Anyone with load cell experience with Prop?

T Chap

I want to experiment with a load cell. A main pulley has a belt with a fixed tension on it for example 50LB. When the main pulley is turning it will lift a load of approximately 125lb. The belt under load will then have 50 + 125 = 175lb on it. I want to measure the force against the idler pulley for both sides of the system and have the prop read the load and evaluate for problems. I see tons of load cells on line, does anyone have any experience with something more industrial for this application? I need something very reliable. I am working on trying to find out the formulas for determining the force against the sensor.

PropGuy2

I don't know what brand of Load Cell you are using, but lets assume it is a 4-wire system / bridge circuit. First, read the instructions very carefully. Second, Use a very stable and repeatable power supply, usually 10 volts DC, and keep the leads short to reduce random noise. Third, test the setup on your workbench, using at least a 4-1/2 digit multi-meter ( I typically use a 6-1/2 digit meter) Alternately build a 10X or 50X op-Amp circuit, to get better resolution of your Load Cell measurements. Finally - mount your load cell on a "substantial" metal frame that is both dead flat, and has no flex in it whatsoever. Do all of this and your project will be reliable and repeatable.
Many reputable Load Cell manufactures (Omega Engineering, for one) sell mating digital readouts, ready to go and calibrated (load cells are not linear devices)

kwinn

I have several years experience installing, servicing, and calibrating tensile testers that used load cells. For your application you can use a single load cell that can measure positive and negative loads. You are probably better off getting one with a digital output since amplifying and converting the voltage output of the analog cells is not a simple task.

DigitalBob

Futek has a good line up of load cells an S type would be a good choice . I have a simple amp setup with excitation using the AMP04. Then use the 0-5 volt output into a mcp32xx to the prop. I can post a schematic if you need it.

T Chap

I called Omega as suggested above and was given this device LC101-200 for a 200 lb max device. In my application I don't yet know the forces on the sensors/idler plate based on approx 175 load on the working side of the belt. But I assume the load is much less than 200lb. The S type allows for compression and tension( push and pull on the device and get + and - output).

http://www.omega.com/pressure/pdf/LC101.pdf

You can give it a 10VDC supply and it gives +/- 30mv output max, linear output at 3 mV/V. I would need to convert the +/- output to a positive output with a center at 5V, then 0 and 10V is each extreme.

I could not get info from the tech on the life expectancy. I would like to get 10 years life.

Any thoughts on this approach? Any better ideas to get the force on the swivel plate into a Prop? This seems workable but I really don't want to spend the time building a PCB and working out the op amp/gain/ d to a, but I see no way around it. He said use the differential input on an op amp, not single input as the device has differential output. I have done some op amp gain on other sensors like current sensors and after some tweaks I can get it but it is not my forte. Ideally I would build a Prop board dedicated to the sensor, and set a max upper and lower limit for it to fire a simple 3v3 output on overloads. Then the main board( already over taxed) can just look at the sensor as a simple limit switch.

dre

Hi,
You can also buy all forms of load cells on e-bay. The ones I have bought come with a specification sheet. This shows the gage resistance and the ratiometic full scale output. You can electronically calibrate the output by shunting a bridge arm with a precision resistor. Micromeasurements [maker of excellent strain gages] has, on their web site, some good information; including calculators for determining the value of shunt resistors. For example: if the bridge uses 530 ohm gages & the stated output of the load cell is 2mV/V, then to simulate a full scale load output a 43,xxx ohm shunt resistor is called for [don't remember the exact value].
I presently use INA 125 amplifiers; and am looking into a HX711 24 bit converter.
Hope this helps.
cheers, David

dre

Oops. Meant 350 ohm gages.
cheers, David

localroger

On life expectancy, the things that usually kill load cells are moisture in the cables/strain gage assembly and mechanical overstress weakening the bond between the gages and metal substrate. I work in the scale industry and load cells which aren't abused in applications like hopper scales where it's impossible to drop a load on them and in dry environments often last 30+ years.

For reading them, you won't do better at the hobby level than a HX711. They're cheap, easy to use, and as accurate as you need for most systems, and you can get breakouts cheap on eBay and (less cheap) from SparkFun. They're not NTEP and don't support sense leads, but it's find as long as the cable between the ADC and the sensor isn't more than ten or fifteen feet long. (Sense leads feed the excitation voltage back from the cell to the ADC so it can compensate for the voltage drop.) Every better ADC I've ever looked into is massively more difficult to use as well as much more expensive.

T Chap

Re the HX711

Looks like someone has put an object here

http://obex.parallax.com/object/731

Here is the sparkfun BOB

https://www.sparkfun.com/products/13879

I could use two of these and not have to deal with +/- output conversion?

https://www.sparkfun.com/products/13331

I will order these and at least get some understanding but I don't see why two of these single sensors wont work as well as the S type with tension/compression voltage output.

pjv

T Chap;

Have you thought of just simply "encoding" the deflection angle of your idler plate?

I presume all you need is biliteral overload trip points, and this could easily be done digitally with two opto interrupters, or several optos per side for several trip points.

All digital, no op amps.... easy peasy.

Cheers,

Peter (pjv)

T Chap

As in holding it at a center point with springs and then have switches or opto sensors of some sort to detect distance traveled?

Yes that was my first idea as I have done that before. However, in this application once installed it will not be accessible, or will be a challenge to access it if springs needed adjusting after the fact. So my thoughts were put a load sensor of some sort where the swivel plate does not move at all, it just puts pressure on a sensor or two sensors as mentioned. Run the wires out of the area to the accessible area, then any changes would be in software, not mechanical.

The idea with springs is, you set the motor in motion lifting the object, set the springs to allow for the switches to not be pressed under normal conditions. But, the springs may be pressed at startup. This is sort the same idea of encoding but using switches.

I'll give that some thought though, I am looking for the simplest most reliable solution.

DigitalBob

I forgot to mention Transcell Technologies had the best price on load cells

kwinn

Take a look at this load cell series. Comes with 5V TTL frequency output, and optional USB, Analog, and Hybrid interface.

Bean

I think Parallax used to sell the iLoad sensors. I think I got one.
If I remember correctly it has two frequency outputs, the delta is the load.

They were pretty easy to use the P1.

Bean

Capt. Quirk

kwinn wrote: »

I have several years experience installing, servicing, and calibrating tensile testers that used load cells. For your application you can use a single load cell that can measure positive and negative loads.

Any thought on how you will calibrate it?

I have a 100 lb S shaped analog load cell from Omega, and
I used to calibrate it with 3 different weights. I don't know
how accurate you need. Mine was lightly preloaded in
compression, but I could have reversed direction and used it
for tension too.


Bill M.

kwinn

@Capt. Quirk
Calibration for small loads (up to 25Kg) in tension was done with a set of certified weights. Higher loads were done with certified load cells that could calibrate for tension and compression. All of the weights and load cells used were calibrated yearly by a NIST certified lab.