1 June 2007
Wave a welcome
Microwaves have particular utility in measurement tasks that require performance in hostile environments: high temperatures, interference of plastic, fluids, dirt
By Nicholas Sheble
Using microwaves in positioning systems has been a topic of discourse in instrumentation circles since at least 2002.
An algorithm for precise position measurement based on microwave/radar technology surfaced that year-emanating from the Georgia Institute of Technology.
It was a new radar sensor with several differences in capability over conventional noncontact position sensors, such as eddy current proximity probes, capacitive gauges, and laser-based systems.
Microwaves have particular utility in measurement tasks that require performance in hostile environments: high temperature or where interference of non-metallic materials, such as plastics, fluids, and dirt, are present.
They could work well at high bandwidth where the frequency response of capacitive and eddy current systems are limited and also at greater standoffs and at greater ranges than eddy current or capacitance gauges could support.
Now commercial and opening new possible applications in harsh environment comes a microwave sensor in a pneumatic cylinder. "This sensor system is the first in the world to offer non-length dependent displacement measurement for pneumatic or hydraulic linear drives," claims the manufacturer. The microwave sensor also provides important faculties such as diagnostics, process monitoring, and plug-and-play.
Previously, pneumatic drives supplied a position feedback signal via proximity sensors mounted in the cylinder barrel or externally. These sensors involved additional assembly and installation work and thus extra costs.
Moreover, it was possible to provide displacement measurements only for a limited range of cylinders, since displacement encoders were available only in discrete lengths.
Microwave sensors now offer an optimum solution. The unique feature is the integration of a sensor into the end face of a pneumatic linear drive, where it continuously measures piston position and speed across the entire cylinder stroke range.
This means this microwave sensor is a non-length dependent displacement encoder for linear position detection.
The continuous detection of status, position, and speed facilitates better monitoring of motion sequences and thus more precise diagnostics and preventive maintenance.
Machine downtime is less because the sensor, thanks to the fact that it is inside the cylinder, is safe from dirt and wear.
Mode of operation
The Festo AG-designed system exploits high technology in the form of microwaves in the frequency range of 1-24 GHz, which also serve in telecommunications and the automobile industry.
A simple antenna system feeds electromagnetic waves into the cylinder barrel, acting as a hollow conductor. These waves reflect back and off the piston. The same antenna receives the reflected signal, compares it to the transmitted signal, and calculates the distance.
"It's not a time-of-flight principal we're using," said Armin Seitz, head of Festo's sensors business unit. "It is more a continuous wave principle, and the distance calculation comes by looking at the wave's phase shifts that occur with varying distances."
The measuring length is unlimited, and the stroke length of the cylinder automatically governs this distance. Projects exist that peg this stroke length at 1,500 mm (59 inches).
Microwave sensors can replace virtually every displacement encoder solution in pneumatics, including potentiometers and magnetostrictive sensors.
Thanks to the high resolution of the sensors, measurement operations can happen during working strokes. For example, one can check a product that is going to process for correct position or for its dimensions.
Thus, there is no need for special sensors. The demands on logistics are also simplified.
There is no need to produce sensors in sizes for individual cylinder strokes, since, on the principle of "one size fits all," the sensor can be installed in cylinders of different lengths.
In comparison with conventional displacement encoders, microwave sensors offer many advantages, such as shorter commissioning times, less need for installation components, and greater design freedom, which drastically reduce the cost of applications.
The ability to detect positions across an entire cylinder stroke enables faster cycle times. Moreover, there is no need for mechanical adjustment and thus no need for access to the sensors from outside.
One of the main beneficiaries of this new microwave technology is the automobile industry. When welding is carried out on vehicle bodywork, strong electromagnetic fields result, which can cause conventional sensors to malfunction. This is why expensive special sensors and elaborate
protective measures were required in order to protect the sensors against welding splashes and mechanical damage. Microwave sensors offer important advantages in applications of this kind, since:
They are immune to magnetic fields.
They are resistant to the build-up of welding splashes.
They require less system design and installation time.
They work with cylinders of any length.
There is also potential for the use of microwave sensors in the food and packaging industry. For this high-growth market, there are corrosion-free and easy-to-clean "Clean Design" components.
With "Clean Design" drives, external sensors are a hygiene risk, since their additional edges and corners act as dirt traps and are hard to clean. Microwave sensors integrated into stainless steel cylinders have full protection against aggressive media such as fruit and lactic acids and against cleaning agents sprayed onto the components at high pressures.
A further area of application for microwave sensors is aluminium smelting. Here, pneumatic cylinders are crust-breakers. In this process, the sensors on the cylinders work in extremely high temperatures and strong magnetic fields. Thanks to their immunity to magnetic fields, microwave sensors can work here without difficulty for displacement measurement. Moreover, there is no need for modification of cylinder piston rods. To provide protection against high temperatures, microwave sensors sit in the retracted end position of cylinders.
About the Author
Nicholas Sheble (firstname.lastname@example.org) is the senior technical editor for InTech.
TERMS AND TECHNOLOGY
Microwave refers to electromagnetic energy having a frequency higher than 1GHz. Microwave signals propagate in straight lines and are not refracted or reflected by ionized regions in the upper atmosphere. Microwave beams do not readily diffract around barriers such as hills, mountains, and large human-made structures. Some attenuation occurs when microwave energy passes through trees and frame houses.
Proximity sensor is a device that detects the presence of an object without physical contact. There are capacitive and inductive versions.
Capacitance: The property of an electric nonconductor that permits the storage of energy.
Induction: The ability of a conductor to produce a magnetic field.
Potentiometer is a technology for position sensors. The function principle rests on the change of the electrical resistance, which is proportional to the angle (rotary) or the travel (linear).
Magnetostriction is a property of ferromagnetic materials that causes them to change their shape when subjected to a magnetic field. Magnetostrictive materials can convert magnetic energy into kinetic energy, or the reverse, and we used them to build actuators and sensors.
Encoder is a technology for position sensing. It is a feedback device, which converts mechanical motion into electronic signals. Usually an encoder is a rotary device, which outputs digital pulses, which correspond to incremental angular motion. For example, a 1,000-line encoder produces 1,000 pulses every mechanical revolution. The encoder consists of a glass or metal wheel with alternating clear and opaque stripes, which an optical sensor detects to produce the digital outputs.
Photoelectric proximity sensor is a device that uses the properties of light-sensitive elements to detect objects.
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