Defining Micropositioning and Nanopositioning Terminology

The terminology used to specify and describe the performance of precision motion control equipment is often subject to significant misinterpretation in everyday usage. Unfortunately, terms frequently employed by both users and manufacturers of micropositioning hardware carry different meanings for different individuals. To avoid a crane vs. crane scenario (see image), it’s essential that we establish a common understanding when discussing terms like repeatability, accuracy, precision, and related concepts in micropositioning and nanopositioning applications.

To help clarify, the accompanying glossary defines the most common terms. However, a few of these terms require more detailed explanation beyond their brief definitions.

One of the most misunderstood — and yet most crucial — terms is accuracy, or absolute accuracy. This term is often confused with others, such as precision, resolution, repeatability, or sensitivity. In simple terms, accuracy is the maximum expected deviation between the actual position and the target (or ideal) position for a given input. To measure accuracy in micropositioning or nanopositioning equipment, an external laser interferometer is the best tool for the job.  Some laser interferometers can measure both linear and angular positions with very high accuracy. An autocollimator is another optical tool for precise angular (error) measurements.

• Hysteresis: A difference in the absolute position of an object after traveling in one direction and then reversing to return by the opposite way. It is due to elastic forces accumulated in various drive train components (a leadscrew windup, couplings, bearings, etc.). Often confused with Backlash.
• Incremental Move Command: A motion command refers to the current position of the controlled object. Thus, if an object is at x = 10 and a command is given to move by +5, the object will move to x = 15 (not to x = 5 as it would in an absolute system).
• Input Shaping:An advanced technique used to modify the input command of a motion system to reduce or eliminate vibrations by applying a series of pre-calculated impulses that counteract oscillations.
• Lead: The distance traveled for each rotation of a leadscrew.
• Leadscrew Pitch: In the Imperial / English system, the number of revolutions required to move an object mounted on a leadscrew by one inch. In metric systems, the peak-to-peak distance between the threads of a screw; see Lead.
• Learning Based Motion Control:An approach that uses machine learning algorithms to improve a system's ability to perform precise and adaptive movements by learning from past experiences and real-time feedback.
• Minimum Incremental Motion: The minimum controllable motion interval that the system is capable of producing repeatably.
• Orthogonality: A measure of how accurately a system’s axes maintain a 90° relationship among them.
• Pitch (Tilt): In a linear stage, an unwanted tendency of a positioned object to move "up and down" (tilt) during a pure horizontal translation.
• Precision: Sometimes used synonymously to accuracy and (falsely) to repeatability. The average deviation from the ideal position for a given input.
• Repeatability: The ability of a system to bring an object to a desired position repeatedly (see Bi-directional Repeatability and Precision). Depending on what norm is used to measure it, it can be the worst result from several tests or provided as standard deviation (1, 2, 3,…  sigma).
• Resolution: Often confused with Minimum Incremental Motion. In a closed loop system, the sensor resolution. In a geared drivetrain, it is often used to specify the calculated, theoretical size of one step (based on the number of microsteps or rotary encoder counts, gear reduction ratio, and drive screw pitch).
• Rigidity: The ability of a servo motor or system to resist external force or torque interference.
• Roll: In a linear stage, an unwanted tendency of a positioned object to rotate about the axis of its translatory motion.
• Runout: In a linear stage, any unwanted motion other than pure translation. Thus, runout may include yaw, pitch, and roll.
• Sensitivity: The minimum input capable of producing an output motion.
• Stability: The ability of a system to maintain positioning accuracies with passage of time in a given ambient. It is a function of wear and tear, lubricants (film thickness may contribute to error), and temperature.
• Straightness: The ability of a system to move an object along a perfectly straight path. Air bearings can provide the best straightness and flatness.
• Translation stage: A precision mechanical device that allows controlled linear movement of an object along one or more axes, commonly used in applications requiring precise positioning and alignment.
• Wind-up (Torsional): An effect whereby a torque applied to the drive end of a leadscrew rotates that end farther than the output end.
• Yaw: In a linear system, an unwanted tendency of a positioned object to move left and right from the motion axis.
• Zero-Backlash Gearbox: A type of gearbox designed to eliminate play, or lost motion, between input and output shafts, allowing for precise, smooth, and highly accurate transmission of torque without delays or positional errors.

Most of the terms of the above glossary apply primarily to single-axis positioning. However, many applications require multi-axis operation, where additional factors come into play. Some of the most critical terms and concepts for multi-axis positioning are summarized below. Cross-coupling, as previously defined, is especially important in multi-axis systems.

The cantilever effect is one of the most significant challenges in multi-axis positioning. This effect arises when a load, specifically its center of gravity, is positioned off-axis. In multi-axis positioners, the center of gravity of each positioner often lies off the axis of another stage, which can lead to flexing or bending and potentially increase backlash. To minimize the cantilever effect, the lowest positioner should have the widest base (distance between bearings). For the best performance, true planar stages (available with air bearings) where both the X and Y axis use the same base, are recommended