Tuesday, April 5, 2011

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.


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.

Tuesday, March 29, 2011

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.

Monday, March 14, 2011

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.


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


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.

Wednesday, January 19, 2011

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…