Load cell for weighing powder

Discussion in 'General Electronics Chat' started by MuzMueen, Dec 18, 2014.

  1. MuzMueen

    Thread Starter New Member

    Dec 18, 2014
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    Hello

    we are working on a project for a machine for capsule filling with powder. The powder will be filled by weight. The fill amount will be 400 , 600 & 800 mg. The error margin which is acceptable is 5 - 10 mg, so my thinking is to go with a load cell of resolution of 0.001g and total capacity of 10 - 20g. I am totally new to load cells and weighing powder side. I have done some research and I am baffled about a few things.

    1. which properties of a load cell should be considered more than others in my case ? (hysteresis, resolution etc)
    2. We would prefer a response time which is quicker (in milliseconds)

    There are some load cells available cheaply in the market, but then there are others which are extremely expensive. Furthermore, for instrumental amplifier & ADC I am inclined towards getting one-chip solution like Texas instruments' ADS1242, so its less hassle.
    If anyone of you have worked with any such situations, I would be grateful if you can share your experience and suggestion for appropriate parts.
     
  2. GopherT

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    Nov 23, 2012
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    You are trying to fill to 1 or 2% accuracy for small pills and better resolution for the bigger ones. What type of screw feeder or other mechanism do you plan do use to deposit gnats eggs to adjust your weight.

    Why are you building a milligram scale when mettler-Toledo and Sartorius and many others are already very good at it? Yes, they are available with an interface.

    Powders are fluffy, how do you plan to get the powder into the capsule without overflowing?

    If you look at what is available in bottles at any drug store, you will see this is done all the time on what must be 100s of thousands per day scale (one machine). Why don't you look into commercially available equipment to do this - much available on the used market too.
     
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  3. MuzMueen

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    Dec 18, 2014
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    Thanks a lot. We are considering screw feeder or a pipette feeder. I checked with mettler and Sartorious and they were very expensive ($ 8k / unit, and we need 5).

    I understand your point, but this isn't a normal capsule which is consumed orally, but a Dental Capsule. Its a plastic container, which has three parts, the bottom part will contain the dental amalgam alloy powder (its metallic powder, so its not fluffy). Moreover, as its dense, so it cant over flow, as the size of the bottom container is way bigger than that. We would intercept the weight long before its fully filled up.

    Bottom part has a middle cap .. it goes on top of the bottom container and has a membrane in the middle to seperate its upper part from the lower container. The upper portion of the middle cap would be filled with the same weight of mercury. On the open end of the middle cap would go in the final cap, a chisel shaped one. When dentists want to use the capsule, he will simply press the chisel down, breaking the membrane and mercury will fall in the lower chamber. the capsule will then be placed on a shaker or vibrater to mix both up, and it would be ready for use.

    I know there are a lot of machines available for general capsule filling, but I couldn't locate that could do this kind of two way filling. The pictures of the capsules to be filled are attached herein too.
     
  4. GopherT

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    @MuzMueen
    Nice, and interesting.
    I hope you are not giving away any company secrets as you describe your project.
     
  5. cmartinez

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    Jan 17, 2007
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    You are in for a long and exhausting project, especially if you don't yet have selected a proper ADC...
    What I can tell you is this:
    Load cells are:
    • prone to drift
    • Can get points of fatigue, especially after being used for repetitive tasks
    • sensitive to temperature changes
    • sensitive to damage if overloaded
    • Are best excited with AC instead of DC
    • And many MANY other things

    The good news is that you're only asking for a 1% accuracy... however, if you need to certify for 1% then you need to get stable readings of 0.1%, which is quite attainable using a 16-bit ADC

    Like I said, your project is going to be a long a arduous one, and you've got to get started somewhere.
    I suggest you start by defining the ADC that you want, and the MCU that you're planning to use.
    An excellent unipolar ADC that I've used successfully in the past is this one.
     
    Last edited: Dec 19, 2014
  6. cmartinez

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    Also, there are virtually hundreds of companies out there that manufacture load cells in that range, you're in for a lengthy selection process.
    Reading speed will depend almost exclusively on your choice of ADC, MCU and amplifier circuit. Keep in mind also that there are some ADCs out there that already have differential instrumentation amplifiers included in them.
     
  7. MuzMueen

    Thread Starter New Member

    Dec 18, 2014
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    No not really... the actually secret is how to make this powder ... this is just filling the product :)
     
  8. MuzMueen

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    Dec 18, 2014
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    Yes, thats the confusion. If you have experience with some good components for load cell, can you recommend me ? It would be a good start for me.
    Moreover, you're right, I was looking more for a OpAmp + ADC package, instead of seperates, none the less, I have noted down your recommendation and would consider it.

    Furthermore, we are planing to use computer, not micro-controllers. Micro controller would be too much of hassle.
     
    Last edited: Dec 19, 2014
  9. cmartinez

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    Ok, first questions first:
    • Are you proficient with an kind of MCU?
    • How much do you now about A/D converters?
     
  10. MuzMueen

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    Dec 18, 2014
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    Not particularly (its been quite a while I worked on any hardware lately). I am currently collecting specs and recommendation and the software and mechanical parts would be assembled by a CNC & mechanical team, who will need my guidance. I have to compile the architecture of the final product, with my recommendations for parts to be used in the final product.
     
  11. cmartinez

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    There are virtually hundreds of options for each component in the architecture that you're trying develop, resulting in many thousands of different combinations. But every journey starts with the first step.
    For your project, personally I would concentrate on the repeatability of the load cell, rather than its hysteresis, since you'd probably be feeding the product continuously until completing each load. Like I said before, there are many manufacturers of load cells. Among the best (and most expensive) is a company named Futek. I suggest you look it up, if only for reference reasons.
    The hard part won't be selecting the load cell, but rather the component selection for your circuit. And most important, designing the PCB layout. The placement, order and orientation of each component in the PCB is critical. This PCB must be of the 4-layer type, with its ground and power planes properly designed.
     
  12. MuzMueen

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    Dec 18, 2014
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    Yes, I have gone through almost all the top notch load cell providers, and you're right. They are very expensive and actually way out of my range. Someone suggested me to get myself familiarized with load cells of these big names and then try to match it with some chinese vendor for a cheaper alternative. I found one load cell with the following specs. They actually looked pretty good, but I am not sure.

    Capacity (grams) ---------- 10,20,30,50
    Comprehensive Error ---------- 0.03 mv/v
    Output Sensitivity ---------- 0.8±0.1 mv/v
    Nonlinearity ---------- 0.03 %FS
    Repeatability ---------- 0.02 %FS
    Hysteresis ---------- 0.02 %FS
    Creep ---------- 0.02 %FS (3 MIN)
    Zero Drift ---------- 0.02 (1min) %FS
    Temp. Effect on ZerO ---------- =0.3 %FS /10
    Temp. Effect on Output ---------- 0.03 %FS /10
    Zero Output ---------- ±0.1 mv/v
    Input Resistance ---------- 1050±50 O
    Output Resistance ---------- 1000±5 O
    Insulation Resistance ---------- 2000 MO
    Excitation Voltage ---------- 6 V
    Operation Temp. Range ---------- 0--+50 C
    Overload Capacity ---------- 120 %FS
     
  13. cmartinez

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    It doesn't look bad (though you didn't mention the price)... although a 1K resistance of I/O is not the 370 standard normally available. Also, the excitation voltage is lower than most. But the repeatability is quite acceptable. You're going to need an instrumentation amplifier of around x100 to get full resolution with the ADC that I suggested.
    In your case, the most important part is to get started with your circuitry, and then buy two or three load cells of different brands to start testing.
     
  14. MuzMueen

    Thread Starter New Member

    Dec 18, 2014
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    Actually thats the thing .. I can't get a midway price & it confuses me, either they are extremely expensive, or like this. This load cell is around 3 $ ... Thats why I get confused that which properties of a load cell should I consider and not compromise on while buying one ... The link is ...
    http://bingfst.en.alibaba.com/produ..._alloy_Weight_Measuring_Sensor_load_cell.html
     
  15. cmartinez

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    Well... three bucks a piece is definitely worth the bet... I'd suggest you buy a few of them, and at least one from Futek for comparison purposes
     
  16. MuzMueen

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    Dec 18, 2014
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    Comparing a 3 bucks load cell and 900 $ one .. sounds a little unfair .. :D ... but , i'll give it a shot ..
     
  17. cmartinez

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    As we say down here... "there has be something in this water for it to be holy" ... meaning that sometimes things look very ordinary in the outside but they might have very special (and secret) things in them that make them extraordinary...
    So it's either $3 dlls or $900.... nothing in the range of, say, $150 ?
     
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  18. MuzMueen

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    Dec 18, 2014
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    well said ...
    No i couldn't find something which falls under my requirement and is also around 150 - 200 .. Thats what I was actually looking for ..
     
  19. GopherT

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  20. cmartinez

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    Where are you located?
     
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