The alkali metal lithium (Li) has particularly mobile ions and therefore enables the production of power storage units with a high energy density. As with smartphones or tablets, the term 'rechargeable battery' should actually be used, but the term 'battery' has become established for the rechargeable unit in electric cars. This consists of thin layers of foil, each with a lithium-coated cathode, a separator foil and the anode, which is usually coated with graphite.

A liquid compound (slurry) is applied to coat the electrode foils. After the drying phase, they pass through a rolling mill (calender), where they are compacted at high pressure and at the same time given their uniform thickness. In order to achieve the desired result here, the films must maintain a defined speed as they pass through the rolling mills. At the end of this process step, they are wound onto rolls. The rotational speed of certain rolls in the calender is measured using encoders. The winding distance and material stock can be derived from the speed. The value is also used to control the material feed and allows the correct web tension to be set.

Precise measurement of the roller speed

The 'ENI90' encoder series from Pepperl+Fuchs is ideal for this task. The devices with IP65, IP66 or IP67 protection use BlueBeam technology for reliable speed measurement: Their blue light with a wavelength of 460 nm minimizes signal flutter (jitter). At the same time, it enables an optimized signal amplitude and therefore a significantly higher resolution than other light colors. The underlying optical effects generate an intrinsic resistance of the signal to mechanical influences such as vibration and shock. The fine structures of the scanning chips also contribute to the efficiency of the signal generation and the high signal quality. The absolute accuracy is marked by the maximum deviation of ± 0.025°. The signal is transmitted via a universal output driver or an RS422 interface.

Recognizing wafer-thin arches

Batteries in the form of pouch cells are predominantly used for electric cars, which enable a high energy density with optimum use of space.

The double sheet control 'UDC-18GS' prevents double layers of one polarity.

© Pepperl+Fuchs

To produce the cells, suitable pieces are punched out of the electrode and separator foils and joined together in the correct order. The cut foil sheets are picked up with vacuum grippers and positioned on an alignment table. In the next step, they are stacked alternately in the correct sequence by a second vacuum gripper. During the entire process, it is important that the gripper only picks up one of the very thin film sheets at a time. However, it can happen that several layers stick together. In this case, or if no film has been gripped at all, stacking must be interrupted and corrected. This is ensured by the automatic layer control on the alignment table. A double sheet sensor from the 'M18' series reliably detects whether there is one, no or several sheets on the table. In the event of incorrect stacking, the process is interrupted. This prevents double layers with the same polarity from reducing the battery capacity.

Ultrasound cannot be irritated

The ENI90PL for precise speed measurement and improved positioning when punching out film sheets

© Pepperl+Fuchs

Detection is contactless using ultrasound and without any impact on the sensitive films. The ultrasonic measuring principle is intrinsically robust: detection is completely independent of surface properties such as color and reflection behavior of the target material, which in this case is often very shiny. The sensor function is not affected by vibration and other mechanical influences from the machine.

The parameters of the measuring process can be precisely adapted to the thickness of the wafer-thin films by simple teach-in. The device can also be integrated into the system control via IO-Link. The IO-Link interface enables access to the sensor parameters as well as all diagnostic and process data. This includes, for example, the detailed evaluation of amplitude values or the simple commissioning and parameterization of the sensors. With the automatic synchronization of the devices, several ultrasonic sensors can be used simultaneously in a confined space.