Force and strain sensors for different press technologies

such as clinching, punching, self-pierce riveting and forming of sheets and profiles

The measurement of strain in the clinching process

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.

The measurement of strain in self-pierce riveting

An alternative option to unamplified strain sensors are amplified strain sensors with an integrated measurement amplifier. One advantage is that the recorded strain is transmitted to the controller as a robust ± 10 V or 4-20 mA output signal. Independent of the transmission length of the cable. Furthermore, there is the option to teach-in the zero point in the sensor itself, depending on the application.

When the X-106 is mounted on the machine frame, it records the expansion or the relative change in length of the support structure. The forces that occur within the application or during the process are decisive for the choice of sensor. The force ranges differ from machine to machine and therefore a suitable sensor is required in each case, which is pre-calibrated to the required measuring ranges. If the measuring range and the nominal strain of the transducer are close to each other, the accuracy and resolution can be recognized more precisely. The X-106 is a universal force transducer that can be easily retrofitted due to its slim design.

For process monitoring, the recorded signals are transmitted to the control system and displayed in an online tool (visualization). The measured signals form a measurement curve that must run within the preset deviations. The deviations from the reference measurement curve can be precisely monitored and documented. If the deviations are too large, it is very likely that deviating signals will be emitted, thus a deviating strain will be recorded and most likely there will be an abnormality or defect on the tool or the joining element.

Normally, strain sensors have a fixed, pre-calibrated measuring range. This nominal strain of the sensor has been designed for the maximum strain that the frame construction will experience. Our strain transducers X-103 with integrated measuring amplifier can be equipped with a calibratable measuring range. This additional function allows flexible use on different types of machines. The preconfigured measuring ranges can be selected via a digital input.

Compression force sensors in the pressing process

Packaging likes to be pressed directly around the product. A constant force is applied for a few seconds to compress the packaging elements and bond them together. Indirect measurement via strain sensors would not be sufficient to measure the low stress. With a force sensor, measurement is possible directly in the force flow, here behind the fixed point. There, the stress is large enough to obtain an evaluable measurement signal. For such processes, it is also crucial to use a cost-effective standard solution. With the XCH-171 compressive force sensor, the measuring forces can be recorded reliably and as a standard solution. In conjunction with a measurement amplifier, which outputs a V or mA signal, the entire measurement chain is efficiently mapped.