The Pneumatic-Electromechanical Connection

The strength of standard pneumatics has always been its ability to provide cost-effective, discrete end-to-end movements which, by employing specific valves (5/3 way), can achieve a mid-position that offers low repetition accuracy at best.

But now, servo pneumatics is emerging as a technology that offers the flexibility of multi-position and force control with position and velocity monitoring. Positioning and force tasks can now be linked and sequenced, reducing PLC I/O requirements and programming complexity. Servo pneumatics makes this possible by employing actuators in conjunction with feedback devices to form a multi-position, servo-pneumatic system.

"The power of servo pneumatics is that it bridges discrete standard pneumatics and the electromechanical solutions available in the market today," says Nuzha Yakoob, product manager - positioning for Festo Corp. "Pneumatics offers benefits such as simplicity, robust operation, compactness and high-force densities, which make the technology desirable with systems that are simple to operate and implement, and have demands that cannot be met by electromechanical solutions."

With the trends in the market today pushing the need for more flexibility and on-the-fly adjustments, there is a move toward transforming pneumatic systems into more electronic and software-oriented solutions. Engineers can implement systems that are 100 percent electric and, since the power source is no longer air, any losses sustained in maintaining pressure in the pneumatic cylinders is eliminated. The tendency to migrate to electric solutions is due to the fact that electric systems consume energy to operate on demand unlike pneumatic systems which require air pressure to be maintained. But, according to Yakoob, somewhere in the middle electromechanical systems often don't meet the speeds and force densities of pneumatics when comparing similar footprints.

When there is a need for balance between cost and flexibility, and the need for precise movements in the five to 10 micron-range is not required, servo pneumatics fits that gap.

It takes the best of both worlds and combines the flexibility and software control of electromechanical systems with the speeds and feed force advantages inherent with pneumatic axes. It brings together the best of both and fits well into the complete landscape of positioning technologies.

Filling System Design Gaps

"The sweet spot for servo pneumatics is in addressing application needs that can't be fulfilled by other existing electromechanical and linear motor technologies," says Yakoob. "When there is a need to move large loads continuously (24/7 or high-duty cycle operation) on positioning systems operating on a low voltage, or when space constraints requiring high feed forces and high dynamics are an issue, the servo pneumatic axis becomes a suitable solution."

One advantage to pneumatics is that it doesn't have the motor overloading and heating issues than can occur with an electrical system. With a pneumatic cylinder, the application has the flexibility that comes with a repetition accuracy of ±0.2 mm compared to a linear motor system that might offer three micron repetition accuracy.

Areas where servo pneumatics fit best include handling of hazardous products such as explosives, where you can't guarantee the surrounding air is free of explosive gases. Also, if there is a need to operate on a low voltage, servo pneumatic systems are appropriate because they can operate on a 24V dc supply.

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Handling large loads definitely differentiates pneumatics from electromechanical solutions because it becomes more cost-effective, and the technology provides high-force density, which helps to create more compact designs. To achieve the same feed forces from an electromechanical system typically requires a much larger footprint and therefore results in a more costly solution.

While servo pneumatics is basically a pneumatic system that uses feedback, the controller offers an algorithm that differs from an electromechanical system and takes many parameters into consideration. For example, temperature has an immediate impact on the volume and quality of air, and the orientation and frictional forces are all factored into this algorithm in the controller.

Yakoob says that servo pneumatic product offerings are very limited in the market and Festo's goal is to offer a range of positioning technologies to meet the various application needs of its customers providing both optimized performance and cost. An axis can do positioning, and also provides force control for clamping and pressing applications.

Festo's system approach comprises a series of technology modules including a software controller that resides on its CPX platform and a CMPX (Soft Stop) module, which is an end-position controller or electronic shock absorber. A third measurement node (CMIX) offers digital measurement of position data with repetition accuracy of ±0.01 mm precise. Velocity data can also be measured.