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I am trying to design a crane load (Weight) measuring unit using motor current measurement technique. For some reasons we can't use load cells here.

Part of schematic I have drawn hereenter image description here

  1. An air core current transformer is used here to convert current into a measurable voltage. I have measured using a DSO 1.4Vrms at no load & 2.1Vrms at full load of crane capacity across 220 ohms resistor. Voltage waveform is sine wave. Points 2 & 3 haven't been tested yet.
  2. A diode rectifier & a low pass filter will convert 50Hz AC into DC. Every crane will be calibrated using standard weights & ADC values will be stored in EEPROM of a micro-controller for ADC value to weight correlation. Linearity of filters & rectifiers won't hamper the accuracy.
  3. A 10 bit ADC with 4.096v internal reference voltage will convert this dc into a value which will be converted into appropriate load (wight) using values stored in EEPROM during calibration in point 1.

I need help in designing a level shift amplifier with following points in mind.

  1. Can convert 1.3v to 2.2v into 0 to 4.096v, non-inverting with minimum opamps (if possible, only 1).
  2. Use of trim pots-resistors combination is required to make finer adjustments at site.
  3. Only 5v VCC supply is available.
  4. If possible, merge 2nd order 5 to 10Hz low pass filter with it. This will chop off AC ripples & provide smooth DC in proportion to CT output voltage.

Even if you have a different approach to crane load (weight) measurement, please share.

Thank you,

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  • \$\begingroup\$ Why don't you choose a more suitable current transformer (or design it differently), so that you don't need amplification, but only do scaling with just two E192 resistors? If the input impedance of the ADC is not high enough, you can use a single opamp for buffering the signal. \$\endgroup\$ Jun 21, 2014 at 15:58
  • \$\begingroup\$ An example of a cheap reliable opamp, lm321, is actually cheaper in my experience as a quad pack rather than a single opamp, in large quantities. \$\endgroup\$ Jun 21, 2014 at 16:03
  • \$\begingroup\$ Aren't you introducing a dead zone in your measurement because of the two 1N5819 diode drops on each phase of the current wave? In other words, until your current transformer's output exceeds approx. 1400 mV peak-peak, you will see no current flow thru the 33K resistor. Thus, no voltage across the holding capacitor. Is this within the bounds of your intended current measurement range? What is the actual AC current range you need to measure? \$\endgroup\$
    – FiddyOhm
    Jun 21, 2014 at 16:25
  • \$\begingroup\$ Is this homework? It looks like a silly design for a real application, but a reasonable h/w question. \$\endgroup\$ Jun 21, 2014 at 18:25
  • \$\begingroup\$ @ hkBattousai ADC needs to be utilized completely. No load motor current is 6A & full load current is about 12A. This will only utilize 50% of the ADC resolution. \$\endgroup\$
    – user46011
    Jun 22, 2014 at 11:48

1 Answer 1

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I wouldn't bother with the analog amplification or the RC filter to DC part. Just turn the AC into full wave rectified DC. I assume that if you have a ADC and an EEPROM that you're using a uController. If so, create a simple peak check algorithm (if current value is less than previous value within time frame, then you're at peak). You could take the last 5-10 or whatever peaks that you've found and average them to get a more precise measurement.

Amplification and level shifting you're trying to accomplish with op-amps will introduce offset errors and multiplication errors, so you won't gain any accuracy. Just use the 1.3 to 2.2 V signal you have now directly into the ADC.

FiddyOhm makes a good point. If you want to full-wave rectify this, you'll gain a lot more accuracy at the low end by using lower voltage drop Schottky diodes.

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  • \$\begingroup\$ Thanks for your answer. Change of voltage between 1.3v to 2.2v won't fully utilize ADC resolution even if we use Vref of 2.2v. 1N5819 is indeed a low Vf (0.34v@100mA) schottky diode. \$\endgroup\$
    – user46011
    Jun 22, 2014 at 12:17
  • \$\begingroup\$ @user46011 Maybe you are getting a full range with your schottky's then. My point was that if you amplify your signal, you'll also amplify your noise so you won't gain anything out of it. Your resolution may 'look' better after amplification/level shifting, but your actual accuracy won't change. The 1-2 bits you gain will likely be nothing more than noise. \$\endgroup\$
    – horta
    Jun 22, 2014 at 15:14
  • \$\begingroup\$ You may even get better resolution than what ypur adc allows, using no op amp, no filter and averaging multiple samples. And better accuracy of course... \$\endgroup\$
    – Nicolas D
    Jun 24, 2014 at 16:12

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