Solving The Problems Of Mechanical Asparagus Harvesting
Sensing The Leaning Spears
Let's begin by examining the sensing system. An
ideal asparagus sensor would give us the location that a spear entered
the ground if it was long enough to harvest. With a straight spear
that does not lean in any direction that is not a problem. Most
spears lean though. Our sensor will need to be able to find the
location of the base of the spear even if it is leaning in any
direction. An Accurate High Resolution Asparagus Spear
Detection System For Detecting Leaning Spears
The sensing system used by our harvester, the Geiger
Lund machine, is a unique concept for sensing asparagus that is far more
accurate than any sensor ever used for sensing asparagus, especially for
leaning spears! Our system utilizes two laser beams
across the bed with a series of lenses looking down at an angle at the
two beams. The upper beam is set to the cutting height and the
second beam is set about 4 inches off of the bed. The sensor
incorporates a multitude of microprocessors to process the data from
photo detectors behind the lenses and to set the correct timing to fire
the cylinders. A speedometer feeds the processors with the speed
of the machine many times a second. The cut timing is accurate
from less than 1/4 mile per hour to over 3-1/2 miles per hour.
When a spear is illuminated by the bottom laser the appropriate lens and
photo detector provide the electronics with the location across the bed
so the correct blade will be fired. But unless the upper beam is
also broken the spear is not long enough to harvest. If the channel
directly above the activated lower channel or the channel above and on either side
is also activated then the system knows the spear is long enough to cut
and fires the blade.
The location is obtained much closer to the ground than
other harvesters so a leaning spear is far less likely to be missed.
Since spears tend to hook or curve as well as lean, the location at 4
inches above the bed is much much closer to where the spear actually
enters the ground than at nine inches above the bed, making our sensing
system by far the most accurate ever used. Since nothing over the bed extends down to
the cutting height there is nothing to break spears as the machine moves
along the bed. It is a totally non-contact system.
Our state of the art opto-electronic sensing system will
sense any spear down to the diameter of a coat hanger wire or smaller.
The sensor is immune to ambient light and works as well in total
darkness as in broad daylight. Dust and dirt have virtually no affect.
As I mentioned earlier our
last machine had a blade width of 2-3/4 inches on 2-1/2 inch centers.
The blades slightly overlap. Our sensors were also on 2-1/2 inch
centers and had much more overlap than the blades.
The sensor
channels were wider than the blades, having a field of view of
about 4 inches compared to a blade width of about 2-3/4". It seemed like a good idea at the time to have the
overlap, but in hindsight it was a terrible idea. It resulted in nearly
always causing multiple blades to fire creating significant collateral
damage to adjacent spears.
Brittle Spear Problems
Minimized
Because the spear sensing
system has no parts that come near the cutting height over the bed it
will never make contact with spears and thus won't break any.
The first thing a spear encounters with the Lund machine
is the pickup unit. The pickup unit consists of longitudinally
oriented shafts at an angle to the ground of about 40 degrees. The
shafts are counter rotating and covered with slim rubber fingers.
When the tip of a spear contacts the rubber fingers they guide it
between the rollers that are exerting an upward pull on the spear.
If the fingers are rotating at too high of a speed the fingers will
exert a rearward pull as well as an upward pull on the spear. If the
rollers are rotating too slowly the pickup unit will try to drag the
spears forward with the motion of the machine. Ideally the roller
speed is adjusted to match the ground speed to exert mostly and upward
pull on the spear.
Brittle spears are much better able to handle a straight
upward pull than being bent over as the incorrect roller speed would
cause. Since the bottom of the finger unit (the rollers) is about 6 inches
above the ground, any spears less than 6 inches tall never contact
anything at all. The harvested spears pass through three sets of rollers
in order to reach the conveyor which takes them off to the back of the
machine to be sorted.
Spears Too Close Together
There will obviously be many instances where adjacent
spears are so close together that if one is too short to harvest it's
going to get sacrificed. To minimize the collateral damage one can
reduce the blade width and optical field of view for better resolution.
We have done that in spades.
We've decreased our blade
and channel width to 1-3/4 inches wide. That gives us the narrowest
blade/channel width of any machine. The haws machine has a channel width
of 3 inches. The Oraka machine had a blade 2 inches wide. We've
also reduced the optical field of view from about 4 inches to 2 inches.
This provides a tiny bit of overlap to send both blades down when a
spear is directly between two blades.
These adjustments
will result in a significant reduction in collateral damage.
Keeping It Simple To Minimize Downtime
The Geiger Lund machine is simple, easy to understand,
and very reliable. If something breaks it will be easy and quick to
repair.
Air Compressor
Let's begin with the air compressor. The air compressor
is obviously a critical part of the machine. We looked at vane
compressors, screw compressors and various others. We decided on an old
fashioned two stage piston type air compressor. The compressor
needs almost no maintenance and has an estimated life of 50,000 hours.
Running your harvester 24 hours a day for 60 days a year would result in
a life of expectancy of over 34 seasons.
Pickup Units
The pickup system with it's 10 shafts covered with
rubber fingers is another example of simplicity. Each of the
shafts passes through two ball bearings and terminates in a gear. A
hydraulic motor drives the gears. Doesn't get much simpler. And
the pickup units are built to last. Since the bearings have
virtually no load on them and they are spinning at a relatively low rpm,
they should last many years. The gears are steel with a 1/2 inch
face. They should out last the compressor.
If for some
reason a shaft, gear or bearing needs to be replaced one simply removes
3 bolts from the cover plate, loosens the shaft collar, removes the push
nut from the bottom of the shaft and slides the rubber fingers off.
Replace the defective item whatever it is, and reverse the process.
Replacing a gear or bearing should only take about 20 minutes, if it
ever needs to be done. The fingers mount on 1/2 inch diameter
steel shafts which are not easy to bend.
Cutting System
The cutting system is another example of simplicity.
The machine has 12 air cylinders mounted side by side across the bed.
Affixed to the end of each piston rod is a blade. Each cylinder has a
single solenoid air valve to actuate it. A 12 volt signal from the
sensor sends the blade down, and when the signal is removed the valve
retracts the cylinder. That's all there is to the cutting system.
These are not ordinary air cylinders though. Not many air
cylinders with a 20 inch stroke are capable of over 5 complete in and
out strokes in one second. These air cylinders are built more like high
pressure hydraulic cylinders. The cylinder barrels are made from
3/16" thick steel tubing welded into the front and rear heads. No
tie rods to stretch or break and no tie rod nuts to work loose.
The piston rods are welded into the pistons.
Rebuilding a cylinder
takes only a few minutes and the cylinder need not even be removed from
the machine.
Conveyors
The machine has only three straight conveyors. One runs
across the front of the machine under the headers and collects the
spears from all four headers, taking the spears to the elevator
conveyor. The elevator conveyor takes the spears up and to the
rear of the machine depositing them on a third conveyor. The third
conveyor is the sorting conveyor where a man stands and sorts the
asparagus spears placing the good ones in boxes and discarding the
culls. All three conveyors are straight plain simple
conveyors. Obviously they never have a heavy load on them, and they all
travel at low speed, assuring a long life. The conveyors all have V-belt
guides to provide positive tracking and problem free operation.
The hydraulic motors all mount directly to the drive roller shafts
eliminating the need for flexible couplings.
Hydraulic System
The hydraulic system is, like everything else, simple
and straight forward. A PTO mounted pump on the tractor turns a
motor on the harvester. The motor is connected to a pressure compensated
pump which provides the hydraulic pressure for the pickup rollers and
conveyors. By using a pressure compensated system it is easy to
control the speed of the rollers and conveyors using a simple flow
control valve.
The system pressure will be kept to a low 1,000 psi and
the flow requirements are quite low typically less than one gallon per
minute for any of the circuits. The hydraulic reservoir is oversized to
keep the oil temperatures low. All hydraulic components are commonplace
and easily obtainable.
Spear sensing system
The sensor electronics are extremely rugged and
reliable. There are no moving parts and no routine maintenance.
There are only three circuit boards and should a problem develop any
board can be replaced in minutes.
The lasers have an expected
lifetime of 100,000 hours. |