CNC machines push linear encoder limits
The bleeding edge limitation for CNC accuracy and reproducibility involves thermal expansion. One solution is to provide a second, shorter, encoder axis at right angles to the main axis. Here's how.
CNC machines need encoders for accurately controlling the traversing axes of a machine tool, according to Ryan Legg, product marketing manager for Sinumerick CNC systems at Siemens Industry. “You’ll typically find a rotary encoder physically on the back of each motor. High-end automated machines also have a linear encoder mounted on the machine axis itself.”
Computer numerical controlled (CNC) machines are specialized multi-axis motion control systems. Unlike the manually operated machine tools they replace, CNC machine tools typically have a separate motor to run each axis.
A horizontal-bed lathe, for example, could have four axes: spindle rotation, longitudinal motion parallel to the spindle axis, cross feed horizontal motion at right angles to the spindle axis, and vertical motion. In a manual lathe, one motor runs the spindle, and, through a transmission and lead screw, the longitudinal axis. The cross-feed and vertical motions were usually (although not always) driven by hand cranks operated by a machinist.
A CNC lathe, however, has four servomotors to separately drive the four axes. Each servomotor has a rotary encoder to track its motion, but those encoders can’t account for wear, slippage, or backlash in the mechanics. To hold the tolerances expected of these machines, which can exceed 0.0001 inches (2.54 microns), the lathe benefits from high precision encoders to track the actual cutting-tool position. Three out of the four motion axes are linear, and only one is rotational, so such a CNC lathe could use three high-precision linear encoders. The encoder technology of choice is the optical glass encoder.
The bleeding edge limitation for CNC accuracy and reproducibility involves thermal expansion. Operating temperatures of these machine tools varies with time. During start up or commissioning, or after an idle period, the machine’s structure is relatively cool. As the machine operates, temperatures rise, and metal components expand, introducing fabrication errors. Depending on design tolerances of the parts being fabricated, thermal expansion coefficients of the materials used in machine-tool construction, and the linear dimensions involved, thermal expansion can easily exceed allowable tolerances for machined parts.
One solution, as shown by the 1D plus encoder from Heidenhain Corp., is to provide a second, shorter, encoder axis at right angles to the main axis. This second axis can be used to sense movement — especially due to thermal expansion — of the cutting tool simultaneously with motion on the main axis.
Suppose a 1 m encoder is used to sense tool movement along the x axis. The actual position will be due to the intended motion, plus a modification due to thermal expansion. The cross-axis reading, however, will be due to thermal expansion alone. As thermal expansion in isotropic materials is, well, isotropic, the CNC controller can use the thermal expansion sensed by the cross axis measurement to calculate a correction to the direct-axis measurement, removing the thermal-expansion error.
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- C.G. Masi is a contributing editor for Control Engineering, www.controleng.com. Reach him at firstname.lastname@example.org.
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