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  • Author Author: colporteur
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Blog 6: Now for some initial data

colporteur

With the python library, misaz shared in his blog post, I can continue with my challenge testing.  My initial tests of the sensor are using the locomotive on the bench. I have been using 3VDC, 5VDC and 12VDC to set some expectations before moving to the rail tests.

I still try and follow the process my electronics lab college instructor tried to instill in us 40 years ago. Have some idea of what the measurement should be, before you go poking around with testing equipment making measurements. I had no success getting current measurements using my Fluke VOM so I only have what the sensor is providing.

The python script generates a current & voltage reading every five seconds. The initial voltage applied to the locomotive motor is 5VDC, then changes (highlite) to 3VDC and then to 12VDC.

image

My question to the sharper electronic knives in the drawer than me is, do the current values look reasonable?

The current reading moves between.05 to .09, the average, throwing out the negative values in the calculation, is .06. I was expecting higher current values at the higher voltages. What I interpret from the current reading is a motor that is an active load where the resistance changes to ensure a steady current. That doesn't seem correct?

The motor speed does change with a change in the voltage.

I confess there is a lot of rust and dust accumulated on my electronic theory. Part of my motivation for doing these challenges is to refresh my knowledge. I would appreciate any feedback members have to offer before proceeding with further testing. What do you see in the values so far?

  • I really appreciate michaelkellett & misaz lending me your expertise. I ran the python script again using the following configuration, to measure both current and voltage.

    max.configure(sample_rate_khz=.5, digital_filter=128, measure_current=True, measure_voltage=True) with the following results. I had to add the sample_rate or the script errored out. This makes it slightly different than what misaz provided.

    image

    The readings are much more stable.

    Break..Break... I could carry this question to another post but I got lazy:o

    Why am I unable to measure the current with a Fluke 77 VOM using the 300mA range?

    I tried using two different meters to read the current. Only when set to the 10amp range on the meters will they produce a reading of 0.07 and 0.06 on the other. If I attempt to use the 300mA on the Fluke meter and 250mA on a second different meter they indicate out of range.

    The reading would suggest the current is within the lower range of the VOM's, Any suggestion on why are they not able to provide a reading?

  • in reply to colporteur

    Error Sample rate must be 0.5 when measurement of both current and voltage is selected. is restriction of sensor. See this section in datsaheet:

    image

  • in reply to michaelkellett

    Both Python anb C Library suppoort configuring digital filter. Maximum value of digital filter (128) eliminated almost whole noise in my case of sensing current flowing simple resistor on breadboard.

    In case of Python Library can try replace configure call by this one:

    max.configure(digital_filter=128, measure_current=True, measure_voltage=True)

  • in reply to michaelkellett

    Just looked as Misaz' latest post - it looks as if library just takes one sample  - this will be very susceptible to noise. So maybe you are  a bit stuck unless he can add some more features to the library.

    MK

  • in reply to colporteur

    I copied your 12V data into MATLAB and plotted it :

    image

    Then I fixed result 17 by setting it to the mean of sample 16 and sample 18.

    image

    The mean of the fixed data is 0.0735 and the standard deviation is 0.0204.

    In measuring science that counts as dreadful.

    The signal to noise ratio is about 11 dB.

    To put it into context - the 12 bit converter in the Maxim chip should easily give you 10 good bits, at 10% of full scale that would be a resolution of 1024/10 = 102 so we should be expecting a standard deviation of about 1% of the mean and a signal to noise ratio of about 40dB.

    There are a couple of reasons it may not be working well:

    1) is the full scale current range of the Maxim chip suitable - it should be about 1A for your set up

    2) is the current really very, very noisy (the Maxim chip should be able to deal with this but I have no idea how its internal settings are set in your experiment.

    You can check out 2 by putting a big capacitor across the loco (but not across the current sensor). It need to be between 1000=uF and 4700uF. This isn't a permament fix  - just for experimentation. If you don't have such a cap handy try the biggest value you do have ! (Must be rated 16V or more)

    Ideally we would like to look at the current with a scope - the Maxim chip can sample quite fast so it should be able to do this but your software library doesn't seem to allow it.

    (If you post any more data can you plonk it as text in the post or as a file attachment - if it's in a picture I have to copy it into MATLAB by hand.)

    MK