Testing An Invention Prototype - How To Do It Yourself With Junk Box Parts

DYI – How to Test a Prototype Air Cylinder for a New Invention

Background and development of our prototype air cylinder

My asparagus harvester invention uses high speed air cylinders to cut the spears of asparagus.  We’ve spent a number of years perfecting the cylinders, and I think we finally have a cylinder that will perform up to my expectations.

There are several problems I’ve encountered with pneumatic cylinders when you try to run them in the way my harvesting machine does.

The cylinders need to have a 20 inch stroke in order to cut the spear below the ground yet be out of the way of spears that are not long enough to cut yet.

In order to get accurate cutting I need to have the cylinders mounted side by side on 1-3/4 inch centers or smaller.  I could find no commercially available pneumatic cylinders that were narrow enough to mount on 1-3/4” centers and still have a large enough piston rod to prevent bending the piston rod during harvesting, so I had to build my own air cylinders from scratch. Years of field testing had taught me that the piston rods need to be at least 5/8” in diameter.  In order to put them on 1-3/4” centers the bore had to be 1” in diameter or less.  I used 1” bore cylinders.

The air cylinders are mounted at a 35 degree angle to the ground pointed in the direction of machine travel.  When the machine detects a spear tall enough to cut, (typically 9”) it fires the cylinder lined up horizontally with the spear.  That is why we need the long stroke, to clear those -7/8” spears, and still be able to penetrate beneath the ground.
 

High speed cylinders are a must

Our target forward speed for the machine is 2 to 2-1/2 mph.  2-1/2 mph is 44 inches per second.  If it takes the air cylinders 1/10 of a second to extend or retract the machine will travel forward 4.4 inches while the cylinder is extending and another 4.4 inches while the cylinder is retracting. 

The spears closer than 8.8 inches and lined up in the same channel won’t get cut because the cylinder won’t be able to cycle quickly enough. Any spears too short to cut but lined up with a harvestable spear is at risk of being struck by the descending blade depending on how tall and how far from the spear being harvested it is.  The faster the cylinder, the closer a spear can be to a harvestable spear and not be damaged as the blade descends.  Spears can also be closer together and still be cut with faster cylinders.

The faster the cylinder cycles the faster the machine can go through the field without missing spears. Asparagus spears tend to grow in clumps, so the spear spacing is critical.  For a machine to be economically viable it will have to get 75% or better of the crop when compared to hand harvesting.  That means out of every 10 harvestable spears the machine encounters it can only lose a maximum of 2 spears due to dropping, miss-cutting, machine damage, or just plain missing the spear.  In other words, every spear counts.

I’ve measured the speed of the air cylinders and they are a little faster than the example above… they actually take about .085 seconds (85 milliseconds) to complete each direction during the stroke.

The average velocity of the cylinder is therefore 22 inches divided by .085 seconds, or about 258 inches per second.  You can’t let the air cylinders bottom out.  They will beat themselves to death in just a few strokes.

In order to not bottom out the cylinder on the down stroke we simply reverse the air flow before the cylinder has a chance to bottom out.  Our cylinders are capable of 22 inches of stroke, but we reverse them at 20 inches.

It’s the return stroke that is the bigger problem.  We tried all kinds of springs, cushions, and even hydraulic shock absorbers.  We’ve found only when we use air cylinders as springs mounted on the rear of the big air cylinders are the cylinders able to survive for more than a few hundred strokes. Die springs, elastomeric springs, and custom made springs never made it to 1000 strokes without some kind of damage occurring. Built in cushions are useless, even special 2” long cushions didn’t last.

Continued on the next page: Invention Prototype Testing