Pneumatic or electric actuators: When and why

September 6, 2013 – 11:09
High Speed linear actuators - Linear Actuators, Linear Motors, TV

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Both pneumatic and electric linear actuators answer a wide and sometimes overlapping range of requirements for linear positioning of components and load movement in a wide range of machinery. Pneumatic actuators, Figure 1, provide defined stroke, quick response, and typical forces from 20 to 4, 000 lb. Electric actuators, Figure 2, provide a wide range of control options for the motion profile. Besides giving the motion designer wide flexibility, both types are clean, compact, and cost effective. But neither is ideal all the time.

Basic factors for choosing one style over another

Once an application is defined, you need to determine factors such as load weight and pressure, speed, acceleration/ deceleration, positioning, mid and end-of-stroke load support, system orientation, external force or bending moments, and actuator stroke length.

All these are important but the prevailing two are force and speed. It’s an exercise in over-simplification to say pneumatic actuators are best for providing lower cost muscle and speed while electric is best for controllable speed and positioning accuracy for intermediate stops. The system designer must consider the advantages of one over the other within a given application. The tradeoffs are many, including obvious ones such as cost, access to air supply, and operating environment. There is a justright answer to almost every situation. The system designer need only make a logical examination of options and tradeoffs to find the answer that is correct for his application.

Classification

Classifying types of actuator configurations helps to understand their application. Here are four common major types of rodless actuators.

The cable cylinder, Figure 3, uses a piston within a cylindrical tube to drive the cable. The cable wraps around end pulleys and provides motion within the actuator stroke length.

These earliest pneumatic “rodless cylinders” date back to the 1950s and continue today to enjoy a growing market, because they do certain things better than their newer pneumatic rodless or electric actuator counterparts. From large wrapping and packaging applications to moving theater stage sets, cable cylinders are preferred. Besides being economical and reliable, they provide motion around corners — motion not practical with other straight-line actuators. Cable cylinders are cost effective in longer strokes. Sheer size of application also gives cable cylinders the edge. Cylinder bores to 8 in. and custom lengths to 60 ft are available.

The band (rodless) cylinder, Figure 4, incorporates a slotted cylinder tube with inner sealing band and outer dust band. The pneumatically driven piston connects to the carrier bracket through the tube slot. Self-lubricating bearing rods on the carrier bracket transfer the load directly to the cylinder tube, making external guides unnecessary in many applications.

Consider rodless pneumatic cylinders when motion is repetitive and reciprocating. Examples include material handling, pick-and-place operations in product assembly, packaging, and paint spraying.

Source: machinedesign.com

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40 corrections per second

2007-12-06 12:27:37 by iamlucky13

What they mean when they say that, is that the computer reads the input states 40 times per second. Each time it reads the input states, it compares the values to expected values, then uses an algorithm to determine the desired control states. We now have computers with processing power to do it hundreds of thousands of times per second...much much faster than the response times of the systems involved and sometimes faster than any actuators can actually respond.
The algorithms are engineered from known factors like lift, drag, center of gravity, etc. Then some poor schmuck of a test pilot takes the plane up and determines if the engineers got it right or if it needs a little tweaking (hopefully not a lot of tweaking)

Fieldbus for steam-drain control  — InTech
Ultimately, when you minimize signals, you improve the overall efficiency and performance of the network.

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