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Sensor Technology

Building block calibration

Yuval Hernik looks at how precision resistors are being used in the shunt calibration of strain gage sensors, the building blocks for many types of transducers


train gages are fundamental sensing devices that function as the building blocks of many other types of transducers - including pressure, load, and torque sensors - used extensively in structural test and monitoring applications. An example of such a transducer is a load cell that converts a mechanical force to an electrical output signal. In these designs, gages are connected as a Wheatstone bridge, resulting in an accurate and rugged transducer that can operate in extreme environments. To achieve accuracy, the Wheatstone bridge is adjusted for manufacturing initial tolerance and ambient and self-heating temperature effects. "Compensation" high-precision resistors are then added to

correct for bridge unbalance, and to adjust the output sensitivity. Other compensation resistors correct for the errors that result when the transducer is used over a widely changing temperature range. Even though strain gages are very common, acquiring reliable data from them can be a challenge. Several factors can affect the measurement performance of a strain gage: the signal conditioning, the construction and location of the Wheatstone bridge (the most common bridge type used to measure resistance), inductance and capacitance, the precision resistors used in the circuit, and the excitation source. Precision resistors have two basic uses in standard strain gage circuits: shunt

calibration of strain-measuring instruments and bridge completion. In shunt calibration, a fixed high-

precision resistor is temporarily shunted across one of the bridge arms to produce a known and controlled resistance change in the bridge circuit. The resulting instrument indication is then compared to the calculated strain corresponding to the resistance change. In bridge completion, a precision resistor is used in the adjacent arm of the Wheatstone bridge, and two additional precision resistors are used in the ratio arm to complete the external half-Wheatstone bridge circuit when a single strain gage is connected in a quarter-Wheatstone bridge arrangement. In each of these applications, the accuracy of the strain measurement is affected, directly or indirectly, by the accuracy and stability of the precision resistors used in the circuit. That’s why only resistors of the highest precision and stability should be used; the choice is usually between Bulk Metal Foil resistors featuring Z or Z1 Technology. When choosing a precision resistor for shunt calibration, the following features are highly desirable:

• Fast field calibration of the pressure transducer and load cell • Ease of use • Low temperature coefficient of


• Low thermal EMF and fast thermal stabilization

• High stability Yuval Hernik Shunt calibration Figure 1. A basic Wheatstone bridge circuit diagram 40 December 2013/January 2014 Components in Electronics

Shunt calibration of a Wheatstone bridge strain gage circuit is a common and convenient method of periodically monitoring the gain or span of a signal conditioner being used in conjunction with a strain-gage-based transducer. A fixed precision resistor is placed, or “shunted,” across one leg of the Wheatstone bridge. This doesn’t amount to a complete calibration, since no mechanical pressure is actually applied. Instead, the shunt calibration provides a simulation of the mechanical input to a transducer by unbalancing the bridge and providing a scenario that shows how to reduce the errors and shifts associated with the electrical characteristics of the strain gages and the connected electrical

Shunt calibration is accepted

throughout the industry as a means of constant calibration of a signal conditioner and transducer between calibrations of known, applied, traceable, mechanical input values. It’s important to remember that the shunt resistor can simulate either a tension or compression input in the Wheatstone bridge. Thermal EMF and TCR errors can affect the process and should be minimised by choosing a proper resistor. The shunt calibration can be applied conveniently and at any moment, and most importantly, during the test programs. Consequently, strain gage and transducer manufacturers supply shunt calibration data with a shunt calibration

components. The shunt resistor that is added in parallel to the strain gages simulates what would happen if a real load were measured by the pressure transducer or any other load cell configuration. This approach works best when done using a high-precision resistor with a tight tolerance, a known resistance, low sensitivity to temperature (especially power TCR), and low thermal EMF. The output in millivolts (mV) can be compared to what would be expected should actual pressure be applied. Then the difference in output signal can be compensated within the monitoring instrumentation.

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