Looking to filter the noise on a Sharp IR sensor
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Posted by Kasterborus on May 26, 2008, 6:45 pm
Please log in for more thread options that the output is very noisy. The sensor updates at 25Hz so I was thinking that if I were to add some kind of 25Hz low pass filter to the output that would clean things up. Datasheet: http://www.philohome.com/sensors/gp2d12/gp2d12-datasheets/gp2d12.pdf The sensor also operates at 0-2.45v so a gain of 2 from the filter would be useful to bring it in line with the 5v range of my ADC. I thought about this circuit http://www.radio-electronics.com/info/circuits/opamp_low_pass_filter/op_amp_lowpassfilter.php But it is unity gain - could it be modified to give a higher gain? Dave | |||||||||||||
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Posted by Bob Monsen on May 26, 2008, 7:30 pm
Please log in for more thread options >
> But it is unity gain - could it be modified to give a higher gain? > > Dave You can use this page to design the analog filter: http://www.analog.com/Analog_Root/static/techSupport/designTools/interactiveTools/filter/filter.html It allows you to set the gain to be whatever makes sense, and computes the right component values. However, if you are interfacing with a microcontroller, then you might want to use a digital filter instead. You can easily average the output over time, and save on the external components. You still need an RC filter to eliminate high frequency noise, since a digital filter will alias frequencies higher than 1/2 the sampling frequency into the baseband if you don't filter them out. You can control the gain with a digital filter as well. There are websites that will design any kind of digital filter for you (i.e., a java app). They usually use floating point, though. For this application, I've used a simple RC filter, along with an averaging loop like this: forever { while (!sample_available()) { do_something_else(); } average = (average << 3) - average + get_sample(); average >>= 3;
// do something with the average here
} This gives each sample an exponential decay. If that simple filter doesn't work, there are lots of others to choose from. The choice of filter will depend on the application. Regards, Bob Monsen | |||||||||||||
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Posted by Tim Wescott on May 27, 2008, 12:50 am
Please log in for more thread options Bob Monsen wrote:
>> I just hooked up a Sharp GP2D12 IR range sensor to a scope and saw
http://www.radio-electronics.com/info/circuits/opamp_low_pass_filter/op_amp_lowpassfilter.php
>> that the output is very noisy. The sensor updates at 25Hz so I was >> thinking that if I were to add some kind of 25Hz low pass filter to >> the output that would clean things up. >> >> Datasheet: >> http://www.philohome.com/sensors/gp2d12/gp2d12-datasheets/gp2d12.pdf >> >> The sensor also operates at 0-2.45v so a gain of 2 from the filter >> would be useful to bring it in line with the 5v range of my ADC. >> >> I thought about this circuit >> >> >>
>> >> But it is unity gain - could it be modified to give a higher gain? >> >> Dave >
http://www.analog.com/Analog_Root/static/techSupport/designTools/interactiveTools/filter/filter.html
> > You can use this page to design the analog filter: > > >
> > It allows you to set the gain to be whatever makes sense, and computes > the right component values. > > However, if you are interfacing with a microcontroller, then you might > want to use a digital filter instead. You can easily average the output > over time, and save on the external components. You still need an RC > filter to eliminate high frequency noise, since a digital filter will > alias frequencies higher than 1/2 the sampling frequency into the > baseband if you don't filter them out. You can control the gain with a > digital filter as well. There are websites that will design any kind of > digital filter for you (i.e., a java app). They usually use floating > point, though. For this application, I've used a simple RC filter, along > with an averaging loop like this: > > forever { > > while (!sample_available()) { > do_something_else(); > } > > average = (average << 3) - average + get_sample(); > average >>= 3;
> // do something with the average here
Ditto all of this, except that if most of the noise is coming from the
> } > > This gives each sample an exponential decay. > > If that simple filter doesn't work, there are lots of others to choose > from. The choice of filter will depend on the application. > > Regards, > Bob Monsen > detector, and if the detector is really updating at 25Hz, and if you can sample synchronously with that 25Hz update, then an anti-alias filter probably won't do much good. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html | |||||||||||||
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Posted by Kasterborus on May 27, 2008, 7:40 am
Please log in for more thread options Thanks for the suggestions - I'll give them a try.
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Posted by Bob Monsen on May 27, 2008, 7:21 pm
Please log in for more thread options > Thanks for the suggestions - I'll give them a try.
Ah, one other thing, those sharp sensors take a big current spike when they power the LED. Depending on your setup, you may actually be seeing the spikes on your A/D input. The solution for this is a big cap (like 100uF) near the power inputs for the sensor, which will smooth the power supply. Note that this only matters if you are using a small supply that can't keep up. Scope the power input to see if it makes a difference. Regards, Bob Monsen | |||||||||||||
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> that the output is very noisy. The sensor updates at 25Hz so I was
> thinking that if I were to add some kind of 25Hz low pass filter to
> the output that would clean things up.
>
> Datasheet:
> http://www.philohome.com/sensors/gp2d12/gp2d12-datasheets/gp2d12.pdf
>
> The sensor also operates at 0-2.45v so a gain of 2 from the filter
> would be useful to bring it in line with the 5v range of my ADC.
>
> I thought about this circuit
>
>