Technologies In Motion

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.

Disney's Integrated Safety Makeover

In an indication of just how seriously North American engineers are taking integrated safety in applications where life and limb are on the line, Walt Disney Imagineering and Siemens Energy & Automation have together been working on a PLC-based safety system for busbar powered rides.

For the past 30 years or so, Disney has entrusted some of safety tasks on these rides to a custom Power Bus Controller of its own design. According to Jody Gerstner, Imagineering’s executive director of show and ride controls, the controller provides a "redundant, testable" way to remove power to all or part of a ride in the event of a safety fault.

Literally a black box located in a ride’s control cabinet, the controller communicates with various sensors–such as pressure mats–located on and around a ride’s moving parts. Gerstner describes the controller as "hierarchical based on the severity of the fault."  Minor faults might disconnect power locally while major faults can quickly remove power from the entire ride.

Currently in its third generation, the system has been used on Disney’s on Toy Story Midway Mania, a newly opened attraction at Disney’s California Adventure (read the full story about it and watch video footage of the attraction). Yet, Disney is now looking at ways to integrate the bus controller’s safety functionality into ride control PLCs. "I believe there are opportunities to simplify our safety systems using integrated safety. It’s something we’re definitely looking at for the future," Gerstner says.

And that future may not be very distant at all. Siemens, one of Disney’s corporate technology partners, has already integrated Disney’s Power Bus Controller functionality into a SIMATIC S7 319 PLC using ProfiSafe safety communication technology. According to  Olaf Scheel, a Siemens engineer who worked on Midway Mania’s design team, the new safety system is essentially a software implementation of Disney’s unique safety requirements. "We replicated what Disney has been doing in standard hardware," he says, adding that the integrated safety system saves the cost of manufacturing and installing the custom safety hardware.

Scheel says the Integrated Power Bus Controller remains in the prototype stage for now, though Siemens did develop it for an older ride that’s undergoing a controls upgrade right now.

Hannover Fair Highlights Energy-Saving Strategies

Reducing energy consumption is a bit like exercise. People tend to agree it’s a good idea and should do it regularly –– but often times they don’t quite make it to the gym. Yet if energy costs keep rising, engineers won’t be able to sit on the sidelines for much longer and will have to work out new ways to trim the energy consumption of the machines they build.

This growing energy awareness was on display throughout this week’s Hannover Fair, where major suppliers and users of motion control and automation equipment showcased their energy-reduction strategies and technologies. The fair also itself featured brand new energy-efficiency displays, including an “Energy Tunnel” exhibit that highlighted ways to make common industrial processes — like pumping — more efficient.

Read more about energy-saving strategies at the Hannover Fair.