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LINEAR MEASUREMENT TECHNIQUES

Often, in industrial motion control and positioning applications, the need for direct feedback of a driven linear position is required.  Though it is quite common, and relatively simple, to gauge the position of a displaced load through direct feedback from the drive motor assembly, inaccuracies can occur using this technique over longer linear distances.

The usual method of using a servo motor's internal feedback encoder or resolver can be complicated by a number of factors. Resolvers are inherently single-turn devices and, as such, cannot be relied upon for accurate positioning beyond the linear translation of a single motor turn.  Even systems based on absolute, multi-turn encoders have their limitations over longer distances, as the method of translation from the motor's rotary motion to the linear motion of the load is not always reliable or accurate enough for the needs of the application.

As typically used in shorter-distance (<1 m) applications, ball or lead screw-coupled drive systems are usually quite reliable if sized and coupled correctly. For these systems, a drive-mounted or integral rotary feedback device is usually sufficient. For distances of a few meters, a linear position sensor, typically a rod or profile-style magnetostricive sensor, is a reliable and economical choice, though response time and accuracy can be challenging near their operational length limit (typically 3 m to 4 m). It is at distances of 5 m or more that optical, laser-based positioning systems become economically justifiable and preferred. Though other means of long-range position detection are possible, usually through reverse linear to rotary translation (translation of the load's linear position through rotary conversion as opposed to that of the rotary drive system to linear motion), they are often costly and lack reliability and durability.

Cable retractor-mounted multi-turn encoders, sometimes referred to as "rope encoders" or "string-pots" are limited by cable length and sag, and are constrained by their installation environments and operating speeds.  Rack and pinion systems, where a pinion gear is attached to an encoder, which then rides back and forth on a toothed rack, are costly to install and maintain, and often jump at higher speeds.  A similar system, often used on overhead crane and gantry systems, has a measuring wheel attached to an encoder, which then rolls along a flat surface.  The trouble here lies in the decreasing circumference of the wheel as it wears, and frequent slippage of the wheel on the roll path.

LIGHTING THE WAY

As demonstrated, few typical, mechanical systems exist that can accurately and reliably encode and transmit linear positions for use in a longer-range motion control or positioning system.  It's only with the introduction of relatively accurate, reasonably priced and easily-integrated optical position measurement systems to the industrial marketplace that the barriers to success in these application have been breached.