Precision and accuracy

Sept. 1, 2004
Motion System Design talks with industry experts to find out exactly what you need to know about precision and accuracy.


Andy O’Connell, V.P. Operation/Engineering, Rockford Ball Screw Co., Rockford, Ill.
Tom Solon, Kerk Motion Products Inc., Hollis, N.H.
John Santos, Mark Lacroix, Intelligent Products and Systems Group, The Timken Co., Canton, Ohio

Define precision and accuracy. What is considered leading edge?

Kerk Motion: Precision and accuracy are the exactness with which something is done in the context of a specified requirement. The requirements are different for every industry. Semiconductors have moved from submicron to nanotechnology and lead the way in positional accuracy and cost. Printing and scanning require uniformity and synchronization throughout their processes, rather than at discrete endpoints. The medical industry has a lower level requirement in absolute terms, but failures come at a higher cost.

Timken: Precision generally refers to system repeatability, which is determined by both resolution and accuracy. Resolution is the number of incremental positions or steps within the control range. Accuracy is how close each incremental step is to its ideal position.

In precision automotive and industrial sensor products, there are typically two types of accuracy — relative and absolute. Relative accuracy is the distance between two adjacent incremental positions as compared to the ideal distance. Absolute accuracy is how close a particular position is compared to its ideal position over the full control range.

Rockford: Precision and accuracy in ball screws is the difference between the nominal travel and actual travel. It’s expressed as inches per foot or millimeter per 300 millimeters. Leading-edge accuracy today is in the range of 0.002 in./ft for ground ball screws. Recent advances in roll-threading technology allow the roll-thread process to produce screws that in the past would be considered ground screw accuracies.

In what applications are precision and accuracy most important?

Rockford: Machine tools (such as CNC lathes and mills) require highaccuracy ball screws for positioning and repeatability.

Kerk Motion: As geometries become more compact, the importance of precision and accuracy increases. In electronics, high precision and accuracy are necessary to maintain function in miniature circuits. In the medical field, accuracy is vital for proper dosing, accurate laser placement, and correct diagnosis of illnesses — which can mean the difference between life and death.

Timken: The importance of precision and accuracy is often proportional to how much users notice their effect. For example, in human transportation systems, accuracy is measured in micro-g levels because the human body is particularly sensitive to motion that includes acceleration. Thus, in automotive and other transportation applications the performance of the mechanical and electrical systems is subjective and based on user perception.

What are limiting factors in a motion system when it comes to precision and accuracy?

Rockford: A number of factors can limit the axial accuracy of a ball screw. Many designers confuse linear accuracy and repeatability. Because of backlash, inadequate preloading of the ballnut can cause positioning inaccuracies that many times are blamed on lead accuracy. Other factors can affect lead accuracy, such as excessive friction from inadequate lubrication and ambient or built-up heat from thermal expansion. Some manufacturers account for this by including ports in the ballnut and hollow screws through which coolant can be pumped to negate thermal expansion.

Kerk Motion: Scale and speed are big challenges: scale because of the need for fine resolution of position and the space to start, stop, and confirm, and speed because the laws of physics constantly work against changing the motion. Changing aspect ratios and shapes helps create space. Reducing mass cuts inertia and improves response time. Eliminating components opens up space, cuts mass, and decreases the tolerance stack.

Timken: Uncertainty about system performance, which is difficult to predict, often results in system over-specification, which results in further subsystem overspecification. In these cases, accuracy and precision requirements are frequently well beyond those ultimately needed. Testing a similar or earlier version of the system can usually eliminate this. Often, a simple system tolerance stack will identify problem areas that can be addressed more cost effectively than over-specifying accuracy and resolution.

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What should engineers know about component interactions and how they affect precision and accuracy?

Kerk Motion: Know that the whole is more than the sum of the parts. Don’t make any selections in isolation. Be sure to evaluate the system as a whole.

Rockford: Many times, linear inaccuracy in ball screws can be compensated for in the machine controller through lead error compensation. Many screws come with a lead error chart that can be used to program out lead error. Machines that use glass scales for the primary feedback system actually measure the travel of the table and don’t rely strictly on the drive screw for accuracy. In this case, a more cost-effective, or less accurate, screw may be selected.

Timken: Tolerances and clearances limit the useful resolution and accuracy of a system. However, system performance can often be improved by careful design of component interactions.

What tips can you provide to help engineers optimize precision and accuracy in their design?

Kerk Motion: Use the resources available. Suppliers have a vested interest in your success: Use their experiences so you don’t repeat the mistakes of others. Manufacturer’s representatives are often good resources as well because they can package components from many sources. The right system of complementary components and technologies beats a bad combination of superstars.

Timken: Look at each component of the system one at a time and consider its sensitivity toward system performance. Then rank them according to the value of upgrading or downgrading it. Also note that with close inspection, some components can often be placed outside the control loop.

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