Real Time Weight Scale with alert

[mtmcd]

Hi Everyone,

I'm trying to accomplish a real-time weight scale that does two things: 1) measures load, and 2) compares the load weight to an inputted value and issues an audible alert if a particular weight is too heavy.

My weight loads are on average 50 lbs. However, due to the application, the weights are dropped onto the scale, so the system would need to ignore the initial shock readings. My readings must also be in 'real time'. If the weight is over a certain value (say 55 lbs), there needs to be an alert to tell the operator the item is too heavy.

I would like a cheap, accurate solution (how's that for an oxymoron?) I'm thinking strain gauges, hooked up to a microcontroller. However, before the weight is applied, I would like the controller to input the spec weight for the system to compare the measured load to. Help please?

Thanks!
Mike

 

[ericgibbs]

Hi Everyone,

I would like a cheap, accurate solution (how's that for an oxymoron?) I'm thinking strain gauges, hooked up to a microcontroller. However, before the weight is applied, I would like the controller to input the spec weight for the system to compare the measured load to. Help please?

Thanks!
Mike

hi,
I would suggest a load cell rather than individual strain gauges, connect the L/C to an instrumentation amplifier, say a AD623 or AD624.

The output of the IA to a PIC's ADC input or if if you need a higher resolution an external A2D converter.

Also you would require a simple keyboard and an LCD display and audible warning beeper.

Program a PIC to drive the keyboard/LCD/beeper and any other peripheral devices.

 

[Reloadron]

I would like a cheap, accurate solution (how's that for an oxymoron?)

OK, this can easily be done for a few thousand US dollars.

As Eric suggest I would consider using a load cell. This way someone has already put a strain gauge bridge together for you. Load cells are pretty common, unfortunately a good load cell in let's say the 100 Lb range isn't cheap or inexpensive.

Something I have never done but given thought to trying is to buy a cheap bathroom scale and hacking the load cell(s) from within. I have seen low priced digital bathroom scales in the $25 to $35 USD ranges. I have read a little about hacking them including this blog. Anyway, for a cheap approach this may be a good start.

If you go with a commercial load cell similar to some of these you need to find one that suits your application. Typically a canister or low profile pancake type. As can be seen, if you want accuracy in a commercial cell they get very pricey. There may be cheap Chinese manufacture clones out there but I haven't seen any. I linked to Omega but there are plenty of manufacturers out there and Lebow Honeywell also comes to mind.

So the front end consisting of the load cell becomes pretty much your call. You don't mention a budget?

The reason I would avoid the use of a strain gauge or gauges is that unless you have worked with them they can be complicated. Not from a mechanical or mounting point but getting calibrated usable data from a setup.

Ron

 

[mtmcd]

Thank you both for your input. What controls how many readings are taken every second? Without diving into the coding, or the instruments suggested below, is there one item that is required for instantaneous readings?

Thanks again!

 

[Reloadron]

Thank you both for your input. What controls how many readings are taken every second? Without diving into the coding, or the instruments suggested below, is there one item that is required for instantaneous readings?

Thanks again!

If the output of the load cell is conditioned then fed for example to an A/D converter the readings / sec will be a function of how fast the A/D can run and the accuracy and resolution will be a function of the A/D converter and the load cell. There is more to all of it but that is a nutshell version. You start with a load cell, the rest is up to what you want to do with the data and what you want this to cost. Don't know what you mean by this:

the instruments suggested below

as I see no listed instruments. The code has little to do with how fast the A/D can run.

Ron

 

[ericgibbs]

Thank you both for your input. What controls how many readings are taken every second? Without diving into the coding, or the instruments suggested below, is there one item that is required for instantaneous readings?

Thanks again!

hi,
Another point that we need to know is area of the base plate [platform of which the object being weigh is placed].

Depending upon the area, the number and placement of L/C's under the base has to be decided.

If you take the ADC readings as fast as possible to produce an average weight which will give a more stable reading.
It should be possible to program the PIC to delay the averaging until after the scale weight overshoots due to the dropped 'object'

With ref the object being 'dropped' on the base plate, if you are using say 30kg L/C's, dont over drive them by more than 10%.
If this could happen then you need 'mechanical' stops under the weighing base, in most commercial scale bases the stops are already fitted.