How to make sure that a certain force is the same everywhere
Force sensors are part of the equipment found in almost every laboratory that deals with mechanical systems. Force transducers are also installed in material testing machines and other test machines to record both the input signal for control, and the force signal that will later be used to evaluate the results.
The two principles of force tranducers
Strain gauge sensors consist of a spring element, on which the strain gauges (SG) are installed. When a force is applied, deformation occurs, which the strain gauges convert to a change in resistance. It is quite common to use four strain gauges wired to make a bridge circuit, which then converts the change in resistance to a measurable electric voltage. An operating voltage (excitation voltage) must always be applied to the strain gauge sensors.
The construction of piezoelectric sensors is based on crystals that give off a charge when influenced by a force. There is a linear relationship between the charge and the force. The charge is converted into a voltage signal by relevant electronics.
Force transducers almost always come supplied with a document containing information about the sensitivity of the sensor. The sensor was exposed to a known force at the factory, and the output signal was measured at this effective force. The result of this ‘calibration’ process is recorded in the relevant document. The output value at maximum force, or several measured values at different forces, are specified here in accordance with the quality and price of the sensor. Calibration is one of the most important steps in the production of a sensor and crucially determines the worth of the force transducer. Of course, later measurement can never be more accurate than the calibration used to establish the characteristic values of the sensor.
Force transducer manufacturers use relevant loading devices to precisely generate this force, and thus create the prerequisite for these calibration measurements. HBM has loading machines available for forces between 10 N and 5 MN.