A modern motion control system typically consists of a motion controller, a motor drive or amplifier, an electric motor, and feedback sensors. The system might also contain other components such as one or more belt-, ballscrew-, or leadscrew-driven linear guides or axis stages. A motion controller today can be a standalone programmable controller, a personal computer containing a motion control card, or a programmable logic controller (PLC).
Below terms you will hear and need to know as a mechanical design engineer both to design good machines and communicate with controls specialists effectively.
Abbe error: A linear error caused by a combination of an underlying angular error along the line of motion and a dimensional offset between the position of the object being measured and the accuracy-determining element such as a leadscrew or encoder.
acceleration: The change in velocity per unit time.
accuracy: (1) absolute accuracy: The motion control system output compared with the commanded input. It is actually a measurement of inaccuracy and it is typically measured in millimeters. (2) motion accuracy: The maximum expected difference between the actual and the intended position of an object or load for a given input. Its value depends on the method used for measuring the actual position. (3) on-axis accuracy: The uncertainty of load position after all linear errors are eliminated. These include such factors as inaccuracy of leadscrew pitch, the angular deviation effect at the measuring point, and thermal expansion of materials.
backlash: The maximum magnitude of an input that produces no measurable output when the direction of motion is reversed. It can result from insufficient preloading or poor meshing of gear teeth in a gear-coupled drivetrain.
error: (1) The difference between the actual result of an input command and the ideal or theoretical result. (2) following error: The instantaneous difference between the actual position as reported by the position feedback loop and the ideal position, as commanded by the controller. (3) steady-state error: The difference between the actual and commanded position after all corrections have been applied by the controller.
hysteresis: The difference in the absolute position of the load for a commanded input when motion is from opposite directions.
inertia: The measure of a load’s resistance to changes in velocity or speed. It is a function of the load’s mass and shape. The torque required to accelerate or decelerate the load is proportional to inertia.
overshoot: The amount of overcorrection in an underdamped control system.
play: The uncontrolled movement due to the looseness of mechanical parts. It is typically caused by wear, overloading the system, or improper system operation.
precision: See repeatability.
repeatability: The ability of a motion control system to return repeatedly to the commanded position. It is influenced by the presence of backlash and hysteresis. Consequently, bidirectional repeatability, a more precise specification, is the ability of the system to achieve the commanded position repeatedly regardless of the direction from which the intended position is approached. It is synonymous with precision. However, accuracy and precision are not the same.
resolution: The smallest position increment that the motion control system can detect. It is typically considered to be display or encoder resolution because it is not necessarily the smallest motion the system is capable of delivering reliably.
runout: The deviation between ideal linear (straight-line) motion and the actual measured motion.
sensitivity: The minimum input capable of producing output motion. It is also the ratio of the output motion to the input drive. This term should not be used in place of resolution.
settling time: The time elapsed between the entry of a command to a system and the instant the system first reaches the commanded position and maintains that position within the specified error value.
velocity: The change in distance per unit time. Velocity is a vector and speed is a scalar, but the terms can be used interchangeably.
Some of these terms are used more often than the others, some causes confusion and mix-up from time to time. However, you master these terms early on, and controls engineers will be delighted.
We will be talking about some of these terms in later posts when appropriate.
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