Jon, I wonder if there is really an advantage to pneumatics. They do have some of the advantages you cite. On the other hand, they are not, I suspect, as controllable as electrical devices. Many years ago my father, who worked at a government lab, thought that hydralics would take over for many applications. They were making the equivalent of control circuits with hydralics. He even broght home some of the machined plates to show me. Well, that never happened. The controllability of electro-mechanical devices will make them a prime contender for some time to come.
The power source has to be compatible with the application. Hydraulics are usually low speed, high torque, and high power. Pneumatics are usually high speed, low torque, and low power. I have seen many machines with pneumatic cylinder actuators. There are also pneumatic logic elements that can be used to control a process without electrical solenoid valves. My Fluid Power courses included both hydraulics and pneumatics.
In the '60's and '70's, fluid logic circuits got quite a bit of press but as far as I know the uses these days are few and far between. Why bother with fluid gates when they're available on a chip as electonic components?
There is alot of work being done on the controllability of pneumatics using proportional control and more extensive use of sensors in systems. Plus, servo pneumatics is emerging as a technology that offers the flexibility of multi-position and force control with position and velocity monitoring. Positioning and force tasks are linked and sequenced, reducing PLC I/O requirements and programming complexity. There are still many simpler applications where the price points of pneumatic systems make them competitive with other technology alternatives especially in apps that have used pneumatic solutions in the past.
Al, I'm curious about the uptake of servopneumatics. Have engineers been able to get past the issue of the compressibility of air? Mathematically, this is a really intimidating subject, which I think desrves more study at the university level.
Chuck, The overall precision of pneumatic axes is still a concern, the compressibility of air being one variable, but many applications don't require high precision. When there is a need for balance between cost, flexibility and the need for precise movements in the five to ten micron-range is not required, servo pneumatics can fit into that gap. The technology is working to take the best of both worlds, and combine the flexibility and software control of electromechanical systems with the speeds and feed force advantages inherent with pneumatic axes.
Other areas where the technology fits are handling of hazardous products such as explosives where you can't guarantee the surrounding air is clean and there is a need to operate on a low voltage since a servo pneumatic system can operate on a 24 VDC supply. Another is where there are space constraints in the machine design, and no high position accuracy requirements.
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.
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 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!
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