May 132012

If you are making some measuring device with display it is always a challenge to select proper rate of refresh. Usually your measurement takes only small amount of time and it is very hard to resist updating display after each one. I saw number of devices that have displays that are just too fast to read.

Slowing rate at which measurement is taken is almost always beneficial for both user comfort and battery life. And that is valid solution, especially if value is relatively stable. However, if measurement fluctuates a bit, that results in jumps between values.

To cure that you should be doing averaging. If your measurement takes 10ms to complete, you can do 10 of them, average the result and still have quite a decent 10/second refresh rate. This is probably solution gets most use but it is not the only one.

My favorite way of slowing display is simplified weighted average. Between two measurements one that is current always carries more weight than newer one. Exact weight is matter of trial but I found small numbers like 23% work the best.

To clarify it a bit, let’s say that we have measurement of 10 and measurement of 20. Our new “average” value will become 12.3. If third measurement is also 20, value becomes 14.1, then 15.4 and so on. Value keeps getting closer and closer to real reading but speed with which it does that is very limited.

If you have measurements that are relatively stable this method works almost like an average. If value jumps occasionally this method will smooth such temporary change. That gives much nicer end user feeling as far as measurement goes. And since we are doing this at much faster rate than actually showing data, if permanent jump does occur, user will see such change relatively quickly.

Code for this might look like:
[c highlight=”6″]
float value = measure();
while (1) {
for (int i=0; i<10; i++) {
float newValue = measure();
value = value + (newValue – value) * 0.23; //to smooth it a little
value = (int)(value * 1000.0) / 1000.0; //rounding
//do other stuff

In this particular code, we show a value to user as soon as we can (to enhance perceived speed). After that we average next 10 values (each new one is given 23% of consideration). Then we display new average to user. Rinse and repeat. Optional rounding step additionally limits small changes.

This code is not that good if measurement takes a long time. E.g. If you have one measurement per second you will find it takes eternity to change a value. For such cases you are probably better off just displaying current measurement to user. Or, if some smoothing is required, using higher values (e.g. 0.79 or similar).

P.S. This method might not work as expected for your measurements. Do test first.

P.P.S. This is intended for human display only. If you are logging values, it is probably best to write measurements without any adjustment. If you average them before writing, you are losing details.

P.P.P.S. If you are doing averaging in full-blown desktop application, ignore this code. You can use proper moving average (linear, exponential, weighted…) that allows for much greater control. This method is just a workaround to get similar results when working on memory-limited PIC.

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