Thursday, December 17, 2009

The Asparagus Harvester Invention – Trouble with cutting.

The Asparagus Harvester Invention – Trouble with cutting.

The harvester invention has been quite a challenge, and here are some more of the difficulties we had to overcome in the development of the machine.

Now that I’ve described some of the problems with sensing the spears and locating their positions on the bed, I’ll move on to the problems we had with actually cutting the spears.

For those of you who are unfamiliar with how asparagus grows, here is a little background. Asparagus is a fern, and the spears we eat are shoots that come up out of the ground and each of the little triangular shaped “brachs” at the tip of the spear will eventually become fern branches, unless you eat the spear first.

Once planted, the asparagus plant will stay in the ground and produce spears for about 15 years or so. Each spring you harvest the spears, and then let the plant become a fern. In the next spring you chop down all the fern, and once the weather gets warm enough the plants start sending up the shoots or spears.

If you cut the spears down before they can become a fern, the root mass or “crown” as it is called, which is about 6 inches below the top of the soil sends up more shoots. As long as you keep cutting the spears, the crown will continue to send up new ones. However, the root mass uses stored carbohydrates to supply the spears with food, and so you don’t want to cut the spears for too long or the crown will run out of food and become damaged.

A bed of asparagus ready for harvest looks like a forest of spears coming up randomly over the bed. The spears are all different heights and can grow more than 6 inches a day which requires you to harvest the field every day when its warm and growth is high.

Often spears are quite close together, and at substantially different heights.

The asparagus harvester can not stop to cut a spear, it must cut the spears as it travels through the field. Speed is critical because unlike most crops that you harvest once and move on to the next field, with asparagus you have to harvest the same field every day.

To avoid damaging spears that are not yet tall enough to harvest and close to a harvestable spear the blades must move very rapidly. Our harvester has blades mounted on the end of the piston rod of an air cylinder with a stroke of about 20 inches. Our cylinders can do the entire 20 inch stroke out and all the way back in less than 2 tenths of a second. It takes less than a tenth of a second to complete the extension part of the stroke.

Even so, since the asparagus harvester will be moving forward at 20 or 30 inches per second, in a tenth of a second the machine will have moved forward 3 inches. The faster the cylinder the less likely it will be to damage a spear that isn’t tall enough to harvest.
Making an air cylinder travel very fast is easy enough in theory, but just try getting an air cylinder manufacturer to build you one. The speed of the cylinders is determined by two main factors, the mass that has to be accelerated, and the maximum flow through the air piping, valve orifices, and cylinder ports.

To accelerate a piston rod assembly you simply need a lot of force. The more force the higher the acceleration. Since acceleration is force divided by mass, ideally we want a light weight piston and rod assembly with a large piston area. The large piston area converts the air pressure into a large force.

It turns out accelerating the piston rod assembly occurs in the first inch or so of stroke, and then you’ve reached terminal velocity. The terminal velocity is limited by the flow rate of the air. We used high CV (flow factor) valves with ½ inch ports, ½” air hoses, to make sure we had the highest flow rate we could achieve. The limiting factor was the size of the ports in the air cylinders.

The first problem we encountered was the inability to find air cylinders with large enough ports. Trying to build our own cylinders was prohibitively expensive due to the low quantity involved and not having the specialized equipment that would help lower the cost. For a long time we used 1-1/2 inch bore cylinders with ¼” cylinder ports. Now we have found a company that will build us our custom designed cylinder for a reasonable price. The latest cylinder is a 1 inch bore cylinder with ½” ports.

Since the spears need to be cut at or below ground level the blade must inter the ground to a depth of several inches to be sure and cut through the spear. But since the machine is traveling forward at 20 to 30 inches per second it’s not difficult to put the piston rod into a bind, and then the cylinder does not retract. This drags the blade through the bed damaging spears and after a few feet bends the piston rod.

At first we tried folding blades on clevis mounts, and spring mounted blades, but those ideas did not work out. We finally put the air cylinders on pivot mounts so if they did not retract quickly enough they would not bind which solved that particular problem.

Another huge problem was keeping the air cylinder from self-destructing. We could reverse the air valve to stop the extend stroke before it bottomed out against the front head, but on the return stroke the piston would hit the rear cylinder head at about 250 inches per second. The result was stretched or broken tie rods. We tried all kinds of springs on the rear of the cylinder and on the front of the cylinder under the blade mount. Springs don’t hold up. Less than a days harvesting would break any spring, metal or rubber that we tried.

We tried hydraulic shock absorbers but they don’t respond quickly enough to handle a cylinder that can fire 5 times in one second. They are way too slow.

We finally found success by mounting an air cylinder in the rear head of the main cylinder to act as a gas spring. Even then the cylinders would still disassemble