very low speed rotation and encoder

15 years 7 months ago #3330925 by sww
I am developing an application that uses the AX2850, however; environmental concerns make using optical encoder problematic. Instead, I was hoping to use sealed hall effect sensors.

The hall effect sensors have the same output as an optical encoder. The problem is the pulse per revolutions (PPR) will be very small. My accuracy requirements are not the limit. The limit seems to be the AX2850 firmware. If I understand it correctly,
for a PPR of 1 the min RPM I can measure is ~228 RPM.
I would like to measure 2 RPM.

Is there a way I can 'magically' increase the Time Base by a factor of 100? Or I am doing the math wrong?

I realize I can increase the PPR, but it increases the cost of the device when the accuracy is not needed.

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15 years 7 months ago #3332087 by cosma
Replied by cosma on topic Re:very low speed rotation and encoder
The maximum time base value is 64. This is equal to a time period of 16ms (64 * 256us).

At 1 PPR (i.e 4 counts per revolution), 1 count per 16ms = 1/4 turn per 16ms = 1 turn per 64ms = 15.6 turns per second = 937 RPM.

The reason the time base value is maxed at 64 is because the controller's PID is evaluated every 16ms and it requires that a new speed value be measured during this interval.

In your case, you would need an increase in the time base that is far larger than 16ms anyways.

2 RPM is extremely slow and you would normally need a sensor with PPR of several thousands.

If the 2RPM is at the output of a gear box, then one obvious solution would be to measure the speed at the motor. Even there, a 1 PPR sensor will probably not give you the resolution you need.

I see no obvious solution to your problem. Other Forum members may.

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15 years 7 months ago #3359259 by sww
Replied by sww on topic Re:very low speed rotation and encoder
Thanks for the reply.

You might want to update the documentation to include the range for the time base. It makes sense that the max is for 16 ms, but I did not see it anywhere. I assumed since the parameter was a byte that it had a range of 0-255.

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