If you’re designing a data acquisition system with Wheatstone bridge-based sensors to be measured, the easiest (if not cheapest) solution comes from National Instruments in the form of the NI 9237 CompactDAQ module. Boston University’s Rocket Team (http://www.bu.edu/rocket) will be using this guy in upcoming tests this academic year to make sure our static motor data is as precise and repeatable as possible.Hooking this system up couldn’t be easier, there are just a few connections to deal with to plug in your standard 4-wire Wheatstone bridge sensor. All the information is in the manual for the plug-in, available at http://www.ni.com/pdf/manuals/374186e.pdf (and the critical bits on the side of the unit itself), but let’s boil it down.
Let’s look at the wiring diagram (this image is actually from the NI USB 9237 system manual – same plug-in, but that kit includes a one-slot USB chassis). You notice the normal Excitation + and – lines, and AL+ and – for direct sensor output reading. What you might find a little more confusing are RS+/- , SC, and T+/-.
RS± are “Remote Sense,” used for Kelvin sensing of the excitation voltage. By using RS± for feedback instead of just applying the desired excitation voltage to EX±, the resistance of the EX± wires is cancelled, increasing accuracy. Normall, the intrinsic resistance of the (potentially very long) EX± leads would effectively decrease the ACTUAL excitation voltage seen by the bridge, thereby decreasing the effective sensitivity (mV output at AL± per volt excitation on EX±) of the sensor. Wiring up RS± as shown (with the ends of the leads tied in to EX± as close to the sensor as possible) negates this issue. Normally, this wouldn’t be a huge problem since lead resistance is small compared to bridge resistance, but with 24-bits accuracy, it will definitely cause the 9237 reading to be more than a few LSBs off.
SC stands for “Shunt Calibration.” For full-bridge applications, hooking these up isn’t extremely useful, but especially for quarter-bridge applications (where only the bottom left resistor changes value with sensor strain), it can be extremely useful. A known resistance is placed in parallel with this resistor, simulating a strain and therefore allowing real-time calibration of the sensor. Further reading is in the manual above.
T+ and T- are for hooking up hardware TEDS, which stands for Transducer Electronic Data Sheet. This standard includes an electronic digital store for calibration values and sensor properties so that when the sensor is plugged into the system, things like calibrated sensitivity, bridge configuration, and element resistance are automatically known to the DAQ system. BURT won’t be using this, since our transducers are normal, raw bridges bought from Omega. More info on TEDS can be found here.
Hookup: The 9237 uses standard, commercially available RJ-50 connectors, so you don’t NEED to buy anything else other than these connectors and a crimper tool to make the system work with your current transducers. Unfortunately, RJ-50 is NOT RJ-11 or RJ-45, and you can’t use these connectors with the 9237 without risking physical damage to the unit – the plastic edges of these connectors will bend the wire contacts in the connectors of the 9237, preventing those pins from working with any future, properly-wired RJ-50 sensors. Again, taken from the manual above, the pinout for the RJ-50 connectors used on the module is pictured at left. Note: RJ-45 connectors are physically able to be used, but WILL FOREVER BREAK the shunt calibration functionality of the module.
If you don’t want to crimp your own RJ-50s, you can also buy RJ-50 cables and RJ-50 female to screw terminal adapters from NI to make your life easy. They’re expensive, though. They can be found here, along with some more info on wiring options.
Finally, a note on excitation voltages. The 9237 can provide excitation at 2.5, 3.3, 5, and 10V to attached bridges (though all four channels share the same excitation voltage). However, it’s only able to supply 150mW, meaning some transducer/excitation combinations are unavailable (and the 9237 will automatically disable them as options). For example, the unit will power four 350Ω full bridges (as we will be using) at no more than 5V, or 4 120Ω half-bridges at 2.5V only. If these limitations are, for some reason, unacceptable, an external excitation voltage can be applied to the four pin connector on the unit.
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