Chapter 6 Electromechanical Systems 125 Copyright Goodheart-Willcox Co., Inc. arm. Pick-and-place (fi xed-stop) robots use this type of control. Programmable fi servo-controlled robots are more complex. These continuous-path robots move from one point to another in a smooth, continuous motion. Open-loop control systems are used almost exclusively for manual-control operations. There are two variations of the open-loop system: full control and par- tial control. Full control simply turns a system off or on. For example, in an elec- trical circuit, current fl ow stops when the circuit path is opened. Switches, circuit fl breakers, fuses, and relays are used for full control. Partial control alters system operations rather than causing them to start or stop. Resistors, inductors, trans- formers, capacitors, semiconductor devices, and integrated circuits are commonly used to achieve partial control. To achieve automatic control, interaction between the control unit and the con- trolled element (load) must occur. In a closed-loop system, feedback is a signal or data that provide information about the interaction between the control unit and the controlled element (load). Feedback can be activated by electrical, ther- mal, light, chemical, or mechanical energy. Both full and partial control can be achieved through a closed-loop system, Figure 6-4. Many of the automated sys- tems used in industry are of the closed-loop type. In a closed-loop system with automatic correction control, Figure 6-5, energy goes to the control unit and proceeds to the controlled element. Feedback from the controlled element is directed to a comparator, which is an element that com- r r pares the feedback signal to a reference signal or standard. A correction signal is developed by the comparator and sent to the control unit. This signal alters the system to conform to the data from the reference signal. Systems that function in this manner maintain a specifi operating level regardless of external variations fic or disturbances. Automatic fabrication methods, packaging, and machining operations have been improved through advances in digital systems. A digital system is a set of components that work together to process numeric information. Digital (numeric) instructions are supplied by variations in pressure, temperature, or electric cur- rent. These instructions are then changed into a series of on/off electrical signals. The signals are processed by the logic circuitry of a computer and are directed to specifi subsystems that perform the necessary operations. fic Electrical power energizes the load device, which performs the work. The load of a system may be electrical actuators or fl uid-power cylinders designed to move fl parts of the robot. When appropriate signals are received, the robot performs the Energy source Controlled element Control Feedback loop Goodheart-Willcox Publisher Figure 6-4. A typical closed-loop system, as shown in this diagram, incorporates a feedback loop.