Calculating a moving average

bruceh

I am using the BS2 to drive a robotic servo using a potentiometer as an input (pretty much using techniques documented by Parallax). Everything works fine except that kids are the source of rotation for the potentiometer. Their imput is jerky, causing the servo to be subjected to rapid acceleration and quick reversals. This is terrorizing the gear train connected to the servo and is causing the spline on the·servo shaft to loosen.

My fix for this would be to calculate a moving average of the last x pot readings and use·this as the input to the servo pulse generation routine. This would create a damping or·shock absorber effect.·(The application doesn't require absolute accurate tracking.)

It seems to me that an array of the previous x readings would be a starting point for the summation, however, I don't see·a way to shift the readings placing the newest reading in the array and sending the oldest reading to the bit bucket each time through the loop.

Anyone solved this problem?

Mike Green

You shouldn't need too many readings to smooth out this movement. Maybe 4 or 5 readings will be enough. In that case, just using simple variables would work, say avg1, avg2, avg3, avg4. The newest is avg1 and the oldest is avg4. To shift the values, just do "avg4 = avg3", "avg3 = avg2", "avg2 = avg1", then get the newest value "avg1 = ???".

Phil Pilgrim (PhiPi)

Assuming that NewReading is less than 8192, you could also use a running average, computed as follows:

AvgReading = AvgReading * 7 + NewReading >> 3

This should give you adequate smoothing. If not, multiply by 15 and shift by 4 instead. In that case NewReading would have to be less than 4096.

-Phil

bruceh

Thanks guys, I'll try both of these methods.

Tracy Allen

Another way to do the low-pass filter takes advantage of the ** operator:

' enter with x = new reading, less than 8192.
IF firstLoop THEN   ' initialize the accumulator
  accum = x * 8   ' accumulator operates at 8 * x.
ELSE
  accum = accum ** 57345 + x   '   57345 = 65536 * 7/8 + 1
ENDIF
  x = accum / 8  ' this is the filter output

--Running the accumulator at 8 times the data input allows it to track small changes smoothly.

--Most filters or averaging schemes do require initialization in order to fill up the pipeline and produce "good" values right away.

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Tracy Allen
www.emesystems.com

Chris Savage

I’ve done like Phil and used a running average, although for simplicity I usually just use a simple add then divide by two scheme. You still have to observe max limits for the variable based on the sum of the max possible values.

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Chris Savage
Parallax Tech Support