ax3500 encoder rpm question

14 years 5 months ago #7929484 by athyssen
ax3500 encoder rpm question was created by athyssen
<P style="MARGIN: 0px">In RoboRun, on the Encoder screen I cannot get the application to report an accurate "RPM Equivalent". This seems to also affect when I try to run in closed loop speed mode.
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<P style="MARGIN: 0px">I have a 400PPR optical encoder attached to my motor axle which is geared 32:1 to the actual wheel. If I rotate the wheel through one full rotation the counter value shows 12800 which makes sense 400PPR * 32 = 12800. I can also apply full power to the motors and the wheels rotate right around 120RPM. After running the motors for 60 seconds the reported counter value is correct. I'm sure the encoders are working fine.
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<P style="MARGIN: 0px">I have chosen a (Time Base = 4) @ (PPR = 12800) which reports a Max RPM of 116 which fits my expected 120RPM. I save the results to the controller. If I then ramp the motor up to full speed the "RPM Equivalent" reports around 45RPM rather than close to the 120RPM.
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<P style="MARGIN: 0px">Am I misinterpretting the RPM Equivalent or is something wrong?
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<P style="MARGIN: 0px">-AndyT
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14 years 5 months ago #7932409 by cosma
Replied by cosma on topic Re:ax3500 encoder rpm question
<P style="MARGIN: 0px">Your problem seems to come from the common confusion between PPR (pulses per Revolution) and CPR (counts per Revolution)
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<P style="MARGIN: 0px">Pulses are the number of pulse that you could physically see with a scope on a single channel output.
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<P style="MARGIN: 0px">Counts are the number of transitions on the combined channels. Since you have two channels and each channel transitions twice per pulse (low to high and high to low), the number of CPRs is 4 x the numbers of PPRs.
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<P style="MARGIN: 0px">See discussion on p.132 of the manual.
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<P style="MARGIN: 0px">So in your case, what you have is a 100 PPR encoder which will deliver your 12800 counts per wheel turn: 100 x 4 x 32. The equivalent PPR at the wheel is then 3200 pulses per revolution (PPRs)
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<P style="MARGIN: 0px">For your time base setting you should plug in 3200 instead of 12800.
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14 years 5 months ago #7940581 by athyssen
Replied by athyssen on topic Re:ax3500 encoder rpm question
<P style="MARGIN: 0px">Thank you. I have set TimeBase=18 and PPR=3200 and it lists the Max RPM=122 which fits my known max speed.
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<P style="MARGIN: 0px">If I try and run the motors in the Encoder screen I immediately get an error dialog followed by the app shutting down. Everything works just fine under then Run screen. Here's the error I'm getting.
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<P style="MARGIN: 0px">RoboConf
<P style="MARGIN: 0px"> Run-time error '6':
<P style="MARGIN: 0px"> Overflow
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<P style="MARGIN: 0px">Thanks again.

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