[En-Nut-Discussion] 60Hz frequency measurment questions..

[Alexander Baranov]

Well, to measure 17ms period with 0.1% you need to have strobe pulse period 
to be about (17us/3) ~ 5 us, which corresponds to 200kHz frequency. You also 
have to use comparator with appropriate sensitivity and response time. You 
can generate this strobe frequency by one timer and use it as external clock 
for another one. I have some doubts about jitter of comparator output but 
you can try.
Alexander
----- Original Message ----- 
From: "Harald Kipp" <harald.kipp at egnite.de>
To: "Ethernut User Chat (English)" <en-nut-discussion at egnite.de>
Sent: Thursday, June 08, 2006 10:23 AM
Subject: Re: [En-Nut-Discussion] 60Hz frequency measurment questions..


> Hello James,
>
> Very impressive project, btw.
>
> I do not have much experience with signal conditioning, which
> may be required here, nor do I have any ready-to-use code.
>
> But one essential hint may be to stick with hardware. For the
> AVR based (8-bit) Ethernuts you have to deal with interrupt
> latencies of about 100 microseconds. This depends very much
> on the interrupt routines / drivers you are using. Thus, using
> interrupts for counting frequencies is a bad idea.
>
> The on-chip timers/counters of the Atmega128 (Ethernut 1 & 2) and
> the AT91R40008 (Ethernut 3) are very versatile devices and can be
> perfectly configured for almost any timing requirement. On the
> other hand they are quite complex and you definitely need to
> read the related chapters in the data sheets more than once,
> in order to select the mode that will fit best. There are several
> ways to start/stop counters by external signals. Then you can
> read the results from the counter registers. This way
> the measurement is done by hardware and the deviation mainly
> depends on the CPU crystal. Btw. the crystals on our Ethernuts
> are no rocket technology (30 ppm at room temperature), but
> 0.1% is no real challenge for a crystal either.
>
> Keep in mind, that timer 0 is used by Nut/OS. All remaining
> timers can be freely used by your application.
>
> Harald
>
> At 00:37 08.06.2006 -0400, you wrote:
>>Greetings All,
>>
>>I'm going to use my EtherNut board as a engine monitoring and control 
>>system for
>>my homebuilt 15KW generator project.  I think I've figured out how to 
>>monitor
>>everything except the line frequency of the generator output (60hz 
>>ideally).
>>
>>While the generator has a mechanical governor on it, it tends to sag about 
>>1hz
>>between no-load and 60% load (10KW).  One goal for the Ethernut board is 
>>to
>>operate a governor assist system to help keep the generator producing 60hz 
>>power
>>even under varying loads (else my UPSes all start getting upset).
>>
>>The problem is I need to measure the frequency fairly accurately, 
>>preferably in
>>the +/- 0.1% range, and be able to update the difference between real 
>>frequency
>>and desired frequency fairly quickly. The diesel motor is a single
>>cylinder,running ideally at 1800rpm (15 power strokes per second, so I 
>>only get
>>to measure few cycles of output to calculate the mean frequency, in time 
>>to
>>update the throttle position in time for the next power stroke)
>>
>>Any thoughts or suggestions?
>>
>>Take Care,
>>James Lerch
>
> _______________________________________________
> En-Nut-Discussion mailing list
> En-Nut-Discussion at egnite.de
> http://www.egnite.de/mailman/listinfo.cgi/en-nut-discussion
>