A differentially steered drive unit for autonomous stage wagon

Abstract: There is more to theatre than meets the eye. Getting the stage ready for a show often means moving ungainly, heavy and irregularly shaped props. Visual Act is an independent company providing theatres, TV-production sets, sporting and cultural events with technology and props. The company has developed and also patented an autonomous stage wagon system that uses proprietary technology to move both people and props across large areas during live shows. These wagons are used in performances to make things travel and are more often than not disguised to fit the theme of the show. Customers might however need to move things backstage as well and are likely to find the current system to be overly complex for that usage, which gave the idea to develop an automatic wagon designated for theatrical logistics. This report presents a project investigating the controllability of a new drive unit for the logistical autonomous stage wagons; a concept based on differential steering that has the potential to double the propelling force of each drive unit whilst maintaining precision and potentially compressing the physical size as compared to the existent drive unit. The new drive unit is to be adapted to the premises of the existent wagon system and hence use the same system architecture to enable it to later on be installed in the wagon whenever customers demand greater propelling force. The final prototype is made in steel and has a u-shaped wheel axle to create stability by placing the center of gravity below the center of the wheels. The wheel axle is approximately 0.4m long and has two wheels with a radius of 0.08m mounted in parallel on the two sides. Each wheel is connected to a 1500WAC servo motor via a bevel gear with a ratio of 1/7.5. As the components of the wagon are made of mostly copper and steel, the tailored prototype wagon with one differentially steered drive unit is heavy and weighs just less than 100kg in total without extra load. The control system is aiming at compensating for heading errors and uses a Gaussian function to model the new wheel speeds required to correct a faulty direction. The control system consistently uses a calculation of the approximated speed that corresponds to the requested path to model new velocities. Theory and tests show a good ability to drive forward, backward and turn on the spot. Hence, the differentially steered drive unit is found to be controllable, but future investigation of its ability to function in a system of multiple, inter-connected differential drive units is necessary. Given certain goal coordinates and a time period to travel there, the test set up showed good repeatability and performance. The tests are however carried out on a small area and possible small errors are difficult to detect.