Design Applications

Sensor 'foils' foil

Unit's enhanced dynamic range eliminates saturation problems at the brightest light levels

David J. Bak, Senior Regional Technical Editor

Tampa, FL--All photoelectric sensors share the same sensing task: respond to a darkest state condition and to a lightest state condition, and resolve the difference between the two. These sensors also share a common problem: how to avoid saturation--the point at which a sensor will not respond to any further increase in received light.

To operate in the bright, received-light conditions that promote saturation, conventional designs require a reduction in amplifier gain. This strategy, however, minimizes the sensor's ability to resolve low-contrast sensing tasks.

Another technique, employed in the COLORMARK II SMARTEYE® uses "enhanced dynamic range" circuitry that automatically compensates during setup to avoid saturation. By avoiding saturation, the EDR circuitry extends the sensor's overall dynamic range to include proper operation in bright received-light conditions.

This range extension proves important when the application requires sensing registration marks printed on shiny foil. Some foil materials are so shiny that they can reflect a "hot spot" glare capable of blinding conventional photoelectric sensors.

Bob Warner, senior vice president of the Tri-Tronics Company, which manufactures the COLORMARK II SMARTEYE, claims the new sensor can resolve low-contrast sensing tasks over a wide range of light intensities. Its design also negates concern for "hot spot glare."

"Repositioning the location of the fiber optic light guide, or changing the sensor angle to combat glare, is no longer necessary" says Warner, "even when sensing printed marks on foils and other shiny materials."

Also beneficial when sensing printed registration marks on translucent or transparent materials, the COLORMARK II SMARTEYE features a 50 µsecond response time, an optional pulse stretcher with a minimum 10 msecond time delay, a 10-LED contrast indicator, and infrared, red, green, or blue LED light sources for improved color perception.

Additional details…Contact Robert Warner, Tri-Tronics Co., Inc., 7705 Cheri Court, Tampa, FL 33634-2419, (813) 886-4000.

Ferrofluid senses differential pressure

External pressure disturbs flux lines,
enabling sensor to output proportional signal

Charles J. Murray, Senior Regional Technical Editor

Grand Island, NY--Measuring fluid flow is no simple task. Until now, conventional measuring techniques have used capacitive differential pressure transducers, each of which may cost as much as $1,000.

Recently, however, engineers at Viatran Corp. developed a simple differential pressure transducer that costs approximately half as much as capacitive devices. The new transducer can perform fluid flow measurement, leak detection, and a variety of other jobs that now use capacitive techniques.

The Ferro-Cell transducer lowers cost by eliminating the manufacturing complexity of capacitive pressure transducers. It cuts the number of welds from about a dozen to two, eliminates numerous components, and requires no special assembly techniques.

Key to the Ferro-Cell's design is the use of a ferrofluid differential pressure sensing technique. The device consists of the ferrofluid, non-magnetic stainless steel diaphragms, permanent magnets, flux concentrators, and a Hall effect sensor. The four magnetic poles and the ferrofluid-filled diaphragm symmetrically enclose the sensor. As a result, flux lines from the magnets pass around the Hall effect sensor, creating a null flux point at its center.

When external pressure is applied to the diaphragm, however, flux distribution changes. External pressure causes the ferrofluid to flow and become non-symmetrically distributed. This change disturbs the path of the flux lines. As the ferrofluid moves to one side, the flux lines on that side are forced away from the sensor. As the flux lines change position, the null flux condition changes and produces an electrical signal proportional to the pressure difference.

Using ferrofluids proved critical to the success of the device, say Viatran engineers. Ferrofluids are super paramagnetic fluids containing tiny magnetic particles. Because the magnetic particles do not agglomerate when a magnetic field is applied and removed, the field remains evenly distributed throughout the volume of the fluid. Ferrofluids also exhibit no hysteresis.

Similarly, the use of Hall effect sensors was also critical to the device's success. "You need a Hall effect sensor to measure the net effect ofthe flux," explains Bob Moss, vice president of engineer-ing for Viatran and inventor of the device. "Other sensors wouldn't work in this application because they don't sense direction of the flux."

Using this technique, the Ferro-Cell can achieve 0.5% measurement accuracy. That's not as good as capacitive techniques, which typically achieve 0.1% accuracy, but the dramatic difference in price could induce many users to opt for the new technology, say Viatran executives. "A core group of users needs 0.1% accuracy," notes Don Joslyn, Viatran's president. "But more than half of the market does not need that kind of accuracy. And there is no other system that offers this kind of cost tradeoff."

By employing the new design, Viatran vastly simplified differential pressure transducer construction. The Ferro-Cell's major components are simple stampings and tubes. Magnets, flux concentrators, and Hall effect sensors are bonded in place, and two welds hold the main sensor assembly to the main body.

Viatran engineers say the system could be used in closed-tank liquid-level measurement, filter management, and fuel-flow systems, among other applications.

Additional details…Contact Bob Moss, Viatran Corp., 300 Industrial Dr., Grand Island, NY 14072, 716-773-1700.