Robotic Tails for Enhanced Mobile Robot Performances

Student thesis: Phd


Maintaining stability is of vital importance for mobile robots working in the field, where they may be in hazardous or confined environments, out of the reach of operators. Robotic tails have already been investigated for use in maintaining robot stability in a number of different scenarios, but the action of transporting a payload is as yet unexplored. This presents unique challenges for robot stability, due to the sudden shift in center of mass (COM). Developing a robotic tail that can respond to this change and maintain robot stability would allow mobile robots to transport heavier and more unbalanced payloads in a safe and stable manner. This research project first investigated the existing uses of robotic tails in terrestrial mobile robots using selected literature from the first systematic search conducted in the field, finding a diverse set of tail functions that maintain stability or enhance the motion of mobile robots. The findings from this research, mainly the rarity of multi-segment tails, went on to inform the direction of later parts of the project. A potential actuation method for the tail joint based on the twisted string actuator (TSA) in an "antagonistic triad" configuration was designed and built, and successfully tested with robust control, able to accurately track a 2 degree of freedom (DOF) joint trajectory within a range of +/- 11 degrees to an accuracy of 1.8 degrees. This TSA was then compared with alternative actuators to characterise its performance, which was found to be dependant on the actuator position. A configurable payload, consisting of a container filled with eight cubes of various materials with different densities, was designed and constructed to be able to generate a variety of different simulated payloads with a range of mass and COM for later experiments. The configurable payload was mathematically defined, and test point sets were created from the extremas of the set of mass and COM that encompassed all possible configurations of the payload, as well as target mass and COM where the closest configuration was found using a search method. Finally, a static test rig was constructed which was able to measure stability using four load cells. A robot arm was then used to pick, hold, and place the configurable payload with and without a single segment 2 DOF tail with a weighted tip, which used the load cell data to minimise the value of the center of pressure (COP) by moving the tail. Overall, the research in this thesis demonstrates a clear improvement in stability of up to 87% when using a robot tail to carry a payload. This research lays the groundwork for the implementation of a robot tail on a mobile robot for simulated tasks involving transporting various payloads. It has also developed a novel new actuated universal joint (AUJ) based on the TSA, which has applications for robotic tails and beyond.
Date of Award1 Aug 2023
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorWilliam Heath (Supervisor), Joaquin Carrasco Gomez (Supervisor) & Andrew Weightman (Supervisor)


  • twisted string actuators
  • manipulators
  • mobile robots
  • robotic tails

Cite this