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Project Help and Ideas » Load cell help

August 17, 2012
by countryguy828
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Hello all, my first post here...

I am new to digital electronics and microcontrollers and all but I have programming experience.

I have completed the 3 projects in the nerd kits guide with relative ease.

I have an idea kicking around in my head. It would require 4 load cells. I have watched the digital scale video several times now and understand the theory behind it. Now heres the question.

What would be the best way to connect 4 load cells to the nerd kit ATmega168? Here is where my in-experience with electronics comes in. Is it possible to wire the 4 load cells together and only use 4 pins from the ATmega168(2 power pins and 2 sense pins)?

Perhaps there is another way to attack the problem, I need to measure weight upwards of 1,000 pounds. Also I will probably need a way to calibrate the scale after the final code is flashed to the ATmega168 so I was figureing adding in a potentiometer for that.

Thanks for the input, Dave

August 17, 2012
by Ralphxyz
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Hi countryguy828, there are a lot of strain gage discussions here in the nerdkits forum.

You would not connect the individual load cells to the mcu you most liketly will need a amplifier first.

I received a lot of help here when I was playing around with load cells (strain gages).

Ralph

August 18, 2012
by countryguy828
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Ok spent some time today drawing... This is my first idea that I am fairly sure won't work but I don't know. At least it illustrates what I was trying to say above.

Schematic

Dave

August 18, 2012
by countryguy828
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Did some re-drawing and this is what my second and better idea look like, will it work?

Schematic idea 2

Dave

August 19, 2012
by pcbolt
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Dave -

Great project. I like the second arrangement simply because it is modular. If you can get 1 cell to work, the rest should be a piece of cake. I noticed in the NK tutorial, they used a voltage divider (with a capacitor) to provide a reference to the AD620 but I don't see it on your drawing (you'd most likely only need 1 divider for all amps - or use a 3.3v regulator). The downside is extra chips (and a bigger bread board) but that's no big deal. Figure 1 would work since the op-amp is used in a similar circuit for audio mixing. Not sure you need the pot though, the "zero" button should work. You could add a second button for a calibration weight, that way you could compute the "start" value and the "slope" value mentioned in the tutorial.

Please keep us up-to-date on your progress...looks like a blast.

August 19, 2012
by countryguy828
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Ok guys, here is the idea, take what you see in the video and modernize it with electronics to take the measurements and do the calculations in an instant to be constantly updated. With the Serial connection to the laptop might as well do data logging too.

Prony Brake

Need to do a little more updating on the schematic...

Dave

August 19, 2012
by pcbolt
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Dave -

I couldn't really tell from the video what the steam engine was doing and what the scale had to do with it but it seems almost a shame to modernize it. Not too many of those old beauties around any more.

One thing I forgot to mention in my last post was if you have ever looked into using Eagle to draw your schematics. It's free as a limited download, widely used, "industry" compatible and has many online tutorials showing how to use it. You can "easily" go from schematics to PCB design as well. You could also try the NK Circuit Lab for some circuit testing.

August 19, 2012
by Rick_S
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Dave, you don't by any chance know K.R. Hough do you?

Rick

August 19, 2012
by countryguy828
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The steam engine is running what is known as a Prony Brake. It is a device for testing horsepower. The Prony Brake shown in the video was built by a friend of mine in 2009/2010. The Steam Engine in the video is owned by some other friends, restoration of that engine was just completed 2 months ago.

I misspoke my words a little, we wouldn't be touching any engines at all. The Prony Brake in the video is owned the Pioneer Engineers Club of Rushville, Indiana and is a permanent fixture. There for we wouldn't really be doing anything to that machine. We would be building a new portable machine for ourselves useing the same concept. The machine in the video has a digital tachometer to display the rpm of the shaft turning within the brake. The brake is transferring power to the scale shown. We know that the arm of the brake is 63 inches from the center of the shaft.

Knowing that the arm is 63 inches, we can use the formula HP = (RPM x POUNDS) / 1000 to calculate the horsepower of the engine. We could use this on machine with any steam engine or tractor with a belt pulley.

I have MS Visio that I am useing to draw my schematics. If there is a place for me to post the files I will post my Visio drawings for others to use. I made one drawing for a starting point that is just the nerdkit that others can use to build off of.

Thanks, Dave

August 19, 2012
by countryguy828
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Can't say as I do Rick, sorry.

Dave

August 19, 2012
by Rick_S
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K.R. owns several steam tractors and brings them to shows and fairs. He worked at the company I work at before he started out on his own with a shop to repair antique tractors. I noticed your video was from Indiana. His shop is in N.W. Indiana that was why I asked. That steam tractor in your video looks very similar to one he owns.

Rick

August 19, 2012
by countryguy828
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Rick, I probably know people that know him. I am from Ohio myself, but not far from the Indiana border.

Ok guys, I updated the schematic a little bit more. I am sure I need to add resisters in for the 7 segment LEDs but that will depend on whether they are common anode or cathode. I added in a 3.3V voltage regulator for the reference for the AD620.

Prony Electonics R1

Thanks, Dave

August 20, 2012
by Ralphxyz
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I am sure I need to add resisters in for the 7 segment LEDs but that will depend on whether they are common anode or cathode

Why would that matter? If you need resissitors you need them rather anode or cathode and I'd say you need them.

I love the sound of that engine.

Still having problems understanding exactly how you get HP from a rotating shaft and a 63" arm.

Ralph

August 20, 2012
by countryguy828
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Hi Ralph

I meant for the location of the resistors on the schematic.

If you like that Gaar Scott, you should listen to a Baker. :) Oh and the fella Operating the Prony Brake, He has a Gaar Scott 2 cylinder, it is a really nice sounding engine.

These two videos do a decent job of explaining the Prony Brake Prony Brake Explained 1 Prony Brake Explained 2

Dave

August 20, 2012
by Ralphxyz
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Man those videos sure make me miss my days hanging out with farmers from Indiana. I used to spend a lot of time down on the farm.

Ralph

August 20, 2012
by JKITSON
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WOW Thanks for the info & videos..I have 8 old hit-miss engines myself, plus abot 15 more from other club members....I think this winter I will build one of these using flat belt drive..Another project added to the list... Thanks again....

Jim

August 20, 2012
by pcbolt
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Dave -

Ah...I see (said the blind man). Wow, there are so many ways you could go with a project like this one. First off, I think your schematic will work. Apart from the LED resistors, you may also need resistors on the SPI bus for long wires (the NK "Multi-Panel Display" has some explanations but you'd have to experiment with values). The good part again is the modular concept. How big would the 7-segment display be? I guess it can be any size as long as you wire it to the decoder chip with some transistors to provide the needed power. Oh and if you are using the 3.3v regulator, make sure you find one with "low dropout" so it can run off the 5v input.

The first video got me thinking of an alternative solution to the problem. If you used a linear actuator to move a constant weight along the lever arm of the brake, you could use the first formula to calculate HP. Not only could you move the weight, you could measure the distance at the same time from the actuator. You could also have an electronic tachometer measure the RPM of the belt wheel. Add a few extra sensors and you could have real-time feedback to adjust the weight precisely.

August 21, 2012
by countryguy828
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Jim, Thanks for the comments. You have inspired me to build a sled monitoring system for another one of my friends garden tractor pulling sled!

PCBolt, I talked with my friend last night about some of the details. He thinks we can do the project with just one load cell. That would definatly make things simpler. We also decided to drop off the last digit as we don't need to display tenths of a HP. I would like to make the 7-segments 6 inch so they can be easily read from about 75 feet away so that the engine operator can read them. I might toss around the idea of piggybacking some 3 inch 7-segments for other spectators to be able to see.

This project will take some serious time and development, we are just now in the concept stage. My friend is going to build the brake from scratch and use 36 inch pulleys that he is going to make in his shop.

Thanks, Dave

August 21, 2012
by JKITSON
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DAVE I posted a schematic of the 7 segment setup for 8 numbers using a atmega 168. It is in the middle of the TRACTOR SLED thread in the 2009 date range. Might be of some help for you..

JIM

August 21, 2012
by pcbolt
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Here is Jim's thread. I've been using those shift registers for many projects (including a 2-wire LCD drive)...work great.

August 21, 2012
by JKITSON
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pcbolt Thanks for helping with the thread ref. I need to learn how to do that...

Dave Use the Dec. 1 2009 schematic.

Jim

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