Here we investigate how certain high-end grinding applications have spurred the pairing of direct-drive torque motors with modular encoders.
Myriad applications require custom torque motors — including hard-machining table drives, mirror millers, and measuring and grinding machines.
In fact, machine builders for these applications are now looking to leverage the high performance of newer direct-drive torque motors to replace hydrostatic bearings in applications where forces approach the motors diagonally — in hard machining or milling of mirrored objects, for example. Why? Some of the units can resist tilt better than comparable hydrostatic components.
As we'll explore, such applications require unique arrangements for feedback — in the form of angle encoders that come in a variety of scale drum diameters — to allow designers to install the encoder as close as possible to the bearing, and minimize error when a bearing tilts.
Motors for high-end grinding
Consider the manufacture of eyeglasses, which demands mill surfaces without vibration marks, or ponder the lithographic processes in the semiconductor industry, which require aspherical lenses up to 500 mm in diameter.The grinding machines used to manufacture these pieces must be very precise. Traditional setups include power-transmission components, but as direct-drive performance increases, torque motors are slowly proliferating in these grinding machines because of their ability to position more quickly and precisely.
More specifically, a new breed of iron-core torque motors is now being used in these applications.
Traditional iron-core units offer greater power density than ironless motors, but their magnets and slotted sheet metal normally generate cogging forces that can be measured as pulses on the motor shaft. These in turn cause short-wave flaws when grinding mineral lenses — which render lenses useless. In contrast, some newer iron-core motors maintain characteristically high power density and incorporate a multi-pole design, an optimized sheet metal and stator winding technology, and special stator sheets and magnets to minimize cogging forces.
Encoder to match performance
To maintain system accuracy, the drives require equally accurate measuring devices. To this end, in some setups, modular angle encoders are used to track angular position to within a few angular seconds. These angle encoders come without an integral bearing, so the scale drum and scanning head are separate components. More specifically, a regular graduation structure (that carries the position information) is applied to the scale drums.
Here, mark homogeneity and edge definition are high, while single-field scanning with the optical filter structure enables consistent output-signal quality over the entire circumference of the drum. (Subdivision accuracy values are significantly better than ±1% of the signal period.)
With other designs, signal interpolations of up to 4,096 are usual, but these encoders have 24,000 lines to enable servo controllers in a optical-lens grinding machine to operate with 16,384-fold interpolation.
In addition, an absolute reference is required to ascertain positions, so the scale drums have an additional track that bears distance-coded reference marks.
These reference marks are individually spaced according to a mathematical algorithm, so the axis rotates only slightly before the scanning unit crosses two successive reference marks and ascertains absolute position.
Modular encoder installation
During its installation, scale drums and motor are aligned with the help of a digital readout. (Dial gauges aren't precise enough for this job, and probing with them risks damage to the encoder.) Scale-drum-to-motor concentricity can be set to 0.5 µm — approximately the graduation accuracy of the encoder.
A second measuring head is applied for alignment and is removed afterwards, reliably transferring the encoder's specified accuracy to the motor. The alignment itself requires a maximum of 20 minutes.
In grinding applications in particular, high-precision alignment is necessary because mounting inaccuracies are immediately visible on the manufactured product.
For more information on modular encoders, call (800) 233-0388 or visit heidenhain.us/rotary_encoders.php. For more information about the motors detailed in this piece, visit schuessler-technik.de.
Bearings must also maintain accuracy
The modular encoder design just detailed allows its scale drum to be placed very close to a motor's bearing, which minimizes the effect of error from any bearing tilt — in turn maintaining measuring accuracy. One caveat: Bearing rigidity also affects measuring accuracy, so stiffer designs allow for more accurate measurements.
After smooth motor operation, bearing quality has the most significant influence on motor-axis pulsation height. Therefore, on its HGE torque motors, Schüssler-Technik installs custom axial/radial roller bearings exhibiting pulsation of less than 0.01 µm — comparable to that of hydrostatic bearings. This minuscule pulsation is attained quality bearing raceways and tightly controlled roller geometry. The result is an iron-core torque motor with high power density, but with total axial axis pulsation (from bearing and motor) of less than 0.02 µm.
Motor bearing tilt resistance is also key, particularly for grinding machines that manufacture mineral lenses with diameters up to 500 mm and weights up to 50 kg for the semiconductor industry. Here, precision bearing geometry restricts tipping error to 0.2 µm thanks to weight transfer during motor operation. The maximum axial and radial runout of the axes is 0.3 µm.
Andreas Fuchs, Dipl. Ing. (FH)
DR. JOHANNES HEIDENHAIN GmbH Traunreut, Germany
Head of R&D for direct drives