ROBOTICS | ARM CONFIGURATION

ROBOTICS

INTRODUCTION:

The popular concept of a robot is of a machine that looks and works like a human being. This humanoid concept has been inspired by science fiction stories and films in the twentieth century. The industrial robots of today may not look the least bit like a human being although all the research is directed to provide more and more anthropomorphic and human-like features and super-human capabilities in these. To sum up, machines that can replace human beings as regards to physical work and decision making are categorized as robots and their study as robotics.

The robot technology is advancing rapidly. The industry is moving from the current state of automation to robotization, to increase productivity and to deliver uniform quality. Robots and robot- like manipulators are now commonly employed in hostile environment, such as at various places in an atomic plant for handling radioactive materials. Robots are being employed to construct and repair space stations and satellites. There are now increasing number of applications of robots such as in nursing and aiding a patient. Microrobots are being designed to do damage control inside human veins. Robot like systems are now employed in heavy earth-moving equipment. It is not possible to put up an exhaustive list of robot applications. One type of robot commonly used in the industry is a robotic manipulator or simply a manipulator or a robotic arm. It is an open or closed kinematic chain of rigid links interconnected by movable joints. In some configurations, links can be considered to correspond to human anatomy as waist, upper arm and forearm with joints at shoulder and elbow. At the end of the arm, wrist joint connects an end-effector to the forearm. The end-effector may be a tool and its fixture or a gripper or any other device to do the work. The end effector is similar to human hand with or without fingers. A robotic arm, as described above, is shown in Fig. 1.1, where various joint movements are also indicated.

ROBOT ANATOMY:

As mentioned in the introduction to the chapter, the manipulator or robotic arm has many similarities to the human body. The mechanical structure of a robot is like the skeleton in the human body. The robot anatomy is, therefore, the study of skeleton of robot, that IS, the physical construction of the manipulator structure. The mechanical structure of a manipulator that consists of rigid bodies (links) connected by means of articulations (joints), is segmented into an arm that ensures mobility and reach ability, a wrist that confers orientation, and an end-effector that performs the required task. Most manipulators are mounted on a base fastened to the floor or on the mobile platform of an autonomous guided vehicle (AGV). The arrangement of base, arm, wrist and end-effector is shown in Fig. 1.5.

LINKS:

The mechanical structure of a robotic manipulator is a mechanism, whose members are rigid links or bars. A rigid link that can be connected, at most, with two other links is referred to as a binary link. Figure 1.6 shows two rigid binary links, 1 and 2, each with two holes at the ends A, B, and C, D, respectively to connect with each other or to other links. Two links are connected together by a joint. By putting a pin through holes B and C of links 1 and 2, an open kinematic chain is formed as shown in Fig. 1.7. The joint formed is called a pin joint also known as a revolute or rotary joint. Relative rotary motion between the links is possible and the two links are said to be paired.

In Fig. 1.7 links are represented by straight lines and rotary joint by a small circle.

JOINTS AND JOINT NOTATION SCHEME:

Many types of joints can be made between two links. However, only two basic types are commonly used in industrial robots. These are

• Revolute (R) and

• Prismatic (P).

The relative motion of the adjoining links of a joint is either rotary or linear depending on the type of joint.

Revolute joint: It is sketched in Fig. 1.8(a). The two links are jointed by a pin (pivot) about the axis of which the links can rotate with respect to each other.

Prismatic joint: It is sketched in Fig. 1.8(b). The two links are so jointed that these can slide (linearly move) with respect to each other. Screw and nut (slow linear motion of the nut), rack and pinon are ways to implement prismatic joints.

Other types of possible joints used are: planar (one surface sliding over another surface); cylindrical (one link rotates about the other at 90° angle, Fig. 1.8(c)); and spherical (one link can move with respect to the other in three dimensions). Yet another variant of rotary joint is the 'twist' joint, where two links remain aligned along a straight line but one turns (twists) about the other around the link axis, Fig. 1.8(d). At a joint, links are connected such that they can be made to move relative to each other by the actuators. A rotary joint allows a pure rotation of one link relative to the connecting link and prismatic joint allows a pure translation of one link relative to the connecting link.

 

The kinematic chain formed by joining two links is extended by connecting more links. To form a manipulator, one end of the chain is connected to the base or ground with a joint. Such a manipulator is an open kinematic chain. The end-effector is connected to the free end of the last link, as illustrated in Fig. 1.5. Closed kinematic chains are used in special purpose manipulators, such as parallel manipulators, to create certain kind of motion of the end-effector.

The kinematic chain of the manipulator is characterized by the degrees of freedom it has, and the space its end-effector can sweep. The number of independent movements that an object can perform in a 3-D space is called the number of degrees of freedom (DOF).

ARM CONFIGURATION:

According to joint movements and arrangement of links, four well distinguished basic structural configurations are possible for the arm. These are characterized by the distribution of three arm joints among prismatic and rotary joints, and are mimed according to the coordinate system employed or the shape of the space they sweep. The four basic configurations are:

(i)  Cartesian (rectangular) configuration - all three P joints.

(ii)   Cylindrical configuration – one R and two P joints.

(iii)   Polar (spherical) configuration - two R and one P joint.

(iv)  Articulated (Revolute or Jointed-arm) Configuration - all three R joints.

CARTESIAN (RECTANGULAR) CONFIGURATION:

This is the simplest configuration with all three prismatic joints, as shown in Fig. 1.11. It is constructed by three perpendicular slides, giving only linear motions along the three principal axes. There is an upper and lower limit for movement of each link. Consequently, the endpoint of the arm is capable of operating in a cuboidal space, called workspace.

CYLINDRICAL CONFIGURATION:

The cylindrical configuration pictured in Fig. 1.13, uses two perpendicular prismatic joints, and a revolute joint. The difference from the Cartesian one is that one of the prismatic joint is replaced with a revolute joint. One typical construction is with the first joint as revolute. The rotary joint may either have the column rotating or a block revolving around a stationary vertical cylindrical column. The vertical column carries a slide that can be moved up or down along the column. The horizontal link is attached to the slide such that it can move linearly, in or out, with respect to the column. This results in a RPP configuration. The arm endpoint is, thus, capable of sweeping a cylindrical space. To be precise, the workspace is a hollow cylinder as shown in Fig. 1.13. Usually a fu11 360° rotation of the vertical column is not permitted due to mechanical restrictions imposed by actuators and transmission elements.

POLAR (SPHERICAL) CONFIGURATION:

The polar configuration is illustrated in Fig. 1.14. It consists of a telescopic link (prismatic joint) that can be raised or lowered about a horizontal revolute joint. These two links are mounted on a rotating base. This arrangement of joints, known as RRP configuration, gives the capability of moving the arm end-point within a partial spherical shell space as work volume, as shown in Fig. 1.14.

Articulated (Revolute or Jointed-arm) Configuration:

The articulated arm is the type that best simulates a human arm and a manipulator with this type of an arm is/often referred as an anthropomorphic manipulator. It consists of two straight links, corresponding to the human "forearm" and "upper arm" with two rotary joints corresponding to the "elbow" and "shoulder" joints. These two links are mounted on a vertical rotary table corresponding to the human waist joint. Figure 1.15 illustrates the joint-link arrangement for the articulated arm.

This configuration (RRR) is also called revolute because three revolute joints are employed. The work volume of this configuration is spherical shaped, and with proper sizing of links and design of joints, the arm endpoint can sweep a full spherical space. The arm endpoint can reach the base point and below the base, as shown in Fig. 1.15. This anthropomorphic structure is the most dexterous one, because all the joints are revolute, and the positioning accuracy varies with arm endpoint location in the workspace. The range of industrial applications of this arm is wide.