Load Cells

[Chris Parks]

Does anyone know anything about these things as I want to pick their brain. I want to set up four of these to weigh each wheel on a superkart. Cheap would be good.

[Pat]

Chris, is this what you mean?

[Vernonv]

What exactly are you trying to measure?

[Chris Parks]

Corner weights on a superkart.

[Calm]

it can be done with 4 bathroom scales but ideally you need something to measure more accurately.

cheers

[Chris Parks]

it can be done with 4 bathroom scales but ideally you need something to measure more accurately.

cheers

I know that, I want to use load cells.

[chrisp]

Mini,

I've used load cells at various times at my work. I'm reasonably familiar with the electrical side of things such as the amplifiers and the excitation required. Neither of these are difficult as it is easy to buy off-the-shelf amplifiers with the excitation supply built-in.

The mechanics is not really my area of expertise, but load cells are fairly simple device that usually consist of a metal body that acts as a spring and a strain gauge that is used to detect the strain (i.e. the change in length) as the metal body is loaded. The stain gauge consists of a number of resistors that change value as the gauge is stretched or compressed. There are also some reference resistors within the gauge that are used for compensation (e.g. temperture variations).

Be parepared to spend. This stuff tends to be expensive.

The hard part will be to mechanically incorporation the load cells in to your design, but if you want the be really clever you maybe able to obtain the strain gauges and attach them to the existing structure.

[ian]

would a spring balance (as used for weighing meat and fish carcasses) and support frame (home made of course) be a suitable substitute?


ian

[joe greiner]

For minimal engineering effort, and probably lowest cost, four digital bathroom scales might work if your loads are within their capacity. Depending on construction and ease of disassembly, you can likely relocate the output screens to a central display panel. For higher loads (up to double capacity), you'll need two such scales with a bridge beam to support your load; your load doesn't need to be perfectly centered on the bridge, because you have to sum the loads anyway.

If your "superkart" (whatever that is) has a rigid frame and suspension, loading could be awkward. Three-point weighing would eliminate that deficiency, if one end is significantly lighter than the other, and still within the scale's capacity. BTDT, but analog, not digital.

Joe

[Calm]

For minimal engineering effort, and probably lowest cost, four digital bathroom scales might work if your loads are within their capacity. Depending on construction and ease of disassembly, you can likely relocate the output screens to a central display panel. For higher loads (up to double capacity), you'll need two such scales with a bridge beam to support your load; your load doesn't need to be perfectly centered on the bridge, because you have to sum the loads anyway.

If your "superkart" (whatever that is) has a rigid frame and suspension, loading could be awkward. Three-point weighing would eliminate that deficiency, if one end is significantly lighter than the other, and still within the scale's capacity. BTDT, but analog, not digital.

Joe

Joe

hope this helps, they weigh less than 100KG and the scales are to get weight balanced equally on all wheels. The problem with bathroom scales is you need a perfect surface to setup on or the readings can be false. Load cells may give the same problem - i ahve only ever used scales and they do work well when you have the right conditions.

http://www.ekartingnews.com/NewToKar.../superkart.jpg

cheers

[damian]

I used to make load cells, I even have a piece of paper that says I'm competant at strain gauging.

Anyway strain gauges are cheap as chips, about $5 each and you need 4 per cell, the thing that will kill you is the amplifier. The cheapest chip I've found is at radio spares and last I looked it was about $200 and you need one for each cell. If you want 1% and better accuracy the technique to attach them properly is tricky and elaborate.

You would be far better off using a beam balance. Rig a beam and fulcrum under each wheel, make it so the kart doesn't roll off. Calibrate each sea saw by putting a common weight on each side and making sure each one balances. Make one leg say 5 times longer than the other, then with the kart in place add weight to each sea saw until it floats. Then swap your weights around between them. If it still floats it's very equal. If not add weight till it does and you can calculate how much the corners are out by. You can make this a woodwork project

[malb]

Joe

hope this helps, they weigh less than 100KG and the scales are to get weight balanced equally on all wheels. The problem with bathroom scales is you need a perfect surface to setup on or the readings can be false. Load cells may give the same problem - i ahve only ever used scales and they do work well when you have the right conditions.

http://www.ekartingnews.com/NewToKar.../superkart.jpg

cheers

Anything that is surface based will have the problems and errors due to surface irregularities etc that you have experienced with scales. It might be simpler to try and build a rigid frame with adjusters beneath, so that you can level the frame properly using the adjusters, place the four scales, and then place the cart and driver to measure/check distribution.

[joe greiner]

Ordinarily, frames with rigid suspension should be flexible enough to accommodate road and track irregularities. But if absolute numbers aren't needed to verify conformance with the rules, balance beams should be satisfactory for any frame or suspension construction.

Joe