Encoder Fights Thermal Drift

Even a little bit of thermal drift can present a big problem for production machines that require dead-on positioning accuracy. HEIDENHAIN has now come up with a solution that can help machine builders cope with errors related to this heat-induced growth of machine elements.

The company’s new “1Dplus” encoders feature a second, shorter set of graduations perpendicular to those found on a conventional linear scale. Those extra marks, plus the addition of a second scanning head, allow the simultaneous measurement and ultimate correction of linear guiding errors in two dimensions rather than the usual one. On a 2D stage, for example, this capability would translate to the measurements of both the X and Y axes using a single encoder.

According to Kevin Kaufenberg, product manager for HEIDENHAIN’s electronics division, the ability to measure two dimensions simultaneously is particularly useful when accounting for thermal drift. “Heat from the motors and guideways of one axis usually affects other axes as well,” he notes.

On the 1DPlus encoders, the long set of graduations would handle the measurement duties of a traditional linear encoder, while the second short set of graduations add a “thermal compensation track,” Kaufenberg says. Users would typically combine the data from the two sets of graduations, creating a map of a guideway’s thermal behavior. “The map can be stored in the controller as a basis for making corrections,” he says.

HEIDENHAIN’s initial 1Dplus products fall into the company’s LIF 400 family of interferential linear encoders. Their measuring standard is DIADUR with a thermal expansion coefficient of 0 ppm/K. Current measuring length is 300 x 2 mm, though Kaufenberg says the lengths will increase over time.

The 1Dplus scale itself is 20 x 4.9 mm. The X-axis measurement of the1Dplus has a accuracy grade of ±1 µm and includes a reference mark. The grating period of the encoder is 8 µm with a signal period of 4 µm.

The company can also supply the new encoders with a third scanning head to measure the angle of rotation of the bracket that houses the scanning units.

Applications include stacked stages, precise gantries, semiconductor wafer processing machines, and large flat-panel-display production and test equipment. “It could really be used by any motion stage user who needs to increase performance by offsetting thermal growth,” Kaufenberg says.