Prototype Testing Results For the Asparagus Harvester Pneumatic Cylinders.

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The results of my initial testing were surprising in that my hypotheses turn out to be wrong on several counts.






Prototype Testing Results For the Asparagus Harvester Pneumatic Cylinders

Continued from the previous page:  Electronic Air Pressure Regulator


Doing the air cylinder testing

Now I have the air supply ready, the optical switches, the counter, the cylinder controller, and the air cylinder ready to go.  Since I now have pretty good instrumentation and I have the time to do it, I'm going to learn all the things I've wondered about but didn't have the time or resources to actually find out.  Like what affect does the weight of the blade and blade mount have on the speed of the cylinder, and what difference would there be in the speed of the cylinder with different types of seals.

I set up my oscilloscope to measure the output pulses of the air cylinder control valve controller.  I began by setting the output pulse period to 50 milliseconds with 30 seconds between pulses.  That would give me time to record the number of counts (milliseconds) the counter recorded each time the cylinder fires, and reset the counter.

I set up the test by putting the laser beam from the first optical switch slightly in front of the end of the piston rod. With the first 1/16 of an inch of extension the rod will cause the switch to send out a pulse.  I placed the second switch 20 inches further down the path of the piston rod.

I decided I would begin with a low pressure, taking measurements as I gradually increased the pressure being careful not to bottom out the piston rod on the down stroke. I would start at 60 psi and increment the pressure in 10 psi steps until I reached 130 psi.

The first series I would do with no blade mount on the piston rod.  The next series I would do with the blade mount threaded onto the end of the piston rod to see what affect mass would have on cylinder speed.  I actually have several prototype blade mounts with different masses to test if need be.

Then I could repeat the procedure with different rod and piston seals, different amounts of lubrication, and I could reposition the optical switches to measure the time hesitation at the end of the stroke etc.  Looks like I may have days and days of testing to do.

The results... what I learned from the initial testing

Well, the results of my initial testing were surprising.  As is often the case my hypotheses turn out to be wrong on several counts.

I found that the mass of the blade mount makes less difference in the cycle time than I expected.  I tried several mounts, the lightest being 3.4 ounces and the heaviest being 8.2 ounces.  Even the heaviest mount increased the down stroke time from about 80 milliseconds to about 85 milliseconds.  That is a good sign. I won't have to worry about making the blade mount overly lightweight.

Another unexpected result was how sensitive the cycle time, and thus the stroke length, were to lubrication.  I don't have low friction seals installed at this point and I will be obtaining some for testing.  I think the low friction seals will reduce the dependency on lubrication significantly.

So far I have only measured the down stroke, but I did take some movies of the cylinder in operation and by reviewing the movies in windows movie maker I can view them one frame at a time. From the movies it appears the upstroke has about the same speed as the down stroke.

From what I understand there are two main elements affecting the speed of the cylinder, the force pushing on the piston which provides acceleration, and the speed at which the air can flow into and out of the cylinder which limits the terminal velocity of the piston rod assembly.

I haven't yet done the acceleration testing, but if the acceleration happens very quickly right at the beginning of the stroke, then the speed at which the air flow would be far more important in so far as the cycle time is concerned.

That is most likely what is happening.  When I designed the cylinder I made sure the air flow path had no restrictions.  I've never seen a commercial air cylinder with a bore equal to or larger than 3/8" and those were custom built for me. Typically on a 1-1/2" bore cylinder the ports would be 1/4" pipe.  Probably 1/8" ports for 1" bore cylinders.

My 1" bore cylinders have full 1/2" ports. To keep the pressure drop and hence the air flow velocity has high as possible I've kept the hoses as short as possible, used a large 1/2" ported directional control valve, All connecting hoses are 1/2" I.D. hoses.  This cylinder can breathe!

Continued on the next page:

Cylinder cycle time and rod speed results


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