What's the difference between repeatability and accuracy in linear motion systems?

Accurate and repeatable,often used interchangeably.When someone says a ball screw is very accurate,they may really mean it is repeatable.In fact,accuracy and repeatability are unrelated.A system can be very accurate,but not very repeatable,and vice versa.Here's the difference:

Accurate,but not repeatable.

●Accuracy

Accuracy is formally defined as "the degree to which a measurement,calculation or specification conforms to a correct or known value or standard".For linear drive systems,this can be understood as the degree to which the final position matches the commanded position.So,if we command a rack and pinion system to travel 535 mm,its accuracy depends on how close it comes to achieving 535 mm of motion.But how accurate is it?It depends on the amount of error you can accept.If the application allows a result of 535 mm ±2 mm,then the ball screw can be considered accurate as long as it achieves a position between 533 and 537 mm.

Accuracy is most affected by mechanical factors such as backlash,"windup"(lack of torsional stiffness)and component bending.On the electrical side,the bandwidth of the control system and the resolution of the measurement system(encoder or resolver)also affect the accuracy of the drive motion,as these components are responsible for commanding,reading and correcting the actual position of the system.

●Repeatability

Repeatability is the ability of a drive mechanism to return to the same position multiple times under the same conditions.Repeatability can be defined as unidirectional,in which points are always approached from the same direction,or bidirectional,in which points can be approached from either direction.Unlike accuracy,which is somewhat subjective depending on the application requirements,repeatability is an absolute value.For example,a ball screw can be described as repeatable to ±10 microns.Just as a system can be accurate but not repeatable,it can also be repeatable but not accurate,For example,if a movement of 535 mm ± 2 mm is specified and the system consistently moves to 537.5 mm on multiple attempts,it is repeatable but inaccurate.

Repeatable,but not accurate

The biggest factor affecting repeatability is the mechanics of the drive system:backlash in the rack and pinion system or lead deviation in the ball screw threads.Repeatability can also be affected by system variations,such as temperature fluctuations that cause components to expand or contract.Although accuracy can be compensated for by programming in the drive amplifier and control,repeatability is usually not possible.

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