

More terminology to follow in weeks to come on rotary positioners and piezo positioners.