Strain gauge

I imagine someone has though I haven't. Coincidentally, there are strain sensors that measure floor joist flexing to detect burglars. A company called Sure Action makes them. They call the products "Pulsors". They give off a tiny electrical current when the joist flexes due to an intruder's weight above.

You could measure the current and build a simple circuit to detect occupancy but it only responds to flexing -- not a static load -- so it still doesn't solve the perennial problem of room occupancy for HA. :(

I've used these sensors on my own jobs a few times. When properly installed they work well. Poorly installed they won't detect anything smaller than Mama Kass.

The company also makes processors to trip an alarm or HA system sensing zone when it detects movement above. The main benefits of these things are they're completely undetectable from the protected area and they are immune to most of the things that cause false alarms with motion or glass break detectors. The disadvantage is they require more effort to install than the lick 'n'stick wireless motion detectors that have become standard fare in the alarm industry.

If you want to experiment with one of these sensors let me know. Although I don't currently list them online, I carry Sure Action's other lines and can provide these as well.

See what I mean?

Thanks for the information about the sensors. I'll look into it.

-- Bobby G.

"Frank Olson" wrote in

Yes. I see. But do you? Bob answered my question precisely and even gave me the name of a product to look for. All *you* did was whine and whine the way you always do. Do you know what the word monomaniacal means? Look it up and then make yourself a name tag. You're a one-trick pony and your single trick has grown quite stale.

Once again, your comments contribute not a single iota's worth of information about strain gauges or anything else HA-related. All your constant carping tells us is that you've got an ongoing vendetta against Bass. We know that. And we don't care. What we *do* care about is you cluttering up CHA with your constant anti-Bass BS. You add nothing to the group except litter. At least Bob answers posted questions and clearly identifies himself as a vendor.

-- Bobby G.

"Frank Olson" ...

Oh horror of horrors, Bob gave someone good information and offered to help him get something that he doesn't normally carry and help him get started with the unit. Boy that is such an evil thing to do.

What color is the sun in your world Frank (Vendetta against Robert Bass) Olson?

Here! Here! Maybe the losers will swim back down their gutters.

In article , snipped-for-privacy@bassburglaralarms.com (Robert L Bass) writes: | > Has anyone here ever mounted strain gauges on | > basement jacking posts to measure the load changes | > of the floor supported by the posts? | | I imagine someone has though I haven't. Coincidentally, there | are strain sensors that measure floor joist flexing to detect | burglars. A company called Sure Action makes them. They call | the products "Pulsors". They give off a tiny electrical current | when the joist flexes due to an intruder's weight above. | | You could measure the current and build a simple circuit to | detect occupancy but it only responds to flexing -- not a static | load -- so it still doesn't solve the perennial problem of room | occupancy for HA. :(

The Pulsors I have at least claim to vary their resistance depending on stress. The delta detection appears to occur in the processors which adapt to the new value with a relatively short time constant. 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. 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've used these sensors on my own jobs a few times. When | properly installed they work well. Poorly installed they won't | detect anything smaller than Mama Kass.

I installed one in my house to see how well they worked (very well) and then gave several to a friend. They are great for houses with cats (and presumably other pets). I bought a bunch more but never got around to installing them. Now the epoxy is way out of date. :(

| If you want to experiment with one of these sensors let me know. | Although I don't currently list them online, I carry Sure | Action's other lines and can provide these as well.

Do you happen to know whether they sell replacement epoxy packs or specify a brand/type that is suitable? (See my complaints about current hardware store epoxy elsewhere...)

Dan Lanciani ddl@danlan.*com

Thanks, Dan, you're right. They don't generate a current as I erroneously stated.

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.

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. 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.

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."

Almost any two-part epoxy cement should do the job. As long as the cement is rigid -- not flexible stuff like RTV -- it should work fine. If you want I'll call and ask them to send you a package to experiment with. It's been a while since I spoke to them but the rep seemed pretty decent -- likes DIYers. :^)

| 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

About 25 years ago I researched using piezo load cells for another application but they might have an application here, mounted as the OP suggested.

I ran the US operations of a French company that made machine tools and I was looking for a way to sense the load on the machine (hydraulic press that came in 30-440 ton capacity) for precise process control and to sense a hand or finger between the dies for safety purposes. I don't recall the name of the company (Swiss) that made the particular load cells I was interested in but I made a trip to upstate NY to visit their US office. Their engineers thought it quite feasible and, to my surprise, were not concerned with the potential liability issues. I also talked to engineers at Battelle Labs and some people from DARPA, all of whom thought it feasible. I passed the idea along to the engineers with my company on a trip to Paris, thinking they could follow up with the Swiss company from there. They never did, but a couple of years later, after the US operation was sold to a former competitor and I was consulting for them, I met with a group of engineers (with whom they were working) at the University of Minnesota. They also saw no difficulty in implementing it other than the need for auto-ranging electronics to deal with the relatively small loads presented by a finger vs. the potentially large loads presented by the workpiece.

I don't think anything came of it as the parent company (KKR) shut down the Minnesota plant soon after that. My health had deteriorated to the point where I had to quit traveling at about that time and had to stop working altogether a little later.

The sensors were not terribly expensive although, >>

After posting that I called an old friend who worked with me years ago and asked about that. You are correct. My bad.

See above response. :^)

I did some more web searching and found that the sensors are pretty expensive - one mailing list suggested $100 each for a sensor and a processing board. That's more than I wanted to spend per sensor.

I want to use something like the strain gauge they use in electronic bathroom scales. The Pulsor seems to work using a longitudinal sensor glued to a joist that can detect deflection. I would rather have a sensor that reported a change in resistance as it experienced an increase or decrease from the "setup" load. Since I am refinishing a section of the basement, I want to mount some sensors in various locations so that I can detect increasing static loads on parts of the support structure.

-- Bobby G.

Thanks for the cite. That's a lot of info to digest but from what I gather, I'm out of luck. Piezo sensors detect deltas only up to a certain point, and it's measured in hours.

What would you do to try to measure both a house settling and/or a serious stress on a support column? I'm a little worried about the power chair that dad's using. He settled on the biggest one we could find because it had the most comfortable chair but with him in it, it weighs close to 400 pounds.

I probably should start looking to seismology because part of their mission is to monitor tiny movements of massive components.

Back to the drawing board.

-- Bobby G.

In addition to the resistive devices I cited, there's been a lot of work on fiber optic load cells but I don't know whether you will find any inexpensive products that you can buy. Here's a paper dealing with them in much larger structures.

These look great and the price is right. The output is exactly what I am looking for. While the readings I get won't likely be calibrated to standard engineering units of measurement, I think they'll provide exactly the kind of information I want!

Not only that but a fourpack is $55 and an 8 pack, $99 so they are less expensive than good PIRs.

Thanks Dave! (108) (-:

-- Bobby G.

The sensors retail for about $41 in single lots. I'm not sure you need their processor.

That's the manufacturer. Alarm manufacturers almost never list prices on their websites (it annoys alarm dealers who are their customers).

From the discussion so far, the Pulsor *might* still be what you need.

Robert,

Check your local building code. You may find that the floors are designed to support 300 lbs per square foot. I seriously doubt your dad plus his wheelchair will affect the structure.

By way of comparison, my waterbed weighed ~3,200 pounds with no one in it when I was in college. During parties there were as many as six people in tit, adding roughly another 1,000 lbs.

The downstairs neighbors didn't always appreciate my music but at least we never fell through their ceiling. :^)