...no matter who measures it!
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.
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 measurements 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.
Force sensors that meet such high standards are based on strain gauge technology. There are two reasons for this. Strain gauge sensors use a Wheatstone bridge circuit. Skillful installation of the strain gauges ensures that a great many parasitic effects, such as temperature, initiated bending moment or lateral forces, are largely compensated. Strain gauge sensors are also absolutely ideal for the static measurements common in calibration, as they have no drift, and demonstrate long-term stability. The characteristics of these sensors are evaluated according to the international standard ISO 376. In this standard, the ability of the sensors to repeat measurements is an important test point. You also find the limits of creep, hysteresis, and also the deviation of sensors from their specified characteristic curve. For ease of orientation, the force transducers are divided into accuracy classes in accordance with ISO 376. Class 00 has the most stringent requirements. This establishes that the mounting and dismounting repeatability (“in varying mounting positions”) may only be a maximum of 0.05% of the measured value. Therefore even class 00 sensors are not adequate for connecting accurate loading machines to a national measurement standard. HBM transfer measurement devices. Depending on their type, the sensors demonstrate repeatability in varying mounting positions between 0.002% and 0.05%. These transfer measurement devices can be used to connect forces between 2.5 N and 5 MN to a measurement standard.
This is why so-called “top-class” sensors are made available. The precision achieved by these sensors is not readily achieved by deliberate production. It is rather that manufacturers check their ongoing production to find suitable examples. These then undergo extensive testing, until their special aptitude can be guaranteed. At first glance, the strict requirements perhaps seem excessive, but you have to consider that precision is lost with every connecting measurement. The machines of an accredited calibration laboratory are initially connected to the national measurement standard. On these loading machines, transfer sensors are then used to connect the machines again, at the next level. For the accuracy of calibrations performed in the field to be satisfactory, you have to make sure that each connection takes place with the maximum possible accuracy. There is a noticeable general trend towards more stringent accuracy requirements for force sensors in production and testing. This trend will ultimately lead to increasing demands being placed on the calibration of force transducers in future.