WALKING MECHANISMS OF A BIPEDAL ROBOT

Under guidance of

Dr T JayarajuProfessor,Dept of mechanical engineering,N I E

ROBOT

“Any automatically operated machine that replaces human effort, though it may not resemble human beings in appearance or perform functions in a humanlike manner”

"A re-programmable, multifunctional manipulator designed to move material, parts, tools, or specialized devices through various programmed motions for the performance of a variety of tasks.”

ROBOT

ASIMOIndustrial Robot Remote camera robot

TYPES OF ROBOTS

Based on locomotion and kinematics

1. Stationary robots (including robotic arms with a global axis of movement)

Cartesian robots Cylindrical robots Spherical robots Articulated robots (robotic arms)

TYPES OF ROBOTS

2. Wheeled robots

Single wheel(ball) robots Two-wheeled robots Three and more wheel robots

3. Legged robots

Bipedal robots (humanoid robots) Tripedal robots Quadrupedal robots Hexapod robots

BIPEDAL OR HUMANOID ROBOTS

A humanoid robot is a robot with its overall appearance based on that of the human body.

A humanoid is a automated machine that replaces human effort and resembles human beings in appearance or perform functions in a humanlike manner.

WHY HUMANOIDS?

• They can work in human environment without the need to adapt themselves or to change the environment

• Everything around us is built to be comfortable for use by human form.

• It is easier for a human being to interact with a human-like being. (operating a complex machine needs more understanding than operating a human-like robot)

HUMAN WALKING

Human walking is defined as series of falls.

HUMAN WALKING

Understanding the parameters of human walking is vital to designing and analyzing walking mechanism of a bipedal robot.

Walking is defined by an 'inverted pendulum' gait in which the body vaults over the stiff limb or limbs with each step.

STABILITY

Stability means the capability to maintain the body posture given the control patterns

Statically stable walking implies that the posture can be achieved even if the legs are frozen / the motion is stopped at any time, without loss of stability

Dynamic stability implies that stability can only be achieved through active control of the leg motion

Statically stable systems can be controlled using kinematic models

Dynamic walking requires use of dynamical models

GAIT

Gait is the pattern of movement of the limbs of animals, including humans, during locomotion over a solid surface.

Standing up and walking appear effortless to us, but we are actually using active control of our balance.– We use muscles and tendons.– Robots use motors.

In order to remain stable, the robot’s Center of Gravity must fall under its polygon of support.

The polygon is basically the projection between all of its support points onto the surface.

GAIT

Human gait cycle

WALKING MECHANISMS

Walking mechanisms can be classified based on type of actuation

Passive dynamic robot Active dynamic robot

PASSIVE DYNAMIC ROBOT

It is called “dynamic "because its movement is characterized by a dynamic stability.

“Passive” refers to the robot's ability to generate locomotive movement without motor input

Utilizes the gravity and the momentum of swinging limbs for efficiency.

Advantages: In contrast to rigidly joint-controlled robots, walking

robots based on passive-dynamic principles can have human-like efficiency and actuation requirements.

Disadvantages: Movements are mostly in sagital plane and in straight

line, being extremely difficult to turn, go back, seat,etc. The motion is mostly symmetrical.

PASSIVE DYNAMIC ROBOT

Frontal plane of Passive Toy Saggital plane of Passive Toy

• A passive dynamic walker with single DOF per leg which walks with a stable gait down a smooth incline

PASSIVE DYNAMIC ROBOT

• A passive dynamic robot with 2 DOF per leg.• It has an extra DOF(knee joint)

ACTIVE DYNAMIC ROBOT

“Active” refers that the robot generates locomotive movement with the aid of motor input.

It requires greater degree of control since all the DOFs of the robot needs to be controlled actively.

These can perform activities running, jumping, hopping, etc.. unlike passive walkers.

Ex: ASIMO by Honda which can walk a stair of steps

ACTIVE DYNAMIC ROBOT

ASIMO Walking down the stairs Robot by Toyota which plays violin

KINEMATIC ANALYSIS

Optimal mass distribution is achieved by understanding the location of the center of mass.

According to the results presented, a cluster region of solutions is found when the function of (c, rgyr) approaches (1,0). (The value c represents the distance of the center of mass measured from the floor).

The distance between the two ankle centers is crucial in maintaining stability. If too large, the distance between the offset between projection of COG and the supporting center will increase. This will decrease the Zero Moment Point stability margin.

KINEMATIC ANALYSIS

• The kinematics analysis is based on the basic equations of link motion where the position of the end effectors point is described by the position (XH,YH).

XH = Ln cosθn + Ln-1 cosθn-1 +….. L0 cosθ0 …… (1)  YH = Ln sinθn + Ln-1 sinθn-1 +…… L0 sinθ0 …… (2)

Region of Solutions

Biped motion side view

KINEMATIC ANALYSIS

Applying equations (1) and (2) relation can be obtained for the position of the end-effectors.

The center of mass (C) is another important factor that must be considered. As previously mentioned, the center of mass is a key component in maintaining stability during biped motion. Equation given above will be used to simulate the motion of the center of mass as the biped attempts each phase

ZERO MOMENT POINT The ZMP (Zero Moment Point) is defined as the

point on the ground about which the sum of all the moments of the active forces equals zero.

ZMP specifies the point with respect to which dynamic reaction force at the contact of the foot with the ground does not produce any moment, i.e. the point where total inertia force equals 0

ZMP is the indicator of the stability of the robot: if it is in the foot shadow – stable, İf not – unstable.

ZERO MOMENT POINT

ZERO MOMENT POINT

First we design a desired ZMP trajectory, then derive the hip motion or torso motion required to achieve that ZMP trajectory.

The figure shows the trajectory that we specify so that the combined motion of the ankle, hip and torso creates a human like walking motion without the chance of tripping.

ZERO MOMENT POINT

The advantage of this method is that the stability margin can be large if the desired ZMP is designed near the center of the stable region.

However, since the change of the ZMP due to hip motion is limited, not all desired ZMP trajectories can be achieved.

Furthermore, to achieve a desired ZMP trajectory, the hip acceleration may need to be large.

In this case, since the torso is relatively massive, energy consumption increases, and control for task execution of the upper limbs becomes difficult. Therefore, it is desirable to obtain hip motion without first designing the desired ZMP trajectory

References:

• Wikipedia (http://en.wikipedia.org/wiki/Robot)

• (http://asimo.honda.com/)

• Tad McGeer. Passive dynamic walking. International Journal of Robotics Research

• Tad McGeer. Passive walking with knees. In IEEE International Conference on Robotics and Automation

• Russ Tedrake, Teresa Weirui Zhang, Ming fai Fong, and H. Sebastian Seung. Actuating a simple 3d passive dynamic walker.