Hi. My name is Mark Bailey. I’m an application engineer here at Agilent Technologies and
today I’d like to talk to you about measuring force, both dynamic force and static force.
So what do I mean by dynamic force? Well, think of a string on a guitar that’s plucked,
and it’s vibrating, going back and forth. You want to measure the force that’s being
generated by that. Maybe the force on an airplane wing, where it’s bouncing around, or
a microphone, where you’re talking to it and it’s picking up the signal.
There’s a couple of different sensors that are used for this. One of them is the piezo sensor.
It’s basically a crystalline structure that generates a voltage as it’s tweaked on.
It’s a little like your old phonograph records, where they used to have a crystal
and a little needle, and as the needle vibrated in the grooves it generated a voltage coming
out of the crystal. Okay, so if you look on the voltmeter, you’ll notice that I’m
measuring basically zero volts while it’s sitting here just dead. As soon as I pick
it up and starting twisting on it and banging it around, you’ll notice that it’s generating
a voltage. But as soon as I quit doing that, it drops back to zero volts.
Okay. I actually captured that. I sat there and kind of did this and just banged on it
and captured it with the multimeter and brought that information in and plotted it out. And
if you look on the screen, you can see that each time I whacked it, it got another pulse
on it. But then as soon as I stopped banging on it, it went to zero. So great for dynamic,
but not very good for static measurement. What do I mean by static measurement?
Well, think about a scale. You step on the scale. It weighs you. In my case, it says it’s
200-plus pounds, and what you get is a constant measurement, even though things aren’t changing.
It’s not dynamic; rather, it’s static. So I’m going to show you a demonstration of making
static measurements using something called a strain gauge. So let me go ahead and set that up.
Okay. So we’re set up to make a static strain measurement or force measurement.
We’re going to do that using something called a strain gauge. A strain gauge is just basically
a bunch of traces laid down on a little plastic substrate going back and forth and back and forth.
And you get enough distance there and it actually turns into a pretty good size resistor.
Three hundred and fifty ohms is a fairly standard value and that’s what
we have here. Usually, when a strain gauge gets elongated, the resistance goes up and
when it gets compressed, the resistance goes down. The way people usually measure this
is they put them in something called a Wheatstone bridge. They have an excitation voltage.
They have a balanced bridge and one, two, or four legs have strain gauges in them and
they measure a voltage across it.
Well, it’s pretty complicated. What I’m going to show you is with the 34411A, we can
take a 4-wire ohms measurement of that same strain gauge and get a similar result.
So, I’ve got my strain gauge here. I’m running four wires back for a 4-wire ohm measurement
on it, and you can see I’ve got a stable reading, around 349. As I push on the cantilever,
it goes into tension on the strain gauge and it goes up. And as I come in and lift it,
now I’m going into compression and the number goes down. And then when I got back to the
stable zero position, it comes back to where it was before. Okay. So think of it being
a weight and I’m standing on the scale and I’m pushing it down, and the number
goes up and I can measure that.
Now, we’re actually able to measure microstrain if we do all the appropriate things with gauge
factors and calculations and stuff, but I’m just going to show you the principle here first.
Now, it shows up even better if I go into null, and now I’ve nulled out that
initial stable position, and now it’s even more clear. As I push down on it, the numbers
go up. As I lift on it, the numbers go down, Okay, it goes negative. What I’d like to
do now is go ahead and set up my computer to take control of the 34411A. The 34411A
has a built-in Web server, and with my Web browser I’m going to go in and take control
of the instruments. Let me go ahead and do that.
Okay. So here’s my Web browser. I’m going to go into the browser control and I want
to allow full control, so I go ahead and click that. There it is, monitoring the multimeter,
and I’m getting readings. I come down here and I set up the control panel so that I can
adjust it. I’m going to want to be in 4-wire ohms. I’ve got Offset Compensation off.
It’s fine to be in auto range. I’m actually going to go pretty fast. I’m going to change
this to 0.006 PLC, which is about 1,000 readings a second, and I’m going to Enable Null,
so sure enough, I’m still in zero. I want to trigger immediately. I’m going to take
about 10,000 counts I’m going to set it up for. I doubt if I’ll take that many counts,
but I’ll set it up for it, and I’m going to go ahead and apply that to the instrument.
Okay. So my instrument’s changed. It’s set up and ready to go on the measurement.
What I’m going to do is start the measurement and then I’m going to bend my cantilever
and then let it bounce back up and down and capture that kind of resonance. So I start
the measurement, I push down on the thing, let it bounce for a while. Okay.
Stop the measurement, and now I’m going to go in and view the data. I should
have a bunch of data now that came from that. Okay, and I’m going to get the data, and
the data fills up the window. I’m going to come in here and capture the data.
I’m going to do a Control-A to highlight it all and a Control-C to capture it. And now
I’m going to go to an Excel spreadsheet and I’m going to populate that spreadsheet with that
information. So I do a Control-V and what I want to do is actually go in and graph it.
So I’m going to go to where my graphing is, grab a graph, do the graph, and lo and behold,
you can see what all was going on. As I expand this out, you can really see what
was going on with the measurement. Okay, went up, I held it, let it go and it vibrated down.
Okay. So we’ve seen both dynamic and static measurements using either a piezo sensor in
the case of dynamic or in static force measurements, we used a strain gauge measurement.
I hope this was clear and I thank you for joining me. I look forward to seeing you again next time.