Measurement and motion become one

Sept. 1, 1999
Combining two technologies makes for a rugged, reliable, and easy-to-install system.

Combining two technologies makes for a rugged, reliable, and easy-to-install system.

The AMSA reduces geometric and deformation errors because the guidance and measurement systems are integrated into one element and thus already aligned.

The AMSA measuring system is based on a magnetoresistive sensor that scans the magnetic grating t of the scale. A relative motion between the scale and the sensor in the measuring direction causes periodic changes of resistance in response to changes in the magnetic flux vector. To identify the relatively small resistance changes and to compensate for temperature-related signal variations, several sensor elements are configured in a bridge circuit. The resultant standard signal outputs are shown at right.

Scanning signals consist of two sinusoidal signals and a reference mark. These signals are phase-shifted to each other by 90°. The signal period, 360°, corresponds to the grating period t of the scale.

The analog current signals of the scanning head are amplified and sent to a digital signal processor, which calculates the current position and, from it, the number of measuring steps covered. Using edge detection, the system achieves measuring steps of 1 µm.

Improving product features through design change is a typical goal for manufacturers. What's not so typical, though, is improving the performance of two separate products by uniting them into a single new one. Schneeberger of Bedford, Mass., has accomplished just that by integrating a linear encoder with a linear guideway to produce an advanced measuring system. It's more rugged, reliable, and easier to install — not to mention the fact that it provides a single system to machine designers that combines two formerly separate functions — guidance and measurement.

The Monorail AMSA (advanced measuring system, analog) stems from an earlier Schneeberger product introduced in 1993. These systems were the first in the metalworking industry to offer the benefits of encoder and guide-way integration. While developing the AMSA, Schneeberger concentrated on two areas — optimizing the magnetization of the strip and redesigning the scan head to increase its robustness and reliability.

Because it uses magnetic rather than optical technology, the new system is more rugged. A magnetic scale is less susceptible to problems resulting from coolants, condensation, oils, and greases than an optical scale. Plus, magnetic encoders, in general, produce a purer signal compared to optical devices.

To create magnetic scales, the system builders apply a magnetic grating along with reference marks to a hard magnetic material that is bonded to a machine rail, and the scan head mounts to the machine's moving carriage. As the scan head rides on the strip, it picks up signals from the scale and generates positional information.

Schneeberger improved scan head reliability by switching from a multi-layer circuit board to a solid state scanning element. Signal quality is also better due to a new sensor chip using advanced semiconductor technology. In addition, the signals of the Monorail AMSA are compatible with current CNC control systems and technology.

A single piece of titanium coated with a lubricating and wear-resistant material houses the scan head. It has no active components or diodes and can easily be detached or mounted from the side. The AMSA systems have resolutions of 1 µm with standard accuracies of 5 µm/m. Typical repeatability is ±1 µm.

In addition to improved performance capabilities, the AMSA also simplifies machine construction. Because two elements are combined into one, all additional brackets and space needed for mounting a separate encoder to a rail are no longer necessary. The system also comes pre-assembled and tested, so no additional installation, adjustment, or alignment is required during machine construction. Plus, machine builders can significantly reduce geometric and deformation errors with an integrated measuring/guideway system because it can be located closer to the machining process.

Currently, some European machine tool builders use the Monorail AMSA. But the popularity of the AMSA is growing in the United States, and the idea of integration in general is gaining ground. While Schneeberger is the only company offering this particular product commercially, other companies have similar — albeit prototype — products that capitalize on the same idea of combining guideways with some form of measurement.

According to George Jaffe, executive vice president and managing director of Schneeberger, the technology will be more prevalent in the future, especially as linear technology gains popularity. "I think it's a product that makes sense, and machine builders will continue to use it," he says.

However, the AMSA is not without its disadvantages. Magnetic fields can negatively affect the system, and it is limited in resolution to about half a micron, which is not acceptable in some applications. Rotary encoders and both glass and taped scales can achieve better resolutions.

Plus, if someone deliberately or accidentally damages the scale, the entire rail needs to be replaced, which is more costly than simply replacing the scale. But virtually all of the replacements Schneeberger performs do not involve new scales. "In our experience over the past six years," says Jaffe, "service issues have been targeted at the scan heads themselves and not at the rails.

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