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Looking to weigh things — Parallax Forums

Looking to weigh things

mlewis54mlewis54 Posts: 7
edited 2010-11-14 04:11 in Accessories
I am interested in weighing items from 2 oz to 5lbs or so and report the results back to a Linux-based server. I would like to use one of the Parallax processors as the middleware between the server and the weighing stations. I was wondering what sensors (cheap) could be used for this project and if someone could point me in the direction of any Parallax-based similar projects. Thanks.

Comments

  • FranklinFranklin Posts: 4,747
    edited 2010-11-12 15:58
    You need to specify your resolution before any valid recommendations can be made.
  • mlewis54mlewis54 Posts: 7
    edited 2010-11-12 16:57
    Sorry. I am looking for 1 gram+- resolution.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-11-12 17:29
    Rather than fiddle with strain gauges or load cells, your best bet might be to obtain a digital scale with a serial output port. The Ohaus SP6000, for example can be accessorized with either an RS232 or a USB adapter.

    -Phil
  • mlewis54mlewis54 Posts: 7
    edited 2010-11-12 18:03
    I need to have 16-20 weighing stations in a single unit rather than a scale situation. Think cups filled with something that reduces. At certain weights the system makes reports. All without human intervention.
  • kwinnkwinn Posts: 8,697
    edited 2010-11-12 21:19
    mlewis54 wrote: »
    I need to have 16-20 weighing stations in a single unit rather than a scale situation. Think cups filled with something that reduces. At certain weights the system makes reports. All without human intervention.

    Then you will probably need 16-20 load cells, their attendant drive and amp circuitry, and 2 or 3 8 channel 12 bit adc chips.
  • mlewis54mlewis54 Posts: 7
    edited 2010-11-13 06:47
    That's what I figured. Back to my original question, is there a Parallax processor-based solution available or similar (even if only a single weigh point)? If not, which processor is probably the best for this type of app? I was leaning towards the BASIC stamp family since speed is not an issue (probably only sampling once a minute or two). I would like the one processor to interface to all the wight points.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-11-13 08:51
    Given the cost of load cells and the low frequency of your measurements, it might make more sense to rig up one load cell (or digital scales) with some sort of rotating carousel for your sample cups, so they can get weighed one at a time.

    -Phil
  • mlewis54mlewis54 Posts: 7
    edited 2010-11-13 09:20
    That's a good idea, Phil. The problem is that this is an inventory control issue and the locations are fixed in place. I have found load cells in the $85 price range with $35 mounting brackets. That's on the edge of my budget but doable.
  • kwinnkwinn Posts: 8,697
    edited 2010-11-13 10:33
    It sounds like you are mounting load cells to the bins the inventory is stored in. If that is the case the microcontroller chosen and how you approach this will depend on how far apart the bins are as well as the load cell output (analog or digital).
  • mlewis54mlewis54 Posts: 7
    edited 2010-11-13 10:54
    It looks like 3 rows of 5 bins. Probably in an area 5 feet wide and 18" deep. Probably analog load cell output.
  • kwinnkwinn Posts: 8,697
    edited 2010-11-13 22:41
    mlewis54 wrote: »
    It looks like 3 rows of 5 bins. Probably in an area 5 feet wide and 18" deep. Probably analog load cell output.

    Then I would suggest using a prop to energize the load cells and two 8 channel ADC's to acquire the data.

    Most load cells require 3-12V AC in and provide a 2-3mV/V AC signal out at maximum load. That is if you put in a signal of 10V AC in to a load cell rated at 5Lbs max you would get an output of 20-30mVAC out with a 5Lb load on it. You would need to amplify and rectify this signal to provide a DC level to the ADC.
    It may also be possible to select the individual load cells by energizing them one at a time and having a single amp/rectification/ADC circuit.
  • mlewis54mlewis54 Posts: 7
    edited 2010-11-14 04:11
    Thanks for the direction That sounds like the way to go.
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