Underwater Sensing, switches and democracy
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Posts: 46,084
I see you are concerned with the reliability of the device(s) used tor
stop the conveyor.
Now, don't be mislead by the popular myth that, "the more complex and
sophisticated, the more reliable and safe". It is usually exactly the
contrary.
I'm sure Chris, Steve and others have a lot to say about safety in machinery.
A switch can (and it must assumed that it will) fail eventually. If this
has serious consequencesor or is life endangering, then you have to put two
switches or more, and have them "vote". Once you get the results of the
voting, you must forget about democracy and adopt the opinion of the
absolute minority. That is, if just one of the N switches says "stop", then
STOP!
Of course you do it by wiring in series your N switches, if they are
"Normally Closed" contacts (the strongly recommended type), or in parallel
if the are "Normally Open" contacts (to avoid). This was called
"interlocking" in the past. The switch (es) in control should open the
conveyor motor contactor directly, as this is your last chance to stop it!
This is just a general suggestion; I agree with those in the list
recommending the "buy - red - apply" the safety procedures and rules, as
there is normally a lot to know and obey in the safety area.
If you are lowering a pearl diver or a set of used uranium rods into your
tank, then surely there will be a lot of mandatory safety rules to follow.
To finish, let me give a real life example:
We recently replaced the lift machinery of our apartment building. It was
30 years old and the relay "logic" was not reliable; now and then the
system would continue to move the cabin past the upper floor, menacing to
bump it into the ceiling. (Very scaring situation indeed).
It never actually happen (the bumping), because an old reliable SWITCH
always stopped the motor at the last minute. These switches are called
something as "limit switches" or "dead limit", sorry but I do not know the
right the word in English.
So we got a brand new computerised control and also a new motor for our
lift. And of course, a new ... mechanical LIMIT SWITCH to stop the cabin
when everytning else fails. (I would rather climb the stairs every day, if
such a switch was not there).
Once you have that set of "limit" switches, you can add all the
sofistication and smartness that a Basic Stamp (or a Cray) computer has to
offer to your design.
Good luck!
Regards,
Jose luis
At 09:26 05/10/2001 -0500, you wrote:
>The only problem with switches, etc... is with the conveyor getting hung up
>before it reaches the bottom of the tank. The sensor is never triggered, so
>the lowering device keeps on running.
>
>
Original Message
>
> > A simple (though waterproof), mechanical switch fixed to the bottom of the
> > conveyor (so that it activates when touching its bottom) would work.
> >
> > Also a reedswitch activated by a magnet fixed on the lowest part of the
> > conveyor (or the other way round, a magent at the bottom and the reed on
> > the conveyor) could do the job, provided the conveyor always land at the
> > same place!.
> >
> > You could use many more complicated solutions, such as load cells, starin
> > gauges, photoelectric detectors, proximity switches of many kinds
> > (inductive, capacitive), depth pressure detectors, even sonar type
>devices.
> > But why bother if there seem to be simple, obvious solutions?.
> >
> > However, if you are asking for help, it might be that you have another
> > problem which has not been clear for those in the list trying to help.
> >
> > Why do you say "I have to detect an object under water" if you already
>have
> > it attached to the "lengthy part" of your conveyor?
>
>
>
>
>To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
>from the same email address that you subscribed. Text in the Subject and
>Body of the message will be ignored.
>
>
>Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
[noparse][[/noparse]Non-text portions of this message have been removed]
stop the conveyor.
Now, don't be mislead by the popular myth that, "the more complex and
sophisticated, the more reliable and safe". It is usually exactly the
contrary.
I'm sure Chris, Steve and others have a lot to say about safety in machinery.
A switch can (and it must assumed that it will) fail eventually. If this
has serious consequencesor or is life endangering, then you have to put two
switches or more, and have them "vote". Once you get the results of the
voting, you must forget about democracy and adopt the opinion of the
absolute minority. That is, if just one of the N switches says "stop", then
STOP!
Of course you do it by wiring in series your N switches, if they are
"Normally Closed" contacts (the strongly recommended type), or in parallel
if the are "Normally Open" contacts (to avoid). This was called
"interlocking" in the past. The switch (es) in control should open the
conveyor motor contactor directly, as this is your last chance to stop it!
This is just a general suggestion; I agree with those in the list
recommending the "buy - red - apply" the safety procedures and rules, as
there is normally a lot to know and obey in the safety area.
If you are lowering a pearl diver or a set of used uranium rods into your
tank, then surely there will be a lot of mandatory safety rules to follow.
To finish, let me give a real life example:
We recently replaced the lift machinery of our apartment building. It was
30 years old and the relay "logic" was not reliable; now and then the
system would continue to move the cabin past the upper floor, menacing to
bump it into the ceiling. (Very scaring situation indeed).
It never actually happen (the bumping), because an old reliable SWITCH
always stopped the motor at the last minute. These switches are called
something as "limit switches" or "dead limit", sorry but I do not know the
right the word in English.
So we got a brand new computerised control and also a new motor for our
lift. And of course, a new ... mechanical LIMIT SWITCH to stop the cabin
when everytning else fails. (I would rather climb the stairs every day, if
such a switch was not there).
Once you have that set of "limit" switches, you can add all the
sofistication and smartness that a Basic Stamp (or a Cray) computer has to
offer to your design.
Good luck!
Regards,
Jose luis
At 09:26 05/10/2001 -0500, you wrote:
>The only problem with switches, etc... is with the conveyor getting hung up
>before it reaches the bottom of the tank. The sensor is never triggered, so
>the lowering device keeps on running.
>
>
Original Message
>
> > A simple (though waterproof), mechanical switch fixed to the bottom of the
> > conveyor (so that it activates when touching its bottom) would work.
> >
> > Also a reedswitch activated by a magnet fixed on the lowest part of the
> > conveyor (or the other way round, a magent at the bottom and the reed on
> > the conveyor) could do the job, provided the conveyor always land at the
> > same place!.
> >
> > You could use many more complicated solutions, such as load cells, starin
> > gauges, photoelectric detectors, proximity switches of many kinds
> > (inductive, capacitive), depth pressure detectors, even sonar type
>devices.
> > But why bother if there seem to be simple, obvious solutions?.
> >
> > However, if you are asking for help, it might be that you have another
> > problem which has not been clear for those in the list trying to help.
> >
> > Why do you say "I have to detect an object under water" if you already
>have
> > it attached to the "lengthy part" of your conveyor?
>
>
>
>
>To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
>from the same email address that you subscribed. Text in the Subject and
>Body of the message will be ignored.
>
>
>Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
[noparse][[/noparse]Non-text portions of this message have been removed]