The HyperLogger and ModuLogger from Logic Beach can easily handle the inputs from strain gages utilizing a Wheatstone bridge design. The capability of the Wheatstone bridge to measure small changes of a resistance makes it ideal for measuring the small resistance change in strain gage based sensor, such as a pressure transducer or load cell. Logic Beach data loggers can easily measure the output voltage from a strain gage as well as supply excitation voltage.

A Wheatstone bridge consists of 4 resistors arranged in a diamond configuration. An input DC voltage, or excitation voltage, is applied between the top and bottom of the diamond and the output voltage is measured across the middle. In our example we are using the 5 volt output from the data logger. Higher excitation voltage will result in a higher bridge output voltage and slightly more accurate measurements.

When the output voltage is zero, the bridge is said to be balanced. One leg of the bridge is a resistive transducer (strain gage in a load cell or pressure transducer.) The other legs of the bridge are simply completion resistors with resistance equal to that of the strain gage. As the resistance of the strain gage leg changes, the previously balanced bridge is now unbalanced. This unbalance causes a voltage to appear across the middle of the bridge. This voltage is measured by the data logger and then normalized in the first math icon for any variation in input voltage. The result is mV of output per Volts of input and is then converted to engineering units of strain by the second Math Icon. See programming example for details.

Note: The standard HLIM-1 and MLIM-1 interface modules are suitable to use with strain gages using 5 VDC excitation voltage or less. Exceeding 5 VDC will cause incorrect measurements because the voltage will exceed the common mode voltage for the A/D converter. For higher sensor excittaion voltage, use the Logic Beach MLIM-7 or HLIM-7 isolated analog interface module.

PROGRAMMING DESCRIPTION

The sample rate clock drives the Warm-Up Icon which cycles power to the sensor. This occurs a few seconds before the reading is taken. Each input is then read, measuring the supply voltage, Vexc, and the bridge output voltage, Vsignal. The measurements are fed into a Math icon which normalizes the signal voltage over the supply voltage to accommodate for any variances in the supply voltage. This signal is then converted with another Math Icon to a force based upon the scaling of the strain gage, then stored in memory and sent to a probe point for real-time viewing.

 

EXAMPLE

The following detailed example illustrates the derivation of calculations required to convert bridge output (mV signals) to standard Engineering Units (PSI) for a 10,000 psi bridge transducer.

Manufacturers typically provide the following information detailing the input and output characteristics of a transducer. Specs for the example transducer follow:

Pressure Range - 0 to 10,000 psig F.S. (full scale)
Recommended excitation voltage (Vexc): 5VDC, Vexc
Full Scale Span : 3.3mV /V per volt (of excitation).
Sensor Output: 10mVDC, Vsig (assumed for example)
Alternatively, manufacturers may specify the FS output at the recommended excitation voltage. In this case, the manufacturer might instead specify:
        Full Scale Output: 16.5mVdc (i.e. with a 10Vdc Vexc, the output will be 5Vdc x         3.3mV/Vexc = 16.5mV at FS.

Armed with these transducer specs we can proceed to develop the necessary equations to convert the transducer output (mV) to Engineering Units (PSI)

Normalization of the Measured Signal (Vsig) to mV/Vexc:
Per the Full Scale Span spec for the above transducer, we know that with 1 Vdc of excitation a 3.3mVdc output represents 10,000 psi. The first step then is to ‘normalize’ the transducer’s output (Vsig) sampled by the logger to an equivalent signal per 1 Volt of excitation. To perform this normalization, we divide the transducer output signal by the transducer excitation voltage. The excitation voltage can be measured with a logger channel (as in this example) or if it is known and stable, then that constant voltage can be used.

       Vsig / Vexc = Vnorm (i.e. the measured signal per Volt of excitation (i.e. normalized to a 1 Volt        excitation))

In the HyperWare Math icon shown in the above sample program NET, the Vsig is fed into the X input and the Vexc is fed into the Y inputs so the equation entered into the Math icon is……

Conversion of the Normalized Measured Signal to Pressure
Since we know that the transducer outputs 3.3mV/Vexc for an applied pressure of 10,000 psi, the normalized output from the first Math icon can now be fed into a second Math icon to perform the conversion to pressure.

For the above transducer, the pressure can then be calculated by
        [Vnorm (in mV) / 3.3mV ] x 10,000 psi

Alternatively, a multiplier constant for the transducer could first be calculated then the normalized measured signal multiplied by this constant to get the output in psi as follows:

K = 10,000 psi / 3.3mV = 3030.3 psi/mV

This constant can then be used in the conversion of the normalized input voltage to psi as follows:

Vnorm (mV) x 3030.3 psi/mV = psi

1) normalization of signal and excitation voltage to mV.
Vexc and Vsignal are fed into the first math icon, where Vsignal is divided by Vexc. Vsignal is 10mV.

Math Icon equation: 0.01/5.0 = 0.002

2) Conversion to pressure. The output of the first math icon is then fed into another math icon that multiples this figure by a scaling factor. In this case the scaling factor is 3.3mV/V for 10,000 psi or 10000/0.0033 = 3030303.0

Math Icon equation: 0.002*3030303.0 = 6060psi.

Mathmatically 10mV is about 60% of 16.5mV, and 60% of 10,000 psi is 6000 psi.

All of the math could be accomplished in a single Math Icon with the use of parenthesis but we used two Math Icons for illustration purposes.

^{_______________________________
1}Full Scale Span is the algebraic difference in output signals measured at maximum and minimum pressures. Full Scale Span is ratiometric to the Excitation Voltage.