[chbot] Filter

[Synco Reynders]

Hi Wallace,
  To get a smooth velocity without adding delay (like an iir filter) you
can simply take the slope, but over more samples. (Eg, 4 samples ahead - 4
samples back divided by 9).
Same can be done for getting a smooth acceleration.
/s


On Tue, 23 Oct 2018, 09:41 Charles Manning <cdhmanning at gmail.com> wrote:

> If you have data with spikes in it then a median filter is a very good way
> to get rid of those spikes. It is better than an IIR or FIR which "cook in"
> the error.
>
> basic algorithm for a 5 point median filter:
>
> Take the five points: x0..x4.
> Sort them into y0..y4
> Select y2 (the median value).
>
> You can then apply an IIR/FIR to the result.
>
> It is pointless going the over way (ie. IIR/FIR) then median filtering
> since the FIR/IIR will cook in the noise and the median filter can't take
> it out.
>
>
> On Tue, Oct 23, 2018 at 8:47 AM Hartley, Robin (Nokia - NZ/Wellington) <
> robin.hartley at nokia.com> wrote:
>
>> Hi Wallace,
>>
>> Something to keep in mind when measuring cam profiles is the shape of the
>> cam follower. Flat bottom followers result in a very different effective
>> profile compared with roller followers. Some followers that appear flat
>> actually have a profile to them.
>>
>> When doing this the old fashion way with a DTI we would do it in situ
>> using the followers, and we used to vibrate the bench at each measurement
>> point to get the pointer to settle. When running the DTI directly on the
>> cam drag on the end of the pointer causes stiction in the bearing that can
>> give this semi cyclic error. A bit of vibration may help
>>
>> For selecting between a set of cams I'd just use Excels curve fitting to
>> smooth the curve. And then do the velocity calcs on the resultant curve.
>>
>> Regards,
>> Robin
>>
>> -----Original Message-----
>> From: Chchrobotics <chchrobotics-bounces at lists.ourshack.com> On Behalf
>> Of Marshland Engineering
>> Sent: Saturday, October 20, 2018 10:30 AM
>> To: chchrobotics at lists.ourshack.com
>> Subject: Re: [chbot] Filter
>>
>> After quite a few chats and emails, the problem has become clearer. In
>> some way it may not really a filter issue. For a 4 stroke engine cam, there
>> are basically 4 components, base circle, up ramp, over the nose and down
>> ramp back to the base circle.
>>
>> So we have 0 velocity, acceleration to the up ramp, constant velocity,
>> deceleration, stop (over the nose) acceleration, down ramp, deceleration
>> and back to 0.
>>
>> The figures to look for are the acceleration. If it is too high then the
>> valve/valve strings etc loose contact with the cam.
>>
>> The figure for acceleration (from memory) is 0.008" per degree or .2
>> mm/degree.  This is what I want to measure/compare.
>>
>> Here is some info on it.
>> http://www.profblairandassociates.com/GPB_4stHEAD_ValveLiftDesign_hmb.html
>>
>> Basically I have several different cams and want to measure them all and
>> choose the one with the most gentle profile.
>>
>> Cheers Wallace.
>>
>>
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