Differentiation of encoder signal results in zero rpm

[ajenga]

Hi.

I am trying to record rpm using an encoder signal.  The data acquisition rate is 20 KHz.

I am using DASYLab v 11 with a WBK 516E with a WBK17 expansion module.  (We are also using a WBK 40 and WBK 16, but I removed those items from the program for trial and testing).  I have a motor running at 1500 rpm with 4096 pulses /rev and a filiment drive running at 9 rev/min. with 1200 pulses/rev.  The differentiation of the filament drive signal is zero.

The theory is that data points are being double sampled resulting in a zero change when diferentiated.  How would I perform a differentiation where there is a larger span between points 1 and 2 for the filament drive?  The screen shot of the program is too large to attach.

Regards,

Amy Enga

[TomRizzo]

Amy,

Are you using the WBK17 for the encoders?

And what is the filiment drive?  If I understand better maybe I can help.

[ajenga]

I am using a WBK 17 and the encoder 4x option.  The filament drive is a motor with a gear reduction.  The filament motor encoder does have a low resolution which could be part of the problem.

Amy

[CJ_Butler__DASYLab_Pro]

I think that I have it... the DBK17 is only measuring the encoder information once every block, and then filling the block with the same value.

As a result, the slope within the block is 0. And then it's a  bigger number from the last value of the block to the first value of the next block, and then back to 0.

Possible solutions:

- use separate to reduce the block to 1 sample, with a block size of 1

- use average (block mode) to do the same thing

to increase your resolution, you may also want to reduce your block size, to reduce the stepping effect, but you shouldn't reduce it to less than 1/10 (one tenth) of the sample rate.