Converting milliseconds to frequency

[weirdjim]

Frequency is the reciprocal of time. For example, if you measure your square wave as 16.66...milliseconds, you have a 60 Hz. frequency.

Now, dependent on whether your milliseconds are floating point (many decimal places), you can measure frequency with concomitant resolution. That is, if time is an integer and you only have choices of 16 or 17 milliseconds, the frequency comes out 62 Hz. or 58 Hz. respectively while the actual waveform is 60 Hz..

Jim

[chips123]

With my millisecond values are integers, I don't expect 100% accuracy. Using PBP, how do I get from say, 16mS to 62Hz ?

Scott

[Kamikaze47]

You can write a subroutine to determine the RPM from the z value. If to the nearest 100rpm is acceptable, this is the relevant data:

z           rpm
-------------------------
<10         out of limits
10          6000
11          5500
12          5000
13          4600
14          4300
15          4000
16          3800
17          3500
18          3300
19          3200
20          3000
21          2900
22          2700
23          2600
24          2500
25          2400
26          2300
27          2200
28-29       2100
30          2000
31-32       1900
33-34       1800
35-36	    1700
37-38	    1600
39-41	    1500
42-44	    1400
45-48	    1300
49-52	    1200
53-57	    1100
58-63	    1000
64-70	    900
71-80       800
81-92       700
93-109      600
110-133     500
134-171     400
172-240     300
241-400     200
401-1200    100
>12000      out of limits

If you need help on how to write a subroutine to do this then let me know and i can help you out.

[chips123]

I am a bit of a rookie. Some direction would be much appreciated.

[Kamikaze47]

I had a few spare mins so i wrote up a subroutine that would work. Im sure its not the most elegant piece of code, but it will do what you want just fine.

It will take the z value and store a relavant value into rpm. The value in rpm will be in hundreds of rpms. So if you write the value of the variable rpm to your lcd, followed by "00" it will display in rpms.

If rpm=0 after the subroutine has run then the input was out of range.

convert_to_rpm:
IF z<10 THEN rpm=0
IF z=10 THEN rpm=60
IF z=11 THEN rpm=55
IF z=12 THEN rpm=50
IF z=13 THEN rpm=46
IF z=14 THEN rpm=43
IF z=15 THEN rpm=40
IF z=16 THEN rpm=38
IF z=17 THEN rpm=35
IF z=18 THEN rpm=33
IF z=19 THEN rpm=32
IF z=20 THEN rpm=30
IF z=21 THEN rpm=29
IF z=22 THEN rpm=27
IF z=23 THEN rpm=26
IF z=24 THEN rpm=25
IF z=25 THEN rpm=24
IF z=26 THEN rpm=23
IF z=27 THEN rpm=22
IF z>27 AND z<30 THEN rpm=21
IF z=30 THEN rpm=20
IF z>30 AND z<33 THEN rpm=19
IF z>32 AND z<35 THEN rpm=18
IF z>34 AND z<37 THEN rpm=17
IF z>36 AND z<39 THEN rpm=16
IF z>38 AND z<42 THEN rpm=15
IF z>41 AND z<45 THEN rpm=14
IF z>44 AND z<49 THEN rpm=13
IF z>48 AND z<53 THEN rpm=12
IF z>52 AND z<58 THEN rpm=11
IF z>57 AND z<64 THEN rpm=10
IF z>63 AND z<71 THEN rpm=9
IF z>70 AND z<81 THEN rpm=8
IF z>80 AND z<93 THEN rpm=7
IF z>92 AND z<110 THEN rpm=6
IF z>109 AND z<134 THEN rpm=5
IF z>133 AND z<172 THEN rpm=4
IF z>171 AND z<241 THEN rpm=3
IF z>240 AND z<401 THEN rpm=2
IF z>400 AND z<1201 THEN rpm=1
IF z>1200 then rpm=0
return

[chips123]

Thanks. A bunch of if-thens came to my mind as well. I just was not sure about it. I think I'll try it.

[Kamikaze47]

Let us know how it goes

[Andy Wood]

With my millisecond values are integers, I don't expect 100% accuracy. Using PBP, how do I get from say, 16mS to 62Hz ?

Scott

Time and Frequency are reciprocal, so you can use a constant (in this case 1000) and divide you mS value into it. For example:

1000 / 16mS = 62Hz.

Or if you had 40mS -

1000 / 40mS = 25Hz etc.

You are probably kicking yourself right about now

Regards,

Andy

[Kamikaze47]

That would work, but that kind of division in PBP would take up a lot more processing time than the IF...THEN statements above.

[RussMartin]

I may not be understanding something here.

For the basic math, I would have said:

rpm=60000/z

where z is in milliseconds.

That would work, but that kind of division in PBP would take up a lot more processing time than the IF...THEN statements above.

My question is: Why would doing that single division take longer than going through multiple IF...THEN statements (40-something of them in the example) until one is found where the condition is satisfied?

Thanks to whomever clarifies this for me!

[chips123]

Thanks Andy.....kick, kick, kick...
And Russ...equally good insight. I'll kick myself once for you too.
Almost too obvious, I have been away from math for too long.

Kamikaze...I tried your solution and ran into an exceeded address 3fff, if I remember correctly. I got sidetracked and have not investigated that further yet.

Appreciate the input.
Scott