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Chapter 12. Industrial Control > Project 12.1. Build a Robot - Pg. 308

308 Electronics Explained: The New Systems Approach to Learning Electronics This particular circuit in Figure 12.30 does the following: when the ON pushbutton is pressed, current is applied to relay coil 1CR through normally closed contacts of SW1. SW1 is a thermal switch whose contacts stay closed as long as the temperature is less than 300 degrees Fahrenheit (300°F). If the temperature rises above that level, the contacts open, thereby breaking the cir- cuits. Relay contacts 1CR around the ON pushbutton latch the circuit on when the ON pushbutton is released. In the next "rung" of the ladder contacts 1CR are on the relay. They close when the relay turns on. This applies voltage to a heating element. This current flows through the normally closed contacts of thermal switch SW2. A green light in parallel with the heating elements turns on to indicate the heater is on. When the heater temperature reaches 200°F, SW2 opens, turning off the heater and the green light. As the heater cools, SW2 will then close again, applying voltage to the heater. The heater will then cycle off and on keeping its temperature near the 200°F value. If the temperature should rise above 300°F due to a failure of SW2 °F then SW1 opens. Relay 1CR is turned off, and its contacts open, releasing the latch on rung 1 and opening the circuit on rung 2. Thermal switch SW3 closes at 300°F, turning on a red light and ringing an alarm bell. This example could be programmed with a graphical ladder logic program or with special commands and syntax of a traditional programming language. In addition to being able do simple off/off tasks common to industrial control, special hardware in the PLC can also implement counters and timers so that external events can be counted and the sequencing of events can be programmed. Project 12.1 Build a Robot Robots are great examples of basic industrial control. While most industrial robots are usually manipulator arms, you can actually buy a mobile robot with wheels that can be programmed for motion and other functions. A wide range of kits are available. It is a great way to get familiar with sensors, a microcomputer control- ler, and various actuators. Typical inputs are switches, photo cells, temperature sensors, and common outputs are relays, lights, and motors. Some recommended kits are those made by Lego and Parallax. Suggested web- sites to learn more follow: Lego Mindstorms NXT: www.mindstorms.lego.com Parallax: www.parallax.com Two magazines regularly cover robots from a hobbyist/experimenter view- point. These are Nuts & Volts and Servo. You may be able to find these at a news- stand; otherwise, go to their websites: Nuts & Volts Magazine: www.nutsvolts.com Servo Magazine: www.servomagazine.com