Improved Position Sensor for Feedback Control of Levitation
- Created: Friday, 01 October 2004
In this application, an incandescent light bulb is preferable to a laser.
An improved optoelectronic apparatus has been developed to provide the position feedback needed for controlling the levitation subsystem of a containerless processing system. As explained below, the advantage of this apparatus over prior optoelectronic apparatuses that have served this purpose stems from the use of an incandescent lamp, instead of a laser, to illuminate the levitated object.
In containerless processing, a small object to be processed is levitated (e.g., by use of a microwave, low-frequency electromagnetic, electrostatic, or acoustic field) so that it is not in contact with the wall of the processing chamber or with any other solid object during processing.In the case of electrostatic or low-frequency electromagnetic levitation, real-time measurement of the displacement of the levitated object from its nominal levitation position along the vertical axis (and, in some cases, along one or two horizontal axes) is needed for feedback control of the levitating field.
In a typical prior position-measuring optoelectronic apparatus for levitation control, a laser beam is aimed through the nominal levitation position to a position-sensitive photodetector, so that the levitated object casts a shadow on the detector face. The output of the position-sensitive detector circuitry is a voltage proportional to the displacement of the shadow from a nominalcentral position and, hence, to the displacement of the levitated object from the nominal levitation position. The main shortcomings of such an apparatus are associated with the laser: Lasers tend to be expensive, and laser beams are often characterized by poor spatial distribution (speckle) and by poor short-term spatial and temporal stability.
The basic principle of operation of the improved optoelectronic position-measuring apparatus is the same as that of the prior apparatus described above, except that the laser is replaced with an incandescent light bulb and associated optics (see figure). Unlike the light from a laser, the light from an incandescent lamp is not subject to mode shifts and, hence, is spatially and temporally stable in the short term. The spatial distribution of light is better for the intended application because there is no speckle and the illumination is approximately constant across the levitation region. Unlike the light from a laser, the light from an incandescent lamp varies smoothly with applied power and is thus more easily controllable. Finally, a light bulb is easily replaceable and costs much less than does a laser.