Mechanically Harvesting
Asparagus - The Problems
Here is a brief description
of the main obstacles to successful mechanized harvesting of asparagus.
Leaning Spears
If all asparagus spears grew
straight up without bending, curving or leaning the task of machine
harvesting it would be far easier. However this is the real world
and asparagus spears tend to lean, curve, and bend randomly all over the
place. This makes it difficult for machines to figure out where
exactly the spear enters the ground.
Most experimental selective
asparagus harvesters detect the spear near the top at the harvesting
height, typically 9" above the ground. Any leaning of the spear can
cause the wrong blade to fire and the machine will miss the spear, or
only partially cut it, or if leaning forward or rearward the leaning can
cause an early or a late cut.
At times when it is very
windy, the wind picks up grains of sand and throws them against the spears
from one direction. The spears react by growing in the direction
of the oncoming sand particles. A whole field can end up with all of the
spears leaning significantly in one direction. The spears also
tend to "hook" or curve as well, which makes it even more difficult for
a machine to figure out where the spear hits the soil.
Spears Too Close
Together
When spears are very close together a new set of
problems develop. Asparagus spears tend to clump together since
many spears come from the same root crown. Sometimes the spears
that are very close to one another are the same height and sometimes one
is much taller than the other. Tightly spaced groupings of two,
three, four and more spears are fairly common, in all combinations
of spacing's and heights. Even hand crews sometimes accidently cut or
damage an immature spear that is in close proximity to a harvestable
spear. When a machine detects a spear it doesn't know
whether or not a short spear is nearby. When the machine harvests
the tall spear or spears, if it even nicks a shorter spear the spear
will hook over and become a cull. The width of the blade/sensors becomes a
consideration. The wider the blade and/or sensor, the
more likely the machine will be to damage a nearby spear. To
minimize this damage we would want to make the blade as narrow as
possible.
However, as you make the blade narrower you also must
increase your accuracy, otherwise you will miss the spear or not cut all
the way through it. If your blades are one inch wide and the spear is
leaning an inch and a half to the side at the sensing height there will
be a problem.
When the adjacent spears are both of harvestable height
things usually work out, but when one is too short you end up with what
I call a "phantom" spear. A phantom spear is one that should be
there in the future to harvest, but now it won't be since the machine
damaged it. The spear could have been ready for harvest anywhere
from the next day to over a week if it hadn't yet emerged from the soil.
The spears grow much more rapidly after emergence.
Brittle Spears
When the asparagus is very cold it can be very brittle
and sometimes after a rain they can be more brittle than normal.
During such times anything bumping into a spear is likely to snap it
off. Especially at high speeds. For machines that use photo
electric beam breaking this can be a serious problem. The support structure for the beams reaches
down to the cutting height. This means that any spears of
harvestable height can break off if they contact the comb like structure
of the sensing mechanism. It's like running a giant comb through the
asparagus. Most of the machines I've seen through the years use this
method of spear detection.
The same holds true for the picking up
mechanism. If the mechanism has to reach all the way to the ground
when grasping a spear then there is a high likelihood that it will
occasionally break and damage spears as it attempts to pick up a
harvested spear. The closer to the ground of anything that can
touch a spear, the more likely it is that it will indeed contact a spear
seeing as how there are more short spears by far than harvestable spears
at any given time.
Picking Up the Harvested Spears
An asparagus harvester needs to grasp a spear before it
cuts it. Once cut it would be unpredictable where the spear would
end up and locating the spear would be technically difficult and time
consuming.
If the machine stops at each spear before cutting it the
picking up isn't too difficult unless the spear is leaning
significantly. However, if the machine has to stop for each spear to
pick it up the machine will be far too slow to be effective. The
picking up mechanism needs to work on the fly.
Leaning spears cause a multitude of problems.
In order to grasp a leaning spear the pickup mechanism must open wide
enough to envelope the whole range of positions the spear might be in.
Both the Haws asparagus harvester and the Oraka machine use a form of
grippers. Making the grippers open wider increases the chance of the
grippers closing on more than one spear. The other spears may or may not
be of harvestable height. As a result spears can be dropped or damaged.
When a gripper arrangement
is used it can have difficulty when it encounters clumps of spears which
of course are very common in an asparagus field. As an example, the Haws
machine has a 3" wide channel, so when the grippers open to grasp a
spear they are spread apart 3 inches. If they then close on a two
spear clump several things can happen. The grippers might try to close
with enough force that they break one or both spears off, or they might
not have enough strength to break the spears in which case the blade
which is affixed to one of the gripper arms may or may not cut through
the spear.
Asparagus Presents A Narrow Harvesting Timeframe
At first it might not seem like a problem, but asparagus
grows so damned fast it can be a very big problem if something goes
wrong with the machine. Asparagus can grow over 7 inches per day.
At that rate if your machine breaks down for 8 hours you have a huge
problem. Much of the asparagus will now have become unusable and much of
the usable spears will be discarded due to the length of the spears.
To be profitable an asparagus harvester will have to be extremely
reliable and easy to repair should something go wrong.
Keeping it simple is a key element. A selective asparagus
harvester needs to be simple enough for a farmer to understand the
mechanisms and to be quick and easy to repair by a person of limited
skill with no special tools required.
Read about how the Geiger Lund selective
asparagus harvester has overcome these obstacles.
Selective Mechanical Asparagus Harvesting - Solving the Problems |