Festo's new "fluidic muscle" has no piston. It is emerging as an alternative to actuators with pistons in many applications. The muscle uses membrane expansion to achieve its stroke length. The maximum stroke length is 25% of the muscle's nominal length. The proprietary membrane consists of a collection of woven fibers. The weave is of a three-dimensional rhomboidal form, which helps give the tube its flexibility and the ability to expand. The two woven layers of the high-tensile supporting material enclose the muscle's polymer core.
As the fluidic muscle contracts, its pulling force decreases. The force changes in relation to the mesh angle of the muscle's fibers. The more acute the angle, the greater the pulling force. The muscle exerts its greatest pulling force in the fully extended position. Its contraction membrane is made from non-elastic fibers oriented in a lattice grid and embedded in rubber material.
Applying either pneumatic or hydraulic pressure to the muscle causes radial expansion. This action produces tensile force in the axial direction, which shortens the muscle and generates an initial pulling force that is more than ten times that of a pneumatic cylinder at the same pressure and diameter.
Acting much like a spring, the force exerted by the pneumatic muscle varies as a function of its elongation. Maximum force is produced at the neutral position.
Although fluidic muscles are used as a substitute for pneumatic cylinders, they cannot be used as a one-to-one replacement in a given application. That's because they are typically twice as long as conventional cylinders for a given stroke length.
Various couplings and attachments facilitate use of the fluidic muscle in a wide range of applications, including assembly tables, presses, lifting equipment, and medical equipment. Descriptions of three applications follow, all of which have different requirements.
|10-mm diameter muscle: 400N|
|20-mm diameter muscle: 1,200N|
|40-mm diameter muscle: 4,000N|
|Maximum acceleration: 50 m/sec2|
Strength in bending application
Wagon Automotive (Waldaschaff, Germany)
Application: Fluidic muscles control the motion of two turntables in a bending machine for automotive door frame profiles. They provide a smooth and repeatable motion without slipping and sticking. The profiles are shaped by the motion of two turntables synchronously rotating around a single pivot point. The upper and lower parts of the bending tool mount on the lower table. Chain drives and servo motors were considered too costly for the application.
Design solution: Each 20-mm fluidic muscle develops high-tensile force coordinated with several other muscles to achieve the right material deformation needed for shaping door frames. The inherent characteristics of membrane contraction technology, where the highest force is generated at the beginning of stroke and then rolled-off as stroke increases, combined with the physical compliance of the membrane material when the muscle is not operating, offer the greatest flexibility in this application.
|Load: 1,000N (225lbs.) per muscle|
|Stroke length: 200 mm|
|Velocity: 10 mm/sec|
|Bending cycle: 20 sec per bend|
Harsh environment, vibration in holding application
DYWIDAG concrete works (Neuss, Germany)
Application: The fluidic muscle is for static holding and tensioning of a toothed belt in a facility that makes concrete railroad toes. An actuator is located next to a concrete vibrator in an environment that subjects it to dust, vibration, and occasional bumps and jolts. Previous attempts to hold a production belt in place with a pneumatic system failed due to dust collecting on the wiper seals of the piston, eventually destroying it over time.
Design solution: To clamp and hold the toothed belt, technicians selected a 10-mm fluidic muscle that was 200 mm long. The fully enclosed system resists dirt, concrete, and other contaminants that are prevelant in this harsh environment. The muscle also flexes and absorbs the occasional jolt.
|Load: 200N (44 lbs)|
|Stroke length: 20-30 mm|
|Holding force: 200N|
Slow, smooth motion for lifting application
D'Fly Jewelry Store (New York City, NY)
Application: The fluid muscle raises and lowers a 48- x 18- x 18-inch glass jewelry display case onto a pedestal base. The application required gradual start-up acceleration with no jerking or sudden movement.
Design solution: The 40-mm diameter fluidic muscle can cycle a 300-lb load from its starting position through a 406 mm (16-inch) stroke and back again in less than a second, but in this application it operates at a slow 0.64 inch/sec. It moves the jewelry case a total of 16 inches in 25 seconds. The muscle is positioned in the middle of the pedestal base. It provides smooth gradual motion within a dynamic range in which standard pneumatic actuators would be plagued by stiction, the resistance to the start of the motion.
As the muscle undergoes radial expansion, there is a roll-off of the linear force generated from angular changes in the energy exerted as the muscle deforms. The contraction force is greatest when the inflatable membrane deflects.
|Load: 1,335N (300 lbs)|
|Stroke length: 406 mm|
|Velocity: 0.64 inch/sec|