Purpose-The installation of industrial robots requires security barriers, a costly, time consuming exercise. Collaborative robots may offer a solution, however these systems only comply with safety standards if operating at reduced speeds. This paper describes the development and implementation of a novel security system that allows human robot coexistence while permitting the robot to execute much of its task at nominal speed. Design/methodology/approach-The security system is defined by three modes: a nominal mode, a coexistence mode and a gravity compensation mode. Mode transition is triggered by three lasers, two of which are mechanically linked to the robot. These scanners create a dynamic envelope around the robot and allow the detection of operator presence or environmental changes. To avoid velocity discontinuities between transitions we propose a novel time scaling method. Findings-The paper describes the system's mechanical, software and control architecture. The system is demonstrated experimentally on a collaborative robot and is compared with the performance of a state of art security system. Both a qualitative and quantitative analysis of the new system is carried out. Pratical Implications-The mode transition method is easily implemented, requires little computing power and leaves the trajectories unchanged. As velocity discontinuities are avoided, motor wear is reduced. The execution time is substantially less than a commercial alternative. These advantages can lead to economic benefits in high volume manufacturing environments. Originality/value-This paper proposes a novel system that is based on industrial material but that can generate dynamic safety zones for a collaborative robot.