Three-legged walking machines

By Nico Julian


Rotated Body Locomotion (RBL) is a new concept in walking robot design and differs from conventional robot design in how the body and the legs move.

  • Conventional walking robot legs propel the level main body of a robot along by leg articulation.
  • The RBL design works in the opposite manner. In RBL, the body propels the legs along by body rotation in an aerial phase.

The RBL approach reduces the complexity of the legs to single-degree of freedom actuators since the necessity for leg articulation is eliminated. The resulting walking robot prototyped by the author employs three linearly actuating electric solenoids mounted rigidly to a simple flat plate body.

Video – Motion Sequence 1: Straight-Line

Wide Angle 1

Wide Angle 2


Video – Motion Sequence 2: Rotation

Wide Angle 1

Close Up

The key to the RBL prototype lies in sequential solenoid activation. Each of the three solenoids is activated with the same force, frequency, and duty cycle.  Horizontal motion is only achieved by activating the solenoids out of phase with each other, causing body rotation.

Two solenoid sequences are used: staight-line motion and rotation motion sequences.  By using these straight-line and rotational motion sequences alternately, omni-directional motion is achieved, where the robot can face any direction while moving in any independent direction. Simple activation sequences such as these trivialize control.

  1. One sequence activates two solenoids together, but out of phase with the third, rotating the body back and forth about a horizontal axis in an aerial phase. This results in a straight-line bounding-like motion towards the third solenoid.
  2. The other sequence activates each solenoid out of phase with the other two, 120 degrees apart, causing the robot to wobble like a dropped coin and rotate about a vertical axis. If the solenoids are activated 120 degrees apart in a clockwise direction, the robot will rotate in a clockwise direction and vice versa.

Control hardware

In the author’s 470-gram prototype consists of a Digital Signal Processor (DSP) that sends on-off pulses to three MOSFET switching circuits. The MOSFET circuits, in turn, activate the solenoids.

The DSP is programmed in the C language and activates the solenoids in an open-loop fashion. The solenoid activation frequency is set at the natural frequency of the robot to minimize power requirements, and solenoid springs aid in aerial phase energy recovery. On-board switches can be manually set to select one of six subroutines that demonstrate various activation sequences. Three on-board 9V NiMH batteries supply all the necessary power to the robot while two separate power supplies run each, the DSP and the solenoids.

Main Advantage

The RBL is a simple design in the mechanical and electrical components and in the software. Simplicity offers reliability, durability, and low cost. Other advantages are omni-directional maneuverability and lower power requirements through energy recovery methods.

Potential Applications

RBL robots can be used in a wide variety of applications including: space exploration, surveillance, and toys.
RBL lends itself well to miniaturization and further application may be found in the micro-world using current technology.


Motion Simulation