Re: Active Filter Design: Motor Controller Current Sensing
- From: "Larry Brasfield" <donotspam_larry_brasfield@xxxxxxxxxxx>
- Date: Tue, 21 Jun 2005 12:38:01 -0700
"Mike" <mep0716@xxxxxxxxx> wrote in message
news:1119380907.826986.11410@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> I'm using a Hall Effect sensor (ACS704ELC) to measure the motor current
> through an H-bridge. The FETs are being driven with an Intersil
> HIP4081 FET driver. I am using PWM (20k Hz)to control the motor.
>
> 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. 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. Filtering the signal to a point where I can
> sample/RMS it within the MCU (in a reasonable manner) is fine. I
> understand that in order to recover all of the remaining (after
> filtering) freq components, I need to sample at atleast twice the
> bandwidth. But what dictates my passband, stop band (cuttoff),
> passband attenuation?
The purpose of the filter is not so much to allow recovery of
what gets thru the filter as to reject components that would
impair signal recovery, due to aliasing, without the filter. The
stop-band rejection needed is determined by how large the
expected problematic components are and what accuracy
is to be maintained in the converted samples. So, for example,
if you had a 16 bit converter and wanted to suppress a fullscale
signal at 0.55 Fs, (so that it does not alias as 0.45 Fs), you
would want something like 98 dB rejection at that frequency,
perhaps more depending on what you will do with the data.
As you can see, you need to know the expected magnitude
of unwanted signals at the filter input and output.
> How do you figure out within what frequency
> range the motor current be calculated from?
What information are you trying to extract from the current?
If your intention is to use current-mode control, then you
need to preserve content somewhat beyond the servo loop
bandwidth and watch the phase response carefully. If you
mean only to detect stalls or overheating, a 10 Hz response
would be plenty, without any phase concerns.
> 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.
>
> I'd appreciate any suggestions, thoughts on how I should go about
> picking the proper filter parameters to meet my goal.
If you are doing current mode control, I would suggest
closing that loop in the analog realm. Sampling delay and
filter phase response response to avoid aliasing will be a
source of much fun or frustration otherwise, unless your
loop bandwidth requirements are lax.
> Thanks,
You're welcome.
--
--Larry Brasfield
email: donotspam_larry_brasfield@xxxxxxxxxxx
Above views may belong only to me.
.
- Follow-Ups:
- Re: Active Filter Design: Motor Controller Current Sensing
- From: Terry Given
- Re: Active Filter Design: Motor Controller Current Sensing
- References:
- Prev by Date: Re: Hardware True Random Number Generator design / concept
- Next by Date: Re: 3 dB bandwidth
- Previous by thread: Active Filter Design: Motor Controller Current Sensing
- Next by thread: Re: Active Filter Design: Motor Controller Current Sensing
- Index(es):
Relevant Pages
|
