Reading Renishaw Linear encoder Atom Dx series (40nm resolution) through NI DAQ 6361

[Venkykkb]

I'm using Atom Dx series Renishaw Linear encoder which has 40 nm resolution, when I connect signal A & B line to the NI DAQ 6361 counter from NI MAX it is picking all unwanted pulses on both lines and counting as the motor moving few million steps (Pulses are unstable). Then I probed the signal A&B using oscilloscope has similar signal but in broader scope it appears Signal A & B line has some noise before steady voltage level. Attached the scope image which i intend to eliminate the circled noise through LabVIEW or NI MAX and read the quadrature input.

Note: Encoder is directly connected to the differential line receiver with 12ohm termination resistor and 220p then A&B signal was probed/Connected to DAQ.

Appreciate your help!

[santo_13]

It looks like you have more than 200mV noise from the linear encoder going into the Transceiver as the AM26LS32A has a 200mV sensitivity for its Schmidt trigger.

Did you try probing the output of the linear encoder? how is the encoder powered? 

[Kevin_Price]

Is it possible that what you're seeing isn't electrical noise but instead is something real, such as mechanical vibration?

Notice how the A & B signals show those extra transitions at different places than one another?   To me, the signal looks plausibly accurate.  Your overall motion appears to be quite slow and your very high resolution could allow tiny vibration amplitudes in the single digits of nm to cause such dithering at each quadrature state change.  

The nice thing about using quadrature is that such one-channel-at-a-time dithering won't accumulate error in your position measurement.

-Kevin P

[Venkykkb]

I don't think that AM26LS32A sensitivity affects on reading because this circuit is part of the PCBA and the signal is connected to the TMS320F series microcontroller EQEP GPIO, this controller eliminates all these noises and counts the pulses correctly. If the controller reads the signal correctly through its inbuilt EQEP function, question triggers why not on the DAQ using LabVIEW .

[Venkykkb]

Yes, that might be possible due to a high resolution encoder it is picking up any small vibration. Perhaps most of the time observed the encoder fluctuates on one signal line either A or B when motor is not moving at all but there is a fan running within the instrument which is closure to the encoder module (approx. 5-8inch distance). 

Does it possible to eliminate those fluctuations in LabVIEW which is similar to the EQEP feature in the microcontroller. This study would be a great help to characterize the encoder module to determine the least resolution that is needed for the application. 

[Kevin_Price]

Sorry, I see that I breezed by your description that you were getting millions of counts during the "dithering".  But you mentioned doing that in MAX.  I'm not at a LV station now to check, but my recollection is that you can only do plain old edge counting in MAX, not quadrature-based position measurement.

If so, then you needn't worry excessively about the millions of counts.  When you do a real position measurement using the device's quadrature decode capability, all that dithering will just give you millions of (+1 count, -1 count) pairs which have 0 net effect.

Try using one of LabVIEW's shipping examples for quadrature-based position measurement instead of MAX.

-Kevin P