Current - Time measurement tool

Discussion in 'General Electronics Chat' started by nerdegutta, Jan 27, 2015.

  1. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Hi.

    What tool do I need to buy to get this kind of reading on a screen?
    ct.png
    I want to measure motor current for a specific motor during operation. Operation time is less than a second one way, and appx the same the other way. The motor is powered from a 12v battery, and is this : http://geareddcmotor.com/dc-gear-motor-jl37b528/

    Can I make it? If I have a circuitry to measure and store data in an EEPROM, and then transfer it to a computer with USB or serial or live? And then make a graph in Excel? Which components do I need?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Does it have to be remote or can you connect the equipment to a PC?

    There are DMM that connect to a PC.
    National Instruments also has DAQ boxes with USB interface.
     
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Check out the cheap hantek usb scopes; under $100. Hantek also sells cheap bnc/scope ac/dc active current clamps; $65 iirc. I know it's expensive just to measure the current of one motor one time, but it's usefulness lives on. I have a hantek 1008; 8 channel scope. Pitiful bandwidth, but for what you're doing it would be fine. And 8 channels is very handy.
     
    cmartinez likes this.
  4. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    I will use it on my workbench, with computers nearby.


    I'll check the Hantek.

    Thanks.
     
  5. Reloadron

    Active Member

    Jan 15, 2015
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    There are several ways to go about it. First you need to sense the voltage and current to the motor. Since the motor will change direction you will either use a switch, relay, or H-Bridge to reverse polarity to the motor for direction change. You mention the voltage will be 12 volts. Will that be a stable 12 volts? Depending on the motor version the data sheet shows a maximum current (stall current) of about 1.7 amps with the normal loaded run currents around 200mA to 320mA and unloaded currents very small around 40mA to 70mA. At this point you need some form of DAQ as MrChips mentions. I would take this route and go buy the old classic ACS712 current sensor. They can be had from Spark Fun Electronics and other sources already on a board. Easy to scale and work just fine for + and - current when you reverse the motor. The output will be a voltage proportional to the current. Measuring the voltage isn't a problem. So we have a current sensor with signal conditioning to measure the low currents.

    As to the DAQ, there are dozens of small units like this one out there from a host of manufacturers. That gives you 4 channels of analog differential input and the basic software to make things happen and chart the data as you record it plus save the recorded data for playback. If you want to get fancy most units like this also include the software to open an Excel Sheet and dump the data into excel so you van make your own charts. The basic 10 bit A to D should be adequate for what you need and the 240 samples per second, though low should be fine. Actually using two channels the sample rate becomes 240/2= 120 samples/second. To get around the +/- 10 volt max input for the 12 volts just make a 2:1 divider using a few 10K resistors and scale the voltage channel accordingly. Depending on what you want to spend you can get much fancier DAQ units and I believe DATAQ (which I linked to) can be had on your side of the pond. You can also write your own software for these modules and they are USB powered. Measurement Computing is another supplier and as you can see more features and the cost goes up.

    These are just a few possibles to consider.

    Ron
     
  6. MrChips

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  7. Reloadron

    Active Member

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    Yeah, the DATAQ starter kits work pretty well for basic task. The old discontinued ones like the DI 158U were nice because they also gave you a few digital in out channels.

    Ron
     
  8. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Thanks guys.

    That di-145 is looking mighty interesting, and I found a supplier in Norway. Yay!

    Great!
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Another inexpensive data acquisition option is the LabJack U3-HV. I love mine. One feature I especially like is that I can access it from any programming language. I use Visual Basic within Excel so I can plot results directly.
     
  10. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Wow... and I thought I was the only one defending Hantek in this forum...
     
  11. Reloadron

    Active Member

    Jan 15, 2015
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    Yeah, you have to love these little DAQ devices. I haven't used LabJack but like LabJack the ones I have used include all sorts of programming examples and basic software to get going. Not being a programmer type I also did all my programming in VB which even I could do. :)

    Ron
     
  12. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    So, after waiting, and going nuts on USPS Tracking - it's finally here.
    dataq.JPG
    Now I need to learn how to get the most out of it.

    Thanks for help, suggestions and links.
     
  13. Reloadron

    Active Member

    Jan 15, 2015
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    Keep us up on how it goes. Funny but they make those things a few miles down the road from me. :)

    Get familiar with the software. Read the manual completely. The software is available online from www.dataq.com. Any questions feel free to ask.

    Ron
     
  14. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Thanks. I've downloaded the software, installed it and started to look around, before I connect anything to the device.

    I need some advice on how to measure current.
     
  15. Reloadron

    Active Member

    Jan 15, 2015
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    Back in post #5 I mentioned:

    I would likely go with a hall effect transducer as mentioned above. You aren't looking at very much current. The ACS 712 is a good choice. Hall Effect current sensor modules should bring up some similar devices. The unit I linked to already has the ACS 712 mounted on a mini board with an amp and the offset. So what you get is a baseline of 2.5 volts. Where 0 Volts = -5 Amps and 5 volts = +5 Amps. Using one of your differential input channels you scale the channel 0 to 5 volts = -5 to +5 Amps. Oh yeah, for the 12 volts. just make a 2:1 voltage divider at the input of the Dataq and scale the channel.

    Ron
     
  16. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Reading the manual, I thought I could do the same thing as on page 12. And adding a voltage divider.
    This is what I had in mind:
    DI-145CurrentMeas.png
    Am I way off track?
     
  17. Reloadron

    Active Member

    Jan 15, 2015
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    Not quite. Make R1 & R2 10K 1% resistors and drop that shunt resistor.

    Dataq Volt Div.png

    The Dataq has 4 differential analog input channels:

    Input Impedance: 1Mohm, each input to ground

    So the 10K resistor won't be effected by the input impedance and won't load down the source voltage. So now in reality the Dataq will read 1/2 the actual input, 12 volts for example will read 6 volts. Then you just scale the channel so 6 Volts reads 12 Volts when you chart things.

    Page 12 of the manual is when you are working with a 4 to 20 mA current loop. Using a 250 Ohm shunt the 4 to 20mA becomes a 1 to 5 volt input to the Dataq. Using a 500 Ohm resistor gets you 2 to 10 volts. Anyway, not what we want here for the motor voltage.

    Ron
     
  18. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Ok, great. I'll try this. Thanks.
     
  19. Reloadron

    Active Member

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    Then we will get to motor current. :)

    Ron
     
  20. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Hopefully we get it on Sunday evening. I'm off to a maneuver in the Home Guard for the weekend.
     
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