4.4 Resistors in Sensors.

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En provenance du cours de Georgia Institute of Technology

Linear Circuits 1: DC Analysis

123 notes

Georgia Institute of Technology

Linear Circuits 1: DC Analysis

123 notes

This course explains how to analyze circuits that have direct current (DC) current or voltage sources. A DC source is one that is constant. Circuits with resistors, capacitors, and inductors are covered, both analytically and experimentally. Some practical applications in sensors are demonstrated.

À partir de la leçon

Module 4

This module introduces additional methods for analyzing circuit problems and how resistors are used in sensors.

4.4 Resistors in Sensors.8:38

Rencontrer les enseignants

Dr. Bonnie H. Ferri
Professor
Electrical and Computer Engineering
Dr. Joyelle Harris
Director of the Engineering for Social Innovation Center
School of Electrical and Computer Engineering
Dr Mary Ann Weitnauer

[MUSIC]

Welcome back to linear circuits.

This is Dr Ferri.

This lesson is on the application of resistors in sensors.

The objective is to examine variable resistors and their use in sensors.

It builds upon the voltage divider law, which we need to build the sensor.

So let's first define what is sensor is.

It's a device that converts a physical quantity to an electrical signal.

A very common example is a sensor in a thermostat in your house.

A thermostat measures the temperature and converts it to voltage.

Other examples are oxygen sensors in cars or

sensors that measure acceleration or velocity.

In particular, a lot of sensors use variable resistors.

These are resistors that vary the resistance based on some sort of

physical quantity.

A force sensor varies its resistance as pressure or force changes.

A thermistor is a resistor that varies with temperature change.

A strain gauge is a device that can elongate, and

as it elongates it changes its resistance.

A potentiometer is a resister that varies with position.

Let's take a look at a resistor that varies with respect to force.

We set this resistor.

It's got these terminals coming out here, and as you press on the end point right

here, you vary resistance if you measure across these two outer pins right there.

So I've taken one of these sensors and put them in the bread board, and

I'm measuring the resistance across those two pins.

So you can see right here on my digital multimeter,

it's over the limit right here.

And if I start pressing on this,

you're going to see that the resistance starts dropping.

So I've got it down to 53, 30, 20k, see how low I can go.

I'm really pressing hard right now on this.

So I can get down to close to 7K.

And then I'm going to start letting it back up,

so I'm decreasing the pressure and I've got 70k.

It's going bigger and bigger.

So you can see that the larger the force, the smaller the resistance.

Let's look at the basic configuration.

If I want to take a variable resister like this, and

I want to make a sensor out of it, because the thing is, a variable

resistor gives you a variation in resistance.

We can't use that in our measurement devices, we need voltage.

So somehow we have to convert this variable resistance into a voltage signal.

And we do that by building a circuit that has a voltage divider law to it.

So this is my variable resistor.

This right here is the output voltage that I'm measuring for my measurement device.

Here's a known resister and this is a known voltage source.

So I can use the voltage divider law here to find the relationship

between this output voltage and the resistance.

Now sensor calibration is what we do in order to

find the relationship between the physical quantity and the V out.

So we're measuring V out and we want to figure out what the physical quantity is.

So after a sensor calibration, after I take a bunch of measurements here and

find this relationship, then I can go back and say, I measured this voltage.

That must correspond to this temperature, or this acceleration, or this pressure.

Let's look at a potentiometer.

This is a variable resistor that varies with respect to position.

What I've shown here is a potentiometer placed inside of a breadboard.

There are three wires coming out of a potentiometer, one we call point a,

the wiper, and point b.

The schematic of that is shown over here,

where the resistance between point b and point a does not vary.

That's the total resistance of that potentiometer.

It's the resistance when we measure the wiper to either of these points,

either this one or this one, that varies.

So, for example, if I took an ohmmeter and measured the difference between

the wiper and point b, I'm going to call that R sub v.

And that varies with respect to, in this particular case,

potentiometers have a dial on there that turns.

If I turn that, then I would find that this measurement varies in resistance.

So let's redraw this particular schematic term and

separate it out a little bit so we can examine the behavior.

So, we've got Rv measure between the wiper and

point b, and then this is what's left over.

So if this is a 100 ohm resistor then this would be a 100 minus Rv.

Now in this particular case, this resistance has to be converted into

some voltage in order for us to measure it and use it in a sensing system.

So I'm going to show that with regard to the rotary type of potentiometer,

which is the one that we're demoing here.

So in this particular case, we'll say the position is theta.

It's the angle here.

If this angle is 0, in other words, the wiper, this is the wiper here,

is pointed directly at the bottom terminal, in other words,

it's way down here, then my voltage output will be 0 because it would be grounded.

If instead my angle is pointed all the way here,

all the way to the other side, V sub out will equal to this Vs.

So, in other words Vs will be varying from 0 all the way up to Vs

depending on the angle that this wiper is at.

Now let's look at the actual behavior of this.

Now this is the operation of a potentiometer.

I'm going to move this to the ohmmeter,

and I've got this in the wiper as well as one of the end spots.

I will call it point b, and you see the dial on the top?

Right now, I'm going to put this screwdriver in here and turn this.

Right now it's at close to 0 ohms, and

as I turn this, I'm going to get larger and larger.

As I go all the way to the other end, I've got close to 100.

This is a 100 ohm pot.

So it varies between close to 100 all the way down to 0 ohms.

Now I want to make a sensor out of this, sensor I've got to hook up a power supply.

So let me go ahead and put this up.

I'm going to put a battery pack in here.

And I've gotta hook up power to the end points,

so what I call point a and point b.

So this goes to point a, and this goes to point b.

So now I'm going to change this to voltage.

So I'm measuring voltage now.

So as I vary this, I start out with 0 volts because I'm all the way at one end.

Basically I've got 0 resistance between these two terminals, and

I am measuring the voltage drop at the wiper.

This is the ground.

That's the wiper.

This is the Vs power supply.

And if I change the position, as I go to the other end,

I get up to about 4.6 volts, and that's full voltage.

That's the voltage I would read if I measured directly across this

set of batteries.

So I've converted the potentiometer into a sensor where

the voltage here varies as I change this position.

Then I should mention that potentiometers are used very commonly in things like

motors where I want to measure the angle of the motor shaft.

Another application is in dials.

A customer has a dial on a product and can turn the dial.

And if the dial's a potentiometer, then based on the voltage reading off of that,

you can tell where the customer placed that dial.

So like an analog dial.

All right, thank you.

[MUSIC]

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