Chuck, During the interviews for an article I did on servopneumatics last year, the parameters they used to describe the applications for the technology is where high precision and positioning accuracy (<0.2 mm) is NOT required. Not sure how they would answer the resolution question. They also identified a good application fit for servopneumatic axes when there is a need to move large loads continuously (24/7 or high duty cycle operation), positioning systems operating on a low voltage, or solutions with space constraints requiring high feed forces and high dynamics. Hope that helps.
Al, what's the best positioning resolution a servopneumatics system can get these days? You mention that servopneumatics isn't being used in high-precision applications. Is that because they can't get the resolution or is it because the resolution is too costly?
An advantage of air being compressible is that provided you use sensible size actuators that give enough force to work but are not over-sized, accidental collisions or foul-ups need not cause serious damage. I'm not recommending frequent foul-ups but they'll happen sooner or later if an operator loads something incorrectly or a work-piece falls out of a gripper. Foul-ups with hydraulics usually end in tears before bed-time. I have also seen buckled lead-screws from even quite modest servo positioners where torque limits haven't been set correctly.
I wouldn't use pneumatic devices for precise positioning, but for devices such as actuators and grabbers they seem to work well. If you have a positioner that moves from one limit to another, pneumatics might do the job quite well.
Unfortunately, for me it was baptism by fire, the school of bumps and hard knocks, etc. My normal role was the controls programmer, trying to get things to work, which forced me to learn.
If there are vendors in your area that sell pneumatic components, I would contact them. They usually have demo equipment, and might put on training sessions. I think you really have to get your hands on the hardware to get a feel for things. It is especially interesting to create situations where the actuators are undersized for the load. You can really see the effect of air as a compressible fluid.
I endorse comments by ttemple. In the right place, pneumatics can do a great job and I have found it quite fun to do the PLC programming, provided the mechanical designer has done his homework and you have the requisite sensors, reed switches etc. However, I also know lots of customers who seriously underestimate the cost of compressed air, with or without air driers and purifiers. I'm glad I never had to pay the running costs...
As for hydraulics, even in the right place they are a serious challenge as well as being even more expensive to run. My recollections of hydraulics projects seem to centre round footwear ruined by contact with Shell Tellus 22 (other brands are similarly corrosive I'm sure). At this moment I'm holding a daily journal notebook for 1969 that still smells of hydraulic fluid!
I have many years (30+) of experience using pneumatic equipment in the industrial automation field.
I will respond to some of your statements:
1) "Air comes in an endless supply". True, but compressed air doesn't. Compressed air is very expensive. Compressors are not efficient, and leaks (all over the place) are inevitable. I have seen studies showing what the equivalent of a 1/8" hole in a compressed air system costs per year, and it is staggering.
2) "the use of clean air...". Clean compressed air is even more expensive than compressed air. Moisture is a huge problem in compressed air systems, and it requires expensive equipment to get the moisture out of the air. When moisture breaches the barriers that are designed in, it can be disastrous to the affected equipment.
3) "They don't dissipate energy...". Virtually all pneumatic devices operate by exhausting compressed air to the atmosphere. That expensive, clean, dry, compressed air is simply exhausted to atmosphere once it has been used to move a cylinder from one end to the other. It might be true that the device is not dissipating energy, but the compressor sure is.
4) "it might seem like it is anyone's guess where engineers pick up the knowledge..." I completely agree with that. I have probably been involved with more bungled pneumatic systems than any other subsystem in the automation field, including servos.
As has been pointed out, the compressibility of air is a problem. I find it to be an almost insurmountable problem with pneumatic equipment. Engineers don't seem to understand how to properly apply components to deal with the problems brought about by the compressible nature of air. The sizing of actuators, fittings, valves, and lines is critical to the proper operation of many pneumatic systems. It is rarely considered in the design phase. (It's a good thing that undersized hoses don't burst into flames like unsersized conductors can!)
When properly applied, pneumatic components have a place in automation systems. Properly applying them is challenging.
From my experience, any attempts to make positioning systems out of pneumatic actuators is an exercise in futility, especially given the availability of similar form factor servo based systems.
I, too, did not have the opportunity to play with pneumatics. But I recognize the power and smooth operation of hydraulics and pneumatics. I think there is a need to expand our experience and education with both electrical and mechanical engineers. We electrical guys need to know how to control the hydraulics/pneumatics that the mechanical guys design into products.
Truchard will be presented the award at the 2014 Golden Mousetrap Awards ceremony during the co-located events Pacific Design & Manufacturing, MD&M West, WestPack, PLASTEC West, Electronics West, ATX West, and AeroCon.
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