Zero in on the Sensor Precision, Accuracy Target

Improvement in Both Areas Increases How Often a Machine Hits a Performance Bull's Eye

By Hank Hogan

It's an analog world, though control systems are today almost entirely digital. That makes sensor precision and accuracy important to machine builders. Accuracy tells us how closely a measurement comes to the real-world value, while precision indicates how repeatable that measurement is. Improvement in both increases how often a machine hits a performance bull's eye.

Recent advances promise to boost accuracy and precision. Some of these innovations involve sensor microprocessors. For instance, the analog-to-digital conversion done in microprocessors is now a 12-bit standard, with 16-bit and higher devices available. These reduce the A-D conversion error at least sixteenfold over 12-bit, upping output accuracy and precision, assuming the greater bit-depth is supported.

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"The sensor manufacturers and the people that build the interface devices have to be in step together," says Tony Udelhoven, director of the North America sensors division for Turck. The company makes a variety of sensors, including presence, inductive or position devices. The latter group measures temperature, pressure, flow and the like. It's these types of measurements that are most impacted by greater precision and accuracy.

In addition to higher bit conversions, sensor microprocessors also can store compensation curves that permit an adjustment for temperature. That effectively improves accuracy and precision. Still more exacting results are possible through use of onboard calibration capabilities.

Recent developments have improved the precision and accuracy of inductive sensors, says Craig Brockman, manager of presence sensors at Rockwell Automation. "For example, greater linearity over temperature has been possible due to the advent of custom ASICs," he says. "In particular, application-specific ICs allow an easy temperature-effect correction. An analog sensor typically produces a signal that is not linear with distance. Instead, it follows an S-curve, with an approximate straight line in the middle and flattened response at the extremes. That can be accounted for, but the shape of the S changes with temperature. It's there that custom ASICs come into play. They are paired with a temperature probe embedded in the sensor."

You get that feedback from the temperature probe in the face of the sensor, and you can compensate for the changing temperature in the ASIC, Brockman adds.

In general, there's a trend toward sensors with an analog output, Brockman adds. Because they provide more than just presence-absence information, they enable machine users to reduce scrap, change a setup on the fly, and improve quality.

Highly precise and accurate feedback from all kinds of sensors is increasingly important, agrees Dean Tyo, chief engineer of vision, measurement and instrumentation at Banner Engineering. For example, late in 2011, Banner introduced its L-Gage LH non-contact measurement sensor that "offered the industrial market something relatively new," Tyo explains. "The LH sensor is a micrometer-resolution laser sensor based on imager technology."

Banner Engineering sees the new sensor, which Tyo says is the company's most precise, as complementary to other products. In the past, tens of microns of precision might have been good enough. Today, the demand could be for precision in the single microns, in part because this greater resolution can help reduce costs.

There's a trend toward using imager technology, Tyo says. The rapidly plunging cost and increasing capability of CMOS imaging sensors is one reason.

Within the past year or so, Pepperl+Fuchs put a different approach in place toward improving accuracy and precision. This helped its analog I/O modules maximize performance over the full industrial temperature range, says Helge Hornis, manager of the company's intelligent systems group.

Pepperl+Fuchs was able to achieve this by redesigning the housing and inserting a dedicated, single-purpose, A-D convertor. These devices offer advantages over the microprocessor ports that normally are used to transform an analog value into a digital equivalent.

"By going with a dedicated device, we were able to offer temperature stability that is at least a factor of three to four, if not an entire order of magnitude, better," Hornis says.

That capability is important because right now there is an increasing push toward temperature extremes, with the low end garnering the most interest. Thus, being able to offer greater precision and accuracy at the coldest temperatures can be a competitive advantage, Hornis says.

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