🚀 go-pugleaf

In article <1989Jan15.035721.14156@utzoo.uucp>, henry@utzoo.uucp (Henry Spencer) writes:
> >	Since the above are all solenoid valves, a position encoder is
> >not required...
> 
> Unless, of course, you want to be sure that the valve has really
> operated when you ask it to, which is usually considered necessary
> in critical control systems.  (If you don't think controlling a
> shower justifies this degree of reliability, consider the consequences
> of turning off cold but not hot in a place with an enthusiastic water
> heater -- non-trivial injury might result.)

	A "reliable" system design would still never trust the valve (and
consequently its position) - it would measure the actual temperature of
the mixed water, which is ultimately the bottom line.

> I think the original poster wanted proportional control rather than just
> on/off, which is why he specified that solenoid valves weren't of interest.
> Short of the (plausible) suggestion of building a 4-bit hydraulic DAC with
> four valves, it looks to me like he's going to need something like a
> motor-controlled needle valve, not normally a consumer item, except perhaps
> in some fancy washing machine.  He'll also need a separate shutoff valve,
> since needle valves aren't usually designed to reduce flow to zero.

	Needle valves are available for process control use which have an
effective shutoff position.  From an idealistic standpoint, a rotary valve
is a poor choice for proportional control applications, since by its very
nature the ON<->OFF transition is only 90 degrees of rotation.  An example of
a suitable valve is a bellows-type valve which uses, in effect, a needle and
seat arrangement; the position of the bellows can be controlled via an
electric motor drive, electromagnet (for small valves, at least), or air
pressure supplied by an I/P transducer (common in industrial applications).

	One _can_ achieve decent proportional control with just a hot and
cold water solenoid valve.  In our darkroom, we have a Wing-Lynch electronic
water temperature controller which uses two solenoid valves; this unit
provides an accuracy of +/- 0.1 deg F.  The electronic circuit monitors
output temperature and _pulses_ the valves accordingly.  The key to effective
control in this arrangement is to have a mixing chamber (can be just a coiled
length of tubing) to mix the hot and cold water BEFORE the output temperature
is measured.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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