Continuous material supply must be ensured in punching machines. As a material buffer, the supplied metal sheets are guided through the system in the form of a hanging loop. This compensates for the time delay between replenishment and the production processes. To monitor this process, an ultrasonic distance sensor is installed above the loop. It checks whether the sag falls below the minimum value or touches the ground. Ultrasonic measurement is not affected by even very shiny plates and dirt such as oil mist or dust.