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Permanent magnet limits rotary motion
High-speed, limited-stroke actuator offers an inexpensive alternative to fully motored solutions
By Norman Bartlett, European Editor (England) -- Design News, February 6, 1995
Keighley, UK-- Many applications for rotary motion control require only a partial rotation. When fitted to a brushless motor, mechanical stops accomplish the task, but waste energy. An alternative solution uses a permanent magnet in the actuator's armature to rotate less than 360 degrees in a single step.
At its simplest, the Permanent Magnet Brushless Torque Actuator (PMBTA) developed by Lucas Control Systems Products, carries two stator poles and four magnetic poles on the armature. In a situation where no power is applied to the stator, the armature poles rest at mid-stroke, where magnetic forces are in balance.
Applying a positive voltage polarizes the stator poles. Two of the armature poles are attracted, two are repelled, and the shaft rotates clockwise through 45 degrees. A negative voltage reverses the polarization of the stator poles. The armature poles change position, and the shaft rotates in the counter-clockwise direction.
By varying the number of stator poles and armature poles, strokes to 160 degrees are possible. Torque is proportional to the angle of rotation. With 45-degree operation, for example, torque is rated at 700 mNm. The torque value decreases in models that specify greater rotation.
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Office automation
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Automotive
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Medical
Besides limited stroke, the PMBTA offers maximum torque-or close to maximum torque-at startup. This characteristic, in turn, results in high starting acceleration and operating speeds as fast as 100 Hz. Lucas design engineer Jim Irwin describes the use of the PMBTA in a spectroscopy camera, where power to a solenoid holds the shutter open against a spring's closing force.
"With the PMBTA's high starting torque, the manufacturer plans on using dynamic braking for controlling the stopping shock," reports Irwin. "Heat can cause image distortion, but our unit will run cooler than the current shutter mechanism because of its size and the fact that it only needs power to change position."
At present, Lucas offers a 50-mm-diameter version of the PMBTA. Other sizes are planned for release later this year.
Additional details, Europe...Contact Roy Whitaker, Lucas Control Systems Products, Ingrow Lane, Keighley, West Yorksire BD21 5EF, England +44 535 661144.
Additional details, U.S....Lucas Control Systems Products, 801 Scholz Dr., Vandalia, OH 45377, (513) 454-2345.
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