|M.Sc Student||Stepura Sergy|
|Subject||Investigation of Techniques to Improve High Weights Lifting|
by Robotic Manipulators
|Department||Department of Mechanical Engineering||Supervisor||Professor Emeritus Yehoshua Dayan|
|Full Thesis text - in Hebrew|
According to manufacturer specification, the maximal allowable payload of most open chained robotic manipulators ranges between 5 to 20 percent of the manipulator’s self weight. Human beings are able to lift weights greater than their own; therefore, it is encouraged to investigate the possibility of improving weightlifting ability of industrial robotic manipulators. A good example for the human ability is the record set by Charles Rigoulot (France) in 1930. The weightlifter had lifted 115 kg, while weighing only 103 Kg. The weightlifting was performed using the One Hand Snatch technique. Utilizing this technique, the human body acts similar to the open chained robotic manipulator. Present research has examined possibilities to improve weightlifting ability of industrial robotic manipulators. Three approaches to improve manipulators weightlifting ability were examined: mimicking the Olympic weightlifter’s strategy; weightlifting along the minimal energy trajectory and manipulators motors overloading.
To obtain the minimal energy trajectory, three optimization approaches were tested: analytical approach (Euler-Lagrange equation, Calculus of Variations); adaptive algorithm (Genetic Algorithm) and gradient based iterative approach (Line-Search). The study was performed on a simple pendulum, i.e. a rod with electrical motor connected to its upper tip. All three approaches lead to the same solution: oscillatory trajectory with growing amplitude up to the weight lifting completion. The analytical approach is the most accurate, but there is significant problems to apply the operational envelop constraints. The adaptive algorithm enables implementation of any constraints; although solution converges slowly (computation duration is very long). Gradient based iterative approach converges the fastest and enables implementation of operational envelop constraints by adding a barrier function (Interior-Point method), although being very sensitive to the initial guess. In order to use Interior-Point method specifically for industrial manipulator, it was suggested that the initial guess would be the reversal of falling trajectory.
In contrast to most of the researches dealing with manipulators weightlifting ability, the RV-M2 Movemaster (selected for the demonstration in this research) is subjected to operational envelops constraints and is unable to perform oscillating movements using its most powerful joints. Implementation of techniques for improving weightlifting capabilities for a manipulator with such limitation indicates that the suggested techniques are applicable for a wide range of industrial manipulators, especially for those that have lesser limitation.
This research has shown that implementation of any of the suggested techniques substantially improves the weightlifting capabilities of the open chained robotic manipulator. Best result is achieved by combination of minimal energy trajectory with electrical motors overload.