Re-examining Precision and Repeatability Criteria in Actuator Selection

One of the main design challenges faced during the early design stages of automated mechanical systems deals with the precision and repeatability needs of components and sub-components.  Sometimes, these two design considerations tend to dwarf all others in becoming the only items considered in the selection of an actuator for mechanical system design.  The temptation is to simply select an actuator rated to the highest possible degree of precision, rated to the highest degree of reliability.  However, this approach is fundamentally flawed and can result in the engineer designing for an actuator with un-needed levels of precision, adding significant, unnecessary costs and complexities to the design.

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MISUMI LS Series Medium Accuracy Actuator

The antidote for this pitfall lies in understanding the fundamental differences between the definitions of precision and repeatability in actuator selection.  Repeatability is a function of the degree to which the actuator is able to duplicate a commanded position, quite literally a measure of the reliability of the actuator in a sense.  Precision, in contrast, is a measure of the smallest incremental resolution that the actuator is able to achieve.  The key to actuator selection lies in understanding when high degrees of precision are truly needed and when more moderate precisions coupled with a machine that can achieve these positions to a high level of repeatability can perform the same job for significantly less cost.  The concept of repeatability, however, requires a bit of explanation due to the fact that the number found on a typical actuator specification sheet is quoted as a plus/minus tolerance.  This number literally means that according to the defining statistical standard, ISO 9283:1998, the actual “true” position of the actuator will fall within this range 99.8% of the time.

Generally speaking, a typical “high-precision” positioning actuator operated by a stepper motor is able to achieve positions in the range of 1 m (~40in), in some cases, even more precision is achievable.  However, key limitations make the device only useful for a narrow range of applications.  Typically devices such as these are only rated for loads on the order of 20-30lb, making the positioning of  large equipment and machinery to these levels difficult and costly to achieve.  These types of actuators are also significantly more expensive than standard single axis actuators; for a standard actuator with a positioning precision around 40 m (~0.0016 in), the cost can be as much as 40% less than a similar micro-positioning actuator.  Actuators rated to these more modest precision levels such as these are also capable of operating under much higher (10-20X) loads.

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MISUMI LX Series High Precision Ball Screw Actuator

Without citing specific numbers, the same rule is generally applicable to repeatability specs.  Given an actuator with a high precision rating, the actuator is de facto placed under higher repeatability demands in order to meet these levels of precision.  For example, a “high precision” actuator with a precision of 40m (0.0016in), possesses a typical repeatability around plus/minus 4m (~0.0016in), which is ten times the precision of the actuator.

In light of these specifications, it is essential that the engineer carefully select an actuator that is appropriate for each desired application.  The “high precision” positioning actuators are typically only employed in industries such as printed-circuit and medical equipment manufacturing as well as precision industrial inspection devices.   Unless the system must operate on these sorts of microinch scales, a “high-precision” actuator will be unnecessary and excessively costly.  In contrast, more traditional uses of mechanical actuators would encompass applications that are designed to achieve simple lifting and positioning functions, akin to something like a screw jack stand.  These sorts of macro-sized automation application needs have driven advances in more traditional actuator technology that has yielded a class of actuators that is able to achieve the levels of precision necessary for such applications very reliably.  In these sorts of applications, reliability actually ends up being the more limiting factor in order to ensure that the actuator can achieve the intended positioning every time that it is required to do so.

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MISUMI MSA Series Belt Drive Actuator

In conclusion, it’s important to remember that today actuators that are fabricated from advanced manufacturing methods such as precision screw thread grinding are available in quality materials such as high strength stainless steels and are able to cost effectively achieve excellent levels of precision extremely reliably, even for high load applications.   Additionally, selecting an actuator solely on precision without carefully considering the application can lead to an over designed, expensive system that could have been achieved with significantly less cost by employing a quality, slightly lower precision actuator.  Admittedly, these sorts of actuators may not be able to match the precision of a comparable “high precision” stepper motor driven actuator; nevertheless, the engineer can be confident that these actuators will be more than sufficient to meet  both the precision and reliability needs of a vast majority of automation applications.

 


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