Optimizing precision and accuracy

July 1, 2007
Remember the dartboard analogy? Darts landing near the bull's eye mean accuracy, whereas misfires landing close together, but far from the bull's eye,

Remember the dartboard analogy? Darts landing near the bull's eye mean accuracy, whereas misfires landing close together, but far from the bull's eye, are more indicative of precision or repeatability. Today's motion systems, of course, usually demand both. Consider these tactics and tools for hitting the mark each and every time.

Compact controller

AMKASYN controllers contain a real-time-capable PLC that ensures precision and accuracy. Integrating visualization into the PLC's development environment via the programming interface allows fast access to all status alerts and variables. A real-time bus system guarantees synchronous operation.

AMK Drives and Controls Inc.

Roller screws

Powerful new roller screws for linear actuation deliver accurate and precise reliability, sustaining static loads to 1,000 tons and dynamic loads to 200 tons. The planetary model has a timing mechanism to ensure reliability at high acceleration, while the recirculating type uses lead threads as small as 1 mm to optimize high-precision positioning.


Linear servomotor

The Linear Shaft Motor is a brushless, direct drive tubular servomotor consisting of two parts — a magnetic shaft and a moving coil assembly, which is controlled by current flow. An ironless forcer and shaft help achieve high precision and zero cogging. No lubrication or adjustment maintenance is required.

Nippon Pulse America Inc.

Compact incremental encoder

With a 1.5-in. diameter, the L15 incremental encoder is ideal for space or weight-limited applications requiring high accuracy. It offers design flexibility, producing a control-ready signal as fine as 40,640 cycles per turn. An internal electronic multiplication boosts output resolution by a factor of 16 with no decline in accuracy.

BEI Industrial Encoders

Low-profile rotary bearings

The FMB series of high precision rotary bearings is meant for rotary positioning systems where precision and height are critical. Specially designed raceways increase load capacity and stiffness, and achieve high precision, long life, and a low coefficient of friction. Nine standard sizes are available with OD's from 120 to 350 mm; custom sizes are also available.

PM Bearings, The Precision Alliance

DBO ϫ list

  • Regarding positioning, accuracy is how closely a system in motion approaches a command position; precision refers to the error between successive attempts to move to the same location; resolution is the smallest move increment that can be achieved

  • Precision means repeatability within a system, and is determined by both resolution and accuracy

  • Precision and accuracy requirements vary by industry; in semiconductor manufacturing, accuracy demands can be in the nanometer range

  • Know thine enemies — errors due to instability, errors in response to external disturbances, and errors stemming from mechanical transmission components

  • The most common transmission errors are from lead screws — backlash and nonlinearity

  • To handle disturbances, designers should make their system stiff and responsive, meaning the control loop must have high gain to make it reject disturbances

  • A system's ability to reject disturbances is related to bandwidth — if the system can follow command signals at high frequencies, it can reject equally dynamic disturbances

  • With regard to component interaction and system accuracy, remember that the whole is more than the sum of the parts; system accuracy can be improved by careful attention to component interactions

  • Transmitting motion over a distance inherently reduces accuracy, especially when motion is transmitted through linkages such as couplings, gearboxes, belts, and screws

  • Proper cabling and shielding can protect against electrical noise, optimizing accuracy

  • Consider materials like ceramics, glass, and engineered plastics for strength and corrosion resistance, which improves precision

  • Think about linear motors and direct drive systems to simplify the transmission chain and reduce system error

  • Integrated rack-and-pinion systems also lessen error from tolerance stack, boosting precision and accuracy

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