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Motion System Design

Interfacing linear feedback

Linear feedback devices are often mounted to critical axes, directly related to part production, so they must be robust. Magnetostrictive sensors fit the bill: They utilize sonic wave pulses to determine position, so they're immune to shock and vibration, temperature variation, and electromagnetic interference.

The classic application is within cylinders. On newer lumber cutting systems, magnetostrictive feedback steers hydraulic-powered curve-shape sawing to edge, trim, chop, and even follow wavy log grains for reduced waste. Other magnetostrictive applications include steel production and on injection molding machines — machines making CD-ROMs for example.

This month's handy tips courtesy of Dave Edeal at MTS Systems Corp., Cary, N.C. For more information, call (919) 677-0100 or e-mail the editor at [email protected].

Q & A

How accurate are these sensors?

Dynamic precision — how fast a sensor produces output in response to actual motion — is often a limiting factor. Magnetostrictive update rates are inherently limited by the speed of sound through the magnetostrictive material, commonly to a respectable 2 kHz (for a 36-in. stroke range) and speed of 400 in./sec. That said, more homogeneous magnetostrictive waveguide materials have improved position linearity. Stable materials and digital electronics have boosted repeatability to ± 0.001% of the full scale — making magnetostrictive sensors suitable even for machine tooling.

Temperature sensitivity is less than 25 PPM/°C. In the past, nonlinearity of magnetostrictive sensors was anywhere from ± 0.01 to 0.02%; now that's typically below ±40 microns.

Soon, users can expect to see overall accuracy below ±20 micron, as internal microprocessing power is used for internal linearity correction and temperature compensation.

How are magnetostrictive sensors best integrated?

Intelligent fieldbus protocols like Profibus DP or CANOpen allow full expression of magnetostrictive capabilities. Because magnetostrictive sensors simply measure the time it takes for a strain pulse to travel from markers to sensing electronics, they can produce multiple marker positions on any one axis. These simultaneous position outputs can then be transmitted on one feedback channel for full process control. For example, in paper slitters, a single sensor can position 15 knives at a time by storing their absolute locations.

Distributed fieldbus-based architecture also simplifies diagnostics and maintenance by making necessary component information available at all times. There is higher investment cost for distributed control, but payoff is usually realized quickly.

Which design allows the most customization?

Customizing digital sensors is easiest. Hardware doesn't have to be physically redesigned; instead, software allows for adjustments and reconfiguration. Too, their diagnotics, LEDs, and real-time feedback allow for on-the-fly programming.

Another benefit is reduced setup time. Say a plant operation is changed. By storing specific production-run configurations, the sensors allow more executed setups in a given timeframe, for boosted machine productivity. (Even shortening a 15-min. setup by a few moments can translate to large savings if many machines are processed.) Small machine builders using automated systems for setup benefit from this most; less setup time means they can design for more flexibility, and be more competitive.

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