Proper DAQ Module for Celesco SP2 or SP3 String Pots

[rowsdower]

Hi! I am currently looking to expand our lab's capabilities by adding string pots to our sensor tool kit. I came across a string pot that will work for us, namely Celesco SP2 or SP3 (either one). Here is the website where more information is listed about these sensors:

http://www.celesco.com/stringpots/

The SP2 has a voltage divider output. According to the Application Engineer, a very clean DC voltage signal should be sent to the SP2 string pot for it to generate accurate measurements while the SP3 will take a 24 V DC signal (less clean signal) and generate accurate measurements making the SP3 more expensive than the SP2. (would prefer to go with the best value option).

Currently we have a cDAQ-9178 chassis. Is there a NI Analog Input module out there that will work with the SP2 or SP3 string pot. The expectation is that this module will be plugged into the cDAQ-chassis and the wires from the string pot will be run to this module.

Would it also be possible to use a USB-6003 with the above string pots? Let me know and thanks for your help! I have also attached the respective data sheets for your reference.

[Blokk]

I would go for the SP3 (request with the voltage signal option, 0-10 VDC). Either the USB-6003 will work, or you can buy a Voltage AI module for your cDAQ, like the NI 9209 ( http://sine.ni.com/nips/cds/view/p/lang/en/nid/213129 ).

How many string pots you want to use? Using the USB-6003 you have 4 differential channels, with the cDAQ module, you get 16 channels.

[rowsdower]

Thank You for the input. NI 9209 is the module that I was looking for. The SP3 string pot has a cable that has three wire leads at the end of it: green, black and red. I saw that the NI 9209 has required accessories (DIN-Rail 37-Pin, horizontal mount, spring terminal) that mounts on the NI 9209 and a shielded female to male cable. Will this DIN-Rail 37-Pin, horizontal mount, spring terminal accept the green, black and red wires? (I am assuming there is a hole where you would insert the screw driver and release the spring to insert the wires). Also is the cable required? What is it used for?

We really only need two string pot as majority of our applications only require two displacement sensors. But it is always good to have the option to have more channels for expansion if needed in the future.

The reason that I listed USB-6003 is because I had found that this device is versatile (please correct if wrong) and could be used for other applications in the future (also being slightly less expensive). I am leaning towards the USB-6003 (and possibly even USB-6002 because 50 kS/s is plenty) right now but is there any reason to possibly go with the NI 9209 (besides having more channels). Thanks!

[Blokk · Accepted solution]

Since the available accuracy of the SP3 device is 1% of full scale (0.1 Volts), I think you can use any of these cheap DAQ devices (USB-6002 or 3). You also need to consider what kind of sampling rate you need. I imagine not too high (maybe 10 or 100 Hz is just enough)? Depends how fast movement you want to check with this spring pot...?

If you use the USB-6002, you get 4 differential channels, and a sampling speed of 50 kHz (but do not forget, if you use for example 2 channels, your max rate will be half: 25 kHz per channel). 16 bits ADC, so you get resolution of 10/2^16 = 0.15 mVolts over the 0-10 Volts range (0.0015% of 10 Volts). This is already an overkill compared to the device's 1% F.S. accuracy.

Cabling: read the 2nd page of your pdf manual. You have three wires. Connect the Red (+24V) and Black (com or GND of the PSU) wires to your PSU (24VDC as you mentioned, use a quality brand like Siemens). You can use a single 24V DC PSU to power both of your sensors (i do not know how much power they draw, but I imagine not much), obviously connect the PSU to the two sensors electrically parallel (so they get the same voltage).

Connect the Green (voltage signal 0-10 Volts) and the Black wires to your DAQ device (use differential pins, check the device spec).

[rowsdower]

Sounds like USB-6002 (or NI Module 9209) does not have the capability to provide power to the string pot. This would have to be done by an external PSU (which is fine because we have a PSU to do this). It would be nice to have an integrated module which will take the input signal from the string pot while at the same time provide power to it. For example, for load cells we use NI 9237 which provides an excitation voltage of 10 V. Is the reason for this because the string pots' power requirements (24V for each string pot) too demanding for an "input signal + power" integrated module?

[Blokk]

There are several reasons, one thing can be the required power. It is better to make hardware components dedicated for one task. Also, I can imagine there is noise consideration (imagine a DAQ module with a built-in higher power PSU). Anyway, a simple good brand 24VDC power supply is cheap and easy to install, or replace if gets broken.

[rowsdower]

Hi Blokk,

Thanks for all the input. You have anwered my question in regards to this stringpot!