X-Y-Z: The ABCs of XYZ
This month marks the 25th installment of ASSEMBLY's X-Y-Z column. During my interviews with suppliers of motion control technology over the past 2 years, I often heard the same complaint: Many manufacturing engineers are confused about certain motion control terms. "If you could teach engineers the difference between accuracy and repeatability," one supplier quipped, "you'd be doing a great public service."
To be fair, suppliers of motion control technology do little to help their cause. For example, the accuracy specification for a multiaxis positioning system rarely states that the figure is for each axis, not all of them. Moreover, the specification may not reflect the system's performance under load.
This month's column is offered in the spirit of fostering communication between engineers and suppliers. And, since there's a direct relationship between the cost of a positioning system and its precision, knowing these terms could save you money on your next assembly system.
Point-to-point accuracy is the ability of a motion control system to reach a target position from a known reference point. Sometimes referred to as "system error," it is the maximum possible difference between the target position and the spot where the system actually moves. Accuracy is determined by measuring the system's ability to reach a target position over several attempts. The specified accuracy is the difference between the target position and the mean of the positions recorded during the test.
Straight-line accuracy is the ability of a machine to accurately travel in a straight line with respect to a known reference plane. It refers to the maximum possible deviation from the desired straight-line path.
Accuracy can be specified as unidirectional or bidirectional. Unidirectional accuracy is the system's performance when it approaches from only one side of the target position. Bidirectional accuracy is the system's performance when it approaches from both sides of the target position.
The accuracy of a positioning system depends on the accuracy of its components, such as the motor, drive mechanism and encoder. It's also affected by backlash.
Repeatability is the extent to which successive attempts to move to a specific location vary in position. A positioning system can be repeatable without being accurate, but it can't be accurate without being repeatable. Thus, if a positioning system has a repeatability of 0.001 inch, any series of identical movements will be within 0.001 inch of each other-but they won't necessarily be within 0.001 inch of the target position.
The repeatability of a positioning system is influenced by friction, torsional stiffness, load, speed, acceleration, deceleration and backlash. As with accuracy, repeatability can be specified as unidirectional or bidirectional. However, because of backlash, it's easier to achieve repeatability in one direction than it is in two.
A positioning system's resolution is the smallest distance that it can travel. Many of the same factors that determine a system's accuracy also determine its resolution. For example, the resolution of a ballscrew-driven system depends on the screw's pitch, as well as the performance of the motor, encoder and controller.
Backlash is the amount by which a driving shaft must rotate, when reversing direction, before it begins to transmit motion in the reverse direction. Sometimes referred to as "play" or "slop," backlash is caused by looseness in the gears, bearings, belts and other interactive elements of the positioning system. This looseness can come from poorly machined components, system wear or inaccurate preloading. When a positioning system is under load, this looseness is removed. Thus, systems designed to eliminate backlash are said to be preloaded.