Making auto weigher. Electronics.

Discussion in 'Workshop Projects' started by Oldmacdonald, Feb 10, 2019.

  1. Oldmacdonald

    Oldmacdonald Member

    Location:
    Scotland
    Afternoon all!

    I sell a bit of veg, but not enough to warrant massive spending.

    I'd like to make an automatic weighing machine with an old potato bagger.

    In effect it'll be a ~ .55kw motor running an elevator into a hopper I will manufacture that will be on a weigh cell. When the predetermined weight is reached (2kg) the controller will tell the motor to stop.
    I empty hopper and the motor restarts, or I press a button to restart the cycle.

    So basically- are there any of that kind of controller available off the shelf, or will I have to design one?
     
  2. Oldmacdonald

    Oldmacdonald Member

    Location:
    Scotland
    Think I've answered my own question- I want a weigh scales with relay output, yes??
     
  3. milkloss

    milkloss Member

    Location:
    East Sussex
    Not sure it will quite be as simple as that if the conveyor has a bit of momentum but I guess you can tweak the weight to sort that. Make sure the relay will have the capacity to switch the motor.

    I would use a weighing thingy with relay (never seen one tbh) to break the current to a contactor, you need a normally closed (NC) relay on the weigher. That way: underweight gives power to contactor momentary on switch so you can turn on. When up to weight relay breaks the power in the contactor coil and makes it dead until you momentarily push the on button again. Gives you a chance to remove bag, put the empty one under and push the button.
     
    Last edited: Feb 10, 2019
    Greenbeast and powerfarmer like this.
  4. powerfarmer

    powerfarmer Member

    Location:
    Cork Ireland
    I was asked to make something similar before for weighing nuts from a hopper into a stream of ingredient that was continuously flowing past. It would have used a loss in weight controller ( the hopper was on load cells) and the nuts were fed by a variable speed auger.
    The plan was to use a controller like this
    https://www.ricelake.com/en-us/prod...-hmi-weight-indicator-controller#/information

    with an analogue output to control the auger Danfoss variable speed drive and hence the feed . The controller had various programmable outputs.

    Anyway the cost of the controller was considered too much by the customer so a basic weight indicator was used instead with manual speed control.
     
    Last edited: Feb 10, 2019
  5. Yeah once upon a time you would have used a PLC (programmable logic controller) for that

    Nowadays you just get your nearest spotty teenager to knock up some code for an Arduino or Raspberry Pi that enables you to do it all for the cost of a a decaf, skinny, vegan frappuccino at Starbucks :p
     
  6. powerfarmer

    powerfarmer Member

    Location:
    Cork Ireland
    Pheasant Surprise likes this.
  7. Good old Ali G eh :LOL:

    Nah seriously, good spot. The only downside, other than usually painfully long shipping times from China, is that the unit only has simple relay output.

    That could be fine, depending on the elevator setup the OP has. For instance if the motor elevator will stop dead when the relay is opened/power removed, or whether it will run on slightly, and hence deposit more taties/veg into the bag.

    Probably not a huge issue, as you could probably adjust the tare weighed amount, to be slightly lower, to allow for any "run on" in the elevator, and hence extra veg/taties being dropped out as the target weight is (almost) reached.
     
  8. powerfarmer

    powerfarmer Member

    Location:
    Cork Ireland
    Yeah,
    I d probably use a cheap variable speed drive on the conveyor motor.
    To allow the belt speed to ramp up or down or stop dead as necessary .
    But as you say it all depends on the set up of the whole thing.
     
  9. Greenbeast likes this.
  10. Dman2

    Dman2 Member

    Location:
    Durham, UK
    Can you not use a simple balance batch weigher, the type that tips when it reaches a set weight
     
  11. Oldmacdonald

    Oldmacdonald Member

    Location:
    Scotland
    So I bought one. Found on ebay that will deliver in a couple weeks. For 35 quid its worth a try!

    Relay is only 3A though. Presumable this means my motor will need to be rated lower than 3, yes? What about start-up, will it need to below 3, or does it matter about start up amperage?

    And I presume my DOL starter will need to have the function of a remote off?

    Probably, just don't have the time just now! If I got the hang of it, I'd have all sorts automated around the farm!
     
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  12. Dman2

    Dman2 Member

    Location:
    Durham, UK
    Just use your 3 amp relay to switch a bigger relay
     
    Greenbeast likes this.
  13. As it’s only a little 0.55 kW rated motor, a 3A rated relay ought be fine.

    I don’t think the startup current would be too bad, but you could measure it with a multimeter on 10A range, to be sure, to be sure.

    Otherwise cascade the controller relay to open/close a larger rated relay as @Dman2 says.
     
  14. Oldmacdonald

    Oldmacdonald Member

    Location:
    Scotland
    So thinking about this project while I wait the two weeks for delivery of the weigh controller.

    Do you think I'd be able to set a negative weight band on the weighing controller? So therefore mount the weighcells under the whole machine, and weigh weight loss, rather than weight gain?

    My theory is it would then be more accurate as wouldn't have to take in the sudden spike of weight as potatoes fall onto the scale?
     
  15. Anything is possible....within reason.

    Why not reach out to the Chinese seller and see if they can email you the instructions beforehand?

    Then you’ll know
    (1) how well translated they are ;)
    (2) if they can do negative offset / trigger weight
     

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