Industrial Robots. Classification

The Robot Institute of America has given the definition of a robot as a reprogrammable, multifunctional manipulator designed to move material, parts, tools, or specialized devices through variable programmed motions for the performance of a variety of tasks (3).

Before going any further, main parameters used for robot classification are briefly introduced, and major benefits of robot installations are addressed. Reprogrammability has made industrial robots a key component of flexible automation. The robot’s motion is controlled by a program that can be modified to change the motion of the robot arm significantly. The programmability provides the versatility of a robot.

The basic geometric configurations of the robots are usually classified as Cartesian, cylindrical, spherical, and jointed arm. A robot that conforms to a Cartesian geometry can move its gripper to any position within the cube or rectangle work envelope. A robot with a cylindrical geometry can move its gripper within a volume described by a cylinder.

The spherical (polar) arm geometry positions the robot through two rotations and one linear actuation. Jointed arm, which are sometimes referred to as articulated robots, have an irregular work envelope. As more flexible and specialized coordinate systems are demanded through time, other robot coordinate systems such as selective compliance assembly robot arm (SCARA), which is particularly used in electronic circuit board assembly applications, have emerged.

Each joint on a robot introduces a degree of freedom. In general, a robot with 6 degrees of freedom is required for positioning the tool to a point in space with any orientation. Although a robot with the highest degrees of freedom can produce the most complex movement, one shall consider other factors such as range and quality of motion corresponding to a given degree of freedom.

The work envelope is a boundary for the region in which the robot operates determined by the extreme positions of the robot axes. The size of the work envelope defines the limits of reach; thus, it is a key characteristic that needs to be considered in robot selection. Although the reach for a Cartesian configuration is a rectangular-type space, the reach for a cylindrical configuration is a hollow cylindrical space, and the reach for a spherical configuration is a hollow spherical space, respectively; the reach for a jointed arm configuration does not have a specific shape.

The basic types of power sources (drives) for robots can be named as hydraulic, pneumatic, and electric. The main advantage of hydraulic actuators is that they can afford large load capacity, but they also have many drawbacks, such as possibility of leaks that may be hazardous in certain applications and a high noise level. An important application of hydraulic robots is in spray painting.

The pneumatic power source is relatively inexpensive; it enables short cycle times, and leaks do not contaminate the work area, but it has limited positioning capability. Pneumatic robots are frequently used in pick-and-place operations and machine loading. Electric power results in uncontaminated work space, low noise level, and better positioning accuracy and repeatability; however, along with limited load capacity compared with the hydraulic power. Nowadays, the electric drive is the most popular type for general purpose industrial robots.

The path control is a means for describing the method that the robot controller employs to guide the tooling through the many points in the desired arm trajectory. The types of path control can be named as point-to-point, controlled path, and continuous path.

Load capacity (payload), which denotes the weight-lifting capacity of a robot, is a key parameter that requires careful analysis. In general, the weights that the robots can hold vary with respect to speed. Furthermore, the shape, surface conditions, and positioning of the object held are also important in terms of load capacity. The user of the robot should pay attention to the conditions under which the load capacity is determined by the manufacturer.

Repeatability and accuracy are the most easily confused attributes. Repeatability is a measure of the ability of the robot to return to the same position and orientation over and over again, whereas accuracy is a measure of closeness between the robot end-effector and the target point, and it is defined as the distance between the target point and the center of all points to which the robot goes on repeated trials. It is easier to correct poor accuracy than repeatability, and thus, repeatability is generally assumed to be a more critical attribute. Repeatability is a vital feature in justification and use of robots because although the accuracy of human workers may be higher, they tend to operate with less repeatability.

Even though robots have numerous advantages compared with humans in the workplace, one shall not consider a robot as a replacement for a worker in performing all manufacturing tasks. For instance, humans are superior to robots for manufacturing tasks that require intelligence and judgment capabilities.

The robots are definitely more efficient, and in certain cases essential, for performing repetitive and highly fatiguing tasks, or for performing applications in environments that are hazardous or dangerous for a human worker to operate. It is also worth noting that a robot can operate three shifts per day for seven days a week with regular maintenance, whereas this schedule would have been impossible for a human worker.

Not only are robots efficient, and in certain cases essential, replacements for humans for performing fatiguing, hazardous, or dangerous tasks, but also they are important for preserving jobs for other workers by increasing productivity.

The major benefits of industrial robots can be named as
- Increased product and market flexibility,
- Increased productivity,
- Improved product quality,

- Shorter cycle times,
- Lower operating costs,
- Higher precision,
- Reduced floor space,
- Elimination of health and safety hazards.

Within the past two decades, the number of robot installations have increased with emphasis on the integration of robots into computer-integrated manufacturing systems.