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Difference between torsion vibration measurement between Analog/Digital Tacho Mode

Creator
Creator

What is difference between torsion vibration measurement by Analog Tacho Mode (Sampling: 24-bit ADC sampling at 204.8kHz) and Digital Tacho Mode (Sampling:820MHz clock).

Explain difference/theory for rpm measurement using 24-bit ADC sampling at 204.8kHz and 820MHz clock by RV4 fronend?

What is importance of 820MHz clock sampling while measuring rpm by RV4 Frontend?

 

 

3 REPLIES

Re: Difference between torsion vibration measurement between Analog/Digital Tacho Mode

Siemens Phenom Siemens Phenom
Siemens Phenom

Hello, here is some information on the analog and digital modes:

 

SCM + RV4 is the preferred solution

16384 pulses per revolution can be measured.

200 kHz in digital mode / 40 kHz in Analog mode. For example:   732 RPM maximum with 16384 ppr.

No minimum rpm for offline processing.

For online processing: the Minimum RPM in Tracking Setup must be adapted.

 

All RV4 modules are delivered with 4 Tacho Inputs and 2 IE inputs. XSI is delivered with two tacho inputs by default. It can optionally be delivered with one IE input as well. When one IE input is used, the two Tacho inputs next to it are disabled. IE cannot be used simultaneously with the 2 tacho inputs next to it.

 

XSI and RV4 Tacho inputs have the same performance. They work in the same way. The only difference is linked to the digital mode of the simple tacho input.

 

  • Tacho Inputs – Analog mode ->  Exact same on XSI and RV4
  • IE inputs ->  Exact same on XSI and RV4 (RS422/485)
  • Tacho Inputs – Digital mode ->  Same performance on XSI and RV4 but the conditioning is different.
    • On RV4 we expect a TTL signal (0 to 5 Volts between signal and ground – 2 wires).
    • On XSI we expect a differential TTL (RS422/485 – 0 to 5 Volts between positive signal and ground, 0 to -5 Volts between negative signal and ground – 3 wires).

 

Digital Tacho on RV4

RV4.gif

Digital Tacho on XSI

 XSI.jpg

 

Accuracy of Digital Mode (Applies to XSI, RV4, IE)

Clock running at 820MHz ->  Accuracy of 1.2ns

For Benchmark made by Cummins, see Steve Seidlitz, Robert J. Kuether and Matthew S. Allen, "Comparison of Noise Floors of Various Torsional Vibration Sensors", presented at the 30th International Modal Analysis Conference (IMAC XXX), Jacksonville, Florida, 2012. 

 

The hardware is capable to detect up to 1000000 pulses per second (1MHz) and transfer up to 200000 pulses per second to the host (200kHz). In practice it limits the real pulse rate to 200kHz, but in case a high pulse per rotation coder is used for high speed measurement, the hardware can skip some of the pulses (pulses to skip parameter in Test.Lab) to stay within the 200kHz. High amount of pulses per rotation are only required for low speed measurement. With the skipping pulse mechanism, the same coder can be used for low and high rotational speed.

 

Accuracy of Analog Mode (Applies to XSI, RV4, QTV, PDTII)

By comparison to the digital mode we could estimate an accuracy of ~10ns. This mode uses an advanced interpolation algorithm from data sampled at 204.8 kHz (XSI, RV4, PDTII). The QTV aligns its sampling frequency (51.2 kHz, 102.4 kHz or 204.8 kHz) of the system ADC bandwidth. 5 samples per pulses are required for a good estimation giving a maximum rate of 40000 pulses per second.

 

I hope that answers your question.

Re: Difference between torsion vibration measurement between Analog/Digital Tacho Mode

Creator
Creator

Suppose I have to measure 4500 rpm using 8000 PPR Encoder for particular application.

Which is more accurate to use Analog/Digital Tacho Mode and Compatible Hardware (RV4)

What is maximum limit for PPR encoder that is compatible to Analog mode and Digital mode for measuring rpm.

Re: Difference between torsion vibration measurement between Analog/Digital Tacho Mode

Siemens Phenom Siemens Phenom
Siemens Phenom

Hello, the math is fairly straight forward as the other example I provided shows. In your case,  neither analog or digital could  be  used unless you use the pulses to skip feature to get below either 200,000 (digital) or 40,000 (analog) Hz.

 

(4500 rpm / 60 ) = 75 Hz x 8000 pulses / rev = 600, 000 Hz.