Accuracy or repeatability - which does my system need?

Question: “I’m designing a new linear motion system. Should I design it for high accuracy or repeatability? Or both?”

Before we answer this question, let’s define accuracy and repeatability for linear systems…

Accuracy

In linear motion, there are two generally categories of accuracy – positioning accuracy and travel accuracy. Positioning accuracy specifies the difference between the system’s target position and the actual position that it reached. Travel accuracy specifies errors that occur during movement – in other words, does the system travel in a straight line, or does it move up-and-down or side-to-side as it travels?

Accuracy is given in relation to a “true” or accepted value or reference. For positioning accuracy, the reference value is the target position. For travel accuracy, the reference value is a defined plane of motion in both the vertical direction (aka the flatness of travel) and in the horizontal direction (aka the straightness of travel). Note that accuracy relates to how closely the target position is reached when approaching from either direction.

Repeatability

Repeatability defines how closely a system returns to the same position over multiple attempts. Repeatability can be specified as either unidirectional, which means the specification is valid when the position is approached from the same direction, or bidirectional, which means the specification is valid when the position is approached from either direction.

Component selection and machine design affect system accuracy and repeatability

Linear systems are made up of four basic components – the base or mounting structure, the linear guide (or guides), the drive mechanism, and the motor – and each of these plays some role in the system’s accuracy or repeatability. Secondary components such as couplings, connectors, mounting plates, sensors, and feedback devices also influence the system’s performance. And even factors that aren’t easily controlled, such as temperature fluctuations and machine vibrations, affect a system’s accuracy and repeatability specifications.

When working to maximize positioning accuracy, the drive mechanism should typically be the area of focus. Ball screws are generally recognized as the best choice for high positioning accuracy, which is specified by their lead error, or tolerance grade, classifications. But lead screws with preloaded nuts and high-precision rack and pinion systems are also capable of providing high positioning accuracies. Flexing and vibrating of the system can degrade positioning accuracy, so the stiffness of the mounting structure, linear guide, and the connections between components is also important for systems that require high positioning accuracy.

In contrast, a system’s travel accuracy is almost entirely dependent on the mounting structure and linear guide system. Most recirculating linear guides are specified by accuracy class, which defines the maximum deviations in height, parallelism, and straightness during travel. But a linear guide is only as “accurate” as the surface to which it’s mounted, so the mounting structure is an important factor. Mounting a “precision” accuracy linear guide to an unmachined base or an aluminum extrusion negates the guide’s travel accuracy performance.

Predictable, measurable factors that influence a system’s positioning and travel accuracy, such as screw lead deviation or profiled rail height, can be compensated to some extent through the feedback and control system. In systems with closed-loop, servo control, it can be more cost-effective to choose lower accuracy mechanical systems and use the servo system to improve accuracy through error mapping in the control.

The repeatability of a linear system is determined primarily by the drive mechanism – that is, the lead accuracy of a screw, the tooth pitch deviation and maximum stretch of a belt, or the backlash in a rack and pinion system. The best way to improve repeatability is to remove play, or clearance, in the drive mechanism. Ball screws are often specified with preload to eliminate backlash, and many lead screw designs also offer zero backlash. Rack and pinion systems inherently have backlash between the gear rack and the pinion teeth, but dual pinion and split pinion designs remove this backlash.

If the system experiences significant temperature fluctuations, the expansion and contraction of components due to thermal effects can also reduce a system’s repeatability. Unlike positioning or travel accuracy, the repeatability of a system cannot be improved through feedback and control. The only way to improve the repeatability of a linear system is to use a drive that has higher repeatability.

Whether a designer or engineer should be more concerned with accuracy or repeatability depends on the type of application. In positioning applications, such as pick and place or assembly, positional accuracy and repeatability are often the most critical factors. But in applications such as dispensing, cutting, or welding, where uniformity and accuracy of the process during travel is critical, travel accuracy should be the primary focus.