correct use of scxi 1126

[FredericR]

Hello,

I have a question concerning module SCXI-1126:

Its datasheet specifies for every ‘output filter bandwidth setting’ the ‘minimum recommended input frequency’

For example: for an ‘output filter bandwidth setting’ of 320 Hz is the recommended input frequency’ 5 kHz.

1)      What is the basis of this restriction?

2)      Does that mean that the shaft speed should be minimal 5000x60/1280 = 234 RPM in case of measuring with an encoder with 1280 pulses per revolution and with an ‘output filter bandwidth setting’ of 320 Hz?

3)      If yes, what is the risk if we measure lower speeds? Does that mean that we can’t measure the speed of oscillating shafts?

Thanks,

Frederic

[ThiCop]

Hello Frederic,

First of all I must say that it has been a long time since I've worked with that specific Module.

I suspect you are browsing through the specifications to see which card you should use in your application.

Can you tell me a bit more about what you need to accomplish in your application?

Maybe there might be more cost-efficient solutions for your project.

Before going to deep into your question I would like to first check if you're familiar with the concept behind these Frequency Input Modules?

For measuring shaft speeds/encoder signals there are probably better solutions.

[FredericR]

Hello Thierry,

Our goal is to combine encoder measurements (speed of rotating and oscillating shafts) synchronously with other measurements (voltage, strain gage) and to do this (if possible) within our current measurement environment. (SCXI-1600, SCXI-1141 and SCXI-1520 in SCXI-1000 chassis)

For this reason we have bought module SCXI-1126 some time ago.

Recently we have done some measurements with this SCXI-1126 module and we have observed strange behaviour of this module.

We have the feeling that we don't understand well the limitations of this module.

In attachment you can find 'fig.png' which shows the measured speed of a rotating shaft.

This shaft was rotating at about 200 RPM.

An encoder with 2048 pulses/revolution was used and the measurement was done with the SCXI-1126 module.

--> red signal = measured speed (in Hz) with the 'output filter setting' set at 40Hz

--> blue signal = measured speed (in Hz) with the 'output filter setting' set at 320Hz

You can see that the blue signal has a high frequency (at 280Hz) component that is not realistic. Can you explain this?

The datasheet specifies for every ‘output filter bandwidth setting’ the ‘minimum recommended input frequency’.

For an ‘output filter bandwidth setting’ of 320 Hz is the minimum recommended input frequency’ 5 kHz which is smaller that the input frequency of this measurement (=2048*200/60=6827 Hz)

It may be that module SCXI-1126 was not the best solution for our objective.

That's why I have posted also another more general question on this forum:

'quadrature encoder measurements with SCXI-1600'

Best regards,

Frederic

[ThiCop]

Hello Frederic,

Can you give some more information about the encoder you're using (eg. quadrature encoder)?

What level of output voltage does it produce? (TTl, 24V,...)

[FredericR]

Hello Thierry,

We are using several encoders:

1) Hengstler RI58 - D

2) SIKO MSK320

3) HBM T12

In attachment you can find their datasheets.

Bst regards,

Frederic

[ThiCop]

Hello Frederic,

My apologies for the late reply.

I have been Out of Office the last couple of weeks (giving courses on-site).

Is this issue still in its original state?

[FredericR]

Hello Thierry,

Yes, this issue is still in its original state

Best regards,

Frederic

[ThiCop]

Hello Frederic,

Thanks for the feedback!

The Hengstler RI58 - D and SIKO MSK320 are incremental encoders.

Single line/channel incremental encoders are supported, absolute encoders not.

If I have understood the document correctly, then these incremental encoders on themselves should be supported.

So as long as they're within the frequency and voltage range, they should work.

Also it could be useful to have a look at the first chapters of this User Manual.

It explains the concepts that are used behind the scenes and also shows where the limitations are coming from:

https://www.ni.com/docs/en-US/bundle/321844c/resource/321844c.pdf

Regarding the HBM T12 (Smart torque):

Which output are you using?

[ThiCop]

Hello frederic,

Were you able to make everything work correctly?

[FredericR]

Hello Thierry,

We still have the feeling that we don't understand well module SCXI-1126.

We are struggling with the limitations of this module.

Let's consider 4 speed measurements to make it more practical:

1) rotating shaft at 'slow speed'

--> mean rotating speed of this shaft = 120 RPM (2Hz)

The speed of this shaft is fluctuating around this mean speed.

We are especially interested in measuring this speed fluctuation (min and max speed within a revolution).

(Assume the frequency component of this fluctuation between 10 and 200 Hz)

2) rotating shaft at 'high speed'

--> mean rotating speed of this shaft = 1200 RPM (20Hz)

The speed of this shaft is fluctuating around this mean speed.

We are especially interested in measuring this speed fluctuation (min and max speed within a revolution).

(Assume the frequency component of this fluctuation between 100 and 2000 Hz)

3) oscillating shaft at 'low oscillating frequency' and 'low speed'

--> oscillating frequency = 2 Hz (120RPM) / max shaft speed = 150 RPM

Assume a simple harmonic oscillation: RPM = 150 x sin(4pi x t)

4) oscillating shaft at 'high oscillating frequency' and 'high speed'

--> oscillating frequency = 20 Hz (1200RPM) / max shaft speed = 1500 RPM

Assume a simple harmonic oscillation: RPM = 1500 x sin(40pi x t)

Can you tell me for each speed measurement whether the speed fluctuation is measurable with module SCXI1126?

And if measurable, what are the recommended settings?

(recommended pulses/revolution of encoder? recommended 'output filter bandwidth setting'? ...)

Best regards,

Frederic