BRIDGE Command Reference Page

Module SENSORM :: BRIDGE

Convert measurements from a balanced bridge network to determine resistance.

Syntax

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

Parameters

VIN: Input data pipe with bridge imbalance voltage measurements; FLOAT PIPE
VS: Nominal or measured excitation voltage driving bridge network; FLOAT CONSTANT | FLOAT PIPE
BALANCE: Nominal or measured balancing network gain; FLOAT CONSTANT
RLOAD: Nominal or calibrated load resistance, ohms at 0 degrees C; FLOAT CONSTANT
RCOEFF: Temperature coefficient of resistance of load resistor, ohms per degree C; FLOAT CONSTANT
LTMP: Input pipe with load temperature measurements, degrees C; FLOAT PIPE
GAIN: The gain used to measure the VIN signal; FLOAT CONSTANT
ROUT: Output resistance measurements, in ohms; FLOAT PIPE

Description

The BRIDGE command converts the differential voltage readings from VIN to measure an unknown resistance in a bridge network configuration. Current from a known voltage supply drives a load resistance, and then passes through the unknown device to the drain voltage. On the other side of the bridge network, two known resistors form a voltage divider to establish a reference voltage. The differential measurements of voltage between the measurement and reference sides of the bridge are used to calculate the values of the unknown resistance, with results placed into the ROUT pipe, one output value for each input value.

The differential input measurement will reject "common mode" voltages, so it does not matter whether the bridge circuit is excited by balanced plus/minus supplies or a single-sided supply, as long as the common mode voltage is within a measurable range. Bridge configurations are useful when the measured changes in resistance are relatively small and ride on top of a relatively large constant level. The differential voltage can be measured with gain to improve resolution. If a gain other than 1.0 is used, either specify it as the value of the GAIN parameter, or correct for the gain prior to sending the VIN data to the BRIDGE command.

If the supply voltage is very well regulated, it can be specified as a constant VS parameter. For best accuracy, measure the actual supply voltage accurately rather than assuming a nominal value. If the supply is subject to small but relevant variations, measure its voltage simultaneously and pass these measurements in units of volts through a VS pipe, with one supply voltage measurement per divider voltage reading. If you have balanced plus and minus supplies, specify the supply-to-supply voltage.

Ideally, the voltage dividers on the reference and measurement sides of the bridge produce exactly the same voltage at a suitable reference operating point within the normal operating range. Obtaining a perfect balance is hard to achieve and not really necessary. Measure the resistances in the balancing network accurately, then set the BALANCE parameter equal to the ratio

 
   BALANCE  =  Rg / (Rs + Rg)

   where   Rs is the balancing resistor on the positive supply side
   and     Rg is the balancing resistor on the negative supply or
           ground side

If measurement accuracy is not critical, use nominal values of the resistors to determine the value of the BALANCE parameter. Good balancing resistors will not cause excessive power supply loading, and will establish a zero differential reading near the center of the operating range.

Resistors in the balancing network are presumed to be located in a controlled operating environment where small temperature variations affect both balancing resistors the same, hence the ratio remains very stable. If the loading resistor is also located in a controlled environment, it too can be presumed to maintain a consistent operating temperature. Specify an accurately measured value of the RLOAD parameter at the stable operating temperature, in ohms. If accuracy is not critical, use the nominal loading resistor value.

When load temperature is not so well controlled and there is significant thermal variation in the loading resistor, use the optional RCOEFF and LTMP parameters. 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 equals the correct loading resistance at 0 degrees Centigrade. Independently measure the load temperature in units of degrees Centigrade, and send these readings to the BRIDGE command through the LTMP pipe. The BRIDGE command will adjust the value of the loading resistor prior to each conversion.

For each input voltage measurement, the reference voltage on the balancing side of the bridge is equal to the BALANCE ratio times the excitation voltage. The voltage on the active side of the bridge equals this reference voltage plus the measured differential voltage. The voltage between the positive source and the measurement point appears across the known loading resistance, so the measurement-side current can be computed. Using the value of this current, and the known voltage on the measurement side of the bridge, the unknown value of resistance can be calculated.

Examples

  BRIDGE(IP2, 5.0, 0.5, 1000.0, R2)

Voltage measurements are taken for a bridge configuration in which nominal values of components are used. The voltage at the active divider junction, relative to the balancing network junction, is obtained from the differential input sample channel pipe IPipe 2. The excitation voltage is a nominal 5.0 volts. The nominal balancing ratio with equal-value balancing resistors is 0.5. The loading resistor is 1K, equal to the nominal operating value of the measured resistance. Default measurement gain of 1.0 is assumed. The computed resistance values are reported in pipe R2.

    BRIDGE(IP2, 4.959, 0.5025, 1001.8, 0.087, TLOAD, 10.0, R2)

The same as the previous configuration, except that all components are calibrated and the loading resistor value is compensated for temperature variations to obtain maximum measurement accuracy. The supply voltage is measured at 4.959 volts. The balancing resistors are not perfectly matched and their gain ratio is 0.5025. The nominal 1K loading resistance is measured at 0 degrees Centigrade where it has the value 1001.8 ohms. The loading resistor value is observed to increase by 8.7 ohms over a 100 degree temperature swing, so the temperature coefficient is 0.087 ohms per degrees C. Independent measurements of the operating temperature of the load resistance are provided by pipe TLOAD. The measurements use an amplifier gain of 10.0. The results of the resistance calculations are returned in pipe R2.

DAPL Operating System | Processing Command