Pneumatic knowledge bank

HOW DO ACTUATOR CHOICES AFFECT PRODUCTIVITY IN MOTION-CENTRIC AUTOMATION ENVIRONMENTS?

Jerry • Numatics: Pneumatic actuators are available in a variety of shapes, sizes, and types, as well as with a multitude of standard options. At first glance, the number of permutations can be overwhelming. The good news is that each actuator type and configuration has a place in today's automation environment.

Pneumatic actuators are selected by their ability to do work and the most common factor that limits productivity is using an undersized actuator. Follow these tips to select the right actuator for the job:

Determine the force.
Subtract the piston rod area, if applicable.
Know the true operating pressure.
Allow for internal actuator friction, i.e. seals, bushings, and wear bands.
Know the true load.
Factor in speed requirements.
Consider the angles.
Design in a margin, anticipating demand for future productivity improvements.
Consider kinetic energy.
Test.

Randy • Norgren: The typical pneumatic actuator application requires two or more positions at high speeds, the key (to productivity) being deceleration performance. Usually, the deceleration technique is the air cushion. Adding an impact-damping seal, however, can help significantly allowing for higher traverse velocities and shorter cushion cycles. Keep in mind that if the bore size is not a correct fit for the application, it can adversely affect the actuator's performance.

John • Parker: Proper actuator sizing and selection is the single biggest factor to ensure high reliability and long-term productivity, especially for applications in extreme conditions. Factors such as seal life, side loading, and operating environment will ultimately drive critical design criteria that will determine MTBF (mean time between failures).

James • Tolomatic: Pneumatic cylinders and systems are a proven, mature technology that has kept up with the times, in large part through the integration of electronics and digital communication protocols in valve controls. Force, speed, and ease of use contribute to their acceptance worldwide. Basically, there are two styles of pneumatic actuators, rod and rodless, the latter accounting for more than 90% of the applications in North America. The most significant difference between rod and rodless style actuators is how the load is guided and mechanically supported.

With rod style cylinders, the designer is required to support and guide the load. And since the cylinder rod extends, quite often the load bearing system increases in complexity and cost. Rodless actuators, on the other hand, incorporate a bearing system of some sort, except for very low cost devices designed for guided loads. The bearing systems can handle moment loads of varying levels in x, y, and z orientations.

Although sizing and selection is a bit more complicated with rodless actuators, the advantages often outweigh the additional effort required. The stroke of the rodless carrier is contained within the physical envelope of the actuator, considerably reducing the size requirements. A typical NFPA tie-rod cylinder with a 2-in. body size and 8-in. stroke has an approximate overall length of 21.75 in. when the rod is extended. By contrast, the equivalent 2-in. body size, 8-in. stroke rodless cylinder has an overall length of 14.3 in., about a 34% smaller footprint. As stroke length increases, so does the space savings.

WHAT ARE THE MAIN CHALLENGES WITH PNEUMATIC ACTUATORS, AND WHEN ARE THEY A BETTER CHOICE THAN ELECTRIC OR HYDRAULIC TYPES?

Jerry • Numatics: Pneumatic actuators for the most part are simple devices. Determining an actuator's theoretical force output is relatively straightforward, although sizing for a specific application can be a daunting task. Specifying an undersized actuator is one of the most common mistakes made when working with pneumatic actuators. The use of pneumatic vs. electrical or hydraulic actuators is an application specific, site specific, and cost specific decision. Additional factors include issues such as safety, reliability, and maintenance schedules. Because pneumatic actuators can operate without electricity, there is no chance of spark generation. This makes them ideal for hazardous environment use, such as methane or natural gas applications.

Randy • Norgren: Some of the challenges that accompany using pneumatic cylinders include controllability (due to the air being compressed), positioning, limited force outputs with respect to physical size, and air consumption. All of the challenges aside, there are times when the only choice is pneumatic actuation. If component cost, cleanliness, or force-speed ratios are important to the application, then pneumatic, rather than hydraulic or electric options, is the way to go.

John • Parker: When using pneumatic actuators, you're dealing with a compressible media that can limit precision control. However, pneumatic technology provides the best value technology for solving common point-to-point applications with high unidirectional repeatability requirements. Electric actuators are completely configurable ? multiple positions, velocity, acceleration control ? and typically offer good bidirectional repeatability. Hydraulic technology can't be beat when it comes to force density and adaptability to mobile and other extreme conditions.

James • Tolomatic: In today's motion control environment, pneumatic systems face two uphill battles: operating costs and controllability. As many users are becoming aware, compressed air is not a free utility. The energy required to compress free air and move it through a plant, with its attendant frictional losses, is high. The size of the molecules in air presents sealing challenges, often resulting in invisible but not inexpensive leaks. Because air is a compressible medium, there are control issues. It takes a calculated critical air volume to move a cylinder; miscalculating air consumption and underestimating pressure losses can result in less than optimum performance.

Pneumatic power is a good choice when speed, ambient conditions, simplicity, and proven technology are required. In addition, pneumatic actuators possess moderate to high load capability at velocities well over 3,000 mm/sec (118.1 in./sec). The critical factor is the size of the air cylinder and the entire pneumatic system feeding the cylinder. SCFM, psig, pressure loss, friction, and control signal scan times come into play when sizing an air cylinder for speed and force. A well designed pneumatic linear system will offer long life and trouble-free service.

Pneumatic cylinders are an excellent choice in certain operating environments, such as high ambient temperatures or when explosion-proof is required. For obvious reasons, hydraulic and elctromechanical cylinders are not the best fit for these types of applications. Other applications, such as leak testing, are perfectly suited for pneumatic actuators as well.

WHAT NEW OPPORTUNITIES EMERGE AS PNEUMATIC ACTUATORS BECOME MORE TECHNICALLY ADVANCED?

Jerry • Numatics: Position feedback is becoming fairly common in pneumatically actuated systems. Many pneumatic cylinders now come with a linear resistive transducer (LRT), which is typically located inside the actuator piston rod. LRTs are ideal for applications where traditional magnetic positioning is not acceptable and where variations in actuator stroke and speed are expected.

Randy • Norgren: As pneumatic actuators become more technologically advanced, new opportunities are emerging. The new technologies can accommodate applications requiring more accurate positioning, higher velocity, and better force control. Smaller footprints, multi-position capabilities, lighter materials and more chemically resistant components continue to emerge.

John • Parker: There are many technical advances in pneumatic actuator technology, such as safety rod locks that help pneumatic actuators solve vertical travel applications. Rod locks provide a mechanical locking system to hold loads indefinitely in place and prevent tooling from being dropped or damaged. Advances in velocity control and mid-point positioning are also opening new applications.

James • Tolomatic: Field bus technologies (serial communication) are a key area in pneumatic controls. Many pneumatic valve systems incorporate networking capability, such as CAN (Controlled Area Network), DeviceNet, and others. The ramifications are significant: Machine controls can coordinate linear and rotary motion regardless of the power technology used. This provides designers with opportunities to select the best power medium and worry less about coordinating various axes.

As for pneumatic actuators, there's a fair amount of development activity in rodless products to offer various types of bearing systems. Solid bearing designs can achieve extremely long life, and the use of profile rail bearing systems extends load capabilities significantly.

WHAT WILL TOMORROW'S PNEUMATIC ACTUATORS LOOK LIKE?

Jerry • Numatics: The actuator of tomorrow will more than likely be lighter, more efficient, and have more of a rounded, aesthetically pleasing look. With such a great appreciation for aesthetics and continuity today, I wouldn't be surprised if pneumatic actuators didn't start looking as if they were specifically designed for OEM equipment. Continuity between components and equipment will add value to all parties involved. In addition to being sleeker and more efficient, the pneumatic actuator of tomorrow will include a whole host of additional features. I believe that onboard actuator intelligence is in the near future, which will provide the benefits of actuator diagnostics and communication modules among others.

Randy • Norgren: Tomorrow's pneumatic actuators will have integrated electromechanical capabilities such as switches, valves, position feedback, control, and wireless interfaces. They will also provide value through advanced, lighter weight composite materials. Much like today's actuators, tomorrow's actuators will continue to produce higher forces via smaller footprints and lighter materials. Faster cycle times along with lower vibrations will be the expectation for the future.

John • Parker: Tomorrow's actuators will be “smarter” across several fronts ? especially in the area of integrated low-cost feedback and sensing. Embedded controls will give rise to smart rod locks and greater access to lifetime and “health” data. New seal technology and lighter weight materials will lower energy consumption. Tomorrow will also bring forth new “crossover” hybrids, blending electromechanical and pneumatic actuation technology.

James • Tolomatic: The future of pneumatic actuators will include a number of improvements:

Integrated valving with wireless communication
Integrated diagnostics that predict and/or detect early maintenance requirements
System air use monitoring with integral controls and valves to decrease leakage
Components that run on lubrication-free air, achieving hundreds of millions of cycles
Improved accuracy and lower cost for pneumatic servo/proportional control
Major engineering schools offering mechanical engineering/mechatronics programs that focus on five disciplines: pneumatics, hydraulics, electromechanical, motors/drives/controls, and robotics