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MSD: Any new developments regarding servopneumatic control systems?

JR • Enfield: As microprocessors continue to become cheaper, faster, and more powerful, low-cost digital controllers will become more common. This will allow for much more capable controllers in smaller packages, which can be easily customized without PCB revisions. There also seem to be promising developments in the field of micro and nanotechnologies, which could make servopneumatics (and servohydraulics) suitable for microfluidic applications. Some companies already sell miniature valves that use piezoresistive elements to control the orifice apertures.

Servopneumatics enable superior swivelling

When motion requires gentle braking, shock absorbers are often preferred. Another option gaining use is servopneumatic components — because they combine the advantages of pneumatic and electric drives.

Compared with standard pneumatics, servopneumatics are up to 30% faster and use up to 30% less air. Servopneumatic drives are also freely positionable, making them up to 50% more economical than electrical solutions, according to engineers at Festo Corp., Hauppauge, N.Y.

The company recently introduced the DSMI-B servopneumatic swivel module that rapidly swivels without vibrating. Shock absorbers are unnecessary, reducing purchasing and maintenance costs.

The DSMI-B is suitable for a wide range of material-handling tasks when combined with the electrical terminal CPX (a modular automation platform for valves and electrical control functions) and its CMPX (end-position controller) or CMAX (positioning) modules.

The sturdy DSMI-B semi-rotary vane drive has integrated measuring for use as a soft-stop axis and for free positioning. Designed for mass moments of inertia of up to 6,000 kg/cm², it swivels up to 270°. In one Festo ball-throwing demonstration, engineers set up the DSMI-B's arm to catapult a ball using a short stroke cylinder. While the ball is in the air, the arm swivels 180° in just 0.5 sec, brakes without vibrating, and catches the ball.

An electronic end-position controller, CMPX, gently brakes the swivel drive in its end positions, even at high speeds. This soft stopping reduces cycle times by about 30% while reducing noise and eliminating vibration. Loads to 300 kg can be dynamically moved; what's more, the system can be programmed to move to two additional, user-defined intermediate positions without a fixed stop.

Pairing the swivel module DSMI-B with servopneumatic positioning module CMAX allows free positioning, which is appropriate where loads exceed 10 kg and accuracy to within a few tenths of a millimeter is sufficient. Using the CMAX, drives can be positioned precisely to within 0.2 mm and force controlled to within 5%.

For more information, visit festo.com or call (800) 993-3786.

Precise positioning requires accurate feedback

Positioning — and its corresponding accuracy — ultimately depends on the validity of the position information provided by sensor feedback. Because servopneumatic linear positioners typically do not contain any rotating elements, rotary encoders cannot be used. Instead, position feedback must come from a linear position sensor, a vital component available in several formats:

• Incremental linear encoder (magnetic or optical)
• Absolute linear encoder (magnetic or optical)
• Linear potentiometer (resistive potentiometric)
• Linear position transducer (magnetostrictive)

All servo positioning systems consist of five main components: A prime mover driving the load being positioned; an energy source for the prime mover; a regulating device that controls the flow of energy to the prime mover; a controller that modulates the energy-regulating device; and a position sensor that reports the position of the prime mover or drive load.

Although incremental and absolute linear encoders can provide exceptional accuracy and performance, they typically have higher initial cost than other alternatives. Their quadrature or serial digital electrical interfaces can also be costly on the controller side, and the operation of optical types can be adversely affected by wet or dirty conditions. Both of these factors can negatively impact the performance-to-cost ratio of these devices.

Linear potentiometers or pots often fit the bill in terms of price as well as simplicity of electrical interface (analog voltage). However, due to mechanical contact wear, their life is limited in high-cycle applications. As the internal contact surface wears, electrical noise increases and accuracy degrades over time. Linear pots often lack the environmental ratings necessary to resist typical industrial conditions such as exposure to heavy dust or liquids, which can greatly accelerate wear and signal degradation. Another note of caution: Linear pots depend on an operating rod that must be mechanically coupled to a positioning system, and sometimes it's inconvenient or impossible to implement such a mechanical connection.

Magnetostrictive linear position transducers solve many of these issues with non-contact operation. Here's how they work: A magnet, acting as a position marker, is affixed to a linear-moving element of the servopneumatic system. The linear position transducer is mounted close to the position magnet, which “floats” above the transducer housing without touching it. The output is directly proportional to the magnet's location over the transducer's entire length. Importantly, magnetostrictive linear position transducers provide absolute position measurement: The transducer always reports actual position without the need to re-home the system, even on power-up. Several electrical interfaces are available, but the most common and least costly to implement on the controller side is analog (voltage or current).

This month's handy tips courtesy of Balluff Inc., Florence, Ky. For more information, visit balluff.com or call (859) 727-2200.