PCI 4472 and PCI 6230 Synchronization questions

[Zhengxin]

Hi, all

I have a PCI 4472 DSC board and a PCI 6230 Multifunction DAQ. There is RTSI cable connecting these two for synchronization. I am using Labview 8.6 and Sound and Vibration Measurement Suite 7.0 and try to build a data acquisition system for a rotor system.

I want to utilize the digital input and counters in PCI-6230 to build a digital tachometer so that I can avoid using one of the analog inputs as the analog tachometer, which will save me a analog channel on 4472. I have searched the previous posts and found some discussions about the synchronization of 4474 and 6601 in PCI platform.

http://forums.ni.com/ni/board/message?board.id=100&thread.id=966

These discussions helped me some. They said unlike PXI platform, in PCI plat form it can not route the Sample Clock Timebase through RTSI (ie, 4472 can only route Timebase to RTSI8 but 6230 has no access to it ). There is one program that makes AI uses its own timebase while the TIO uses the 20MHz timebase. And then, use the AI start trigger to start the TIO. This method is not strictly synchronized but since 20MHz is very high, the error can be ignored.

When considering my case, I have some questions

1. Is there any other route I can pull out the SampleClock Timebase of the DSA as the input timebase of the counter?

2. There are analog inputs on the 6230 board, what is the difference of these analog inputs from those on 4472 or other DSA devises? I have checked the specification of those two. I found the ADC is different, 24 bit for 4472 DSA while 16 bit for 6230 AI. Other difference, bandwidth? Higher Sampling rate? What is the advantage of the DSA analog channel?

Thanks 

Zhengxin

[preston_johnson]

take a look at these:

the first is an example for synchronizing the MIO with the DSA.  The MIO AI channels do not have anti-aliasing filters that are necessary for measuring vibration.  However, the analog inputs are good for measuring the tachometer or the encoder.

http://zone.ni.com/devzone/cda/epd/p/id/3541

If you want to use the counter timer, you will have to make the tachometer signal to be TTL.  If the tachometer is a proximity probe, this can be a problem as the trigger point on the analog signal will change as the shaft rises and lowers in its bearing housing.  However, you could use the example here to generate a time base for the counter.

http://zone.ni.com/devzone/cda/epd/p/id/3547

hope this helps.

[Zhengxin]

Preston,

Thanks for your replying.

I can use one of the analog channel to make a analog tachometer. The objective is to make the program supports digital tachometer.

I have been following the example “Even Angle Reference Signal Processing (Digital Tacho, DAQmx).vi”. It is developed for PXI Platform that. It outputs the Sample Clock Timebase from the DSA to PXI_Star and also uses the PXI_star as the Counter Timebase. The pulses generated by the rotor per revolution will act as the gate of the counter while the PXI_Star transfers the DSA SampleClock Timebase to the source of the counter. At each gate pulse, the counter will output the number of the edges that has been passed. By knowing the frequency of the timebase, the rotor RPM can be calculated. I think that is how the digital tacho works.

When using PCI, there is no such route like PXI_Star can transmit the timebase but the RTSI cable. 4472 can only output its timebase to RTSI8, but 6230 can not access to it.

1.Is there any other route in PCI platform I can synchronize these two by sharing the same timebase?

2.  How big the error will be if I use different timebase? such as use 20MHz for the counter. Will it affect the test result significantly? I will use the speed profile to do other analysis.

Thank you.

[preston_johnson]

you are correct in noting how the digital tacho VI works. 

I do not think you will get much error from using the 20Mhz time base as compared to the DSA oversample clock timebase, so give it a try. 

Let us know here how this works.

[Zhengxin]

Hi, Preston

I have been trying to make the digital tachometer working. It turns out that I use a 20MHz Time base for the counter time base is working. I can get the correct RPM, but there is a phenomenon that I can not under and I am sure it must be wrong.

I used the speed profile that generated from the digital tach vi to get the even angle signal and then calculated the magnitude and phase of the first order. I am using a function generator to give a sinusoidal waveform as the vibration signal, meanwhile a synchronous TTL out put as digital tach input. The result shows the Magnitude is right, while phase lag is keep increasing. I changed the digital tach to analog tach which is simply input the TTL synchronous signal to a analog channel. Every thing works fine and the phase lag kept at a constant as expected.

In my opinion, the digital tach method seem accumulating some kind of error that makes the phase lag increasing.  I attached the digital tach program, hopefully some one can help me out..

Platform: PCI

Devices: PCI 4472 DSC board,  PCI 6230 Multifunction DAQ.

Software: Labview 8.6, Sound&Vibration Measurement Suite 7.0

[Doug_L._Bear]

I am assuming the analog input in coming in on the 4472 whereas the digital tach is on the 6230.  This sounds like it could be an issue with synchronization.  If the two devices aren't synchronized perfectly its very plausible that the phase delay would keep growing over time.  This issue does not appear with the analog because it is on the same card.

[Zhengxin]

Hi, Doug

Yes, I am using one of the counter on 6230 for the digital tach. The LabVIEW examples are mostly focus on PXI platform and 4472 can route its sampling clock though PXI_star. However, I am using PCI platform and those two cards are connected by RTSI cable. The sampling clock of 4472 can only route its clock to RTSI8 while 6230 does not have access to it. Is there any way I can work around to avoid this problem?

Thanks

[Doug_L._Bear]

Even though it does not list the sample clock as connectable to RTSI 8 it still is, the reason it does not show up is because it is reserved for internal routing by the driver.  If you set the source of the SampleClock Timebase as /4472/SampleClockTimebase it should autoroute for you.

[Zhengxin]

Hi Doug，

I route the SampleClockTimebase of 4472 to the CI Edge Term which is the source of the counter, the error occurs as shown in the attachments. It doesn't auto route. I also included the device routes which shows the counter source has no access to RTSI8 while 4472 SampleClockTimebase can only route to RTSI8. I got confused, any suggestions? Thanks

Zhengxin

[Doug_L._Bear]

You are correct, I tried the same thing over here and we were getting the same error.  The options we have are to try using the Sample Clock instead of the Sample Clock Timebase (which should work if your sampling rates are evenly divisible by each other) or you can handwire your RTSI bus to connect RTSI 8 from the DSA to another pin accessible by the counter.

If you could post your code including the DSA and counter tasks, we would like to try and see if we can come up with another solution for you as well.