STRAIN GAUGE INSTRUMENTATION

To be operational, a force transducer must be linked to a device providing its electrical supply and the measurement of the output signal. Specific instruments are used for these functions, commonly known as conditioners, measurement bridges or digital compensators.

The voltage of the electrical cable between the force sensor and the measuring bridge

• The electrical power of the transducer

  can be a DC voltage generally comprising between 4 and 20 V, or an AC voltage, comprising between 60 Hz for static forces and 1,000 Hz for dynamic forces.

• The transducer's output signal voltage

  depends on the sensitivity of the transducer. A 2 mV/V sensitivity and an excitation voltage of 10 V corresponds to a maximum output voltage of 20 mV. Thus, due precaution should be taken so as to measure such voltages with precision.

• The electrical cable connecting the transducer and the measurement bridge

  is an integral and very important part of the measurement system. In fact, variations in room temperature induce resistance variations on the cable conductors, which appreciably modify the signal to be measured. Therefore, for three-metre lengths and above, it is preferable to use six wire cables. Within these connections, two wires serve as the supply for the transducer, two wires are used to measure the transducer's output voltage, and two additional wires are used to adjust voltage supply to the transducer terminal. The results of the dynamometer calibration depend on the connection cables used. Therefore, it is necessary to perform the calibration with the cables that will be used later with the load cell.

Measurement bridge adjustments

Measurement bridge adjustments are the setting of indication to zero when the transducer is at zero load and adjusting the amplification gain of the output signal of the force transducer :

• The zero setting

  Sets the indication and / or output signal equal to zero when there is no force applied to the sensor.

• Adjustment of the gain

  allows the ratio between the force applied to the transducer and the relevant indication to be set in order to obtain a simple ratio or indications enabling the force to be read directly. This adjustment is generally made during the calibration of a dynamometer. If this adjustment remains accessible to the operator it can cause a serious inconvenience given that, in the absence of a calibration device, any subsequent action on the gain will render the calibration results invalid.

• The calibration device

  is an essential element of accurate measurement bridges. It allows for ensuring the stability and continuity of adjustments to the measurement bridge between two calibrations. It comprises either a very stable resistance connected in parallel with one of the four resistor of the Wheatstone bridge, or a reference voltage source enabling the electrical calibration of the measurement amplifier. The accessibility of this device to the operator allows for the measurement point to be reset at all times to the reference conditions obtained during dynamometer calibration. In particular, it allows for the suppression of the evolution of the metrological characteristics constituting the reading electronics over time.

NOTE : Spanish version /en/tecnologia/instrumentacion-sensores-de-fuerza/puente-de-medicion