Re: Strain gauge

[ddl@danlan.*com (Dan Lanciani)]

In article <pr2iv11p9jqtkhq18oe8htehuardbp2n3l@xxxxxxx>, sales@xxxxxxxxxxxxxxxxxxxxx (Robert L Bass) writes:

| Thanks, Dan, you're right.  They don't generate a current as I
| erroneously stated.
|
| > It might be possible to do something over a longer period if you
| > could factor out various non-occupancy-related noise, though
| > thermal changes in the structure to which the Pulsor is attached
| > could be a big problem...
|
| There's more to it than that.  The resistance change is momentary
| and occurs when the joist flexes.  It returns to its previous
| state once the joist stops moving.  In short, they detect a
| moving load on the floor above.  They do not report the existence
| of a static load.

No, that is not how they work.  The resistance changes when the
Pulsor is stressed.  It remains changed as long as the stressing
force is applied.  The detection of (only) a moving load is entirely
a function of the processor electronics.

You can see this for yourself by simply connecting a Pulsor to an
ohm meter.  Apply stress by pushing the middle and pulling on the
ends.  (This can be done easily by placing your thumb on the middle
of one side and your first two fingers on either end of the other
side.  The two "sides" are the one with the potting compound visible
and the one opposite.)  Observe that the resistance changes and remains
changed until you release your grip.  Now apply stress in the opposite
direction by switching sides.  Observe that the resistance changes in
the opposite direction and again remains changed until you release
your grip.

| > Given the phenomenal signal processing power that you can
| > throw at a problem for relatively low cost these days (even
| > compared to a few years ago) it may be worth looking at.
|
| I wouldn't discourage anyone from researching possible solutions
| to the problem of occupancy sensing.  However, before starting it
| helps to understand the limitations of the sensors themselves.
| These devices detect joist movement.

No, Pulsors really are static strain transducers.  It is the processors
which are in effect "AC coupled" (really they probably use some sort of
self-balancing bridge, though I may be abusing the terminology slightly).

| A small kerf is cut into
| the joist and the detector is cemented in place under that cut.
| When the joist flexes the detector is stretched, causing a
| momentary electrical change.  It's not like a pressure switch
| that closes when you stand on it and opens when you leave.

Actually, it is a lot like a pressure switch--at least an analog
version.  The real problem is that live loads typically produce
much more joist deflection than static ones.  That may be the
cause of confusion about how the sensors work.  I can't say whether
it is possible to pull useful static stress information from the
inevitable noise, but this would be true for any stress-based
detection approach.  It is not a limitation of the Pulsor sensors.

| Also note that the sensors can't tell in which direction a load
| is moving -- into or out of an area.  They only know that a load
| has been placed upon the floor somewhere within a certain radius.
| TTBOMK, the transducers don't differentiate between "load placed"
| and "load removed."

The resistance changes one way when stress is applied and the other
when it is removed.  The sensors can definitely tell the difference
between load placed and load removed.  Again, do not be fooled by
the fact that the processors discard most of the available information
and give you a momentary motion detect output.  They are merely trying
to emulate an alarm-style motion detector to make life easy--and they
do a very good job.

				Dan Lanciani
				ddl@danlan.*com