Try to find the RMS of an input sine wave using a microcontroller

[Minda]

I am trying to find the RMS of an input sine wave using the ARM 7 LPC2119 micro-controller(http://www.kynix.com/Detail/299612/LPC2119FBD.html). I know the theory behind finding the RMS of such a waveform but I am having trouble implementing this in code, especially the sampling at equally spaced mid-ordinates. I also have just used timer interrupts to find the frequency of a sine wave using capture inputs, so I am aware of the basic functionalities in terms of programming an ARM 7 LPC 2119 micro-controller.



I am testing this functionality using a 3 V peak-to-peak sine wave with a frequency of 50 Hz. I have shifted the waveform upwards by 1.5 V so to avoid having any negative values going into the ADC pins of the micro-controller.

To sample at equally spaced intervals I am using timer interrupts, In a very similar way to this example here. Instead of switching on an LED, I am doing an ADC conversion.

Before using the following formula:


I need to make sure that I am sampling at given intervals, and to test this I have set my timer clock to provide a 20ms delay before entering the ADC function by doing so:

Code: Select all

T0CTCR = 0x0; //Set Timer Mode
T0PR = 60000-1; //Increment T0TC at every 60000 clock cycles
//60000 clock cycles @60Mhz = 1 mS

T0MR0 = 21-1;   //Zero Indexed Count-hence subtracting 1
T0MCR = (1<<0) | (1<<1);//Set bit0 & bit1 to Interrupt & Reset TC on MR0  

Once this is done, I am enabling my timer interrupt:

Code: Select all

  VICVectAddr0 = (unsigned )timer0ISR; //Pointer Interrupt Function (ISR)
  VICVectCntl0 = (1<<5) | 4; //(bit 5 = 1)->to enable Vectored IRQ slot 
  VICIntEnable = (1<<4); // Enable timer0 interrupt

The above code gives a 20 ms delay. With a sine wave at 50 Hz, this should give the same value on every reading, since the period of such a sine wave is also 20 ms. However this is not the case and I can see no visible pattern in my results.

The ADC function I am using works well on DC voltages and I can also confirm that the timer is indeed giving a 20 ms delay before entering the ADC function.

Am I missing something obvious here or could there be some other variables I am not taking into consideration? Any ideas would be appreciated.

[QMESAR]

Hi ,

I am just shooting out of the shoulder here
However 50Hz sine = 20msec 1 complete cycle positive + negative halve so my guess is sampling at 20mSec you possibly have 1 sample and depending where it happens it could be on the zero point so not much sample to get a RMS from, secondly you need to sample at least at Nyquiest which is min 2 time the Frequency so in your case at 100Hz sampling you might start to see something
so 100Hz = 10msec = 5mSec per half wave so about 2 samples at 50Hz signal .push the sample rate higher my guess would be 1K samples and you will get the RMS properly

[Benj]

Hello,

If it were me doing it then I would probably do something along the lines of this, assuming you are working with a pure sine wave.

Sample at least 12 x the frequency so at least 600Hz. The faster you sample the more likely you are to find the very peak of the wave.

Take a number of samples over say a full period and find the largest value. Multiply this largest value by 0.707 to give you the RMS value.

Repeat, maybe filtering the RMS with a low pass filter so that spurious values are filtered out.

Here is a simple low pass filter calculation you can use.

NEWRMS = LargestReading * 0.707
FILTEREDRMS = (0.1 * NEWRMS) + (0.9 * OLDRMS)
OLDRMS = FILTEREDRMS

If the Sine wave is not a pure sine then the 0.707 rule no longer applies and you probably have to go back to the calculations you were using.

[Steve001]

Hello,

Is this application note any help?

Steve

[Steve001]

another here ..

Steve