Normally, force or displacement measurement is used to monitor pressing procedures. There are two different methods available for force monitoring:
- Direct force measurement in the press tool's force flow
- Force shunt measurement, for example, using strain transducers
In both cases, it is essential that press forces are reliably and quickly recorded by sensors and transmitted to the press control system using adequate amplifier electronics. Sensors and amplifiers need to meet particularly high requirements in terms of ruggedness and operational reliability. Easy mounting and integration of the components with existing machine concepts is a critical factor that needs to be taken into account when choosing test and measurement equipment.
Both force measurement methods have specific advantages and drawbacks. In general, when measuring force directly in the force flow it must be kept in mind that the force transducer is an integral part of the tool and thus significantly impacts on its properties. Furthermore and, in particular, with huge forces, the force transducers need to have sufficiently high nominal (rated) forces. Such sensors are very heavy and, in addition, expensive. It is beneficial that HBM force transducers are calibrated at the factory and therefore need not be calibrated in the system where they are used. The force in N can be directly determined after assembly.
U5 flange-type force transducer. Overhead mounting of the force transducer is possible and, thanks to the flange, it is particularly user-friendly. Nominal (rated) forces up to 500 kN are available. Tensile and compressive forces can be measured.
Strain transducers based on the following operating principle are available as an alternative: Every press uses a structure that is mechanically stressed during the pressing procedure. This causes minimal, invisible deformation. Strain transducers, for example, SLB700A from HBM measure this deformation which, in the end, is proportional to the press force. The advantage of this method is that the same sensor can be used, irrespective of the pressing power. Measurement using strain transducers is a cost-effective alternative, in particular with high-performance presses. The accuracy of force measurement with a calibrated force transducer cannot be achieved using this method, however, HBM strain sensors easily meet the regular accuracy requirements for press applications. Calibration is required in any case to determine the relationship of the output signal and the applied force.