arduino and LC-305 LOAD CELL

The loadcell spec tells you that the output is 2mV/V by which it means that with, for example, 5V excitation it will have a full scale output of 2 * 5  =10mV. So you will definitely need an amplifier with a gain of about 500 to get a signal for the Arduino to read.

What are you trying to weigh (it's relevant to the kind of signal processing you might need).

This is  a very expensive load cell - was there a special reason for choosing it ?

MK

Hi I used this kind of load cell as I need a cell to be able to measure the impact force that requires a split second. Initially I was using a normal strain gauge (those found in bathroom scales, but realized they register the force too slowly).

Oh can I find out for the example u shared with me in the reply email, may I know where the 5V excitation come from? From battery or arduino or from the PC? I am quite confused here. Noted with the need to amplify! I found this circuit which I could use to amplify as attached in this email. It works right?

Are there any things to watch out when it comes to using it with arduino and coolterm? Any tips? Thanks!

Best of regards,

Robin

Sent from my Windows Phone

Attachments:

The chip is quite a good choice, I'm not going to check the breadboard against the chip data sheet but you can.

When you say an impact force which requires a "split second" can you be  a bit more precise - either by describing the mechanical set up or by specifying the response time you need for the measuring system.

For future reference - strain gauges can be as fast as you like, the response time is limited by what they are stuck onto.

MK

Like when I drop sth on the load cell, the impact is like 1 second or less thus that's the reason I bought this expensive load cell as the inexpensive strain gauges are not able to register the impact too quickly and they take 2-3sec.

Also, what do you mean by "For future reference - strain gauges can be as fast as you like, the response time is limited by what they are stuck onto"

Regards,

Robin

Sent from my Windows Phone

You need to separate the load cell form the strain gauge when you are thinking about this.

The simplest load cells are made from a metal beam (usually Aluminium or Stainless Steel) with a strain gauge stuck to it. The strain gauge is a pattern of very thin metal wires on a thin plastic film. As the film is stretched (strained)  the resistance of the wires changes and this can be measured. Almost always the strain gauge is wired in a bridge arrangement, often wih all four arms of the bridge on the same plastic film.

Have  a look here Vishay Precision Group - Micro-Measurements - Home Page to learn a lot more about strain gauges.

The frequency response of the basic foil and film strain gauge glued to a metal substrate extends up to 10s of kHz. The frequency response of  a load cell made using such gauges will be determined by the mechanical arrangements. If you make the load cell very small and very stiff it will have  a good frequency response but low sensitivity.

A quick Google found this:

SparkFun SEN-10245 | Load Sensor - 50kg | Proto-PIC

They don't quote the resonant frequency of the load cell  - it will be quite high unloaded but much lower with a 50kg mass on it.

What sort of impacts are you trying to measure - I don't know what  'sth' is.

MK

oh no help! my load cell can't work when i drop an object on it.

before dropping the object i put the object on the load cell to see the analog output value and it showed 55 for e.g. and when i dropped the object on the load cell, it also showed 55. i think it only measures weight and not impact force

How are you measuring the output from the loadcell.

MK

Hi,

Happy new year firstly! Basically i used arduino and the serial monitor shows the analog input of the signal which is amplified from an ina125p on a breadboard. However I just realized this load cell does not measure impact forces! It only measure the weight of the object being thrown on the load cell. Silly me for buying the wrong load cell!! Or is this lc-305 load cell able to measure? Need some help. My circuit is good though as when I pressed the button of the load cell with increasing force, the analog input value on the arduino serial monitor Increases proportionately too! Calibration gives a straight line. Now im worried whether I bought the correct kind of load cell

Robin

Sent from my Windows Phone

I think you have some issues with the basic physics of this measurement.

Impact force is a rather vague term and you may need to think quite hard about what you are measuring - I did ask before but you didn't explain.

Suppose the effective mass of your load cell is 50gm  (that's the mass of the bit that moves to strain the force sensor). If it has hard steel surface and you drop a very small ball bearing on to it the acceleration on the ball bearing will be of the order of 50,000 m/s/s. If the ball bearing weighs 1gm the force on the load cell is 50N. How long the force is applied for will depend on how fast the bb is moving - suppose it's doing 4.5m/s (from being dropped 1m) the impact force will be there for (very roughly) 4.5/50,000 * 4 = 360uS. This very fast pulse will excite all sorts of vibration in the load cell and the actual output from it is only predictable by using Finite Element Analysis and having access to precise design information for the load cell.

On the other hand if you drop a tennis ball onto the load cell you will get quite nice half sine pulses from the load cell.

Step 1 is to ignore the Arduino for now and look at the output from the amplifier and load cell with an oscilloscope. You may need to do some basic sums first to check that the amplifier bandwidth is adequate.

Once you know what you are trying to measure you can make sure that the Arduino is sampling fast enough and possibly move on from there.

If you could explain exactly what you are trying to do it would be much easier to advise you.

MK