May 2019

43 www.drivesncontrols.com May 2019 LINEAR MOTION n Actuators are on track for electrified public transport boom T he increasing use of electric vehicles in public transport networks is driving a need for higher-performance linear electric actuators. Actuators allow trams, buses and trains to connect and disconnect with power sources intermittently. The demands on these actuators are limited in vehicles that rely on a single power source to which they connect once a day and and then stay connected for the rest of the day. Hybrid vehicles, on the other hand, may use economical overhead power in built-up areas, but switch to on- board sources beyond the city limits. These vehicles are becoming increasingly important as the number of passengers increases, but this mode of operation places higher demands on the actuators. One area where actuators are being used is to control spring-loaded pantographs – mechanical assemblies on top of vehicles that are raised to connect with overhead power lines. A spring maintains contact with the line, while an actuator pulls the pantograph away from the line when switching to battery power. In other designs, an actuator may pre-load the spring that raises the pantograph to the overhead power line. Intermittent switching to battery power reduces the need to install costly, unattractive and potentially dangerous overhead wiring in urban environments. Some pantographs use pneumatic actuators, but the air compression process gives off moisture that can cause clogging and, in low temperatures, icing. Pneumatic systems also need many more components, such as pumps, pipes and compressed air systems. Increasing passenger numbers are compounding such issues, making pneumatic systems less attractive as traffic levels increase. Actuators can also be used to control switching at the recharging stations where hybrid buses top up their batteries. And for trains that get their power from third rails, actuators can control the extension of the contacts from the train to the powered rail. While existing applications can be handled by actuators with lifetimes of 20,000–30,000 cycles, the increasing use of hybrid public transport vehicles is expected to require actuators with lifetimes of 500,000–1,000,000 cycles. Extending actuator lives Creating longer-life actuators requires at least four areas to be evaluated: ballscrew selection; motor design; braking; and environmental resistance. Actuators translate rotational motion As transport networks introduce vehicles that can swap between different power sources, electric actuators are playing an increasingly important role. As Anders Karlsson, an linear actuator specialist at Thomson, explains, actuators are having to be developed that can handle up to a million operating cycles. In Sweden, electrified trucks which obtain power from overhead lines have been tested on a 2km stretch of motorway. The Scania trucks use electrical technology supplied by Siemens.