Electrical Contacts and bounce
By John Rhodes
In 1976, I selected a metal-contact keyboard for a new Hewlett-Packard terminal. In the course of that investigation, I uncovered several items which may be of interest to the automatic-music subscribers.
A monograph by Kenneth E. Pitney titled "Ney Contact Manual - Electrical Contacts for Low Energy Uses" was published in 1973 by the J. M. Ney Company. This ~200-page book contains a wealth of information on the general theory of contacts; interactions of alloys, atmospheres, surface oxides, contact voltages, erosive arcs, and contact pressures.
One of the author's interesting observations concerns the production of insulating "friction polymers" where sliding contact systems utilizing platinum-family materials catalyze the production of polymers from the vapors of organic hydrocarbons. (Gold and silver contacts do not produce friction polymers.)
Another observation concerns threshold voltages for puncturing oxides and other contaminating layers. In general, TTL logic levels are marginal unless used with silver or gold contacts. Direct operation of high voltage, inductive loads (read: "solenoids") caused considerable erosion if sufficient thermal mass was not present in the contacts.
I also came across a paper published by researchers at Bell Telephone Labs which discussed the problems of designing coin-operated pay phones. Their challenge was to produce reliable contact closures using the energy of a falling dime (a "thin" one, at that). The toughest environment was a remote phone booth situated on the New Jersey Turnpike! They achieved reliable operation by use of multiple/paralleled palladium-gold alloy contacts.
The keyboard I selected (produced by the Hi-Tek corporation of Santa Ana, CA) used quadfurcated Au-Pd contacts for high reliability. The contacts were similar to the tines of a fork, and designed such that dust and dirt particles under one of the individual contacts would not prevent the other three from closing.
These mechanical contacts suffered from contact bounce. Our debounce solution was implemented digitally in the keyboard-scanner integrated circuit (as >100 resistor-capacitor networks were not practical for space and cost reasons). The debounce algorithm recognized the fact that a typist cannot repeat a single key faster than 8-10 times per second (pianists have some tricks for higher repetition rates, however). Therefore, key down/up events with shorter than 100ms duration had to originate from contact bounce.
We implemented a simple 3-bit counter clocked at 80 Hz. The counter was reset (and reported KEYDOWN) for any contact closure, and reported KEYUP only after 8 successive non-keydown clock cycles.
This type of algorithm is easily implemented in today's digital computers. The algorithm can be made symmetric such that upon change of state -- after a predetermined stable period -- the new state is immediately reported. Resistor-capacitor networks introduce a propagation delay for one (or both) state changes. A final advantage of the computer algorithm is that its time constant is a program variable; you don't have to get out the old soldering iron to twiddle things!
John D. Rhodes - AA7HL Vancouver, WA [jrhodes@teleport.com]
"We used to be able to do that, but the technology's improved
-- and it's no longer possible." |
(Message sent Sun 31 Dec 1995, 10:02:44 GMT, from time zone GMT.) |
|
|
Mechanical
Music Digest™ Archives
