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Piezo Actuator Resolves To 10-6 Meter
Ultra-precise screw positioner simplifies alignment of optical elements
By Mark A. Gottschalk, Western Technical Editor -- Design News, March 27, 1995
Sunnyvale, CA- Photonics engineers measure success with the finest of rulers-wavelengths of light. In the laboratory, these measurements require optical elements of the highest precision positioned to sub-micron accuracy. Aligning such optical-bench setups can be nightmarish. Simply touching an adjustment knob may ruin the alignment of a lens or mirror that previously required minutes of tedious tuning.
"Everything is rubber at the scale we're talking about," says Frank Luecke, vice president and head of engineering at New Focus. Inspired to ease the adjustment process, Luecke invented the Picomotor, a patent-pending piezo-driven actuator that provides sub-micron resolution without backlash, hysteresis, or creep.
It consists of an 80-pitch, 3/8-inch-diameter screw clamped between two spring-loaded, aluminum-bronze jaws. A piezoelectric transducer drives the jaws in opposite directions when voltage is applied-the greater the voltage, the larger the displacement. Cutting voltage relaxes the transducer, and spring tension returns the jaws to their original positions.
To cause the screw to rotate, and not simply oscillate, the jaws move slowly in one direction and then quickly in the other. Slow movement turns the screw, while fast movement-due to inertia-slides the jaws against the screw and causes no rotation. The underlying principle is the same as that which allows a magician to yank a tablecloth off a table without disturbing the china.
To control the Picomotor, an electronic driver module generates 120V waveforms. A waveform with a fast rise time and slow decay turns the screw counterclockwise, while one with a slow rise and fast decay turns it clockwise. Holding position requires no energy.
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Telescope optics
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Lab instruments
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Precision mechanisms
Each waveform rotates the screw approximately 115,000th of a turn and moves the screw less than the rated 0.1æm. At its maximum rate, the screw turns about 3 rpm. The waveform shape that produces the best motion is not obvious. After accounting for the spring force, friction, inertia, and viscosity of the lubricating grease, what began as a simple sawtooth evolved into a complex and proprietary shape. "We've seen a tenfold increase in speed over the sawtooth with the current waveform," says Luecke.
A sophisticated power supply interacts with the piezo transducer to reduce voltage and power requirements. It exploits the piezo's capacitance to flush energy in and out of the Picomotor with each waveform pulse.
New Focus isn't the first company to offer fine trim for optical mounts. But backlash, creep, limited travel, or the need for continuous power handicapped previous solutions.
New Focus offers Picomotors with screws from 0.5-inch to 2-inches long and has incorporated the motor into pure-rotary stages as well.
Additional details...Contact Frank Luecke, New Focus, 1275 Reamwood Ave., Sunnyvale, CA 94089, (408) 734-8988, FAX: (408) 734-8882.
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