Configuring a reed sensor/switch to monitor RPM

[Ralph@NES]

Hello all

I'm still amidst a project to develop a system to read RPM on a bearing cage via small rare earth magnets embedded in the cage.

I've sourced some sensors that seem to fit the form factor I need, and they are reed style two-wire open/closed switches:

http://www.cherrycorp.com/english/sensors/mp2017.htm

What I need to do is measure RPM (roughly 12000 on the shaft and 4-5k on the cage) in three locations (two cages and one shaft) to detect 'skidding' on the cage when a failure occurs.

Being that these are simple open/closed switches and have no logic whatsoever, can I use these with digital input as-is?

Here's my configuration:  PCI-6221, SCXI-1001 chassis, a few temp modules, a switch module, and an 1180 feedthrough module.  I believe I can access my digital lines via the 1180 and the terminal block.

I'm aware that I can purchase an SCXI-1126 module and do frequency measurements via a TTL sensor or proximity sensor.  I'm unable to use a prox sensor due to temperature, size, and material constraints, however I'm not opposed to purchasing the 1126 if it is needed to make this work.  For that matter, I have approval for a new DAQ card as well, if needed.

Cliffs notes:  Can I use a simple on-off switch direct to my digital IO lines and measure up to 24khz pulses (two pulses per rev)?

If so, do I provide the switch with a voltage or does the digital line just use its own reference signal?

And on top of all of it, what kind of Labview task would be used to process this information?

OR

Do I need the 1126 to approach any of this and will it work with a simple on/off switch rather than a TTL sensor?

Thanks in advance!

Ralph

[Sandra_T]

Hello Ralph,

Please clarify if what I am going to say isn't what you are trying to do.

You are trying to measure RPM, meaning you need to measure time and position of rotation. Correct?

If you would like to make angular position measurements, you typically use counters and an encoder.

Options for Measuring Displacement and Motion

The sensor you are planning to use seems to only switch, open and close. For counter and digital type measurements it is expecting TTL signals (0-5V or 0-3.3V) depending on the hardware you decide to make the acquisition. The sensor will not work with this setup, unless by switching the sensor, it closes the circuit on additional extrenal circuitry creating the TTL signal that the DAQ card needs to make a angular position measurement.

If I have misunderstoon, please correct me. Let me know if something is not clear.

Regards,
  Sandra T.

Applications Engineer | National Instruments

[Ralph@NES]

Would simply introducing a 5v power supply to the switch (with a resistor of course) so that when the switch closes, 5v passes through, but when open, nothing?  Or does the TTL signal actually have a closed but 'zero' voltage value?

I can go with a TTL circuit, that's absolutely fine, just the reed switch seems so simple and applying a voltage would be easy.

Here is the scope of what I'm trying to do so that perhaps you can offer some direction:

One shaft will be spinning at 12,000 RPM.  On each end of that shaft is a bearing in a housing.  In each bearing are several rolling balls and a 'cage' that separates them and prevents the balls from contacting each other.  On that cage will be small magnets recessed into it which will pass a magnetic sensor and activate it twice per revolution (per bearing).  The cage speed should be around 5000 RPM, so it would be a 166hz signal from each bearing and a 200hz signal from the shaft (one sensor, one reading per revolution).  As the physical and temperature constraints are VERY restrictive, a true rotary encoder is impossible on the cage and impractical on the shaft.  It's also a very oily environment so an IR-reflective or interrupted solution is also not realistic.  The only way to get a measurement off the cage is with a magnetic sensor of some type, hence the question about the reed switch.

I will need three 'channels' of RPM reading, and each location must be read and recorded only twice per hour.

As noted, my system has a PCI-6221 DAQ card which has several digital IO lines and two counters.  At the frequency I'm trying to measure, is it realistic to use digital lines or just counters?  Since I only have two counters, the three readings is a problem.  I have a PCI-6602 which is not in the system which has 8 counters, though, and I could re-implement that if need be.

So:

1.  Can I make a TTL signal with a reed switch and a power source (off topic a bit but a big part of what I'm doing)

2.  Can I use a digital line along with my counters and just use the PCI-6221 or should I put in the PCI-6602 which has more counters

3.  Since it's only two readings per location per hour, can I somehow 'share' a counter channel among all three locations (just a thought)

4.  If connecting directly to the digital or counter inputs with a TTL signal, are there any additional pre-connection processing needed other than a pull-up resistor and the correct voltage?

Thanks for your input, it's greatly appreciated!

[AnalogKid2DigitalMan]

Ralph:

Not regarding your DAQ setup, but have you tested to make sure the switches can open and close that fast? I know reeds are pretty quick, but this may be pushing it. They may also exhibit contact bounce (multiple open and close states before settling down) and this will yield erroneous readings. If you have an oscilloscope available, I would suggest that you first verify the reeds will function at full motor speed before attempting anything with your DAQ hardware. You could tie one end to 5V with a pullup resistor and monitor that signal with the scope.

[Ralph@NES]

I've been wondering that myself.  The problem is that until I actually get these in the unit and run it, I won't be certain, and at this point I'm not able to test it at that RPM.  I guess I'll have to go on the manufacturer's spec, which I'll go look up now.

[Sandra_T]

Hello Ralph,

Your application sounds very interesting. I love to hear about customer applications. It is interesting to see how National Instruments' products are used.

With regards to your questions:

1) additional circuitry is a must to allow your switch to be able to generate a TTL/CMOS signal. You need to get a pull up that will ensure that your high is 5V when the switch is closed. You also need a pull down network to pull the signal to gnd when the signal is low, when the switch is open. The network will consist of resistors in addition to the supply and gnd. You can refer to this KB to find out more on what is expected of your TTL.

What Is the Definition of a TTL (Transistor-Transistor Logic) Compatible Signal?

2) Use the 6602 with the additional counters.

3) Additional circuitry is required in addition to the supply, see question 1.

Regards,
  Sandra T.

Applications Engineer  | National Instruments

[Ralph@NES]

Alright Sandra!  That helps a lot.  I'll use the 6602 then.  I have actually decided to go with an actual sensor, a Melexis US5581LUA hall effet magnetic sensor  (http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=413&M=US5881LUA).  It's an open-collector sensor that I should only need a pull-up resistor for, and is nice and tiny and fits where I need it to.

Please, be on the lookout for more interesting questions as I also have to connect flowmeters, load meters, and thermocouples to this system to monitor oil pressure, flow, and temperature during the course of the run.

*note* I have had -0- labview training and -0- signal conditioning training at any time, and this is my fourth or fifth VI in this sort of setup.  I'm blown away by the learning curve on this system and software, and frankly my brain hurts.  I think the boss is going to spring for LV and SC courses come September though.

Thanks again for the input on this, it's greatly appreciated!

[AnalogKid2DigitalMan]

Ralph:

You beat me to it, I was going to suggest Hall effects if the reeds were not fast enough.

Let us know how it goes.

-AK2DM