Tutorial

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Tutorial#

LC601-5 constant moment beam load cell. Excitation and signal wire pair colors match Fig. 1.

C-Therm manual next to wooden box containing reference samples and glove

Fig. 1 C-Therm manual and reference samples#

The load cell calibration sheet, dated 2019-01-21, excited at 10.000 Vdc. The balance is -0.012 mVdc, sesnitivity 20.023 mVdc, in and out resist are 404.00 Ω and 351.70 Ω. The 59k shunt is 14.77104.00 mVdc. Calibration factors are 2.002 mV/V, 59K shunt, 1.477 mV/V.

With a sensitivity of ~2 mV/V, if the excitation voltage is 10 V, the full-scale load of 5 lb would correspond to a reading of 20 mV on the white/green signal wires. See general equations for details.

The DAQ needs sufficient sensitivity to read the smallest increment you’re interested in. In the wiring for raw bridge measurements guide, the built-in excitation voltage for an x201 gain setting and 1.25 V offset and the excitation voltage supply selection suggests we have a 2.5 V excitation in this configuration.

At 5 lb load, then we should expect a signal of 5.0 mV. Let’s assume we want to distinguish between 0.1 lb difference, this means the DAQ needs to measure 100 μV. Our gain of 201 provided by the instrumentation amplifier means that the 100 μV signal we’re interested in is amplified to 20.1 mV.

From the T7 datasheet, on the +-10V channel, with 16-bit resolution, we get about 1e6*20/(2^16-1) or 300 μV resolution. Since we want to discriminate between 20.1 mV increments, then we have about ~70 digital “steps” corresponding to 0.1 lb difference.

A sensitivity of 2.002 mV/V, multiplied through a gain of 201 is 402.4 mV/V, meaning a 5 lb weight with a 2.5 V excitation would cause a 1.005 V shift in the reading. In other words, the sensitivity is 4.975 lb/V or 2257 g/V or 443 μV/g.

The reading is offset to 1.25 V, so at maximum load, the value should swing from 0.25 V to 2.25 V.

To-do#

  • Consider the U6-Pro which has a 24-bit ADC to the T7’s 16-bit. There’s a software compromise here, but the resolution is better.

  • Run these same calculations for a 0.4 V offset, 201 gain, on the +-1 V range for U6-Pro and T7

  • Also check the flexible IO channelo on the T7, which may provide higher effective resolution

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