An access control apparatus has a blocking element which is moved by a motor or similar actuator controlled via a control means. The reference object may be scaled in proportion to the sonographic image. Messages can be transmitted in real time without any significant delays introduced by the telecommunication network to effect the encryption functionality. Moreover, the movement of the safety link is stopped indirectly by the safety piece, thereby preventing the deformation of the drive plate. The system may also include manual, timed, or conductivity-controlled flush systems for ridding the enclosures of waste fluids under a grate that is easy to clean.