measure the current of a motor dc

1) estimate your maximum current for this motor
2) calculate max. voltage across yout measurement resistor based on this max current (in item 1)
3) compare to you ADC max input range, also consider your OpAmp buffer gain, they all must fit in
4) calculate resolution of ADC, compare to changes you want to measure
5) when you send lower PWM value do you measure current/voltage on resistor ? is it hardware or firmware problem ?

Hi tictac,

I'm a little unclear as to the problem you're having. Would it be fair to say this is a summary:

When you measure the current at high duty cycles (for example 95%), you get a steady result, but at lower duty cycles (like 25%) you get un-predictable results?

If so, that's probably just because the motor control input isn't DC (i.e. it's a sinusoid that is effected by the square wave you're putting into it).

Let me know if this is the problem and I can suggest a fix for it.

Hi BoBtHePlUmBeR
thank you for your answer.
yes ,you guess correctly.I havnt fix current measuring at low duty cycle.perhaps it is because of pwm .I dont know

by regards.

Added after 2 minutes:

Hi Zman
perhaps it is a noise.I dont think it is a firmware problem,because I can read fix number of adc at high duty cycle.

by regards.

OK, well I think I know what your problem is: You're PWM'ing the motor, so you are reading the current either when it is on, off, or some transient in between. This will even out more as you get larger duty cycles (say >90%), which is why you're observing the current as being unstable.

My suggestion is to filter your signal. This will smooth it off significantly, but will add some extra lag into your system, so you will have to have a tradeoff between the filter speed and the filter ripple voltage. You can also improve this to some degree by increasing your PWM frequency.

You've already done most of the hard work. From your 1 ohm resistor you have an op amp which feeds into the ADC. Rather than feeding it in directly, make up an RC lowpass filter before feeding it into the ADC. If you choose a cutoff value of say 10 times less your PWM frequency, you should notice a MUCH smoother waveform. If you need help getting values, let me know what frequency your PWM is running at and I can work out some values for you.

I send this circuit

Hi
I use fast pwm top=03FF with 10.800 KHZ .
I write C code with code vision.

Hi tictac,

I'm not familiar with the LM741 without the BC560A on its output, so I'll submit a simplified solution and maybe you can extrapolate what you need from it. Basically I've added a resistor and capacitor at the output of the amplifier to smooth out the waveform. As I said before - this will create some lag in the signal, but with a cutoff of around 400Hz, it shouldn't be too bad...

Hi BoBtHePlUmBeR
Thank you so much
does it need special opamp?
at first I study AN894 from microchip about high-side and low side resistive current measurement,I try both of them,but I cant take any good response,in this app note ,notes that a special opamp(such as auto-zero) must be used,I didnt find any of this opamp.
by regards

On Page 7 of that app note there is a specific reference to removing emi noise, where on page 8 (figure 9) they have incorporated emi filtering, as well as signal filtering and gain - this is probably the best option for you if you have the components and can work out the R and C values. If not, just go with the circuit in figure 8 and add the passive RC configuration that I put on the diagram previously (should do the same sort of job).

I have this experience driving an array of leds with pwm controller and constant current. I fix it with a 2th order Pasive RC filter like BoBtHePlUmBeR is suggesting.

Calculate the filter with the pwm frecuency and make a media in the ADC Microcontroller because the filter is not the best for the velocity of the clock as you will see in your oscilloscope.

Eng. Canó
Dominican Republic.
2007

thank you BoBtHePlUmBeR so much
thank you for your answer.
but it doesnt fix, LSB=200
I couldnt find such opamp that work well

I now test one hall effect sensor ,because I dont have time,later I should test previous way (low side,high side)
by regards