A drill crown used for drilling purposes, especially for rotary percussion drilling of a mouth of a blast furnace. The lighting system is powered by rechargeable batteries that are recharged by a solar energy system. The hinge assembly includes a flexible strap and biasing element capable of tensioning the flexible strap to maintain the first and second base portions with respect to each other in an open configuration. The data communications device includes a port that couples to a network, and a processor coupled to the port. The magnetic-pole position computation mechanism computes the estimated magnetic-pole position so that the alternating current amplitude coincides with the alternating current amplitude instruction. By attaching two substrates, each of which is provided with a light emitting element, displays are provided on the front and back of the display device, thus a high added value can be realized.