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…