Thursday, December 23, 2010

Electrical System Design - Experimental Selective Asparagus Harvester

Designing the Electronics and Electrical System for the Model 2010
Selective Asparagus Harvester

I’ve found that when I am designing things it helps me to write about the process. When I try to describe to my readers, (if I have any readers), the how and why for doing something it often helps
me improve and perfect my designs.

Today I’m taking a look at the overall electrical system including the source of power, without going into a lot of detail about the “harvesting circuitry”.  The functions of detecting the spears and providing a time delay before activating the cutters will not be included.

I’ve seen a number of my competitors’ asparagus harvesting machine prototypes, and not everyone uses 12 volts for their electrical systems.  Kim Haws has a prototype harvester that uses 24 volts.  I don’t like the idea of needing two 12 volt batteries for the machine.  Regulating 24 volts down to 5 volts is hard on the voltage regulators as well.  I imagine a 24 volt alternator is more expensive and harder to find than 12 volt alternators.

Another experimental harvester, the ORAKA machine, uses 220 volts.  They need the 220 volts AC for the servo motors they use.  I think using 220 volts as the primary system voltage is a crazy
idea.  220 volts can electrocute a person!  I’ll use 12 volts dc for the primary system because
it’s readily available, safe, easy to get parts for, and the components are inexpensive.  The harvester electronics operate primarily on 5 volts which is easy to provide by using a 5 volt three terminal regulator on
each circuit board.

The valves are all 12 volt dc solenoid valves.  A simple open collector Darlington transistor is the only interfacing I need between the electronic circuitry and the valve solenoids.  I figure the electronics will draw about 3 amps average while it’s operating.  The solenoid valves are 6 watts I think, for the air valves, which draw about ½ amp then they are drawing current.  The on pulses last for about 45 milliseconds, so the average current draw is very small.

The hydraulic valves have 12 watt solenoids so they draw about 1 amp when on and they too are very intermittent, although the on and off times will be quite a bit larger than 45 milliseconds of the air valves… more like a few seconds at a time.

Since the harvester will be used at night it will need lighting at some point for night operation.  By having a car
battery and a 100 amp alternator I should have plenty of current to run lights without interfering with the electronics. I will belt drive the alternator off of the motor being driven by the tractor’s PTO pump.  That way whenever the harvester is operating the alternator will be producing current.

I will mount a pressure switch in the hydraulic line between the PTO pump and the drive motor that connects the field terminal on the alternator to the battery.  That way when the pump is not running and the alternator isn’t spinning the battery won’t drain through the field windings of the alternator.

I am considering having a voltage monitoring circuit that would sound an alarm if the battery voltage drops below 12 volts.  When the voltage drops below 12 volts the solenoids may start having problems and we don’t want that. 

By having a totally isolated electrical system with its own battery and alternator the farmer does not have to tie into the tractor for electricity. It makes hooking up to the tractor very easy. All you need to do is hang the PTO pump on the PTO shaft and hitch up the harvester.

The risk of damage to the electrical system due to improper hookup to the tractor is eliminated, and the possibility of electrical noise getting into the electronics from the tractors electrical system is eliminated.
I haven’t decided how to “turn on” the harvester electronics.  Should I have a switch?  Should I have it come on when the alternator pressure switch signals the tractor is turning the PTO pump?

I’m leaning toward having a specific switch for turning on the harvester.  I think for safety reasons it’s probably best if the machine needs to be “turned on” by the operator.  I do plan on incorporating a speed switch that won’t let the blades fire unless the harvester is moving forward at some minimum speed.
I’m also thinking of having the harvester produce a “beep” from the alarm periodically if the electronics are “on” and the PTO pump is not spinning.  That way you won’t accidently leave the harvester on when you finish harvesting for the day and shut off the tractor.

The header tracks the height of the bed and remains 8” above the bed for proper cutting.  Two hydraulic cylinders raise and lower the header tracking the bed height.  A metal rod drags on the bed and two inductive proximity switches provide feedback about the bed height.  The bed height cylinders which raise and lower the header are controlled through two hydraulic directional control valves with flow controls installed.
One valve is used for tracking the bed height and is adjusted for a low flow, and the other valve is for raising the header rapidly.

If an air cylinder extends its blade down to cut a spear and doesn’t retract for some reason it will be damaged as the machine travels forward with the blade stuck in the ground.  The air cylinders are front pivot mounted so at first they just begin rotating, but after about 2 or 3 feet of distance the cylinder comes to a stop and the piston rod will probably get bent shortly after that.

A photo electric beam will be placed so that if any of the cylinders begin rotating about the front pivot the beam will be broken triggering the header to raise to it’s full up position and an alarm will sound telling the tractor driver to stop.

When the harvester reaches the end of the row the header needs to be lifted to turn the machine around. A pendant control at the end of a long cable is provided to the tractor driver with three buttons. One button raises the header and sounds an alert. Another button returns the header to its harvesting position, and the
third button resets the alarm when the system detects a stuck air cylinder.

The electrical system also includes a shaft encoder which obtains the machine speed through gearing to one of the wheels.

In order to maintain a highly accurate air supply pressure to the cutting cylinders I will be using electronic air
regulation as well.

The air compressor will be driven from a hydraulic motor.  The air compressor is a piston type compressor with an 80 gallon tank.  The compressor will operate just as it normally would, starting up when the pressure drops below 125 psi and shutting down at 175 psi. 

The compressor’s air pressure switch will control a 12 volt hydraulic valve which controls the hydraulic compressor motor.The electrical system will consist of the following components:

ON-OFF power switch
12 Volt 100 amp alternator
Car Battery
Hydraulic Pressure switch for alternator circuit
Air pressure transducer for the air regulator
Air pressure switch for the compressor motor
Two inductive proximity switches for bed height
Twenty four 12 volt dc air valves for the cutters
Two 12 volt hydraulic valves for the bed height
One 12 volt hydraulic valve for the compressor
One 12 volt air valve for the air regulator
One Audio Alert
Three time delay circuit boards
24 Optical sensor circuit boards
One switch and relay for the lights.
Lights for night harvesting
One shaft encoder
One controller circuit board for the air regulator, bed height system and alarms

All of the controls will be implemented using Microchip microcontrollers.

Wednesday, December 22, 2010

Computing the Air Consumption of a Reciprocating Air Cylinder

In order for me to properly size the air compressor on my asparagus harvester I need to determine how much air is consumed each time an air cylinder is extended and retracted to cut a spear of asparagus.

My air cylinders have a one inch bore and a 5/8” diameter piston rod.

The stroke is 24 inches.

To extend the cylinder requires filling the cylinder until it is fully extended, and vice versa for retracting it. Therefore a first step is to calculate the total cubic inches of air contained in the cylinder when it is completely extended and again when it is completely retracted.

The area of the piston is .785 square inches. Multiplying the area times the stroke gives us a volume of .785 x 24 = 18.84 cubic inches.

The area of the piston rod is .306 square inches. We subtract the area of the rod from the area of the piston and multiply times the stroke to get the volume of the retracted cylinder. .785 - .306 = .479 square inches x 24 inches = 11.5 cubic inches.

Adding the two volumes gives us 30 cubic inches per stroke of the air cylinder. Converting to cubic feet: 30/1728 = .017361 cubic feet per cycle.

This is the free air volume, in other words the air is not compressed. I now need to determine how much “compressed” air is used.

The formula is ((100 psi) – (14.7 psi))/14.7 psi. This is known as the “compression factor”. Here the factor is 5.8.

Multiplying the cubic feet per minute times the compression factor gives: 1 cubic foot of air per stroke at 100 psi.

Friday, December 10, 2010

Sizing an Air Compressor for My Asparagus Harvester Invention

Well, I’m starting to get orders for my asparagus harvester invention. In fact, I have an order to build one right now from a grower in Australia.

I’ve had the harvester in mothballs for the last few years waiting for the asparagus growers to get brave enough to try mechanical harvesting. It looks as though that is now happening.

Asparagus growers to not all grow their asparagus using the same cultural practices. Different growers use different row widths and different center-to-center spacings for the beds. That means I have to basically custom design the harvester for each grower to match the row spacing’s and bed widths.

Without knowing the cutting width and row centers of the harvester I could not establish a number of needed parameters for the machine, like how much compressed air I would need. Sizing the air compressor that powers the air cylinders that cut the spears is one of the design parameters I need to address.

Here is how I have gone about sizing the air compressor for my harvesting machine.

I went to Google of course, and researched the yields of asparagus. I found studies by the University of California and others which included yields for a number of asparagus varieties. The studies even provided the number of spears per acre that were produced.

The largest number of spears per acre was about 70,000 spears. I decided to use 70,000 spears per acre as the basis for my calculations.

My machine uses air cylinders with a one inch bore and a 24 inch stroke to cut the spears. There are a number of these cylinders side by side across the bed. Each cylinder has a blade 2 inches wide. If a spear is tall enough to cut, the appropriate cylinder is selected and fired at the right moment to sever the spear as it is grasped by a set of rollers.

If a spear is lined up between two blades then both blades are triggered to be sure and cut the spear completely. I anticipate that about 25% of the time two blades will fire.

Anything above the cutting height of the spears and located on the bed will trigger the blades to fire. Hand crews cut down the culls but don’t pick them up. The machine may or may not pick up a cull, but it will fire at. I figure that will be result in another 20% of blade firings.

Adding the valid cuts, culls and weeds, and double blade firings I come out with about 100,000 cylinder actuations per acre per season.

Early in the season when it is still pretty cold the spears only need to be harvested every 2nd or 3rd day and as the temperature rises you have to harvest more often until you are harvesting every day. A spear of asparagus can grow over 7 inches in a day. A typical harvest can result in anywhere from 45 cutting days to 60 cutting days. So the 100,000 strokes need to be spread out over the number of cutting days.

I am going to figure on 50 cutting days. So that 100,000 cuts per acre per season becomes 2,000 cuts per acre per day. The machine I will be building is a one row harvester and will cut at a rate of about 1.25 acres per hour at top speed. This of course means the machine will be cutting at a rate of 2,500 cuts per hour, or about 42 cuts per minute.

The air cylinders that do the cutting consume 0.9 cubic feet of air per stroke at 100 psi. Multiplying the .9 cubic feet per cut times the 42 cuts per minute gives us 37 cubic feet per minute.

Now I know that I need an air compressor that can deliver right around 35 – 40 cubic feet per minute of compressed air at 100 psi.