DIVIDER Command Reference Page

Module SENSORM :: DIVIDER

Convert measurements from a voltage divider network to determine resistance.

Syntax

DIVIDER( VIN, VS, RLOAD, [RCOEFF, LTMP,] [GAIN,] ROUT )

Parameters

VIN: Input measurements of divider voltage; FLOAT PIPE
VS: Nominal or measured excitation voltage driving divider network; FLOAT CONSTANT | FLOAT PIPE
RLOAD: Nominal or calibrated load resistance, ohms at 0 degrees C; FLOAT CONSTANT
RCOEFF: Temperature coefficient of resistance for load resistor; FLOAT CONSTANT
LTMP: Input pipe with load temperature measurements, degrees C; FLOAT PIPE
GAIN: Gain used to measure input signal VIN; FLOAT CONSTANT
ROUT: Output resistance measurements, in ohms; FLOAT PIPE

Description

The DIVIDER command converts voltage readings to measure an unknown resistance. The unknown device to be measured is placed in a voltage divider network with an accurately-known loading resistor. The divider network is driven by an accurately-regulated voltage source, and the voltages VIN are observed at the junction between the unknown element and load resistor. The results, in ohms, are returned through the ROUT pipe, one result per divider measurement input value.

The input divider voltage measurements are received through the VIN pipe. These measurements have units of volts. By default, this command assumes that the voltage measurement are obtained using an amplifier gain of 1.0, but if you use a higher gain, specify it as the GAIN parameter.

If the supply voltage is very well regulated, you can measure it once and specify the value as a constant VS parameter. If you use the regulated voltage from the data acquisition processor for this excitation, you could specify 5.0 volts and omit the calibration measurement. If the excitation is subject to small but potentially relevant variations, make simultaneous voltage measurements of the supply voltage, in units of volts, and provide this second input stream to the DIVIDER command through the VS pipe, one supply voltage measurement per divider voltage reading.

Specify the loading resistor value RLOAD parameter in units of ohms. Sometimes a nominal resistor value is close enough, but for better results enter an accurately- measured value.

Most of the time, variations in operating temperature are insignificant, and the load resistance can be presumed constant. For these cases, omit the RCOEFF and LTMP parameters. However, if significant temperature variations are anticipated, the DIVIDER command can adjust the effective load resistance to compensate for temperature-dependent changes. Set the RCOEFF parameter to the temperature coefficient of load resistance in ohms per degree Centigrade. Adjust the RLOAD parameter if necessary so that it reports the correct loading resistance at 0 degrees Centigrade. Measure the load resistance temperature in units of degrees Centigrade and send these measurements to the DIVIDER command through the LTMP pipe, one temperature reading per divider measurement.

For processing each input value, the difference between the known excitation voltage source and the measured divider junction appears across the known loading resistor, hence the divider current can be computed. This known divider current passes through the unknown element to produce the measured voltage, so its resistance can be computed, producing the results placed into the ROUT pipe.

Examples

  DIVIDER(IP2, 5.000, 10000.0, ROUT)

Read the voltage at the junction between a load resistance of exactly 10K ohms and an unknown resistive sensor when a 5.000 volt excitation voltage is applied across the divider network. The voltages across the resistive sensor element are measured with gain 1 and received from input sample channel pipe IPipe 2. The computed resistance values are reported in pipe ROUT.

  DIVIDER(IP2, 4.968, 10008.0, 0.42, PAmb, 4.0, ROUT)

This example is the same as the previous configuration, except calibrated for maximum measurement accuracy. The pre-measured supply voltage level is 4.958 ohms is specified as a constant. The measured load resistor value is 10008 ohms. The nominal load resistance is observed to increase by 42 ohms over a 100 degree temperature swing, so a temperature coefficient 0.42 ohms per degrees C is specified. The measurements of divider voltage are captured using an amplifier gain of 4.0. The operating temperature is separately measured and provided in the PAmb pipe. The computations use a temperature-adjusted value of load resistance for each conversion.

DAPL Operating System | Processing Command