Force and strain sensors for different press technologies

Clinching is a special joining process for joining different materials such as sheets or profiles made of steel, stainless steel or aluminum. The main feature of this joining technique is that the form-fitting joining elements (joining joints) are formed from the material of the sheets to be joined. No further auxiliary joining parts or filler materials are required.

Since the clinch joints are formed from the materials of the parts to be joined, there is a direct correlation between the mechanical parameters of the joining part materials and the joining part thicknesses and the achievable holding forces. In addition, the holding forces correlate with the clinch point diameter and the realized base thickness.

For quality control of the clinching process, the company uses Eckold GmbH & Co KG uses computer-aided online process monitoring. The process monitoring is based on the so-called envelope-curve principle. In this process, the real mold element is enveloped by an idealized mold element. For this purpose, a measurement curve is generated according to the ideal envelope curve with the measured signals. The software checks whether the envelope conditions or the defined deviations from the envelope curve were observed. Through this evaluation of the recorded signals, it is possible to detect process and machine errors, as well as to monitor and document the entire clinching process.

Indirect force measurement with strain sensors on the machine frame or even machine frame is particularly suitable. There is sufficient space available to mount the sensor. The strain transducers are thus mounted indirectly in the force bypass of the machine structure. The compact dimensions and easy retrofitting outside the working area are important advantages. In addition, the forces that occur can be measured in an overload-proof manner and the dynamic properties of a system are not impaired. Indirect force measurement with strain sensors is thus a reliable method of monitoring and continuously controlling process forces.

Force and form closure of the punch are measured indirectly at the machine. Since the c-shaped yoke back of clinching machines is quite rigid, the sensor must be able to absorb even the smallest strains. The greatest importance is therefore to use a sensor with sufficient sensitivity. A high sensitivity on the sensor side is a crucial criterion to get a sufficient signal and to be able to perform accurate measurements.

For this special application we have developed unamplified strain transducers, which measure smallest strains precisely, have a flat design for space-saving mounting and are suitable for cyclic measurements. The sensor is mounted on a flat structure in the force bypass on a machine part and from there, via an input card, directly connected to the control system. This allows strain to be reliably recorded in any application and converted into a usable output signal.

The X-103 unamplified strain sensors differ primarily in their various sensitivities. Our X-103-S10, with a sensitivity of 1.0 mV/V, is the standard solution for direct and simple connection with an M12 connector. In some applications, however, a higher sensitivity is required. The X-103-S30 with its significantly higher sensitivity of 3 mV/V is designed for this purpose. The high output signal enables accurate resolution, even of small strains on stiff structures such as a clinch bracket. The surface strain sensor picks up the strain between the screw-on surfaces and amplifies it mechanically. The measuring bridge with four strain gauges picks up the strain concentrated at the measuring point and converts it into an electrical signal using a resistive measuring bridge.