Pneumatic or electric controls?

I have some air circuits that are controlled by cams that actuate manual air valves that look like limit switches with roller arms that follow the cam. Would I be better off with electric limit switches on the cams and use electric air valves? The cam rotates at 120 rpm. The dwell on the switch is 45 degrees.

I do have to replace the air switches every 2 years or so.

I will NOT install a PLC or anything more complex.

I have a similar application coming up and will need to use electric relays to control air valves or air piloted valves to control air valves. Which is more reliable? I do have really clean, dry air.

Bill's post made me think of this.

Reply to
Buerste
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I will have to guess, but, since electrical relays bounce when making a transfer, the air valves will be more reliable.

Paul

Reply to
KD7HB

Why don't you replace the cams with discs with notches (the existing cams might actually still work) and use optical sensors to sense the up/down of the cam? This will have no wear at all except the shaft bearings of the cam. The sensors can then drive a single transistor to control DC solenoid valves. Some of the better valves can operate for millions of cycles. And, if they wear out, you don't have to adjust the cam timing when you replace them. I have salvaged sensors from old floppy drives and used them for a bunch of projects.

Jon

Reply to
Jon Elson

Dust problems? Optical isn't reliable long term in a dusty environment.

Reply to
IanM

WAY dusty atmosphere and I don't want to complicate a simple function. A mechanic can replace an air switch in 5 minutes. The time to troubleshoot anything more complex would cost $7/min in lost production.

Reply to
Buerste

As an Electrical Engineer I always think that electrical is the best solution. In a dusty environment magnetic sensing of the cam will be pretty reliable, think anti lock brakes. Magnetize the cam, Hall effect senor driving a transistor, driving an air solenoid. Add a couple of LEDs for trouble shooting. Output of hall not blinking replace the hall, output of the transistor not blinking replace the transistor, else replace solenoid. The solenoid will be 90 % of the reliability issues. The hall effect sensor and transistor are reliable if the design is good.

CarlBoyd

Reply to
Carl

CLIP

As a pneumatics guy most of the day, I was thinking that his set-up is pretty reliable now with a 2 year replacement schedule... Just schedule in a swap out of the switch every 1.5 years. :)

However, what about a reed switch-like device that senses the cam? Do they respond quickly enough to make it work?

Reply to
Joe AutoDrill

Use electrical. It's far easier to service and you should get at least

5 years (probably more) out of the cams/micro-switches. This was a standard Honeywell method and I always found it worked well, was easy to adjust, and fault finding with a meter was easy. Don't screw around with optical / position sensors - you'll spend more time fixing those.
Reply to
N Morrison

One difference between the approaches is that the cams operate the mechanical switches gradually compared to switches and electric air valves, so changing to electric might introduce some "hammer" when they abruptly open and/or close. The difference may also affect system timing, but once set up right it would tend to survive repairs involving replacement of switches and/or valves.

Reply to
Don Foreman

On Thu, 02 Jul 2009 09:47:54 -0500, the infamous Don Foreman scrawled the following:

I've seen air-damped solenoids, so I know they're available in at least some form factors, FWIW. Only the curious will learn and only the resolute overcome the obstacles to learning. The quest quotient has always excited me more than the intelligence quotient. -- Eugene S. Wilson

Reply to
Larry Jaques

This doesn't help on the switch end but they are some neat air operated valve actuators for retrofit on hermetic type valves, Asco, Skinner etc. see:

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Reply to
oldjag

Solenoids or solenoid valves?

Reply to
Don Foreman

No, these MicroSwitch pneumatic valves are snap-action, just like an electrical MicroSwitch. I have some of them in my pick and place machine, and they definitely do not respond slowly.

Jon

Reply to
Jon Elson

Have you carefully considered the profile of your cam? Adjusting things for minimum movement, and ramp angles the minimize the speed of the roller arms could help.

How many cycles do you make in two years? What is the pressure and diameter of the air line. I would think a roller tip micro switch and a quality relay should last for millions of cycles.

Reply to
Tim

Hmmm, I'd say an average of 28,800,000 in 2 years. Air line is 1/4" Nylon at @100 psi.

Reply to
Buerste

Right. They may be noticably quicker than mechanical valves operated by cams.

Reply to
Don Foreman

How accurate and repeatable does the trip point need to be?

I'd be tempted to use an aluminum cam sensed by an electromagnetic non-contact proximity sensor of some kind. This will work through most kinds of dust and dirt, and can be hosed down as needed.

What is the dust and dirt mainly made of?

Joe Gwinn

Reply to
Joseph Gwinn

Replace the valves with proximity switches and solenoid valves. I've done it a number of times on packaging machines and it has gotten rid of the reliability problems completely

Reply to
Grumpy

Dust is wood dust from drilling one 9/32" x 1/2" deep hole in Beech every rotation. The cam is "soft" activation and the trip point is +/- 10 deg or so.

Reply to
Buerste

This ought to be easy. Wood dust will not affect an electromagnetic prox sensor at all. A cam with sharp ramps will easily manage ten degrees. The absence of contact means that the cam profile need not be that gentle.

A cam made of aluminum with some kind of adjustable collet lock on the shaft ought to work just dandy with a prox sensor with circuit closure output.

One can buy adjustable ready-made cams where one can set the duty cycle by adjustment of the cam halves. Perhaps one of these will suffice. Failing that, one can always cut a cam with a scroll saw. As there is no physical contact, the surfaces need not be smooth.

Joe Gwinn

Reply to
Joseph Gwinn

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