Air Pressure

Air Pressure

Inventors Notes – Switching Electronic Air Pressure Regulation

My Asparagus harvester invention utilizes pneumatic cylinders to cut the individual spears, and the stroke length has to right on the money every time.  If the pressure goes up the stroke length becomes longer, and if the pressure goes down the stroke length shortens.

Too much pressure and the piston rod will bottom out against the front cylinder head, and not enough pressure will reduce the stroke length an cause the blade to not cut all the way through the spear or even not reaching the spear at all. Allowing the piston to bottom out against the front head will eventually damage the cylinder.

The asparagus harvester has 14 air cylinders mounted on the header arranged across the asparagus bed.  Each piston rod is equipped with a sharp blade with a slight bit of overlap with the blades next to it. The cylinders are angled down toward the ground and when they extend the blade severs the spear slightly below ground level requiring a stroke length of about 20 inches. Typically the extension stroke takes around 35 to 40 milliseconds.

An optical detection system locates the spears and sends a signal to open the air valve for the cylinder corresponding to the co-ordinates of the spear to be cut.  The harvester is moving forward at between 20 and 30 inches per second, and so the blades must cut the spear and get back up out of the way of any spears that are not quite tall enough to harvest.

Asparagus spears emerge from the bed in a random pattern with random heights.  At any moment during harvesting there may be as many as 5 or 6 cylinders operating at the same time, or none at all. You might have 10 feet with nary a spear, and 18 spears in the next 24 inches.

Because these cylinders are very fast acting they require high flow rates at a constant stable air pressure.  While stroking, the cylinder will be consuming around 165 cubic feet per minute of air. Six cylinders operating at once would require a whopping 990 cubic feet per minute.

With such large swings in flow and rapidly varying air consumption the mechanical air regulator will have a significant variation in the pressure drop, which will have a detrimental affect on the stroke length of the cylinders.

We can, however, use another approach to regulating the air pressure.  We can use a switching electronic air pressure regulation scheme.  With this approach we replace the mechanical pressure regulator with an on or off electric air valve with a high flow rate.

We can then use an accurate analog pressure transducer to open the valve whenever the pressure drops below the set point, and shut off when the pressure is at or above the set point.

The valve has a very low pressure drop unlike the mechanical regulator. The valve can handle the flow required by multiple cylinders without the air pressure drooping that the mechanical regulators end up with.

There will be small pressure spikes or what is known in electronics as a ripple in the pressure.  By properly sizing the manifold I can filter out the small pressure ripples.

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How can you convert absolute air pressure readings into relative readings?

I've been asked a question that I can't answer. Someone has a weather station that measures absolute air pressure. Is there a way of converting this into relative air pressure?

I have never heard about relative vs. absolute air pressure, sorrry. This is what I know about it:

The Standard Atmosphere (SA) pressure at sea level is 29.9 inch Hg or 1013.25 hPa.

That pressure is called QNE and what all aircraft above a certain altitude calibrate their altimeter to.

The pressure at a certain place, corrected for the altitude is called QNH and it is what pilots set their altimeter when taking off or landing. They need then to know their altitude above mean sea water because all maps are written with those obstacle elevations.

There is also something called QFF, which is the same as QNH but instead of being corrected for the altitude using QNE, it is corrected by a monthly or weekly value that takes into consideration also the local variation of temperature. QFF is used e.g. to draw isobaric lines on maps.

Last is the QFE which is simply the pressure read at a certain place, not corrected for altitude.

So, you question might be (not sure, though!) how to convert relative (local pressure or QFE) to pressure over the mean sea level (QNH).

Well, you'll have to add the altitude over the sea as pressure. I don't have the exact equation but it is one hectoPascal per 8 meters, at sea level, and increasing with altitude so about the double when 5 km up in the atmosphere. If your elevation is not very high, use 8 meters.

Air Pressure - Science Theater 4