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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?

  • in reply to michaelkellett

    Great suggestion MK. I don't have a scope but I did tinker with the sample rates.

    The python library issued a warning about frequency sample rate when measuring both current and voltage.

    raise ValueError("Sample rate must be 0.5 when measurement of both current and voltage is selected.")

    For the above readings the following was used.

    max.configure(sample_rate_khz=.5, digital_filter=1, measure_current=True, measure_voltage=True)

    For the next set of readings, I modified the config to only measure current and that enabled me to use the default sample rate 15.

    image

    I have the locomotive flipped on its back sitting on the bench for these tests.

    image

  • The readings seem to make sense, although timing of conversions and filtering may affect results.  If you multiply voltage times current you will get an idea of the power consumed in each state. If the load is more or less constant (no load except friction), the speed will be proportional to power. Back EMF of the motor, which is proportional to speed will reduce the effective applied voltage, which tends to reduce current through the motor.

  • Do you have a scope - might be worth checking the nature of the applied voltage and the current ?

    Looking at your results the current measurments fluctuate too much to be happy with them

    Can you measure a a lot faster ?

    (I would have hoped for >100 readings per second.)

    Looking at your 12V readings the voltage is quite stable for the six readings but the current is still at the end dropping  - would be really nice if you kept going with more readings until it settles.

    Having said that, 6 samples would span 25 seconds which is a long time constant for the current to vary over .......

    More data needed.

    Suggested step 1.

    Repeat experiment but take at least 20 readings per voltage.

    BTW - what is th loco doing (on rails, upside down etc)

    MK