Microwaves for cylinder sensing

June 7, 2007
Position sensor works with actuators of any length.

Edited by Kenneth Korane

A sensor housed in the cylinder endcap measures piston position with accuracy to about 0.1 mm.

A simple antenna inside the cylinder transmits electromagnetic waves and receives the reflected signal. Onboard electronics generates position and speed data. The microwave sensors are suited for pneumatic and hydraulic drives.

A reported breakthrough in sensor technology from Festo Corp., Hauppauge, N.Y., (festo.com/us) and Astyx GmbH, Ottobrunn, Germany (astyx.com) is opening up new fluid-power applications, particularly in harsh environments. Engineers have developed the world's first sensor for position measurement that isn't restricted by the length of pneumatic and hydraulic linear drives, says Armin Seitz, head of Festo's Sensors Business Unit. The sensors also offer built-in diagnostics and process-monitoring capabilities.

Traditional proximity sensors inside or outside the cylinder barrel send position feedback signals to a controller. Installing such sensors involves additional assembly and installation work, such as boring out cylinder rods or installing mounting brackets — and thus entails extra costs. Moreover, standard sensors only provide measurements for a limited range of cylinders, because displacement encoders are only available in standard, discrete lengths.

Microwave sensors offer a better alternative, says Seitz. They mount into the end face of linear actuators, where they continuously measure piston position and speed across the cylinder's entire stroke, he explains.

The system relies on microwaves in the 1 to 24-GHz range, the same used in telecommunication and automotive industries. A simple antenna transmits electromagnetic waves into the cylinder barrel, which acts as a hollow conductor. The piston reflects the waves, and the same antenna receives the reflected signal. Onboard electronics compares and evaluates the transmitted and reflected signals, and generates position, speed, and status data. Virtually any length can be measured, which is governed by the cylinder stroke length. Accuracy is about 0.1 mm, and near-instantaneous response lets it make measurements during working strokes.

The sensors are housed inside the cylinder, protected against dirt and damage. And they have no moving parts to wear out. Because there is no need to size sensors for specific cylinder strokes, designers have more flexibility and OEMs can reduce inventory and simplify replacement. Costs are expected to be at or below competing sensors.

Microwave sensors have the potential to replace potentiometers, magnetostrictive sensors, and most every other type of displacement encoder used for fluid-power applications, says Seitz. One beneficiary would be the automotive industry. For instance, when welding vehicle bodies, strong electromagnetic fields can make conventional sensors malfunction. To date, the solution has been expensive, special sensors and elaborate measures to protect against EMI, weld splatter, and mechanical damage.

Microwave sensors, on the other hand, are immune to magnetic fields from ac and dc welders and electric motors. And they are well protected from weld splatter, are easier to design, require less installation time, and are insensitive to vibration.

Other applications include food and packaging, where stainless-steel cylinders and sensors must withstand aggressive media such as fruit and lactic acids, and high-pressure sprays of cleaning agents. And they are suited for aluminum-smelting operations, where cylinders are exposed to extremely high temperatures and strong magnetic fields.

The technology is also suited for accurate sensing in hydraulic cylinders. In fact, it is the only practical means for position sensing in very long cylinders, says Felix Hoehne, engineering manager at Astyx.

Water in hydraulic fluid, and variations in fluid contamination and temperature, can affect precision. Developers feel sensor electronics can be designed to compensate for such variations and maintain accuracy.

Festo plans to offer microwave sensors for a variety of pneumatic actuators. To date, the company has used the sensors in pneumatic cylinders with strokes to 1.5 m.

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