RPM module? 1126?

[Ralph@NES]

Hello All!

Here is what I'm trying to do... 

My current configuration is PCI-6221 into a SCXI-1001 Chassis with a 1349 Cable Adapter.  The only modules I have in this chassis right now are a 1102/b/c with a 1303 terminal block and a 1180 feedthrough with a 1302 terminal block.

I need to obtain a signal from a magnetic sensor of some variety (reluctance I'm assuming) and convert it to an RPM measurement. 

What we are doing is testing failure of bearings at 12,000 RPM.  My requirement is to take RPM measurements from the bearing CAGE (the metal part inside the bearing that prevents the balls from impacting each other) and accurately enough that when the cage fractures and the inscribed 'diameter' grows, that I can measure the RPM drop and shut down the system.

This bearing will be in oil and as such, we have no optical way of doing this.  At the moment I'm pretty sure a magnetic solution is the only that will work, and my questions are as follows:

1.  Can someone recommend a magnetic sensor that will be friendly to connect to my SCXI system?

2.  Will I be able to connect these sensors via the 1302/1180 to my PCI DAQ or will I need another terminal block and SCXI Module (1126)?

3.  Any proven or recommended setups for doing precise RPM monitoring via SCXI?

Any input is greatly appreciated, I need to have equipment purchased and on it's way in a few days.

[Luke_G]

Hi Ralph,

I’d like to get a little more information from you regarding your project.  

1. What material is the cage made of?
2. What material is the outer casing of the bearing made of?
3. Do you need to measure the RPM directly from the cage, or could you measure the rotational speed of the shaft that is driving the bearing?  
4. I’m not clear on what the criteria are for shutting down the system.  When the cage breaks, are you looking for a change in reluctance to trigger the shut down, or are you looking for a change in rotational speed?  

The SCXI system that you are using basically just passes the signal to your DAQ card.  This configuration would be able to work with any sensor that outputs a clean voltage signal of sufficient magnitude. (The PCI-6221 has a programmable input range that can be configured as low as +/- 0.2V, so as long as the signal is at least on that order of magnitude it would not need to be magnified.)  If your signal is particularly noisy, you may need an SCXI module for filtering.  If you could find a suitable proximity switch (a common form of magnetic sensor that outputs a digital signal) then no conditioning would be necessary.  You would be able to use one of your counters to calculate the frequency of the digital pulses.

Thanks,

[Ralph@NES]

Hey Luke!

a-b.  The material of the cage, bearing, and housing are all variants of steel (hardened 52100 and 4340).

c.  The customer requires cage speed be read as well as shaft speed.  Figured I'd get the tough one out of the way (cage) then address the shaft!  The cage is fairly smooth and uniform on the faces so a proximity switch is out...  That's a bummer!

d.  When the cage fractures it will induce a bit of a drag on the rollers and slow down an amount--this is how we plan to know there is a failure.  The inner ring and shaft will still be spinning at 12k and the rollers/cage will 'drag' and slow somewhat from the speed they had been running (haven't calc'd what they should be turning yet).  At the point of 'drag' is when heat and friction build up and can mar/gall/discolor the components that we want to inspect for the failure we're testing for--cage fracture.  Hard to tell the customer what happened when you get a pile of magma out of your test rig!

So that is our scope; measure RPM at shaft and cage, and be able to shut system down as soon as cage/roller drag is perceptible.

Most of the sensors I've seen run in the TTL/CMOS output style, which seems to work well with the 1126, and have voltages from 0-24 which work well with the 1327.

Started messing with the VI: http://forums.ni.com/ni/board/message?board.id=170&message.id=245168 .  Simulated hardware is weird.

Initially I had hoped to just use throughfeed on my 1180 and use counters/inputs on my card, but it's become apparent that I need more channels (8) and the two counters on my card just won't cut it, unless I'm missing something.

Once again, any input is greatly appreciated.

Ralph

[Matt_A]

Hello Ralph,

I am not too well versed in the realm of magnetic sensors, so I can't really comment on that question. But I would like to make several notes regarding your overall application.

First of all, the PCI-6221 has two 32-bit counters. If you can find a sensor that converts your measurement into a TTL digital pulse train, you could use the counters on the PCI-6221 to determine a frequency of this pulse train. By configuring a Frequency measurement task with the Measurement Method configured to 1 Counter (Low Frequency), you are able to utilize each counter to determine the frequency of the digital pulse train. Using this method, you should be able to measure both the shaft frequency and the cage frequency using the PCI-6221.

Next, I just wanted to note that if you are attempting to use the PCI-6221 to control an SCXI chassis, you will not have access to all 8 digital lines on the PCI-6221. You said in your post:

---

Ralph@NES wrote:

 ...

Initially I had hoped to just use throughfeed on my 1180 and use counters/inputs on my card, but it's become apparent that I need more channels (8) and the two counters on my card just won't cut it, unless I'm missing something.

...

---

As stated in the M Series User Manual here:

"When using your M Series device to control an SCXI chassis, DIO lines 0, 1, 2, and 4 are used as communication lines..."

This means that you will only actually have access to lines 3, 5, 6 and 7 of port 0 on the PCI-6221. If you need more than 4 digital lines, you may need to consider another configuration or device.

[Ralph@NES]

Thanks for your input Matt.  Other discussion on here has brought me to the point where I'm aware I'm going to need an 1126 module and a 1327 terminal block to pull this off.

Going up to eight channels was what did it 🙂

Thanks again,

Ralph