|Rotary table construction|
Torque motors are also called "frameless" motors. They don't have housings, bearings, or feedback devices. In this sense the motor is a kit and meant to be part of the machine structure. Torque motors are designed as direct drives. They eliminate the need for gearboxes, worm-gear drives, and other mechanical transmission elements and directly couple the payload to the drive. This makes rotary drives with high dynamic responses and no hysteresis possible. The large inner diameter of a torque motor is a big advantage in machine tool design. Large hollow shaft allows designers to design rotary table to support bearings, feedback devices and payloads.
Torque motors are available in a wide range of sizes, with diameters from smaller than 100 mm to larger than 1 m. Motor diameter is similar to frame size for conventional brush-less servomotors. And for a given diameter, several axial lengths are available.
Torque motors have a large number of magnetic pole-pairs with many permanent magnets on the rotor. Torque motors can be built as thin rings. They have smooth velocity modulation with low ripple. Eddy-current losses in torque motors constrains the maximum practical number of pole pairs and speed. As a result, torque motors are primarily designed for low/mid-speed range applications, generally below 1,000 rpm.
Torque motors produce high torque at stall and can have high dynamic stiffness. However, the motor alone does not determine dynamic stiffness or precision. To exploit the full benefits of direct-drive, an integrated rotary table must also contain precision feedback device and high stiffness bearing to provide a high-performance rotary stage.
High-precision, high-resolution feedback is essential for optimal performance of rotary tables. Because loads are directly coupled to the drives, higher accuracy is possible. Rotary table positioning resolution is also in direct relation to the feedback's resolution, so it takes an encoder with a high line count and a high resolution interpolation factor. System resolution below 1 arc-sec is achievable in direct drive rotary stages..
An important difference between direct-drive rotary tables and those driven by conventional dc servomotors and gearboxes is that torque motors are found inside the rotary table axis and are part of the machine. Most of torque motors include provision for liquid cooling which effectively increases the continuous torque rating of the motor and higher power rating of the rotary table. Air cooling, while an option, is much less effective than liquid-based cooling.
A torque motor is only one element in a complete rotary stage. It still needs a mechanical structure with high rigidity and precision bearings. It is an overall integration of these elements that determines complete rotary stage performance.
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