Robot Control Systems

     ROBOT CONTROL SYSTEMS

The actuations of the individual joints must be controlled in a coordinated fashion for the manipulator to perform a desired motion cycle. Microprocessor-based controllers are commonly used today in robotics as the control system hardware. The controller is organized in a hierarchical structure as indicated in Figure 7.9 so that each joint has its own feedback control system, and a supervisory controller coordinates the combined actuations of the joints according to the sequence of the robot program. Different types of control are required for different applications. Robot controllers can be classified into four categories [6]:

(1) limited sequence control, (2) playback with point-to-point control, (3) playback with continuous path control, and (4) intelligent control.

Limited Sequence Control. This is the most elementary control type. It can be utilized only for simple motion cycles, such as pick-and-place operations [i.e., picking an object up at one location and placing it at another location). It is usually implemented by setting limits or mechanical stops for each joint and sequencing the actuation of the joints to

Playback with Point to Point Control, Playback robots represent a more-sophisticated form of control than limited sequence robots. Playback control means that the controller has a memory to record the sequence of motions in a given work cycle as well as the locations and other parameters (such as speed) associated with each motion and then to subsequently play back the work cycle during execution of the program. It is this playback feature that gives the control type its name. In point-to-point (PTP) control, individual positions of the robot arm are recorded into memory. These positions are not limited to mechanical stops for each joint as in limited sequence robots. Instead. each position in the robot program consists of a set of values representing locations in the range of each joint of the manipulator. For each position defined 111 the program, the joints arc thus directed to actuate to their respective specified locations. Feedback control is used during the motion cycle to confirm that the individual joints achieve the specified locations in the program

Playback with Continuous Path Control. Continuous path robots have the same playback capability as the previous type. The difference between continuous path and point-to-point is the same in robotics as it is in NC (Section 6.1.3). A playback robot with continuous path control is capable of one or both of the following:

1 Cremer storage capacity. The controller has a far greater storage capacity than its point-to-point counterpart, so that the number of locations that can be recorded into memory is far greater than for point=to-point. Thus, the points constituting the motion cycle can be spaced very closely together to permit the robot to accomplish a smooth continuous motion. In PTP. only the final location of the individual motion elements arc controlled. so the path taken by the arm to reach the final location is not controlled. In a continuous path motion, the movement of the arm and wrist is controlled during the motion.

    Interpolation calculation. The controller computers the path between the starting point and the ending point of each move using interpolation routines similar to those used in NC These routines generally include linear and circular interpolation (Table 6.1).

The difference between PIP and continuous path control can be distinguished in the following mathematical way. Consider a three-axis Cartesian coordinate manipulator in that the end-of-arm is moved in xyz spare In point-to-point systems, the .r, y, and z axes are controlled to achieve a specified point location within the robot's work volume. In continuous path systems. not only are the .r, Land z axes controlled. but the velocities dx/dt. dy/dt.and dzldl are controlled simultaneously to achieve the specified linear or curvilinear path. Servo-control is used to continuously regulate the position and speed of the manipulator. It should be mentioned that a playback robot with continuous path control has the capacity for PTP control.

Intelligent Control. Industrial robots are becoming increasingly intelligent. In this context. an intelligent robot is one that exhibits behavior that makes it seem intelligent. Some of the characteristics that make a robot appear intelligent include the capacity to:

   interact  with  its environment

   make  decisions  when things  go wrong  during  the work  cycle

   communicate  with humans

   make  computations   during  the motion  cycle

   respond to advanced sensor inputs such as machine vision

In addition, robots with intelligent control possess playback capability for both PTP or continuous path control. These features require (1) a relatively high level of computer control and (2) an advanced programming language to input the decision-making logic and other "intelligence" into memory.