Testing device 485/422

Discussion in 'The Projects Forum' started by Wavelength, Apr 28, 2010.

  1. Wavelength

    Thread Starter Member

    Mar 2, 2006
    13
    0
    I have this load sensor which have 5 wires. From a piece of paper it said the first 2 wire are for excitation + and - ( max 15dc or AC). Another 2 are for +out and - out. Last a wire is for shield. I'm trying to test the sensor using my rs232 port on my laptop to see the output on hyper terminal.

    So is it safe to assume that the I need to use a RS485 or 422 to see the output of the sensor?

    What is excitation is it a just supplying +15vdc and -15vdc or I need a sine wave AC supply?

    I tried asked for more detail from the company but still no response.

    Thank you in advance. :)

    Product model SB00-10t
    Product web site http://www.sewhacnm.co.kr/
     
  2. polleke

    New Member

    Oct 30, 2009
    3
    0
    excitation is just 10 volts
    and the output is on other 2 wires.
    max is 10mV so very low.
    just push the cell and read the change.
    in these small household scales is also a cell you can use.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
  4. Wavelength

    Thread Starter Member

    Mar 2, 2006
    13
    0
    Thanks for the answers.

    Still how do I supply an excitation voltage? So i just connect a +10v to the positive point and -10v to the negative point?

    I think I need to make a circuit or find a rs485 converter to rs232 so I can plug it to my laptop. Probably I just use max485 IC to change the RS485 signal to ttl voltage and from that ttl voltage I connect it to a max232 for RS232 signal to my laptop.
     
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    A normal load cell is nothing to do with RS485 or any digital system.

    All it consists of is four strain gauge resistor elements attached to a metal support.

    Look at figure 1 in this data sheet:
    http://www.mip.sdu.dk/~legobot/am36/Delta/proj_2_files/strain_gauge_data.pdf

    'Ve' is the 10V exitation, 'Vo' is the output voltage.

    The output from the load cell is intended to connect to a differential amplifier such as an 'instrumentation amp'.

    In principle it is nothing more than four very slightly variable resistors connected in wheatstone bridge style. Physical load on the cell causes the resistors to change value slightly and a (very) small voltage is produced across the outputs.
     
  6. Wavelength

    Thread Starter Member

    Mar 2, 2006
    13
    0
    Oh thanks a lot for clearing that for me :)

    So the sensor will give out an analog output and thus I need to build an ADC for a digital output?
     
  7. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Correct. You can set the output levels of the strain gauge to the specs of the ADC using and opamp. This will allow you to get the most accuracy out of the ADC range.

    If the output from the bridge is 3.4v to 4.85v You will want to get that as close to the ADCs range as you can. (typically 0-5v)

    So the 3.4 would reference down to ~0v and the 4.85 would reference to ~5v.

    If you are going to use a uC onboard ADC, they are usually only 10bit so you want the largest sampling range you can.
     
  8. Wavelength

    Thread Starter Member

    Mar 2, 2006
    13
    0
    Thank you.

    Any simple ADC circuit or using UC for me to check the output.
     
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