An Electronic Air Pressure Regulator For Highly Stable Consistent Air Pressure To Power The Air Cylinders.

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Continued from the previous page:  Prototyping an Air Cylinder – Measuring the Speed

Air pressure supply problems solved with electronic air pressure regulation.

Previously I had used air cylinders with a 1-1/2” bore mounted on 2-1/2 inch centers.  These new cylinders however only have a 1 inch bore.  That makes for a huge difference in the force on the piston.  With a 1-1/2 inch bore cylinder the force on the piston in the down stroke was 1.7 times the air pressure, and the retract stroke has a force of 1.2 times the air pressure.

The one inch bore cylinder however only has .78 times the supply pressure on the down stroke and .48 times the air pressure for the retract stroke.  The acceleration of the piston rod is a direct result of the force on the piston.  In order to achieve the acceleration I needed with the smaller bore cylinder I had to up the air pressure.

While doing my preliminary testing I noticed that the stroke length was highly dependent on air pressure, much more so than the previous larger bore air cylinders.  In fact, I found that a air pressure difference of 5 psi could change the stroke length of over an inch.

On previous machines I used 100 psi of air, but for the new cylinder I would need about 130 psi.  I found that with my little hobby air compressor there were problems due to the slowness of the mechanical pressure regulator.  As the air pressure from the compressor approaches the regulator set point pressure the flow gets slower.  I found it took several minutes for the air pressure to reach the set point.

This made it difficult to get accurate results because it was very hard to get a consistent air pressure without waiting about 5 minutes for the pressure to stabilize.

Electronic air pressure regulation

Before I began my testing I knew I had to address the air pressure problem, not only for the testing but for the actual harvester as well.  With the real machine the problem would be worse. Each of the four headers on the machine has 12 air cylinders which in theory could all fire at once.  As I mentioned previously asparagus tends to clump.  The root crowns are typically spaced several feet apart, and each root crown sends up multiple shoots. You end up with little patches of asparagus.

As a result, the cylinders tend to be activated in an overlapping manner, and often three or four cylinders can be operating at the same time on one header.  Having a small bore as they do, this doesn’t consume all that much air, but because the cylinder is so fast the flow rate of that air is high.  High flow rates cause pressure drops.  Any cylinders having a common air supply line will suffer pressure drops that are higher as more cylinders simultaneously operate.

On the machine I simply plumb each air valve into a large tank, thus avoid common air supply lines altogether. In my garage it’s another story. 

My air compressor has 1.8 horsepower and can pump 6.3 cubic feet per minute at 90 psia. It starts up at 120 psi and shuts off at 145 psi, and has a 20 gallon tank.

I still have the tank from my previous air compressor, a 5 gallon tank I believe.  I burned up the previous one testing air cylinders a couple of years ago.

I turned up the regulator on the compressor to its maximum, and ran a hose from it to an air valve, and from the air valve to the small 5 gallon tank.  Next I installed an analog pressure transducer in the small tank. 

I then built a little air pressure controller from yet another 12F675 chip with a pot to control the air valve between the tanks, and the input from the transducer.  The chip compares the pot setting to the transducer output and turns the valve on and off as needed.

If I set the pot for 100 psi, when the pressure transducer voltage drops to below the pot generated voltage the valve opens and lets the air into the small tank at a very high flow rate. When the voltage rises above the cut off the valve closes.  The transducer output is near instant and the air valve can respond in about 30 milliseconds.

The result is a very stable air pressure in the smaller tank.  I run a ˝” hose from the small tank to the air cylinders’ control valve.  With this setup I can run the cylinder several times a second and the air gauge on the small tank barely moves. The pressure drop is very small and very short.  However, as I raise the pressure setting to near the 130 psi mark the performance begins to go down due to the inability of the mechanical regulator to provide sufficient flow.  Not a problem for the machine but definitely a problem in my garage.

Introducing electronic air pressure regulation to the asparagus harvester will definitely improve its performance in the field due to the improved cut timing accuracy. All my previous harvesters have had long air supply runs and most shared a ˝” diameter (pipe size) air manifold.  The new machine will not have manifold mounted valves, each valve will have its own line straight from the tank, which will have the electronic air regulator.

Continued on the next page: Electronic Air Pressure Regulator