Basic procedure for a bridge circuit with a U6/T7
We will assume a typical bridge made up of 4x 350 ohm elements, and thus the overall bridge resistance is 350 ohms. Our example bridge will be a load cell with a rated output of 2 mV/V. The bridge has 4 wires. Referring to the Wikipedia diagram, A=Vexc+, C=Vexc-, and D & B are Signal+ & Signal-.
The output of a bridge is directly proportional to the excitation voltage. If we excite our 2 mV/V load cell with 2.5 V, it will produce an output signal of about 5 mV at rated load. If the excitation source has +/-10 mV of noise, our signal will have +/-20 uV of noise.
It is always recommended to use feedback to measure the actual value of the excitation voltage in real-time. The analog inputs on the U6/T7 are better (more accurate and more stable) than all but extremely expensive excitation sources. Make a connection from Vexc+ to an AIN terminal and take a reading from that whenever you take a reading of the bridge signal.
1. Choose an excitation source. Connect the source to Vexc+ and connect GND to Vexc-. Also connect the source to AIN3 for measurement of the actual value in real-time.
VS: This is the 5V power supply from the U6/T7. It is not particularly stable or low-noise, but as long as you use real-time feedback (see paragraph above) it works fine for many applications. Decent option.
DACx: The analog outputs on the U6/T7 are quite stable and low-noise. Each can provide perhaps 20 mA max, but also consider that they have 50 ohms of source impedance so at 20 mA the output will be about 1 V less than specified (real-time feedback required). Set DACx to 4.0V as the power-up default, then power cycle the U6/T7 and use a DMM to confirm that the DAC does power up to 4.0V with no load. U6 - Use "Config Defaults" in LJControlPanel. T7 - Use "Analog Outputs" tab and then "Power-Up Defaults" tab. Note that for good noise performance you want to keep the DAC voltage at least a few tenths of a volt below the power rail (VS). Better option.
LJTick-VRef-41 or Vref from other LJTick: Provides a very stable and low-noise reference capable of substantial current drive. The LJTick-InAmp and LJTick-Divider also provide excellent 2.5 V references that can be used. Because of the accuracy and stability of these references real-time feedback is optional. Best option.
External Vexc: Advantage is that these are usually 10 V, and thus you get more output signal which is generally good. Disadvantages are that a good one is expensive, and it is very difficult to find an external supply at any price that will result in lower noise than use a DAC output or LJTick-Vref. Comparing an external 10V source to the LJTick-Vref-41, if the external source has >2.4 times the noise (versus LabJack GND) of the tick, which is likely, you have not gained anything by using the larger voltage source. If you use an external power supply, you need 1 connection from the common/negative of the supply to GND on the U6/T7, and then connect positive to Vexc+ on each bridge and common/negative to Vexc- on each bridge.
Note that many load cells and strain gages (or gauges) specify a recommended excitation voltage of 10 volts. This might be where the manufacturer calibrated, but the output will scale linearly with any excitation voltage. A maximum excitation voltage is often specified (15 volts is typical), but generally no minimum voltage is specified and there is no reason to. Using normal load cells with lower excitation voltage such as 2.5 or 4.1 volts is fine, and when considering the low noise of our reference sources the 2.5/4.1 excitation usually provides superior performance.
2. Connect Signal+ to AIN0 and Signal- to AIN1. Take a differential measurement of AIN0-AIN1 to acquire the signal voltage.
Put on a known load, and confirm that you get the expected output from the bridge. In our example, the output at 100% rated load is Vexc * 0.002, so if we measure Vexc as 3.5 V and are at 50% load we expect a signal of 0.0035 V.
Range: In our example the load cell has a max output of 5 mV or 8.2 mV for an excitation voltage of 2.5 or 4.1 volts, so we can use the smallest analog input range on the U6/T7 which is +/-0.01 V (Gain = x1000).
Resolution Index: The default of 0 equates to 8 on a U6/T7 and 9 on a U6-Pro/T7-Pro. This will work great, but for the best results possible on a -Pro you would set this to 12.
T7: Use the Analog Inputs tab in Kipling to view the reading. Click the "+" under Options for AIN0, set Range to +/-0.01 V, set Resolution Index as desired, and set Negative Channel to AIN1. Use the "Power-Up Defaults" tab if you want to save these settings so they will work in LJLogM.
U6: Use the test panel in LJControlPanel to view the reading. Set the Range of AIN0 to "BI 0.01V", check the "Diff" box for AIN0 to make it differential (which will be AIN0-AIN1), and set Resolution Index as desired.
2b. Troubleshooting the signal voltage measurement if wrong or too noisy.
Wrong value: Put a known load on the load cell and check the bridge voltages with a DMM. Measure from Vexc+ to Vexc- to confirm the excitation voltage. Measure from Signal+ to Signal- to confirm the signal voltage is as expected.
Right average value but too noisy: Remove your signals from AIN0 & AIN1 and instead jumper both inputs to GND. Look at the noise level and compare to the expected levels for the U6 & T7 (which are the same in this regard). If the noise level of the readings with your actual bridge signals connected are much higher, the most likely culprit is the excitation voltage, but you can usually get rid of most of that later since you will use the real-time Vexc reading (feedback) in your scaling equation.
3. Apply scaling to the voltage readings.
Using the bridge output spec: In our case, the example load cell spec of 2 mV/V tells us that the signal output in volts at 100% rated load will be 0.002 * Vexc. Thus we can say:
Load = RatedLoad * Vsignal / (0.002 * Vexc)
Using a system calibration: A system calibration is the best option because it includes all sources of error. If you can put the system in 2 known conditions, you can get 2 pairs of points and fit a line to get a slope & offset. In our load cell example, 2 likely known conditions would be 0 load and 80-90% rated load (use a reference weight for example). This will give you a slope & offset (y = m * x + b, or Load = Slope * Vsignal + Offset) that is valid at the Vexc during the time of that calibration (Vexccal), so to make it valid with the real-time reading of Vexc you would write it as:
Load = ((Slope * Vsignal) + Offset) * Vexc/Vexccal
Note that both of these equations rely on 2 real-time readings: Vsignal & Vexc. With most of the excitation sources above, simply getting a single reading of Vsignal and then Vexc will provide excellent results, and if using VS for Vexc that will provide decent results. For the absolute best results, you want to get a bunch of readings that are done Vsignal->Vexc->Vsignal->Vexc->... and so on. That way you have a bunch of readings of both that are spread across the same time period and you can average these.
4. Acquiring & logging data with LJLogUD/LJLogM.
On Windows, an easy way to quickly view and log data is with LJLogUD.exe (U3/U6/UE9) or LJLogM.exe (T7).
When LJLog first opens, by default #Channels=4. That means it is using the first 4 rows, which can be set to acquire any channels you want.
The first 2 rows are taking single-ended measurements of AIN0 and AIN1. With a typical bridge circuit, you should see that these both read about 0.5 * Vexc.
In the 3rd row (row 2 or c), set +Ch = 0 and set -Ch = 1. If using LJLogUD, set Range = LJ_rgBIPP01V ("BIPolar Point 01 Volt" is the +/-0.01V range). As of this writing, LJLogM does not do analog configuration, so if you are using a T7 you need to first use Kipling to do the analog configuration as described in step #2 above (setting the small range is important). You should now see your raw bridge voltage in this row.
The 4th row (row 3 or d) is taking a single-ended measurement of AIN3. This is Vexc.
Modify the scaling equation in the 3rd row (row 2 or c) as desired. Assume you are using the 1st style of equation from step #3 above and the rated load of the cell is 200 kg:
kg = 200 * Vsignal / (0.002 * Vexc)
Vsignal is from row c and Vexc is from row d, so you would enter this in LJLog as:
Find more detail on scaling equations here.
To start logging data to file, click "Write To File".
For more software options, see "Software Options" on the support homepage:
U6 Support Homepage
T7 Support Homepage