Hi, my name is Mark Bailey. I'm an application engineer here at Agilent Technologies, and
today we'd like to talk about rotation, speed of rotation, how you measure that. Speed of
rotation in things like motors, fans, engine speed, maybe even the rotation of wheels in
an ABS or assisted braking system. So first let me talk about the sensors that are used.
One of the most common ones is a variable reluctance sensor, or VRS. Essentially this
is just a small coil wound inside of here, and this is set up against a ferrous rotating plate,
maybe a cog wheel on a car or something like that. And as the magnetic thing passes
it'll actually induce a little pulse inside the coil, which is then, you can measure it.
Now the problem is the faster you go, the bigger the pulse and the more narrow the
pulse is, and sometimes that can be a problem. Sometimes you have to amplify it because they're
very small. Other times they make it so big that you actually have to clamp it. But, it's
a very common sensor; it's used a lot in ABS systems like I'm showing here on the computer.
Where the wheel's rotating, there's these cogs in this plate and the sensor is picking
that up and sending it to the control module and deciding what it needs to do with each
wheel as you're trying to brake. Okay, a second kind of sensor for RPM is something called
an optical sensor. And if you think of a light beam, as the light beam gets broken, it's
sending out a pulse each time that that happens. Now it's not always a light beam; sometimes
they actually use infrared sensors, and infrared senders, and that signal gets broken.
The third kind, and actually much more popular nowadays is the Hall effect sensor. A Hall
effect sensor also works on magnetic induction but instead of having a variable amplitude,
it's fixed at whatever the voltage is across the Hall effect sensor. So that's a real plus for it.
So I'd like to show you this in practice. What I've got on the circuit board here is a motor.
It runs off of 12 volts so I've got +12 and ground. And then there's a third line,
this yellow line coming off that's actually a little output that sends a pulse based on
the rotational speed of the motor. So let me show you that on the scope to show you
what I'm talking about. So when I turn it on we see a series of pulses on the scope,
and if I slow the fan down a little bit, don't try this at home by the way, but I'm going
to put my finger in here and actually slow it down a little bit, and you can see that
the pulses slow down and so forth. Okay that's great, and in fact we could measure it in
this, the scope's saying it's around 135, 140 hertz or cycles per second, pulses per
second. That's great but it's not RPM. So what I'm going to use instead is the
Agilent 34972 here to measure that pulse and we're going to turn it into RPM. So first I have
to set up my measurement; I'm going to turn the channel on. I can measure temperature,
AC volts, DC volts, a bunch of stuff. Finally I get to frequency -- well that's what I want.
So I'm going to measure frequency. Rather than being on auto range, I'm going to set
it to like a 10 volt range. I'm going to go to 4-1/2 digits and it's done. And sure enough
I'm measuring frequency. It's saying 139 hertz, or 139 cycles per second. Well, that's great,
but it's not RPM. So what I need to do is scale. I'm going to come in here to MX+B,
which is a scaling. I'm going to turn it on, I'm going to set the gain. And the gain is,
I want to set it to 60, since it's 60 per minute.
Then there's no offset that I want to enter.
I can put in a label, so I'm going to go ahead and do that. I'm going to actually
change this to say RPM. Done. So now it's measuring and it says it's 8400 RPM.
Well, that's great, except the motor's actually running at about 4200 RPM according to the spec sheet.
So it's going twice as fact. Well, that makes me realize is that actually it puts out two
pulses for every rotation of the fan. So what I need to do is to back and change my gain.
I'm going to set the gain again. In this case I want to now set it, instead of the 60 that
I had, I want to set it as 30, and everything else should be the same. Just leave the label,
and sure enough now it's reading 4200 RPM, which is what it ought to be reading. Okay,
so we've talked about measuring the speed of rotation. We've talked about the sensors,
we've talked about how you can use the scaling factors on the 34972 to actually convert it
over to a measure that we understand of 4200 RPM. So I hope this has been informative to
you and shown you how simple it is to set up and measure RPM. I look forward to talking
to you again soon. Thanks.