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Old Feb 25, 2014, 08:35 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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Arduino Prop Testing Thrust Gauge

I want to see if it is practical to make the sensor in the image work to measure the thrust produced by motor and prop combos on a static test stand.

The sensor was bought on eBay as a "5Kg Electronic Scale Weighing Sensor Load Cell Aluminum Alloy 3-12V DC" and specs there were:

Product Description
Rated Load: 5Kg
Rated Output: 1.0mV/V0.15mV/V
Zero Output: 0.1mV/V
Creep: 0.03%F.S./30min
Input End: Red+(power), Black-(power)
Output End: Green+(signal), White-(signal)
Recommended operating voltage: 3 ~ 12 VDC
Maximum operating voltage: 15 VDC
Input Impedance: 111510%O
Output Impedance: 100010%O
Protection class: IP65
Total Size: 3.16 x 0.51 x 0.51 inch
Material: Aluminum Alloy
Weight: 30g

I assume that I would put the input voltage on the red and black wires and see a resistance across the other two wires (green and white) that will vary with the load. But I know nothing about these and welcome enlightenment on what I think.

And any suggestions for how to display the value, what the range of would be, and how to convert that to a useable value that would indicate thrust.

I have to note the thrust (taken from a hanging laying vertical scale now) so I don't mind having to note a displayed value and cross reference it to a thrust value on a look up table or something like that.

Any help or suggestions are welcome and appreciated.

Thanks,

Jack
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Old Feb 26, 2014, 12:53 AM
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Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
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This is the same type of sensor that is used in cheap electronic kitchen scales.

Its rated output is 1mV at 5kg, but the excitation voltage for this sensitivity is not specified! Assuming it is 5V, you need an instrumentation amp with a voltage gain of around 1000~5000 (depending on how much input voltage your display needs).

Here is an example: SGAU Universal Strain Gauge Amplifier

The display could be as simple as a digital multimeter, with one lead connected to a potentiometer which 'zero's the no-load reading.
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Old Feb 26, 2014, 01:29 AM
OlliW
Joined Sep 2009
1,797 Posts
Hey Jack

that's a nice project. I have seen on the web several guys doing that, including Arduino code, I just can't remember a page, but it's probably worth to google.

As Bruce pointed out these gauges usually implement a resistive Wheatstone bridge. The voltages are low so a "good" electronics is needed for accurate results, which is a bit hard to build from individual parts (you don't want just to have high amplification, but also the more sophisticated types of offset balancing etc. pp). But fortunately, ICs doing this are available, and also very cheap: see e.g. the HX711 module: http://www.ebay.com/sch/i.html?_trks...at=0&_from=R40

Good luck,
Olli

EDIT:
- a recent thread on a Gemran forum: http://fpv-community.de/showthread.p...er-Pr%FCfstand
- https://www.google.de/search?q=HX711...CwDw&gws_rd=cr
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Old Feb 26, 2014, 10:03 AM
Jack
USA, ME, Ellsworth
Joined May 2008
17,012 Posts
Thanks for the replies and details and links guys. You've probably got me off and running here at least.

The HX711 board on eBay looks like a good next step in the process then, and a display too of course. This eBay seller shows one of the boards hooked up and some schematics. A lot of it is in Chinese unfortunately...

http://www.ebay.com/itm/1set-x-Weigh...-/251384697621

I'll leave this open and follow up here as any progress is made. In the mean time any and all are welcome to chime in in the of helping the hobby and making me schmarter!

Jack
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Old Feb 28, 2014, 03:06 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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The page where they are selling this HX711 board I am contemplating buying shows the block diagram in the first attached image. And, as an example, they show it set up and working in the second image.

That shows a Wheatstone bridge connected to the HX711 board. There are four wires:

E +
E -
A +
A -.

Is it fair to assume that the four wires from come from the load sensor I have now are the same four wires and the components of the Wheatstone bridge is what is embedded in the patch of epoxy on the side of the sensor?

When I put power (6.7V) on the red (VDC+) and black (VDC-) leads of the sensor, I get a display of 0.3 mV on a RMS DMM across the green and white wires on the load sensor.

And with the load sensor clamped to the bench top on one end and the other end hanging over the edge, if I lay a piece of lead that weighs 220g on top of the free end (the end marked for down force), the reading increases from 0.3 mV to 0.5 mV.

I don't have anything that measures voltage with higher precision but I assume that if I could measure microVolts I would get better resolution and see smaller amounts of weight. Does that sound right?

And I assume that the HX711 will take care of all of the details as far as talking to a MCU and the MCU is where I can add a display to all of this.

The description on the HX711 AD Module reads:

"This module uses 24 high-precision A / D converter chip hx711, is designed for high-precision electronic scale and design, with two analog input channels , a gain of 128 programmable integrated amplifier. Input circuit can be configured to provide an electrical bridge bridge voltage ( such as pressure , weight ) sensor model is an ideal high-precision , low-cost sampling front-end module.

I looked through the other listings from the seller that has the HX711 for items with the name "MCU" and don't seem to see an MCU there either with a display or that be something that can take a display.

Would a arduino "UNO" MCU or something like that work for both a programmer and a display when it was in use? I would not mind having a programmer and display combo from the viewpoint of long-term usefulness.

Jack
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Old Mar 04, 2014, 09:07 AM
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Dan Baldwin's Avatar
United States, CA, Norwalk
Joined Apr 2004
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Quote:
Originally Posted by Bruce Abbott View Post
This is the same type of sensor that is used in cheap electronic kitchen scales.

Its rated output is 1mV at 5kg, but the excitation voltage for this sensitivity is not specified! Assuming it is 5V, you need an instrumentation amp with a voltage gain of around 1000~5000 (depending on how much input voltage your display needs).

Here is an example: SGAU Universal Strain Gauge Amplifier

The display could be as simple as a digital multimeter, with one lead connected to a potentiometer which 'zero's the no-load reading.
The rating on Jack's load cell is 1 mv/volt meaning that at the rated load (5 kg) the load cell will put out 1 mv for every volt of excitation (power supply). At 5 volts it will put out 5 mv, at 10 volts it will put out 10 mv, etc. The output of the load cell is differential, and the output leads will be at approximately half the excitation power supply. If you want to amplify the signal and turn it into a single ended output (say 0-5 volts) you will need to use an instrumentation amplifier.

Dan
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Old Mar 04, 2014, 06:44 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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Hi Dan! Thanks again for your genius work on the A123 charging modules!

I still have and use a couple of them for A123 1100 and 2300 packs and the VPX tool power packs for a drill and a flashlight. I even still have a fair stash of new 1100 and 2300 A123 cells, a lifetime supply I'll bet (I'm 72).

There is a dfrobot page here that tells how to make a weighing scale using a weight sensor module that is based on the HX711 and the same load sensor that I have:

http://www.dfrobot.com/wiki/index.ph...nsor_Module_V1

That uses a DFROBOT UNO microcontroller board. So I assume the Arduino or other UNO microcontroller board would work too.

And I could hook a display to the UNO as shown there and just use it like that couldn't I?

I think the info there will work for me if I get a UNO and a display, does that sound right?

And this zip file has the libraries for programming it:

http://www.dfrobot.com/image/data/SE...ple%20code.zip

Jack
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Old Mar 05, 2014, 10:43 AM
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Dan Baldwin's Avatar
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Joined Apr 2004
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Apparently the HX711 processor is designed to be directly connected to a mv analog source like a load cell, so that should work fine. If you're looking for a project, and want be able to use the load cell you already have, this sounds like a fun project, but it would be much easier to use a scale. I've seen several pivot type adapters that convert horizontal thrust into vertical force to check thrust with a scale. 5 kg scales are cheap.

I'm glad I was able to help with the terminators. It was fun working on the them, and it's always fun to see people using my designs.

Dan
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Old Mar 05, 2014, 10:57 AM
Jack
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I have two scales now and the problem with them is that they have too many features!

They time out and shut off, they quit measuring when the reading stabilizes, etc.

They don't like to just turn on, measure a weight that varies up and down, display the changing weight continuously, and simple things like that.

I am using hanging scales laying flat and, while my persistence has let me get pretty good and useful data from them, it has not be easy or pleasant. So I don't mind fooling around with this load sensor if I can make it work.

Jack
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Old Mar 05, 2014, 02:28 PM
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Dan Baldwin's Avatar
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Joined Apr 2004
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I know what you mean. I have 2 scales; one turns off after 30 seconds, and one locks in the weight after it stabilizes for several seconds. Both could be used to check static thrust, but it would be a hassle, and you couldn't use them to check thrust over time.

Dan
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Old Mar 05, 2014, 05:46 PM
Jack
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Joined May 2008
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I use a thing called a "pulling station" that I got from R2 Hobbies for my testing. It is attractively priced at a little over $10 and with some tweaks and mods I have it working well for me.

I visualize that beam as located vertically and to one side back somewhere near that Power Panel display with a lanyard that attaches to the "sled". And the display would be somewhere handy too. The sled only moves 1/4" or so and the tension on the load sensor beam should give me a weight in grams is what I hope. And it will just go up and down with the RPM and that it. No other tricks or features are wanted. The sensor was only about $5 on eBay and they have them in a number of ranges (I got the 5kg one).

As it is now, the eLogger records all the important numbers and I just have to note down the thrust and the PWM rate as displayed on the HJ Servo Tester that is used at a throttle.

The PWM rate is consistent as far as values at zero and full throttle so I have a look up chart that let's me pause at 25%, 50%, 75%, and full throttle.

That pulling station is an OK setup. I had to work on it a little to get the motor mounting to go faster and easier and also to reduce the friction on the fibreglas or CF rods it slides on. I had a little store credit for a piece of defective merchandise I got from R2 and I choose that pulling station as a way to "spend it". I certainly have no regrets but you have to be a bit of a tinkerer to get the best out of it.

Jack
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Old Mar 22, 2014, 08:41 PM
Jack
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Joined May 2008
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I have now acquired some more bits and pieces for this project.

I have the following in hand:

Freaduino UNO board - http://www.ebay.com/itm/Freaduino-UN...-/221207173255

1602 LCD Board Keypad Shield - http://www.ebay.com/itm/1602-LCD-Boa...-/321315121753

HX711 module - http://www.ebay.com/itm/Weighing-Sen...-/221342716344

And my plan now is to, proceed to get smarter about Arduino and this stuff, using any help I can get here and from this link:

http://www.dfrobot.com/wiki/index.ph...nsor_Module_V1

That dfrobot HX711 module is not available as far as I can tell so I bought the one above. But they have the same connections and will work the same way.

So now I'm waiting to signed up on the Arduino forums to get smarter and will welcome any help I can get here.

I've gotten some of the hardware mounted to make a mock up of sort and will sort my way through the rest of this soon.

Jack
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Old Mar 22, 2014, 09:19 PM
Don L.
Old Lyme, Connecticut
Joined Feb 2006
939 Posts
Quote:
Originally Posted by jackerbes View Post
Hi Dan! Thanks again for your genius work on the A123 charging modules!

I still have and use a couple of them for A123 1100 and 2300 packs and the VPX tool power packs for a drill and a flashlight. I even still have a fair stash of new 1100 and 2300 A123 cells, a lifetime supply I'll bet (I'm 72).

There is a dfrobot page here that tells how to make a weighing scale using a weight sensor module that is based on the HX711 and the same load sensor that I have:

http://www.dfrobot.com/wiki/index.ph...nsor_Module_V1

That uses a DFROBOT UNO microcontroller board. So I assume the Arduino or other UNO microcontroller board would work too.

And I could hook a display to the UNO as shown there and just use it like that couldn't I?

I think the info there will work for me if I get a UNO and a display, does that sound right?

And this zip file has the libraries for programming it:

http://www.dfrobot.com/image/data/SE...ple%20code.zip

Jack
Jack,

The diagrams, Arduino code, and library (hx711.h) at this site should get you started. There is no provision in the code for an LCD display, but you can use the Arduino IDE's built-in serial monitor to view the output.

I suggest you wire it up, upload the code and use the serial monitor to verify that it works. Then, you can add your display and keyboard, knowing that the scale portion is already functioning properly.

As for the library, after you download and unzip the file, move the folder "Hx711" into the "Libraries" sub-folder in your Arduino folder.

-Don
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Old Mar 23, 2014, 05:22 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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Hi Don,

Thanks for dropping in!

I realize now that the libraries I asked you about in email are not for this scale thing, they had to do with your work on using the Arduino LCD shield as a replacement for the eLogger's Power Panel display. So that is something I'll probably work on after I get this going. I would be nice to be able to all of the data from the eLogger and have the thrust display there too.

As it is right now, when testing motors, I write the thrust down from a hanging scale laying flat, and note the PWM rate on the HJ servo tester throttle to get the throttle percentage. Then I can annotate that info on the screen shots when I plot the eLogger data.

The attached images are examples of how I am using the eLogger data now and adding the thrust and throttle info.

Jack
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Old Mar 23, 2014, 05:33 AM
Jack
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Joined May 2008
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It has been some years since I messed with anything RS232. I vaguely remember some of the details of reading from and maybe even sending to or talking to GPS receivers from PC keyboard.

I am going to do most of this from an XP Pro machine.

"..I suggest you wire it up, upload the code and use the serial monitor to verify that it works. Then, you can add your display and keyboard, knowing that the scale portion is already functioning properly..."

Is there a serial monitor on an XP machine or is that an software I need to add? And if the latter, do you have a recommendation for the software?

Added note: And now it comes back to me! Hypernet! And it is still there on XP Pro but not on my W7 laptop.

Jack
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