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Author Topic: Recommendations on high speed encoder?  (Read 1025 times)

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Offline GrooveHolmesTopic starter

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Recommendations on high speed encoder?
« on: August 11, 2010, 03:31:25 PM »
Having trouble finding an AxonII friendly wheel/wheel encoder (sensor and wheel) that can detect RPMs up to 100,000. (one hundred thousand)

In short, the plan is to use my AxonII to monitor the rpm of a rotor, however the rotor spins at very high speeds. Up to, but not exceeding, 100K RPM.

Suggestions? Thanks folks!
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Offline Webbot

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Re: Recommendations on high speed encoder?
« Reply #1 on: August 11, 2010, 04:11:29 PM »
Think you are asking for advice on the hardware..... but

100K RPM is going to mean 1 RPM every 0.6ms.

So I'm not sure what you want the encoder to do - just measure entire revolutions or fractions of a revolution.

EG an encoder with 32 stripes will generate 4x32=128 interrupts per revolution. ie every 0.6ms/128 = every 4.6uS. So the interrupt handler must be faster than 4.6uS - and thats just for one motor. Its impossible. Also you may want the controller to be doing other stuff like communicating with something else regarding the current readings.

What you may want to consider is some extra components such as a counter that is clocked by your encoder and can then read periodically by the Axon. ie off load all those interrupts onto another device.
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Offline futmacl

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Re: Recommendations on high speed encoder?
« Reply #2 on: August 11, 2010, 04:21:59 PM »
Do you seriously need continuous positional information at 100k RPM (which is what encoders are for) - or do you just need to count the number of turns / synchronize something with a single, specific angular position?

If the latter, then any slot photointerruptor will do - Toshiba TLP841 or whatever. The signal would be barely 1.6 kHz, while a typical photointerruptor has a response time of 3-30 Ás.

If the former, then yeah, depending on the resolution needed, this may be hitting the limits of the speed your ATmega chip may be able to run at.

Offline GrooveHolmesTopic starter

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Re: Recommendations on high speed encoder?
« Reply #3 on: August 11, 2010, 09:03:41 PM »
Sorry, I should have phrased my question differently. I really only need to monitor the RPM of the rotor, not its position.

So any solution that can do this is welcomed.

« Last Edit: August 11, 2010, 09:08:50 PM by GrooveHolmes »
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Offline Webbot

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Re: Recommendations on high speed encoder?
« Reply #4 on: August 11, 2010, 09:28:04 PM »
ok - so use a photo interrupter connected to an INT pin on the processor so that you get an interrupt every revolution which increments a variable.

If you need to detect forward and reverse movement then you may need two sensors - bit like a quadrature encoder.


Then have a timer that is running some code every second, or some other fixed time interval, that reads, and clears, the variable. It can then be multiplied by a constant to give the current RPM.

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Offline GrooveHolmesTopic starter

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Re: Recommendations on high speed encoder?
« Reply #5 on: August 12, 2010, 07:06:20 PM »
Thanks folks, that is what I will do.   Since i'm designing this dohickey from scratch I can easily add a fly wheel like disc with a hole or notch. I'll get the sensor and tinker.

:)
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