Microwaves keep cylinders under control

Integrated microwave sensors provide positional feedback in pneumatic systems. Dean Palmer reports

Pneumatic linear actuators that incorporate a microwave sensor offer continuous, stroke-independent positional feedback on the piston. While microwave sensors are nothing new, placing them inside pneumatic cylinders to provide continuous feedback on the position of the piston certainly is. Most pneumatic actuators require some sort of sensor, but most use proximity sensors, LVDT potentiometers or ultrasonic sensors. However, customers must work with the pneumatics supplier or the sensor supplier (or both) to customise a sensor solution for their particular application - which can be time consuming and costly. Now German company Festo has developed pneumatic actuators with integrated microwave sensors that solve these problems. The units were first shown to the public at this year's Hannover Fair in Germany. The sensor is integrated into the face of the cylinder end cap. An electromagnetic wave (in the GHz range) is radiated into the waveguide structure of the cylinder barrel via a simple antenna structure. This wave reflects off the piston. The signal received via the same antenna is then compared with the coupled signal and evaluated. The cylinder's metallic screening prevents radiation into the surroundings. A patented evaluation method was developed in order to obtain a unique position signal. This delivers a signal with constant high resolution and accuracy, even at close range with a 0mm stroke. The measuring length is unrestricted and is automatically determined by the stroke length of the cylinder. Current projects have been realised with stroke lengths of up to 1,500m, although the majority of Festo pneumatic actuators have a stroke length of less than 3m. Festo has been researching and developing microwave sensors for pneumatic devices since 1996. The units should be ready as 'off-the-shelf' solutions within two years, but for now Festo is only supplying the units to large OEM customers who require large volumes for certain specific applications. The units will be produced by Festo's subsidiary AM-Tech, based in Switzerland, a company with experience in micro technology applications. Armin Seitz, head of the sensors business unit at Festo told Eureka: "Similar microwave technology has been used for years in the military and satellite communications industry, but we needed to develop it for shorter-range applications - sub-micron range rather than kilometres. We had to develop appropriate antenna technology to sit inside the cylinder. To transmit microwaves in a cylinder, we had to find the right frequency mode and develop the software and algorithms." Pneumatic linear drives are used in countless applications in automation technology. Requirements for position sensors and displacement encoders for feedback are increasing due to the increasing need for system availability, accuracy and faster cycle times. Positional feedback from pneumatic drives (cylinders) is normally provided via magnetic, field-sensitive proximity sensors. These are mounted on the cylinder barrel at the position on which feedback is required. Each position requires an additional sensor. "This increases the effort required for assembly, installation, PLC inputs and programming, and also increases costs," explained Seitz. If the system is optimised or reset, the sensors need to be readjusted. The surface-mounted sensors must therefore be arranged in such a way that they can be accessed at any time for adjustment and fast replacement. "Outside the switching position the cylinder is flying blind," said Seitz. "This can be solved using analogue displacement encoders - the most popular being potentiometers and magnetostrictive ultrasound sensors." He explained that the individually encapsulated displacement encoders, which are only available in discrete lengths, are generally installed parallel to the cylinder. The sensors are then actuated by an additional mechanism or via the cylinder piston's permanent magnets. "The field strength of the magnets must be adapted to the measuring system to facilitate reliable operation," says Seitz. "This increases costs significantly." Displacement encoders can only be integrated or customised with considerable effort, while optical systems can fail due to contamination by grease, oil and general wear. According to Seitz: "Ultrasound sensors cannot physically generate the required resolutions and are negatively influenced by the rapid pressure and temperature fluctuations inside the cylinder." With externally mounted proximity sensors, the installation costs increase - as do the requirements on the sensors. Apart from the extra installation space, a key consideration is the higher failure rate of sensors exposed to ambient conditions such as detergents, humidity or dust. The high resolution of the microwave sensor (< 0.02mm) means that the working stroke can be combined with measuring tasks. So the presence of the product to be clamped be monitored and confirmed, the correct orientation or dimensions of the product can also be determined at the same time. Seitz said the system offers further benefits in optimising the speed profile of standard pneumatic components. Power valves, for example, can be switched over and back-pressurised as a function of position and the current speed, before the end position is reached. "This facilitates adaptive cushioning and means that external shock absorbers, which are intrinsically susceptible to wear and complicate system design, can be omitted," he stated. Festo's 'concealed' solution makes the microwave technology ideal for food and beverage applications. The cylinder offers high protection against aggressive media, including fruit and lactic acid, but also against detergents that are sprayed onto the components at high pressure. "This, combined with the use of cost-effective M12 plug-in systems, approved for the food industry, means that high resistance systems up to IP67K can be provided," said Seitz. Because microwave sensors are not influenced by electromagnetic fields, the units can be used near transformers, motors and welding guns. In the automotive industry, for example, electromagnetic fields are present in assembly lines and in welding areas. As Seitz told Eureka: "Normally, systems manufacturers have to use special, high cost sensors here and take further protective measures against weld spatter and mechanical destruction." Currently, the system is implemented in standard cylinders from the company's 'DNC', 'DNCB' and 'CDN' series and includes all necessary interfaces for servo-pneumatics. "We can offer this system to our customers on a project-specific basis," he said. The sensor is fully electronic and wear-free. It will outlast the service life of the drive. Festo intends to sell the drive with sensor as a single component - as a rival to current alternatives. "In the past 40 years, genuine innovation has been rare in the basic design of pneumatic devices," said Seitz. "But innovation in sensor technology has been very encouraging. If we combine the genuine innovation from both these areas, we can develop the next generation of drive technology."