Patents, Prototypes, Manufacturing, and Marketing New Inventions
Electronic Air Pressure Regulation For Precise Pressure Regulation.
I ran across an article about regulating air pressure with a two way valve instead of a mechanical pressure regulator in one of the trade journals that I read on a regular basis. Perhaps it was a case of serendipity or synchronicity or something similar, or maybe just coincidence. But it appears to be just what I need for my asparagus harvester invention.
My harvester uses pneumatic cylinders to do the cutting, and they have to be very quick. The stroke length has to be repeatable to a high degree of accuracy. Since the stroke length is determined by the acceleration of the piston rod assembly due to the force acting on the piston, the air pressure acting on the piston is critical.
Too much pressure and the piston rod will bottom out against the front cylinder head, and not enough pressure will cause the stroke to be short and the spear will not be cut and harvested.
According to the article, mechanical air pressure regulators can consume up to 38 percent of the horsepower required to compress the air. An interesting benefit, but not my primary concern at this point.
The problem with the air cylinder pressure is that since there are 14 air cylinders mounted on the header arranged across the asparagus bed. Asparagus spears grow randomly on the bed and often grow in clumps. The number of cylinders in operation at any one time could vary from zero to perhaps 5 or six. Some extending, some retracting, and all at various stages of their cycles.
Being as they are only 1 inch bore cylinders they don't consume all that much air per stroke, but they consume the air at a very high flow rate. These cylinders will do a 20 inch stroke in less than a tenth of a second.
High flow rates cause large pressure drops. This is true of the regulator as well as the piping, hoses, valves etc. By looking at the pressure drop curves for the regulators it becomes obvious that if several cylinders are operating at once, the pressure drop through the regulator will increase dramatically, changing the air pressure pushing on the air cylinder pistons.
However, there is another way to regulate the air pressure without having the variable pressure drop that the mechanical pressure regulator has. It's called an electronic switching regulator... at least that is what I am calling it. It uses an air valve that is either switched on or switched off.
If we mount an analog pressure transducer or transmitter in the manifold that supplies the air to the cylinders valves, and then use a large high flow two way valve between the pressurized air source and the manifold. The air source is the large tank connected to the air compressor.
The pressure transducer reacts nearly instantaneously, typically a few milliseconds. The valves I have looked at typically require a pulse of about 30 milliseconds to get the valve to shift. The pressure transducers output is used to trigger the valve to come on whenever the pressure drops below the set point.
The transducer is accurate to 0.5%. I'm using 150 psi for my air pressure setting, so the transducer could be off by 3psi which isn't enough to cause a measurable change in stroke from the air cylinders.
When the pressure drops below the set point the valve opens letting air into the manifold. When the air pressure rises above the set point the valve closes. Since the valve has almost no pressure drop like a mechanical regulator does, it can handle the flow required by multiple cylinders without the air pressure drooping.
This arrangement will however cause many small pressure spikes or what is known in electronics as ripples. With proper sizing of the manifold, it should act as a filter just as a capacitor would do in an electronic circuit, and smooth out the ripples. The larger the amount of air it will hold the smaller the pressure ripples.
Now I need to go about the task of assembling one and testing it on my selective asparagus harvester invention. Perhaps I will write about it after I finish my prototype and testing.
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