Monday, February 8, 2010

Inventing a Selective Asparagus Harvester

For the last week or so I have been trying to do some life cycle testing on the pneumatic cylinders that we are going to use for the next asparagus harvesting machine we build. The cylinders should be able to do about a million strokes before the need to be replaced.

The machine has a row of pneumatic cylinders, or often referred to as air cylinders, arrayed across the asparagus bed. As the machine moves forward a sensing system locates the spears and tells the air cylinder lined up with the spear when to cut.

The cut signal causes the piston rod to extend from the cylinder at high speed with about 18 inches of stroke. It takes less than a tenth of a second for the cylinder to extend to full stroke. On the end of the piston rod is mounted a blade that cuts the spear.

I set up a fixture out in my garage for testing the cylinder. I’ve got it mounted to a frame similar to how it will be mounted on the asparagus harvester, pointed down at the ground at around 45 degrees.

I filled an asparagus crate or lug box, lined with plastic, full of dirt from the back yard. I placed the crate of dirt so that when the cylinder is extended the blade goes about two inches deep into the soil.

I actually went to the grocery store and bought a bunch of asparagus to test the cutting ability of my blades. I wanted to see if I could detect a difference between a blade with a V notch in it, a slanted edge like a guillotine, and an arrowhead type blade.

I tamped the soil down till it was nice and firm, and then used a dowel to make a hole just big enough to get an asparagus spear into. Then I pushed a spear into the hole and tamped the dirt down around it. I lined up three spears so the blade would contact the first spear while in mid-air, the second spear right at ground level, and the third spear would have the cut line about an inch below ground.

I tried this with all three blade types, and I could find no difference at all in the cutting ability or anything else. The blades sliced through all three spears like they were made of butter. There was no deflection or twisting of the blade, so my new secret method of preventing blade rotation seems to work well.

Since revealing details about an invention online would compromise my patent rights I can’t go into details about the new method I am using to prevent the blades from rotating out of position.

I would like to do the life testing at 150 psi, but my compressor only goes between 120 psi and 135psi as it cycles. So I set the air pressure for the testing at 120 psi.

I’m interested in the life of the seals, and whether the piston rod ends up breaking due to metal fatigue. The load placed on the end of the piston rod by the blade and guiding assembly is offset from the center of the piston rod.

On the down stroke the pneumatic valve reverses the direction of the air to the cylinder before the cylinder reaches the physical end of its stroke to prevent damaging the cylinder. On the return stroke the piston hits the rod end of a smaller cylinder screwed into the rear head of the cutting cylinder to act as a spring and absorb the shock loads.

My compressor can just barely keep up with the cylinder if I fire the cylinder every 20 seconds. It’s going to take a long time to get anywhere near a million strokes. I need a much bigger compressor.

To cycle the cylinder I used a 12f675 micro controller chip, an 8 pin chip with a microprocessor, memory, and various interface modules like analog to digital converters, comparators, and counters all included. Even an accurate clock is built in. Learning to program and use these microcontroller chips should be in every inventor’s toolbox.

I programmed the chip using a basic language. I used a breadboard, a couple of pots and a voltage regulator etc along with the chip to create an automatic cycling controller. It has two pots. One pot controls the time between firings and the other determines the length of the pulse sent to the air valve. The longer the pulse the longer the stroke produced by the air cylinder.

I’ve tested a whole lot of air cylinders with this method and I’ve yet to find one that would even go 10,000 cycles without developing a problem. I think this time I’ve got an air cylinder that will hold up for that million strokes I need.

These new cylinders I’m using have a 1” diameter bore. The cylinders I’ve used previous had a 1-1/2 inch bore. There is a big difference. The smaller surface area of the piston means the force is much smaller. The acceleration is determined by the force, and the new cylinder is much more sensitive to variations in pressure. That is something that the asparagus harvester invention will have to address.

In a future article I will describe in some detail the pressure problems and the special electronic air pressure regulation system I intend to use for the machine.

To learn more about my selective asparagus harvester invention visit: Selective Asparagus Harvester