Using strain transducers to compensate for bending moments
With strain gauge-based strain transducers, potential bending moments can easily be compensated for when symmetrical components are involved. This requires that two sensors are mounted at the same height, however, opposite from each other. Bending causes one sensor to be loaded with a positive and the other sensor with a negative strain. When the sensors are connected in parallel, the total output signal is "zero". All strains loading both sensors in the same direction are measured without any problems. In press applications, this guarantees that tensile/compressive strains only are measured.
Low height of construction and uniform sensitivity of the sensor are technical prerequisites. For this reason, HBM strain sensors are adjust to a uniform sensitivity using a special procedure. Furthermore, the output resistance is balanced, which is a prerequisite for parallel connection. The bridge resistance of 700 ohms is selected to prevent overloading of the bridge amplifier as a result of the decreasing overall resistance. Even four sensors connected in parallel have a resistance of 175 ohms. Modern amplifiers enable the force or strain signals of both single force or strain transducers and sensors connected in parallel to be measured and processed.
Strain gauges installed on a symmetrical component. Bending causes both SLB700 to be loaded with the same absolute strain value, however, with different signs. This gives no output signal, when they are electrically connected in parallel. Bending is compensated for in the measurement, only tensile or compressive strains are included in the result.
As already mentioned above, strain transducers need to be calibrated to enable a force to be derived.
The calibration procedure requires that force is measured at two load steps at least; for example: force = zero and maximum force. It is important to know that it is not absolutely necessary to measure at maximum force. It is absolutely possible to calibrate the measuring chain at half the process force, in particular, when accuracy requirements are low.
The process is as follows:
- Measurement of the strain transducer's output signal at zero force. The amplifier should be set to prevent scaling, i.e. the output signal is displayed in mV/V.
- Measurement of the output signal at the force to be used for calibration. In this case, as well, measurement is to be in mV/V.
It is now easy to calculate the measuring chain's sensitivity:
S = (output at meas. force - output at zero) / (meas. force - force at zero)
This gives a value in pC/N when a piezoelectric sensor is used. The same applies for SG-based strain transducers, however, with the value being specified in mV/V/N.
In many cases, the sensitivity at a specific strain has become accepted as the scaling value for instruments in SG measurement technology. This only requires the sensor's output signal at nominal (rated) force to be known. You can directly enter the pair of values of force and output signal in your device. Many instruments facilitate these settings requiring only the four above mentioned values that can be allocated in a table:
Force = 0 N Signal = xy mV/V
Force = measuring force Signal = AB mV/V
Amplifiers then perform the setup. Scaling is carried out afterwards. This "Teach-In" process is implemented in modern amplifiers which enable automated calibration.
The following tips are important:
- Verification is always recommended, i.e. the system should be loaded again and the display values compared.
- Load the system three times with maximum load prior to the calibration procedure to ensure settling of the sensors.
- To achieve optimal accuracy it is ideal to measure exactly at the same force at which measurements will be taken later.