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.
I/we mostly deal with hydraulics. Pneumatic servo control is a very small part of our business because there are usually better solution. However, we use pneumatics as a means of testing hydraulic algorithms. If one can control pneumatics the hydraulics and servo motors are even easier.
Here is an example of what can be done.
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We have controlled pneumatics in some low pressure testing applicaitons.
Besides the energy lost in compressing and decompressing air the second problem is that feedback devices are still needed and the controller can't be a simple cheap PID controller. It takes some complicated math to do the control shown and this isn't cheap. For small jobs little linear motors are tough to beat.
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.
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?
The last pneumatic positioner I worked on was in a vertical orientation, which created additional challenges for the actuator. The systems I have worked on used "magnetostrictive" position feedback ("Temposonics" would be one trade name). This feedback system is based upon timing pulses transmitted down a rod that get reflected back by a magnetic ring, or something like that. The time is converted to an analog output of some resolution. The resolution doesn't change as the length of the transducer gets longer, so the system resolution is typically affected by the transducer length.
The controls on this system were essentially analog in nature, comparing setpoints to the transducer position, and throttling air valves to try to keep the actuator in the correct position, and move it to new positions, etc.
The end result was less than what I would have desired (Fortunately I didn't pick the equipment, I was just called on to get it to work.). There was a tendency to overshoot significantly (large fractions of inches at least), especially when moving down. I don't know what the final position tolerance was, but it was not comparable to an electric servo actuator. It is also relatively noisy, due to the air valves constantly fighting with each other to try to position the load. I couldn't help but think that there would soon be a mechanical failure in the valves because of the frequency of the switching, etc.
Having worked with literally hundreds of servo systems of many different flavors, and a small number of pneumatic positioners (less than 10 probably), I would favor an electric solution unless there is some overwhelming reason to not use electric (explosive environment?).
Under ideal circumstances pneumatic positioners talk about 0.005" positioning resolution. That might be achievable under some circumstances, but if I were designing the system, I would be thinking a quarter of an inch and designing in energy absorbers and positive stops at the desired stopping points.
As a general substitution for this type of actuator, I would favor the belt drive style slides offered by a number of vendors. They offer high speed capability in a similar form factor, and the precision of a completely digitally controlled electric servo system.
It sounds as if you've had a frustrating experience with a poorly designed pneumatic servo system.
Modern, well designed systems are able to rapidly position heavy vertical loads without overshoot, without high levels of valve control activity, and to high levels of repeatability. The 0.005" accuracy you mentioned is readily achievable in many systems.
I would compare moving a load with air to using a rubber bat to play baseball, or a rubber stick to play hockey. You could do it, but why would you? ("Because I can" is not a good reason.)
What are the selling points for such a system?
If the system is not exponentially less expensive to purchase and operate, I don't see the advantage. I doubt if the systems are much less expensive to purchase than a comparable electric solution, and I seriously doubt whether the operation cost is lower, when the cost of clean compressed air is factored in. Then have one incident where the dryer floods the air line with contamination and see what the cost of repair is.
For explosive environments I can concede that air is compelling. Otherwise, I don't get it, and I wouldn't design it in over an electric servo system.
For grippers, clamping cylinders, and a host of other short stroke, bang-bang actuators, I'm all for pneunatics. It is also great for certain counterbalancing systems. I just don't see it for positioning systems.
You could play hockey with a chop stick too. Maybe you'd do so to handicap a much less capable opponent? But it's a false analogy to the pneumatic - electric - (hydraulic) debate.
In applications with high cycle rates and high masses, pneumatic servos can be 25% the cost of a linear motor solution, and 30-50% of the cost of an electromechanical solution - while requiring less space.
For many applications that require a combination of controlled, rapid translation (12" in 100ms) coupled with a high force joining operation (think resistance welding, hot melt, etc.), servo pneumatics again has a similar big cost advantage.
Generally speaking, if you look into many of the positioning, general automation applications in packaging and similar industries, you can replace a linear electric axis with servo pneumatics with the benefit of 50% less installation cost, and a 50% increase in cycle rates.
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.
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.
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.
Pneumatics is becoming an important component in the field of Mechatronics for efficiency and motion control precision. Festo is a leader in pneumatic based robotics with their gripper and arm products. I remember taking a Pneumatics class at community college back in the early 80s. I didn't see the importance of this class because of my fascination with electronics. I which more emphasis on system integration was discussed in class as it relates to Mechatronics, maybe I would have considered working for engineering companies like Festo who makes great and cool pneumatics based robots instead of just electronic/semiconductor jobs.
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 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.
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.
Chuck, Forgot the second part of your question. I don't think pneumatics is too costly. The precision limits are a reflection of the underlying technology, especially when compared to electromechanical solutions which can achieve very high precision, resolution and repeatable accuracy.
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