Weld volume, geometry and quality standards in structures in the wind energy sector demand automation. Conventional industrial robots are often infeasible due to safety factors, cost, work space available and the time-consuming programming involved. On the other hand, typical machining tasks such as cutting, arc welding, and ultrasonic testing are so complex that they cannot be mechanised using simple appliances. Small, inexpensive modular machines on rails, known as crawlers, bridge the gap between simple mechanised equipment on the one hand, and industrial robots on the other. They inherit the easy handling and versatility for use even in difficult site conditions from the former, together with the possibility of programming and sensor-controlled movement from industrial robots. The article discusses the possibilities and limitations in this concept by reference to a series of examples from applications.