Re: Active Filter Design: Motor Controller Current Sensing
- From: kensmith@xxxxxxxxxxxxxxx (Ken Smith)
- Date: Wed, 22 Jun 2005 04:20:03 +0000 (UTC)
In article <1119380907.826986.11410@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Mike <mep0716@xxxxxxxxx> wrote:
[...]
>The Hall Effect sensor outputs a 2.5v DC signal at 0 Amps, and 2.5v
>+133mV/A as current increases. My goal is to read the RMS motor
>current using an ADC channel on my MCU.
I assume you mean the that RMS will be taken over some small finite time.
> It is clear that I have to low
>pass filter the signal first, but I am not sure how to decide on the
>proper cut off frequency.
I'm not sure why you say "it is clear" that you need to filter it. Have
you looked at the spectra of what is coming out of the sensor. It may
already have most of its energy at low frequencies. The windings of the
motor are fairly inductive so the high frequencies of the voltage waveform
will be reduced.
Also, remember that any high frequencies that are really in the motor
current do add to its RMS. You don't want to filter too much before you
take the square.
The best way to attack this sort of problem is usually to start with the
highest sample rate ADC you can reasonably deal with. Your filtering will
mostly happen in digital land.
>filtering) freq components, I need to sample at atleast twice the
>bandwidth.
A small point:
Actually, you need to sample at a little more than twice. Nyquist
frequency is the first frequency you can't fully characterize not the last
one you can. Imagine a sine wave at Nyquist that just happens to pass
through zero at the sampling point. Your ADC won't see it at all.
If the RMS reading doesn't need to be fast enough to get results within a
cycle or two, you can take advantage of that fact. You can do like a
sampling scope does. Allow the sampling time(s) to move over the repeated
waveform so that you digitize what it is at many places within the cycle.
Effectively what you are doing is aliasing the repeated waveform down to a
lower frequency.
[...]
>I am in the process of setting up a sweet FFT module on my scope which
>will allow me to view the actual motor current (voltage output of hall
>effect) in f-domain.
Look at what you really get from the Hall sensor before you go very far in
the design. There may be huge spikes electrostatically coupled into the
signal or some other dreadful thing to deal with.
--
--
kensmith@xxxxxxxxx forging knowledge
.
- References:
- Prev by Date: Re: test
- Next by Date: Re: Bizzare behaviour from SG/UC3525
- Previous by thread: Re: Active Filter Design: Motor Controller Current Sensing
- Next by thread: 3 dB bandwidth
- Index(es):
Relevant Pages
|
