Electric Valves
By Robbie Rhodes
John, I'm glad you enjoyed the translation of the article:
Juergen Hocker, Horst Mohr, Walter Tenten: "Computer Control and
Synchronization of two Ampico Self-playing Grand-pianos", in
"The Mechanical Music-instrument", Nr. 63 (August 1995) p. 42-48,
published by the Society for Self-playing Music Instruments (GSM),
Germany.
The writers say that many valve designs were studied, including yours, but none met all their requirements. I think that they rejected your solenoid valve design because of concerns about power consumption, repetition rate, consistent response from valve to valve, and adjustment stability.
Consider the difference in armature masses between (1) a solenoid, (2) an ordinary relay, and (3) an organ "magnet" valve, such as the Reisner design. ("Armature" is the moving piece in any magnetic device like these.)
In order to realize similar transit acceleration, the force of the return spring must be proportional to the armature mass. Recall the familiar equation, "force = mass x acceleration".
The return-spring of a solenoid valve does not have very good stability because of high force and large compression percentage -- after many cycles of operation, and heating and cooling, the spring force is likely to change. This will degrade the consistency of the valve-to-valve response times. Also, the sliding armature of a solenoid is very susceptible to contamination, hence varying friction. I can't recall solenoid valves being used in any industrial application which requires long-term stability like in a reproducing piano.
The basic requirement of an electric valve for a reproducing piano is only to open a tiny aperture to a hose, from 4 up to 40 (Ampico) or 70 (DuoArt) inches-of-water vacuum. There is no need for the long stroke and "brute force" of a solenoid, nor for the large port area which a relay armature can cover. Therefore, the organ-style "magnet valve" (such as supplied for many decades by the Reisner company) is adequate. It has other deficiencies, though.
The organ valve design sacrifices speed and magnetic path efficiency in favor of simplicity. The valve assembly is bulky, but the armature is tiny, and the return-force is gravity. The low efficiency results in high power dissipation, but this is not a problem due to the great area of the exposed coils, and in an organ there is plenty of space for convection cooling. (And _inside_ the wind chest there's plenty of moving air!).
The valve for a reproducing piano, as you correctly point out, must be both low-power and small. It must also be quiet! Author Walter Tenten told me that their valve assembly had to be contained in a solid, felt- lined box, in order to reduce the noise. Most of the air vents ultimately were covered, too, because of the noise. Fortunately, he said, the temperature of the relay valves didn't rise so high that a fan was necessary.
The magnet valve designed by Wayne Stahnke for the IMI Cassette Converter improves the Reisner valve in these ways: (1) it's considerably smaller, (2) power dissipation is reduced, because a strap-steel frame forms a complete magnetic path with the armature disk, (2) the mass of the armature is minimal, and (3) a rather "long" return spring is used, which experiences only about 15% active compression.
The extremely rare DuoArt Concertola has a magnet valve with even better efficiency, due to the steel cup enclosing the coil, but without a return spring it is slow compared to the Stahnke design. Also, all 98 valve components are assembled into single wooden board, much like a primary valve set from an Ampico, and this assembly is difficult to adjust.
Mike Ames inquired about the cost of duplicating the Concertola valve, and was told it would be about $10 each in a small quantity (1000 units, I believe). A few years ago Ampico technician Randy Cox and I got a similar quote for a short run of Cassette Converter valves.
All I know about the "new" Reisner valve is that the company is talking about modifying their old vacuum valve design to make it smaller (and less expensive, we hope). The "old" design costs only about $4.00 in 100 quantity, which is appealing, but otherwise it fails to meet other important requirements.
All of the potential designs for a cheap reproducing piano valve seem to cost the same, and $10 per valve is still too much for me. The only apparent way to reduce the cost is through production in large quantities, like 10,000 units. We (that's Mike Ames, Randy Cox and, with advice from Wayne Stahnke) are studying all the possibilities.
I agree with you, John, that a market is developing right now for good quality valves. You and I and many subscribers in this group are definitely planning to someday connect the computer to the piano, just as described in the German Society article.
I want to see all the options before I commit my meager funds!
-- Robbie Rhodes
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(Message sent Sat 30 Dec 1995, 22:11:46 GMT, from time zone GMT-0800.) |
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