Actuators

Actuators are the means by which robots move and change their body shape. In order to understand the main features of the design of actuators, it is necessary to first consider the abstract concepts of motion and form, using the concept of the degree of freedom. As a degree of freedom, we will consider every independent direction in which either a robot or one of its executive mechanisms can move. For example, a solid-state free-moving robot, such as an autonomous underwater vehicle, has six degrees of freedom; three of them, (x, y, z), determine the position of the robot in space, and the other three determine its angular orientation along three axes of rotation, known as swing (yaw), turn (roll) and tilt (pitch). These six degrees of freedom determine the kinematic state or pose of the robot. Dynamic state
the robot includes one additional dimension for the rate of change
each kinematic measurement.

Robots that are not solid-state have additional degrees of freedom within themselves. For example, in the hand of a person, the elbow has one degree of freedom (it can bend in one direction), and the brush has three degrees of freedom (it can move up and down, side to side, and also rotate). Each of the hinges of the robot also has 1, 2 or 3 degrees of freedom. To move any object, such as a hand, to a specific point with a specific orientation, you must have six degrees of freedom. The arm shown in figure a) has exactly six degrees of freedom, created with the help of five rotary hinges, which form a rotational movement, and one prismatic joint, which forms a sliding movement. To make sure that the hand of a person as a whole has more than six degrees of freedom, you can conduct a simple experiment: put the brush on the table and make sure that you still have the opportunity to turn the arm at the elbow, without changing the position of the brush on the table. Manipulators that have more degrees of freedom than is required to translate the final actuator to the target position are easier to control compared to robots that have only a minimum number of degrees of freedom.

Design features of the robot manipulator: Stanford Manipulator (Stanford Manipulator) is one of the first robot manipulators, which uses five rotary hinges (R) and one prismatic joint (V), which allows a total of six degrees of freedom (a); the trajectory of the movement of a non-four-wheel vehicle with steering from the front wheels (b)

In mobile robots, the number of degrees of freedom does not necessarily coincide with the number of actuated elements. Consider, for example, an ordinary car: it can move forward or backward and also turn, which corresponds to two degrees of freedom. In contrast, the kinematic configuration of the car is three-dimensional - on an open flat surface, you can easily transfer the car to any point (x, y), with any orientation (see Figure b). Thus, the car has three effective degrees of freedom, but two controlled degrees of freedom. A robot is called nonholonomic if it has more effective degrees of freedom than controlled degrees of freedom, and holonomic if these two values ​​coincide. Holonomic robots are easier to manage (it would be much easier to park a car that can move not only back and forth, but also to the sides), but holonomic robots are also mechanically more complex. Most robot manipulators are holonomic, and most mobile robots are nonholonomic.

In mobile robots, a variety of mechanisms are used to move in space, including wheels, tracks and legs. Differential-drive robots are equipped with independently activated wheels (or tracks as in an army tank) located on both sides. If the wheels on both sides rotate at the same speed, the robot moves in a straight line. If they rotate in opposite directions, the robot turns in place. An alternative option is to use a synchronous drive, in which each wheel can rotate and rotate around a vertical axis. The use of such a drive system could well lead to chaotic movement if such a restriction were not used, that all pairs of wheels turn in the same direction and rotate at the same speed. Both differential and synchronous drives are non-holonomic. Some more expensive robots use holonomic drives, which usually consist of three or more wheels that can rotate and rotate independently of each other.

Legs, unlike wheels, can be used for movement not on a flat surface, but on an area characterized by very rough terrain. Nevertheless, on flat surfaces, legs as vehicles are much inferior to wheels, and the task of creating a mechanical design for them is very difficult. Researchers in the field of robotics have attempted to develop designs with a very different number of legs, ranging from one leg to literally dozens. Robots equipped with legs for walking, running and even jumping (as shown in the example of a walking robot in Figure a) were developed. This robot is dynamically stable; this means that it can remain upright only moving continuously. A robot that can remain upright without moving its legs is called statically stable. A robot is statically stable if its center of gravity is above the polygon covered by its legs.

In mobile robots of other types for movement other, extremely various mechanisms are used. In aircraft, propellers or turbines are commonly used. Robotic airships are kept in the air due to thermal effects. Autonomous underwater vehicles often use thrusters similar to those installed on submarines.

In order for a robot to function, it is not enough for it to be equipped only with sensors and actuators. A full-fledged robot must also have an energy source for driving its actuators. Electric motors are most often used for actuating the manipulator and for moving; Pneumatic drives that use compressed gas and hydraulic drives that use high-pressure fluids also have a specific application. In addition, most robots have some kind of digital communications like a wireless network. Finally, the robot must have a rigid case on which all these devices could be hung, and, figuratively speaking, keep a soldering iron with you, in case his equipment stops working.



Examples of robots moving with the help of legs: one of Marc Raibert’s walking robots in motion (a); Sony AIBO robots playing football (© 2001, RoboCup) (b)