# New Project for a Newbie to take readings and do simple math

Discussion in 'General Electronics Chat' started by hartparr, Oct 14, 2009.

1. ### hartparr Thread Starter New Member

Oct 14, 2009
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I've dabbled in electronics, but nothing major.

I am currently building a Prony Brake, which is an old style way of measuring horse power on engines.

To calculate horsepower in my device, you need to measure torque and RPM and put it in the following formula...

Hp = Force X rpm / 1000

I will be measuring Torque with a load cell and RPM with an electronic tachometer.

Can someone help me out with standards I need to following when purchaseing my electronic tachometer and load cell?

I also would appreciate some direction on what I need to purchase on the microcontroler side and some resources to program it to take the Torque and RPM reading and make the calculations on the fly, then display them via LCD screen.

Thanks!

JTB

2. ### wr8y Active Member

Sep 16, 2008
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I just wanna know what a load cell is!

3. ### studiot AAC Fanatic!

Nov 9, 2007
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Did you mean load cell or tension ring?

Direct measurement of Brake Horse Power used to use a standardised brake pad applied by pulling harder and harder on a spring. The extension in the spring indicated BHP.

It would also be useful to indicate the range of HP to be measured.

4. ### studiot AAC Fanatic!

Nov 9, 2007
5,003
523
A load cell is a block of material containing bonded strain gauges that distorts (deflects) in response to applied force. Usually this force is compressive. Since the gauges are bonded to the block they distort with it so we can use their change of electrical characteristics to measure the distortion and thus calculate the applied load.

Such cells are used as the basis for weighing machines, large and small.

Apr 20, 2004
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6. ### BMorse AAC Fanatic!

Sep 26, 2009
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I found that purchasing a small digital scale and scavenging the load cell out of it is cheaper than buying the actual load cell... Just make sure the scale is using a load cell and it has a large measurement range (i.e. a scale that can handle 4 pounds is better than one that can only weigh stuff in ounces....)

Something like this one >>> http://www.digitalscalescenter.com/product/AMW-810-5K

My .02

Last edited: Oct 14, 2009
7. ### hartparr Thread Starter New Member

Oct 14, 2009
8
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I am building a large brake drum that will take the rotational force from the brake and press on a load cell at a single point.

If you google "Prony Brake" and look at the pictures, you'll get the idea. In all of the pics however, you will see a large scale, I want to replace the mechanical scale with a load cell.

8. ### hartparr Thread Starter New Member

Oct 14, 2009
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I found a picture of a guy who had my same idea.

http://www.98dodge.com/jerry/bluebrake002.jpg

As you can see on the left, the S bar load cell in place.

Looks like his only takes the readings, outputs on the 2 digital read outs and the calculations are done manually.

I want mine to do the calcs live and have a horsepower output screen.

9. ### wr8y Active Member

Sep 16, 2008
232
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hartparr may not have learned anything in this thread so far, but I have.

10. ### hartparr Thread Starter New Member

Oct 14, 2009
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So...To recap.

Looking for direction on something that can take a reading from a load cell and a tachometer, do some simple math with it, then show it on an LCD display.

Ideas?

11. ### beenthere Retired Moderator

Apr 20, 2004
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There is some more hardware than that. The cell has to have a stable and accurate excitation voltage across it. Then you will need an instrumentation amplifier to bump the load cell output up to the range for your A to D converter.

A microcontroller usually has a 10 bit A to D. That is pretty inadequate for good resolution from a load cell - 14 bits is much better, meaning a converter external to the microcontroller. You need other inputs as well - force is just force, and can't be made into horsepower without some other forces and variables being measured.

Look at the link - http://en.wikipedia.org/wiki/Dynamometer