Humanoid Robots: Planning, Sensors and Control

Humanoid robots do not yet have some features of the human body. They include structures with variable flexibility, which provide safety (to the robot itself and to the people), and redundancy of movements, i.e. more degrees of freedom and therefore wide task availability. Although these characteristics are desirable to humanoid robots, they will bring more complexity and new problems to planning and control. The field of whole-body control deals with these issues and addresses the proper coordination of numerous degrees of freedom, e.g. to realize several control tasks simultaneously while following a given order of priority.

Another characteristic of humanoid robots is that they move, gather information (using sensors) on the "real world" and interact with it. They don’t stay still like factory manipulators and other robots that work in highly structured environments. To allow humanoids to move in complex environments, planning and control must focus on self-collision detection, path planning and obstacle avoidance.

To maintain dynamic balance during the walk, a robot needs information about contact force and its current and desired motion.

The stability of walking biped robots on the surface is of great importance. Maintenance of the robot’s center of gravity is the goal of control.

The essential difference between humanoids and other types of robots is that the movement of the robot has to be like that of humans, using legs, especially biped. The ideal planning for humanoid movements during normal walking should result in minimum energy consumption, as it does in the human body.

Planning and control

Pneumatic actuators operate on the basis of gas compressibility. As they are inflated, they expand along the axis, and as they deflate, they contract. If one end is fixed, the other will move in a linear trajectory. These actuators are intended for low speed and low/medium load applications. Between pneumatic actuators there are: cylinders, bellows, pneumatic engines, pneumatic stepper motors and pneumatic artificial muscles.

Ultrasonic actuators are designed to produce movements in a micrometer order at ultrasonic frequencies (over 20 kHz). They are useful for controlling vibration, positioning applications and quick switching.

Piezoelectric actuators generate a small movement with a high force capability when voltage is applied. They can be used for ultra-precise positioning and for generating and handling high forces or pressures in static or dynamic situations.

Hydraulic and electric actuators have a very rigid behavior and can only be made to act in a compliant manner through the use of relatively complex feedback control strategies. While electric coreless motor actuators are better suited for high speed and low load applications, hydraulic ones operate well at low speed and high load applications.

Actuators are responsible for the robot’s movement or locomotion. Mainly, rotary actuators are used to achieve the same effect as humans’ movement. They can be either electric, hydraulic, piezoelectric, ultrasonic or pneumatic. These actuators act as the muscles and joints of the robot, but with the structural arrangement unlike the human body.

Actuator

Arrangement of tactels, or tactile elements are used to provide data of touch. For instance, The Shadow Hand uses an array alignment of 34 tactels arranged strategically on each fingertip. Vision sensors can also be included under this category. These sensors work in similarity to the human eyes. Most robots are equipped with CCD cameras as vision sensors.

Exteroceptive sensors

These sensors sense the orientation, position and the speed of the humanoids’ limbs. Humanoids also use accelerometers, from which the velocity can be calculated by integration, tilt sensors to measure inclination, force sensors placed in robot’s limbs to measure contact forces with environment.

Proprioceptive sensors

Frequently used sensors in robots are proprioceptive sensors and exteroceptive sensors.

Three primitives of robotics are: planning, sensors, and control. Sensors play an important role in robotic paradigms. They can be classified according to the type of information they give as output.

Humanoid robots have been in the headlines since a long time. Many researchers are working on different applications of these robots. Today, a humanoid robot is capable to display only a limited subset of skill. These efforts by the researchers are motivated by the vision to create a general-purpose robot which will work in cooperation with the humans. It is suggested that very advanced robotics will facilitate the enhancement of ordinary human beings, in other words, transhumanism will prevail.