Practical D.I.Y. Wheatstone Bridge

I was working on my M.Sc. project and it includes using strain gage as the sensor. So I built the circuit on my own to experiment with them. Strain gage is generally a resistor made from metal foil. The resistance of strain gages change proportionately to its cross-section area when compressed or stretched. Their relationship is displayed in the equation below.

Engineers usually use strain gages to measure strains and tensions experienced by a material under certain circumstances. It is an excellent sensor that can detect the smallest deformation in the material that it is bonded to. More about strain gages can be searched in other websites. 

However, the resistance change is small; in range of 1 milistrain - 1000 milistrain. This is where Wheatstone bridge come into the scene. Wheatstone bridge is a differential circuit that captures only the voltage change of a network. Then, the voltage change is amplified using differential amplifier; I prefer to use three Op amps-configuration instrumentation amplifier. After the voltage is measured using a voltmeter, the strain can be calculated and estimated using the appropriate formula depending on the network configuration you used. Below is a schematic diagram of a decent Wheatstone bridge network for quarter-bridge and half-bridge configuration. 

Notice that one of the resistor is actually a potentiometer. Potentiometer is an adjustable resistor. It is used in the Wheatstone bridge to manually balanced the network to zero or set an offset to the network. When I was assembling my circuit, I noticed that my potentiometer is very vulnerable to temperature change  and vibrations. 

I found some interesting tips to mitigate the problem and like to share with others. One of the solution to reduce temperature change is using a potentiometer model that is covered with insulators. My previous potentiometer is exposed to the air. It could contribute change in the voltage output up to 10 mV. Meanwhile, my strain gage resistance change is less than that. Potentiometer that is equipped with insulator can help to slow the effect.

Vibrations can cause the potentiometer dial to rotate undesirably. Even small movement can affect the output significantly because of the sensitive nature of strain gages. Therefore, the potentiometer is replaced with a smaller value potentiometer in series with resistor to solve the problem. 

A common strain gage value is 120 Ohm. Each arm of the Wheatstone bridge have to be 120 Ohm in order for the network to be balanced. If we were to implement the vibration resistance method, we replace the 120 Ohm potentiometer with a 10 Ohm potentiometer paired with a 110 Ohm resistor. Therefore, if vibration caused the dial to move, the change will be less significant that it might be neglected. This method also reduces the temperature effect. The smaller the potentiometer the better resistance it had with vibrations and temperature change.

Temperature change can also affect the strain gage itself. Conventionally, this can be canceled by connecting a dummy gage to its opposite arm and place it in the same area. Therefore, one strain gage will experience positive change and the other is negative and they will cancel each other out. Another way is to use strain gage that is made from constatan alloy. This alloy has a constant resistivity in a wide range of temperature. This special feature help the strain gage resistance to remain steady when temperature change occurs.

I hope this entry will help engineers that is working on D.I.Y. strain gage. Good luck!

My Hand Made FM transmitter

Today, I'm just gonna show you the FM transmitter that I made during my EECE252 lab. This course is about signal and processing. We were given two choice wheather to build FM transmitter or AM receiver. I chose to build FM transmitter because FM is way cooler. huhu

Basically the circuit is consist of input (microphone), mixer (BJT) and antenna (wire). There is also a tuner (blue device) to tune the transmitter to desired frequency.

FYI FM signal consume huge bandwidth but as a trade off the signal is almost imune to inteference while AM signal is vice versa.

This is how it works: there is two different signals, one called information signal and another is called carrier signal. The info signal which contains our desired info i.e. voice, music, data etc. is then modulated to the carrier signal which is basically a simple sinusoidal wave range 88MHz to 108MHz (for standard FM radio spectrum) that carry our info wirelessly accross the medium (air) to any radio (FM receiver).

We also learn the mathematics behind it. For FM, after modulation the frequency respond has Bassel Function property and it is depends on the modulation index which is entirely depends on the designed circuit. For AM simply find the product of two wave function.Here calculus play a big role.

So I did it within 45 minutes but only desparation I get. I tested it using a conventional radio but no sound coming out from it. I went home and try to troubleshoot it using multimeter but failed. Sigh!.... I saw others manage to tap the microphone and heard results on the radio which is so awesome. This is my first time building a real life FM transmitter (although I suck). Now I'm seeing my future as an electrical engineer become brighter and brighter. :)

Engineering Week

nass_sare: masuk eggdrop jom

are_nan1611: fuh bile?

nass_sare: 16 feb ar

nass_sare: isnin

are_nan1611: isnin??

are_nan1611: hurm ok2..bleh kot

nass_sare: 1230

nass_sare: ha

nass_sare: cool

are_nan1611: ok2

That was my my instant massage with hanan before the start of engeneering week. certainly "the heat" was felt. For us Malaysian here in vanderbilt everyone was affected by it regardless we participate or not. 

Before further reporting about Engineering Week here at Vanderbilt let me tell you a bit background about Vanderbilt University and also our School of Engineering. Vanderbilt University is ranked 18th place nationally while our school of engineering is ranked 42nd. This is quoted from US News & World Report 2009 edition. If you don't believe me you can search at my search engine-ering. There is about Malaysians 130 undergrads here and all of them took engineering. Interesting isn't it? Done with the briefing, if you have more question you can shout at my box or leave a comment.

Below is some of activity we have done in the Engineering week. well... one picture can tell thousands stories.

Egg Drop Competition

 i want to buy buble tea if i win $400

poor egg

rest in peace

azlan and baba on top of the world

iCreate Robots Competition

the main actor

the obstacles

Fastest Geek

Haziq with his special pose

try again next year naser

muhaimin strugling to be the one

the grand prize DELL XPS 1530

Tower Building Competition

zed in action

Thats all for now. see ya next week!!

p/s: sorry zed... hehe

Engineer

ding! I entered the lift

"How are you?" said the African American male with his girlfriend with his unique accent.

"I'm good, how are you?" I replied.

"good! what are studying here?" he ask out of the blue.

"Engineering"

" what kind of engineering?"

"Electrical and Computer Engineering"

"man! you gonna make a lot of money I tell you!" suddenly he get fired up

"yeah, I think so....but I'm thinking of becoming a lecturer though" I replied.

ding! I walk out from the lift

"have a good night!" I said.

"you gonna make a lot of money I tell you!" he repeated the same thing for the third time.

and then I left thinking about the huge amount money that I'm gonna make.... ha! ha! ha!

Who is an engineer?

The well known definition of engineer is the person who applied science and mathematics to solve problem, issue, crisis, need of people in daily life.

In my perspective that definition is not wrong at all, others always thought that engineer is the person who makes a lot of money. In some way it is true but the main idea is engineer is a problem solver.

In the 20th century there are a lot of invention and majestic building that has shape the world physically and culturally were made such as radio, computer, cellphone, fast cars, bullet train, airplane, WTC, Petronas Twin Tower and the marvelous of the world that we took for granted. This are all made by engineers and I am going to be one of them.

Our Priority

Energy crisis is our main concern. Now everything that is made has to be energy conserve and renewable. The scarcity of oil and higher demand of energy consumption is the main cause. Then, whoever enter this field is the one the has the highest probability to make huge amount of money.

Another field of interest is nanotechnology. This is a new field which only starts less than 20 years but was predicted 50 years ago by Richard Feynman, some say he is the father of nanotechnology. Now most of the stuff is going nano from microchip to nanochip, microscope to nanoscope. Observing and moving a single atom is common these days. After we manage to manipulate the material in nanoscale we can change their property (i.e. density, hardness, conductivity etc.).

One of the wonder material of the engineering field is the carbon nanotube (CNT). Its cheap yet strong and light. Engineer in this field is highly demanded and I recommend you to learn about it.

Cool Stuff

Look at this video; speaker made from array of carbon nanotube shown during my signal processing class. It was made in China, that explain the music selection.

• The speaker was stretched yet the sound quality still remain excellent.
• Very thin and can be placed anywhere you could think of.
  

Bismillah

Subhaanaka Laa 'ilma lanaa illa ma 'allamtana innaka anta 'aleemul hakeem.
In the name of Allah I start this blog.