Editorial Feature

Introduction to Robotic Locomotion


Image Credit: Leo Pakhomov/Shutterstock.com

Article updated on 24/02/20 by Mihaela Dimitrova

Locomotion seems to be performed seamlessly but the complex central nervous system controls it. The CNS generates commands at cortical and spinal levels and integrates these commands with the sensory feedback it receives from the environment and the body. All movements and coordination processes are produced without the need for conscious control.

Damage to the central nervous system (CNS) has a direct impact on pattern generation and balance in locomotor function. Unfortunately, assessment of these sub-functions is difficult owing to technical limitations and hence their rehabilitation is also affected.

Mimicking the Brain

Finding effective locomotion strategies in robotics is challenging. To account for the always-changing environmental conditions, various methods have been utilized. One of the most effective locomotion mechanism that deals with changing terrains is called leg-based motion. However, legged locomotion is difficult to be modeled and controlled.

Recent technological advancements have led to new locomotion approaches that incorporate synergies and focus on the cerebellar contribution to the movement. The cerebellum improves the accuracy of movements through motor learning and adaptation of the motor commands from the motor cortex. It stores information about body-object interaction through long-term synaptic plasticity (LTP). This way it can generate internal models of movements. This functionality has been implemented for adaptable gain control for robotic manipulation tasks. These paradigms incorporate and error-dependent signal that is self-adaptable and operates over different time scales and improves learning accuracy.

Professor Grégoire Courtine, Chair in Spinal Cord Repair along with his team discovered a robotic interface, which is flexible, efficient and capable of assessing and training pattern generation and balance while observing natural walking behavior of rats in experimental conditions.

The key benefits of the robotic interface are:

  • It makes clear assessments of dynamic equilibrium and pattern generation in evaluation mode once spinal cord injury and stroke occur.
  • The robot acts as a prosthetic limb.
  • Restoration of weight-supported locomotion and coordinated steering and balance takes place in rats during training mode along with recovery of SCI.

This novel robotic technology yields the desired result in both humans and animals.

Principles to Robotic Locomotion

Wheeled locomotion is an interesting concept that merges the inspiration from man-made wheels and the biological system that forms the basis for developing legged locomotive robots. However, the environment and ground contact are key factors in challenging these mechanisms.

The following are attributes of the type of the environment:

  • Medium (e.g., soft or hard ground, air or water).
  • Structure of medium (e.g., rough or flat in case of hard medium).

The attributes of ground contact are:

  • Friction between the surface and the robot
  • Angle of contact to ground
  • Type of contact point (e.g., footprint in case of legged locomotion)

The attributes of stability are:

  • Inclination of terrain
  • Centre of gravity of the robot
  • Geometry and number of contact points

Types of Locomotion

Wheeled Locomotion

Wheeled locomotion is the most popular mechanism integrated into the design and development of mobile robotics, with the main reason being that this locomotive capability is efficient and simple to control. Moreover, balance is not a major problem with this locomotion type as the wheels are in contact with the ground at all times. Sufficient balance is guaranteed in the case of two- and three-wheeled robots. However, a suspension system is necessary for balance in robots with more than two wheels.

Issues of maneuverability, control, and traction of the wheeled robot are far more important to resolve than focusing on its balance because a wheeled robot needs to be capable of maintaining full functional capacity when performing on varying ground surface conditions. The following video is by DFKI Bremen – Robotics Innovation Center and demonstrates Sherpa, a wheeled rover with an integrated suspension system. In this video, the rover displays wheeled locomotion with effective use of energy whilst being capable of flexibility and adapting to varying terrains and high-range surfaces.

Sherpa: Expandable rover for planetary applications

Wheel Design

The kinematics of mobile robots is largely based on the selection of wheel type. The standard and castor wheel are highly directional, whereas the standard wheel can achieve steering motion without side effects since the rotational center passes via the contact point of the ground. However, the castor wheel imparts a force to the robot chassis while rotating around and offset axis during steering.

There is a less directional constraint with a Swedish wheel and a spherical wheel. The Swedish wheel can move along different directions with less friction as the wheel is rotated along the one primary axis.

Applications of Mobile Robots

Some of the major applications of mobile robots are:

  • Wheeled robots with tricycle-drive configurations are suitable for AGV applications.
  • The automotive and military industry commonly use the wheeled mobile robots with Ackerman steering (car drive) configuration.
  • Robots with tracked vehicle configuration are used for explosive ordnance disposal.
  • Multi-degree-of-freedom (DOF) vehicles are suitable for naval forces. e.g., an 8-DOF vehicle built by Unique Mobility, Inc.

References and Further Reading

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