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Motion System Design

Hollow actuator precision

Many precision applications are moving away from actuators with multistage gearsets and belts toward integrated motor mechanisms. These are especially useful for rotating large tables or swiveling robot arms — typical movements in semiconductor manufacturing, electronics assembly, and visual measurement systems. Though expensive brushless permanent-magnet motors (with high magnet pole counts) are used in many new direct-drive systems, hollow-rotor actuators accurately and quickly position and swivel. The hollow inside diameter also helps simplify mechanism wiring.

This month's handy tips provided by Nick Johantgen, Oriental Motor, Torrance, Calif. For more information, visit or contact [email protected].

Q & A

How do hollow actuators withstand heavy loading?

The key to higher load capacity and rigidity is the bearings. Smaller actuators use deep-groove ball bearing systems, while larger units (80 to 130 mm) employ higher-capacity cross-roller bearing systems inside the output table structure. These cross-roller bearings include mutually perpendicular rollers between inner and outer rings, so because the rolling surface is a line instead of point contact, it exhibits higher load capacity and rigidity.

What's their precision?

Precision is boosted with single-stage gear trains, particularly if they're preloaded to reduce backlash; in some cases, this is less than 2 arc-min. Unidirectional repeatability is to 15 arc-sec. On step motors under closed-loop control, built-in rotor position sensors provide velocity and position information, as well as driver commands, eliminating the most deadly problem a traditional open loop step motor can experience — loss of synchronism when subjected to an abrupt load fluctuation or jerk acceleration. The built-in rotor position sensor constantly verifies rotor position. When the motor is about to lose synchronism, the closed loop control is used, correcting the motion problem.

How do other technologies compare?

Sometimes belts transmit motor torque to the equipment axis. Traditional timing belt/pulley mechanisms handle high load, possess large hollow I.D.s, and use an offset rotation axis away from the motor axis. But belt expansion and contraction cause position error and lost rigidity, and tension adjustments are often required.

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