Designing the Hydraulic Fluid Power System for the Selective Asparagus Harvesting Machine

The harvester utilizes hydraulic power to run the air compressor motor, the conveyor motors, the pickup roller motors, the alternator, and the header lift cylinders.


I will be using a pressure compensated pump to provide the hydraulic fluid that powers all of the above items. That way I can use simple flow controls to control the speed of each of the aforementioned items. The pump only puts out as much hydraulic fluid as required by the load. If you were to shut off the flow controls the output from the pump would drop to zero.

I will use a PTO pump hanging off of the PTO shaft on a tractor to drive a motor that in turn drives the pressure compensated pump.

Let’s determine the horsepower, pressure, and flow rates that will be needed by the various components. I would like to keep the system pressure below 1,500 psi.

Pickup Roller and Conveyor Motors

The hydraulic motors used to power the pickup rollers and the conveyors are all identical Char-lynn hydraulic motors with a displacement of 1.95 cubic inches per revolution. I’ll be running the conveyors and pickup rollers at around 100 rpm. That’s about 12 inches per second for the conveyors.

At 100 rpm the motors require about 1 gallon per minute of hydraulic fluid. I’ll be running the three pickup roller motors in series and the three conveyor motors will be in series. That means I will need 1 gpm for each of the two circuits.

However, to be on the conservative side, I will use a figure of 2 gallons per minute for each of the two circuits. So far… 4 gallons per minute needed. Very low horsepower though. The pressure needed for any individual motor is probably only 200 – 300 psi. That is why I am running 3 motors in series. If I keep the system pressure below 1,500 psi I won’t need case drains on the motors.

The next item to consider is the motor that runs the compressor. I’m replacing the electric motor on an air compressor with a hydraulic motor. The motor will be controlled with a hydraulic solenoid valve just as the electric motor was controlled with a pressure switch. Now the pressure switch will activate a solenoid valve to run the compressor motor when the air press drops to less than 150 psi. The switch shuts the valve off at 175 psi.

The motor is a 7.5 horsepower motor and runs the compressor at about 900 rpm through a belt and pulley arrangement. I will be using a 2:1 ratio so I will want the hydraulic motor to run at 1800 rpm to keep the compressor at 900 rpm.

If I choose a motor with a displacement of about 1.25 cubic inches per revolution the motor would require about 10 gallons per minute. At 8 horsepower it would require about 1,500 psi.

So now I have 4 gpm + 10gpm = 14 gpm.

The last item is the header lifting mechanism. The two lift cylinders that provide the lifting power for the header are 2 inch bore cylinders. With a system pressure of 1,500 psi each cylinder will lift over 4,000 pounds and the header only weighs about 1,000 pounds. There are two modes of operation for the lift cylinders.

One mode is when normal harvesting is underway. In that mode the lift cylinders are operated by the”slow” direction control valve. The slow valve has flow controls installed and limit the flow so that the header will only change height at about ½ inch per second. For 2 inch bore cylinders plumbed in parallel that will require less than 1 gallon per minute.

The other mode of operation is when the header must be raised quickly because of a cylinder fault. We would like to have the header lift up quickly enough to prevent binding of the cutting cylinders. That means we need to lift it about 24 inches in about 2 seconds. A flow of 8 gallons per minute into a 2 inch bore cylinder produces a speed of about 10 inches per second. So ideally we would like to have a flow of 10 gallons per minute.

This mode of operation will hopefully rarely occur if ever. The extra 10 gallons a minute would require much larger pumps, motors, and hoses too. What I will do is have the compressor motor shut off when the fast header up command is encountered. If the header is being raised because of a cylinder fault or because the driver is going to turn around at the end of the row, then the compressor doesn’t need to be running. I can program that easily into the controller chips on the circuit boards.

That leaves me with the 14 gallons per minute plus 1 more gallons per minute for the lift cylinders bringing the total needed maximum flow rate from the pressure compensated pump to 15 gallons per minute at 1,500 psi or less.

For now I will select a pump with a displacement of 1.71 cubic inches per revolution. (A Bosch AV10VSO Pump - size 28). To produce the 15 gallons per minute I will need to run the pump at about 2,000 rpm. The maximum rpm for this pump is 3,600 rpm.

Now I need to select a motor to drive the pressure compensated pump. Since I need to run the pressure compensated pump at about 2000 rpm for the maximum needed flow, I will select a motor that will spin at 2,000 rpm with a flow of about 17 gallons per minute. A motor with a displacement of 2 cubic inches per revolution will work nicely.

The last size choice is for the PTO pump that will hang off of the PTO shaft on the tractor. I’ll be using a Prince Model HC-PTO-9A which produces a maximum flow of 21 gallons per minute at 2,000 psi. When the tractor is at full throttle the PTO shaft rotates at 540 rpm. That is the rpm the pump will put out 21 gallons per minute at. Since the PTO pump is a positive displacement gear pump it will force 21 gallons a minute out when running at 540 rpm, so the drive motor for the PC pump will have to handle that flow without exceeding its own maximum rpm rating and also without exceeding the PC pumps maximum rpm rating. At 21 gpm the drive motor I selected will spin at about 2,600 rpm which works for both the motor and the pump.

Hydraulic Reservoir Selection

A commonly followed rule of thumb for selecting a reservoir size is to use the same size reservoir in gallons as the pump puts out. In my case I am using a much larger reservoir, about 50 gallons. It fits nicely on the machine and it will provide a lot more heat radiation area to keep the oil temperature down.

Hose Selection

Selecting the right hoses is important. The suction lines have to be big enough to prevent cavitation in the pump. But as the hose diameter increases the costs go up fast. Typically the hydraulic flow through hoses should be kept below about 15-20 feet per second for pressure lines and oil return lines and below 4 feet per second for suction lines for the pump inlets.

The highest flow for my system will be the 21 gallons per minute going into and coming out of the PTO pump. A 1-1/2 inch diameter suction hose from the tank to the PTO pump is a good choice producing a flow velocity of a little less than 4 feet per second at about 22 gallons a minute. For the pressure line from the PTO pump to the drive motor I can use 1” I.D. hose and stay below the 20 ft/sec.

The PC pump will use the same size hoses for the inlet and for the outlet to the pressure manifold connection as the PTO pump, 1-1/2” suction and 1” pressure lines. The next largest flow is from the manifold to the compressor motor. A ¾” line will be fine for that connection and a ¾” line from the motor to the reservoir. The lift cylinders will only rarely have high volume flows so we can undersize the lift cylinder hoses. I’m going to use ½” hoses for the lift cylinder plumbing. For the conveyor and pick up roller hydraulics I will use 3/8” hoses.

Hydraulic Valves

There are three control valves and two separate flow control valves in the system. The two flow control valves are in series with the conveyor motors and with the pickup roller motors. They are used to set the conveyor and pickup roller speed.

There are two closed-center directional control valves that control the lift cylinders. The valves each have an up and a down solenoid. The third control valve is a solenoid controlled on/off valve for turning the compressor motor on and off.

Filtering

I’ll use a spin-on type filter on the pressure return lines from the two pumps.

And there you have it… the basic design of the hydraulic fluid power system for the selective asparagus harvesting machine.

Bringing A New Invention to Market in Real Time – I got the first order… Now What?

Well, it seems after only about 37 years of working on my selective asparagus harvester I’ve finally managed to sell one. I got my first order yesterday.


I receive it with mixed emotions. Finally! A real Order! But there are some hurdles to overcome.

To begin with I don’t actually have the machine… I have to build it. This is a custom machine built to match the bed width and row spacing of his asparagus crop. Asparagus growers plant their asparagus in a variety of row spacing’s and bed widths. I’ve seen asparagus planted in rows as close together as 36” and as far apart as 72” with bed widths from 22 inches wide to 48 inches wide.

So we are about to build our first commercial version of our experimental selective asparagus harvesting machine. There are no commercially available “selective” asparagus harvesters on the market. There are asparagus harvesting aids for the hand crews such as little carts you can ride on to hand pick the spears, but there are no machines that I know of that will selectively harvest only the ripe asparagus spears leaving the not-yet-ripe spears for the following days.

My partner in this venture is a machine shop over 600 miles away in another state. The way it works is I do all of the design work, send them the blueprints, and they build the machine. When they get the machine finished then I drive to the machine shop, a lovely 9-10 hour drive, do the electronics work and supervise the installation of the hydraulic systems.

Then we will rent or borrow a tractor and do some parking lot fine tuning and debugging… always a few bugs with the new machine. And this new design has a whole bunch of new stuff I haven’t done before from the vertical lifting of the header which used to be on swinging arms to completely new circuit boards. We’ve never tried 1” bore cylinders… in the past we have always used 1-1/2 inch bore cylinders.

The new cylinders use gravity for keeping the blades in the correct position while in the past we’ve always used guide rods. There are just too many changes to mention here. The point is there will certainly be some debugging.

Once we are satisfied with the parking lot testing we will take the machine out to a local asparagus field and run it over a few rows of real asparagus for a week or so to do any final tweaking.

I have my fingers crossed that we don’t run into to some big expensive problem. I am however quite confident in my latest design and I am expecting smooth sailing ahead.

Another hurdle that may cause us problems is timing. It’s now the end of March and asparagus harvesting season is underway. If we want to be able to run a machine on some real asparagus beds and really harvest asparagus then we need to get this machine built quickly. The end of the season is usually at the end of May.

Cost is as always a hurdle. We have done our best to anticipate what everything will cost accurately but so many things can go wrong. We won’t make any money on this machine. In all likelihood we will lose money. But since we can’t afford to build our own machine this is about the only way we can get a machine out in the field where asparagus growers will be able to see for themselves how well the machine works. So cost is definitely a hurdle.

Speaking of hurdles, guess where the asparagus grower who is ordering the machine has his farm… Australia! Another reason we want to do thorough testing and debugging… a service call to Australia is probably not in the cards.

I’ll be spending a lot of time looking at the drawings of the machine trying to make sure I haven’t made any mistakes before sending them to the machine shop. We won’t be starting until the money is transferred to our bank account which will be in a couple of days I believe. Then it’s race time.

In case I have any readers out there, and in case any readers are interested in this project to bring the selective asparagus harvesting machine to life, I will blog frequently about the whole process.

Now please excuse me, I must begin going over those prints.

Design Changes for the Geiger Lund Asparagus Harvester Master Controller

In a previous blog I described how I came up with the design for the circuitry to control various functions of the asparagus harvesting machine. I built a bread boarded circuit and worked on programming it.

I decided the circuitry had a lot of unnecessary redundancy, and there were ways to simplify the circuitry further still. After I’ve worked on programming for a while I often think of ways to improve the hardware. In this case I decided to move the air regulator functions to a separate 12E675 PIC chip. It greatly simplifies the programming.

Functions of the controller:

Turn on electronics by hydraulic pressure.

Lock out air valve when the machine is not moving.

Provide for cut timing adjustment.

Regulate air pressure for air cylinders

Sound alarm horn when air pressure drops below minimum setting.

Sound alarm horn in if there is a cylinder malfunction.

Provide the tractor driver with up and down buttons for the header.

Control the lift cylinders valves to float the headers 9" above the bed.

Raise the header fast in case of an air cylinder malfunction.

Buffer the shaft encoder and send encoder output to optics boards.

Outputs

Lift cylinder "slow" valve Up Solenoid

Lift cylinder "slow" valve Down Solenoid

Lift cylinder "fast" valve Up Solenoid

Lift cylinder "fast" valve Down Solenoid

Alarm Horn

B+ for air valves

Encoder signal for optics boards

Output for air regulator valve

Inputs

Up proximity switch - open collector output

Down proximity switch - open collector output

Driver pendant up button - momentary contact to ground

Driver pendant down button - momentary contact to ground

Shaft Encoder output - open collector output

Photo electric switch - cyl. fault detector - open collector output

Air pressure transducer - 0 to 5 volt = 0 to 250 psi analog output

B+ from Hydraulic Pressure Switch

Explanation of the circuit


The circuit board has a couple of relays, one of which operates without the aid of any microcontrollers, but consists primarily of interfacing between the input sensors and the microcontrollers and providing high current output drivers for the microcontroller outputs.

All of the inputs and outputs use screw terminals. I’ve tried pluggable connectors on my harvester in the past and I’ve had problems with them. I never have any problems with screw terminals.

J1 and J2 are 3 terminal blocks for connecting the proximity switches that determine the bed height. They provide ground, +12 volts and provide 4.7k ohm pull-up resistors for the open collector outputs of the proximity switches.

J3 is a 4 terminal connector for the driver pendant. It provides a ground, a terminal for the up button and a terminal for the down button. A spare up button terminal is also provided.

J4 is a 3 terminal connector for the photo switch that detects air cylinder malfunctions. The connector provides a ground, +12 volts, and a signal terminal which ties to the up switch terminal on J3.

J5 is a 4 pin terminal, with two ground terminals, +12 volts, and a signal pin tied to a 4.7k pull-up resistor. The signal pin ties to the input a ULN2067B driver for buffering, and to the 16F627 chip.

J6 is a 3 terminal connector supplying ground, +12 volts, and pin for connecting to the pressure transducer. The output of the transducer is an analog 0 to 5 volt output and so doesn’t require a pull-up resistor.

J7 is a 3 terminal connector that sends the encoder signal and the timing voltage to the optical board. It has a ground pin, encoder output pin, and a 0-5 volt dc analog signal pin. The 0 to 5 volt timing signal pin connects to the wiper of a 5 k pot forming a voltage divider. The encoder out pin connects to input of the UL2067B logic driver.

J8 is a 6 pin connector provides terminals for the slow and fast hydraulic cylinder valves, the alarm, and the air regulation valve. The inputs of the drivers are driven by the microcontroller pins.

J9 is a 6 pin connector which having two terminals for connection directly to the battery and a terminal for the hydraulic pressure switch. It has output pins providing the B+ for the optical sensor and hydraulic valves.
The terminals to the battery pass through a fuse, and goes on to provide 12 volts to everything that uses 12 volts.

The alternator needs a field connection to the battery to begin putting out voltage and needs to be disconnected when the alternator isn’t spinning. A hydraulic pressure switch connects the positive battery terminal to the alternators field. The field side of the switch also drives relay K2s coil directly.

Relay K2 provides a connects the fuse to the B+ terminals for the optical board, alarm horn, hydraulic valves and to the relay that provides B+ to the air valves.

The relay that powers the air valves is driven by a npn transistor which is in turn driven by an output pin on the 16F627 chip. The two relays both have suppression diodes across their coils.

The fused 12 volts from the battery feeds to a 3 terminal +5 volt regulator, and bypass and filter caps provide 5 volts for the chips and the pull up resistors.

Chip U2 is a 16F627 PIC chip and provides the functions of floating the header at a pre-determined height above the bed, enabling the air valves when the machine is in motion, raising the header rapidly if a cylinder malfunctions or the up button is pushed activated. It also provides an for an alarm horn signal for a couple of seconds whenever the header is raised rapidly.

Chip U3 Provides the air regulation function and sounds the alarm horn if the air pressure drops too low. A pot is connected from ground to +5 volts with the center lead connected to the analog to digital converter in the chip to obtain an air pressure set point.

In the future I will blog about the programming of the chips.

Figuring Out the Control Functions for a Selective Asparagus Harvesting Machine

The main functions of the circuit will be to control the header position above the asparagus bed, provide for emergency raising of the header in case of cutting cylinder failure, providing an audio alarm when the emergency header lift is activated, controlling the electronic air pressure regulator, providing a low air pressure audio alarm signal, implementing a safety feature that prevents the blades from firing unless the machine is harvesting, and finally a method for automatic start up and shut down of the electronics.

Automatic Electronics Start Up

Previously I had decided to provide a power switch for the electronics portion of the machine, but I’ve changed my mind.

I need a switch to provide excitation current to the field of the alternator that charges the battery so the battery won’t drain through the alternators field when the machine is not running. I decided to use a pressure switch in the hydraulic system to do this. I’m driving the alternator off of the motor I use to turn the main pressure pump. The motor is driven by the PTO pump on the tractor.

The main pressure pump runs the conveyors, pickup motors, header lift, and air compressor. As soon as the tractor driver engages the PTO, the hydraulic pressure rises to 1,500 psi nearly instantly. This closes the contacts in the pressure switch and supplies 12 Volts DC to the control circuit board where it closes a relay. The relay connects the battery to the various hydraulic valve solenoids, and to the control circuit board.

When the PTO is engaged the alarm microcontroller will turn on the audio alarm for 1 second to alert anyone present that the electronics are going live.

There is a shaft encoder driven by the left tire which is used to determine the ground speed of the harvester. The encoder must produce an output indicating a speed of ¼ mph for 1 second before the air cylinders will be operational. This is a safety feature to prevent anyone from being injured by accidentally triggering a blade when someone’s body parts are in the line of fire.

Header Height Adjustment System

There are up and down inductive proximity switches that are used to detect the height of the header above the asparagus bed. The proximity switch outputs are connected to a microcontroller chip on the control board. The microcontroller controls the “slow” hydraulic valve and lift cylinders to maintain the header at a pre-determined height above the bed.

When the machine reaches the end of the asparagus bed the tractor operator pushes a button on a two-button pendent which raises the header to its maximum height so the driver can turn around without worrying about the cutters firing while he is turning. If for some reason the cutters do fire, the blades won’t reach the ground.

Once the driver completes his turn he presses the down button and the header lowers to its set height above the bed and returns to normal operation.

Alarm System

The harvester is equipped with an alarm system to alert the driver of problems.

There is an optical beam breaking sensor mounted above the rear of the air cylinders. If for any reason one or more of the air cylinders does not immediately retract after being extended it will begin to rotate about the front nose mount. This will cause the beam to be broken and send a signal to the control board telling it to raise the header to its maximum height and sound an alarm for 5 seconds.

The header will be raised by a second set of hydraulic valves, the fast valves, which have a much higher flow than the slow valves used to maintain the header height above the bed.

Once the problem has been resolved the header is lowered by pressing the “down” button on a two button pendant control used by the tractor driver.

If the air pressure in the air valve manifold drops below a pre-set point the alarm will sound continuously until the air pressure returns to normal. The low pressure alarm will not sound unless the machine is moving forward at ¼ mph or faster.

There will be a switch located in the sorting area which when pressed will cause the alarm to sound a series of short beeps alerting the driver that the sorting crew wants him to stop the machine.

Air Pressure Regulator

Another microcontroller is used to monitor the air pressure in the air manifold that supplies air to the air valves. A pressure transducer is mounted on the manifold and is fed to an analog to digital converter in the microcontroller.

A potentiometer provides another analog voltage to the microcontroller which is used to fine tune the air pressure. Changing the air pressure changes the length of stroke of the blades.

The microcontroller turns on and off a large air valve that connects the compressor tank to the manifold. The valve takes about 30 milliseconds to activate and allows full flow through a 1 inch diameter pipe to the manifold. The manifold holds about 12 gallons of air. This arrangement ends up providing a very tight pressure range holding the air pressure in the manifold within about 2 psi of the set point pressure whether one blade is activated or 5 blades all at once.

As mentioned earlier the microprocessor also sounds an alarm if the air pressure drops more than 2 psi below the set point pressure.

Air Cylinder Safety Interlock

A shaft encoder is mounted on the left tire to obtain the speed of the machine for the time delay circuit. The control board also has a microcontroller monitoring the shaft encoder output. The control board will not provide the 12 volts for the air cylinder valves unless the shaft encoder is showing the machine to be going at least ¼ mile per hour for 1 second.

I believe I’ve covered everything at least in general.

I think in my next blog I’ll detail the specifics of the electronics as I design the control circuit board itself.

Now go invent something…

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
sensing
Twenty four 12 volt dc air valves for the cutters
Two 12 volt hydraulic valves for the bed height
mechanism
One 12 volt hydraulic valve for the compressor
motor
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. 

http://www.asparagusharvester.com/

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.

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.