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.