This error occurs because the NI 9237 is scaling the transducer input before the excitation value is applied. The maximum Signal Input Range is +/- 25mV without a custom scale, regardless of the amount of excitation used. Once a custom scale is applied, the Signal Input Range becomes the upper and lower limits of the scale still excluding the excitation value.
Slope and the Y-intercept define the new values, where the Slope converts from volts to the scaled units and the Y-intercept adjusts for any offset in the scaled units.As mentioned, the NI 9237 reads input in mV (notice the default Pre-Scaled units below are volts). There is not a mV setting; therefore, remember to convert the reading from mV to volts.
Figure 1: Linear scale configuration
Slope can be determined by the maximum value of the transducer divided by the transducer's rated output (R.O).
Ex: 100 lb tension/compression load cell with 2 mV/V R.O., an offset of 5 lbs, where compression displays a negative value.
Note: The linear scale is the only scaling option that enables the use of a negative slope.
slope = 100 lbs /.002 V/V = 50000 lbs/(V/V)
The equation for this situation would be y = -50000 * X + 5
Since the maximum/minimum values for X, the measured value, are +/- .002 V/V, the new Signal Input Range is:maximum = 50000 lbs/(V/V )* .002 V/V + 5 lbs = 105 lbsminimum = 50000 lbs/(V/V) * -.002 V/V + 5 lbs = -95 lbs
Remember that you will no longer have a symmetric Signal Input Range, shown below in red, when using an intercept. Also verify the custom scale is applied and the new Scaled Units are displayed correctly, shown below in blue.
Figure 2: Task configuration with custom scale applied
Map Ranges Scale
This scale is convenient for directly assigning Pre-Scaled Values to their corresponding Scaled Values, where the maximum and minimum Pre-Scaled Values are +/- the transducer's Rated Output (R.O.) in volts.Ex: A 100 lb tension/compression load cell with a 2 mV/V R.O.
Figure 3: Map ranges configuration
Note: Remember to verify a custom scale is selected and the new units appear as expected in the DAQmx task as shown in blue in Figure 2.
A Table Scale can be used similar to a Map Ranges Scale by directly assigning the Scaled Values. However it can also be used to input the calibration values from the Certificate of Calibration provided with the transducer. Ex: The Certificate of Calibration for a load cell lists the load as -100 lbs and the mV/V as -1.95, as well as, 100 lbs and 1.98 mV/V. Select Add and enter - 0.00195 for Pre-Scaled and -100 for Scaled. Repeat for the next set of values with .00198 for Pre-Scaled and 100 for Scaled.
Figure 4: Table scale configuration
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