Copley Controls: Low inertia boosts linear motor versatility

The virtues of direct-drive linear (DDL) motor technology are becoming better known, albeit slowly. Adding a low-inertia feature to these motors opens new possibilities in applications that need fast, accurate, short-distance moves—notoriously difficult to do via a lead screw or similar alternative.

By Control Engineering Staff October 23, 2003
Micro Linear motors come with thrust rod lengths from 62 to 510 mm (2.44-20 in.)

The virtues of direct-drive linear (DDL) motor technology are becoming better known, albeit slowly. Adding a low-inertia feature to these motors opens new possibilities in applications that need fast, accurate, short-distance moves—notoriously difficult to do via a lead screw or similar alternative.

Advanced polymer construction helps deliver light weight and low inertia, critical for fast response, in Micro Linear motors from Copley Controls Inc . These DDL motors produce continuous force in the range of 3.1-8.7 N (0.7-1.57 lbf), and peak forces developed are about five to six times higher. Peak acceleration ranges from 15 to 25 m/s

Yet the motor construction remains simple. Each Micro Linear motor consists of a tubular thrust rod containing high-energy permanent magnets and a forcer , which surrounds the thrust rod and carries the payload directly. The forcer contains encapsulated electrical drive coils and is supported by one set of low-friction bearings. A 0.016-in. symmetrical air gap between rod and forcer eliminates friction and wear, according to Copley. It allows the digitally controlled magnetic fields to propel the forcer along the thrust rod’s length without backlash. Magnetic symmetry of the tubular design keeps drive force constant throughout forcer’s travel.

Micro Linear motors reportedly make repetitive 1 mm X-Y movements with 1-micron accuracy in 0.025 seconds. Cost is comparable to lead screw drives. Other notable features include:

  • Simpler servo control and braking for rapid starts and stops, without detrimental stick/slip;

  • Relaxed forcer-to-thrust-rod alignment and concentricity specs, due to the 0.016 in. air gap; and

  • Better cooling as the tubular construction allows heat transfer from the thrust rod’s full circumference.

According to Copley Controls, the requirement to provide simultaneous rapid, accurate, and short-distance travel raises the technical challenges exponentially. Pricing of Micro Linear motors (forcer and thrust rod) starts at $235 for quantity 1–9. Delivery from stock is 4 weeks ARO.

—Frank J. Bartos, executive editor, fbartos@reedbusiness.com