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'[EE] getting stepped on....'
2007\05\23@175839 by alan smith

picon face
This is more of a proactive question, because I am afraid I may have to come up with a better solution that whats currently being used.  I am being asked to come and evaluate an existing design where "stepping stones" are used to initiate a waterspray sequence.  In other words, as you step on particular "stones" it triggers a controller input.  
 
 Presently the sensing is done using a load sensor that sits underneath a fake stone so stepping on it causes an analog voltage to change and thus triggers.  I've been there once under contract to repair, where the load sensor took a voltage spike and had to replace (about 1K ea) it and also fix wiring issues, realign things, and basically re-engineer some of the control features.
 
 That was last year, and now I am getting a call from the end user (it was the design house that contracted me the first time) to come and look at it again.  Can you tell I'm excited about this?  So I am wondering....anyone have other ideas other than a load cell that would stand up to lousy enviormental conditions (being flooded, baked, froze, etc) and would keep working more than a year.  The "stepping stone" is attached to a plate that has the load cell attached to it, so whatever is done has to be easily retrofitted.  Cost isnt such an issue since its not a production run but a one-off deal.


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2007\05\23@182505 by Jinx

face picon face
Magnet and Hall sensor. Analogue/digital, water-proofable. Cheap

2007\05\23@185145 by Robert Rolf

picon face
>Magnet and Hall sensor. Analogue/digital, water-proofable. Cheap

What he Jinx said ^^^.

Also available as a 'turn key' industrial grade 'limit switch' (waterproof).

Or a 'mat' switch, as used in security systems under carpets
to detect intruders, or at supermarkets to open doors (not the
X-band radar openers now more common).


alan smith wrote:
> This is more of a proactive question, because I am afraid I may have to come up with a better solution that whats currently being used.  I am being asked to come and evaluate an existing design where "stepping stones" are used to initiate a waterspray sequence.  In other words, as you step on particular "stones" it triggers a controller input.  
>    
>   Presently the sensing is done using a load sensor that sits underneath a fake stone so stepping on it causes an analog voltage to change and thus triggers.  I've been there once under contract to repair, where the load sensor took a voltage spike and had to replace (about 1K ea) it and also fix wiring issues, realign things, and basically re-engineer some of the control features.
>    
>   That was last year, and now I am getting a call from the end user (it was the design house that contracted me the first time) to come and look at it again.  Can you tell I'm excited about this?  So I am wondering....anyone have other ideas other than a load cell that would stand up to lousy enviormental conditions (being flooded, baked, froze, etc) and would keep working more than a year.  The "stepping stone" is attached to a plate that has the load cell attached to it, so whatever is done has to be easily retrofitted.  Cost isnt such an issue since its not a production run but a one-off deal.

2007\05\23@190111 by Brent Brown

picon face
Simple and cheap idea worth trying out: Piezo disk (cheap, pull one out of a piezo
buzzer), series resistor into base of transistor. Mount to steel plate to get some
deflection when weight applied, or direct to stone to pick up "sound" of footstep.
Cover with silicon. Direct to stone may or may not be sensitive enough. Polarity may
effect sensitivity and whether it operates on foot-press or foot-release. De-tune
sensitivity by adding parallel R.

--
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: 027 433 4069
eMail:  spam_OUTbrent.brownTakeThisOuTspamclear.net.nz


2007\05\23@191449 by Dr Skip

picon face
Take a look at the failure mode first. That'll give you info on what
needs to be addressed and overcome. Without that, the new idea may fail
for the same reasons.

-Skip

2007\05\23@222557 by Stephen R Phillips

picon face

--- alan smith <.....micro_eng2KILLspamspam@spam@yahoo.com> wrote:

{Quote hidden}

There is an old story "there are many ways to skin a cat" so if you
qualify what it is you need more precisely that might help.  "Load
Cells" can consist of anything from an semi driven piezo transducer
(this is often used in measuring scales) to stainless steel tranductor
(check out ABB for these) (measures strain inside a chunk of
stainless).  The latter being highly indestructible but not suited for
high temperatures, often these are used in steel rolling mills or just
rolling mills in general.  So you probably need to be more specific
about what the criteria are.  Actually you might answer your question
by doing so. :D

Stephen

Stephen R. Phillips was here
Please be advised what was said may be absolutely wrong, and hereby this disclaimer follows.  I reserve the right to be wrong and admit it in front of the entire world.



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2007\05\24@030859 by Russell McMahon

face
flavicon
face
Many answers already, but:

Puematic closed system where pressure is increased by user standing on
rock. Then detect pressure rise with anything from a sprinf loaded
diaphragm and a microswitch to a 'pressure sensor'. Complete electrics
can be housed in sealed unit.

       Russell


2007\05\24@103516 by alan smith

picon face
wow...couple of ideas I hadn't thought of..so thanks!  The only one I was thinking was using an accelormeter, where its used more of a vibration sensor.
 
 In as much as analyzing the mode of failure, the orignal failure was from a close proximity lighting strike, and not sure on the second since I just sent the failed unit back to the design house. They may not want to replace it with something more robust or less prone to failure but the biggest problem I have with the design is its transmitting over cat5 analog voltages from the load sensor to the interface, so losses in the cable, broken cable, poor connections all can affect the system.  I'd rather just see an on/off signal to the control with the local A/D sending it.  But in the end im just the fixit guy for this project.
 
 I've sent an email to the end user asking to describe what isnt working exactly.  Of course I expect an answer...it doesnt activate when stepped on....hmmm...guess its broken.  I'd rather not make too many trips to site, waste of time and fuel (but I charge these guys mileage at least)

     
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2007\05\24@105208 by Mike Hord

picon face
>  So I am wondering....anyone have other ideas other than a
> load cell that would stand up to lousy enviormental conditions
> (being flooded, baked, froze, etc) and would keep working
> more than a year.

Something based on a capacitive switch?  Either moving plates
closer together or perhaps a large plate under the stone to
detect the presence of the person?

Mike H.

2007\05\24@113148 by Alan B. Pearce

face picon face
>In as much as analyzing the mode of failure, the orignal
>failure was from a close proximity lighting strike,

This is probably where the pneumatic idea could probably help, as well as
with keeping moisture out of the electronics. Use a suitable diaphragm
mechanism under the pad with the pipes from multiple pads going to a central
electronics points where the pressure sensors are all in one box. Using a
metal box would give suitable shielding and remove the cabling runs that
allow ground spike potentials to get induced.

OTOH from what you said it sounds like all the electronics is in one box, so
maybe all you need is a network where it plugs into the box with telecoms
type spark gaps and series resistors and all the other bits for such
protection. May involve a re-calibration of the electronics against the load
cells, but may be the minimum modification route and the filter network
could probably be built into the load cell connector at the electronics end
(what do you mean they use RJ45 ...).

2007\05\24@123921 by engineer

face picon face
A capacitive sensor will be adversely affected by weather, rain, water, etc.

A metal detector WOULD work, but you will need to install a non-metallic cover
to protect from water as well, but it could be potted (A larger  
version is buried in concrete or asphalt and used to detect when cars  
are at a
ticket dispenser; these have an almost unlimited lifetime). You would need
to have a plate that would move down slightly when body weight steps  
on it, and
so detected by an RF metal detector as a change in frequency (changing  
the inductance, i.e. the tuned loop).

--Bob



Quoting alan smith <micro_eng2spamKILLspamyahoo.com>:

{Quote hidden}

> -

2007\05\24@130932 by Bob Blick

face picon face
Just wait for the new immigration bill to pass, then
hire guest workers to observe the foot traffic and
push buttons to activate the fountains.

Seriously though, the reliability of most methods is
more dependent on the underground electrical
connections than on the method itself. Direct your
focus to that area and find out what is causing the
failures.

Cheerful regards,

Bob

2007\05\24@142318 by Dr Skip

picon face


alan smith wrote:
>   I've sent an email to the end user asking to describe what isnt working exactly.  Of course I expect an answer...it doesnt activate when stepped on....hmmm...guess its broken.  I'd rather not make too many trips to site, waste of time and fuel (but I charge these guys mileage at least)
>
>  
That's kinda why I suggested looking at failure modes. If it's wet, and
muddy, there's lots of possibility for the pressure sensor not having
anything to press against underneath as mud softens, if I understand the
design right, and any design that depends on up/down motion will
probably fail. Mud will get in the gap, cavities can form under the
stone, grit can seize moving parts, etc. The design has to be able to
return to the start position under all mud conditions most likely, so
any motion has to be sealed. I would also guess the travel is small,
since stepping on stones that move a lot would be annoying, and this
makes the affect of grit or shifting worse.

How about a light beam across each stone? The foot breaks the beam.

Or embed a hall sensor or better, a reed switch, in the top of each
stone and embed some rare earth magnets in the soles of the stone
walker's shoes?

If the step can be made loud enough, as in hard shoes vs sneakers, an
embedded electret element might work too, but will have to be adjusted
so thunderstorms and lawnmowers don't set it off. Same idea as the piezo
one, but more easily waterproofed I'd think.

-Skip

2007\05\24@143248 by M. Adam Davis

face picon face
Consider also doing hydraulic.  The plates won't have to move as much
vs pneumatic to sense a valid input (in fact the user may not notice
the rock moving at all), and hydraulic will be less susceptible to
crushed lines and water infiltration (as pressures should be similar).
It should also be fairly cheap overall, the components are
ubiquitous.

Hydraulic oil in not generally conductive, so lightning strikes would
pose less of an issue.

You could sell them on changing one to a hydraulic unit with a local
pressure sensor, and if it works ok over time, swap the others out and
extend the lines for lightning protection, as well as central
maintenance.

If well put together, the hydraulic rams and tubes should never need
maintenance.  Consider using cheap brake cylinders for this end, since
the seals are included.

If a pressure sensor needs to be replaced, it's done in one box.  You
could even double up and have two sensors per line for added
redundancy.

-Adam

On 5/24/07, Bob Blick <.....bbblickKILLspamspam.....sbcglobal.net> wrote:
{Quote hidden}

> -

2007\05\24@172303 by Dwayne Reid

flavicon
face
At 03:58 PM 5/23/2007, alan smith wrote:
>
>   Presently the sensing is done using a load sensor that sits
> underneath a fake stone so stepping on it causes an analog voltage
> to change and thus triggers.  I've been there once under contract
> to repair, where the load sensor took a voltage spike and had to
> replace (about 1K ea)

I'm quite curious about the failure mechanism.  Load cells are
usually extremely robust devices so long as the strain gauge elements
are protected from physical damage.

Quite frankly, my first thought would be to use load cells if I were
asked to design a similar system.  I've simply not had unexplained
failures in all my previous experience with them.

My suggestion would be to visit the site and see just what problems
currently exist.

dwayne

PS - please mention what problems you find.

dwayne


--
Dwayne Reid   <EraseMEdwaynerspam_OUTspamTakeThisOuTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2007\05\24@202330 by Bob Axtell

face picon face
Dr Skip wrote:
> alan smith wrote:
>  
>>   I've sent an email to the end user asking to describe what isnt working exactly.  Of course I expect an answer...it doesnt activate when stepped on....hmmm...guess its broken.  I'd rather not make too many trips to site, waste of time and fuel (but I charge these guys mileage at least)
>>
>>  
>>    
> That's kinda why I suggested looking at failure modes. If it's wet, and
> muddy, there's lots of possibility for the pressure sensor not having
> anything to press against underneath as mud softens, if I understand the
> design right, and any design that depends on up/down motion will
> probably fail. Mud will get in the gap, cavities can form under the
> stone, grit can seize moving parts, etc. The design has to be able to
> return to the start position under all mud conditions most likely, so
> any motion has to be sealed. I would also guess the travel is small,
> since stepping on stones that move a lot would be annoying, and this
> makes the affect of grit or shifting worse.
>  

Yes. I imagine the problem to be ice crystals  over time breaking  the
mechanism apart.  But it
needs to be researched carefully before choosing a workable method.

--Bob
{Quote hidden}

2007\05\25@104212 by alan smith

picon face
so...they called me.....and asked when I can come look at it.
 
 me: so...are any of the sensors working?
 him: no
 me: have they been working at all? when did they last?
 him: hmmm...im pretty sure they didnt work after winter.
 me: you know, these are on an incline where mud and debris can flow into them, have they been cleaned out?
 him: no, I dont think so.
 me: why dont you clean them out and then see if anything works....
 him: OK, I'll let you know.
 
 So, could be that the cavities in the cement are filled with...stuff. I hope so but at least but it buys me some time that I don't have to take a day to go and find it was that simple....

     
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2007\05\25@110750 by Alan B. Pearce

face picon face
>So, could be that the cavities in the cement are filled
>with...stuff. I hope so but at least but it buys me some
>time that I don't have to take a day to go and find it
>was that simple....

I can see that you haven't yet worked out how to charge $2000 for a days
work ... $100 for doing it and 1900 for knowing what to do ... plus
travelling of course ... ;))))

2007\05\25@113538 by Dr Skip

picon face
Alan B. Pearce wrote:
> I can see that you haven't yet worked out how to charge $2000 for a days
> work ... $100 for doing it and 1900 for knowing what to do ... plus
> travelling of course ... ;))))
>  
Or he doesn't want to give up the $3000/day work he's got at home and
take a cut for the day... ;)

2007\05\25@124442 by Rich

picon face
Reminds me of the story about the consultant that charged Henry For $10 for
tweaking and $ 9,990 for knowing where to tweak
{Original Message removed}


'[EE] getting stepped on....'
2007\06\01@131705 by alan smith
picon face
Well...looks like I get to visit them.  Email back says....we cleaned them out of all the gunk, and now one works all the time, one works some of the time and one doesnt work at all.  Of course its nice he never asked how much my rates are.....just wants me there to fix it.
     
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2007\06\01@135951 by Dr Skip

picon face
Watch out. He might think it's part of the 'service' or guarantee... I
would suggest a fixed price to come out to look at them and give an
estimate, to be credited towards the repair charges if he goes ahead at
your rates or estimate. If your estimate comes out too high and he says
no, your trip is paid for. If he says no to the charges for the estimate
and trip, it was going to get ugly by the end anyway...

-Skip

alan smith wrote:
> Well...looks like I get to visit them.  Email back says....we cleaned them out of all the gunk, and now one works all the time, one works some of the time and one doesnt work at all.  Of course its nice he never asked how much my rates are.....just wants me there to fix it.
>        
> ---------------------------------
> Yahoo! oneSearch: Finally,  mobile search that gives answers, not web links.
>  

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