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CHAPTER 7 Sequencing and Programs
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The machine would cycle. A cycle is a round of events that happens repeatedly in the same order, a loop of action. The cycle described above would have the machine shift to the right until it hit the switch, shift back to the left switches, and then repeat until the power was turned o . Using nothing more than mechanical switches, you can create long chains of actions. For example, imagine the robot arm shown in Fig. 7-3. Assume that it has four actions: the arm can retract (get shorter), extend (get longer), grab, and drop. Each motion has a limit switch labeled RETRACTED, EXTENDED, CLOSED, and OPEN. We assume that it begins in the retracted and open position. What will the arm do if it is hooked up to the control system in Fig. 7-4 Note that these systems also assume that the machine starts in a position that forces a switch closed. While we can t normally rely on the initial position of a machine, we can ignore the complexities of the real world for the sake of illustration. So, in this perfect universe, the arm should extend to its outer limit, close the gripper, retract, and then open the gripper again. This four-step, or four-state, cycle will repeat forever. A state, by the way, is a combination of attributes in this case, the position of the switches and the power applied to the loads. Only the signi cant states, where there is a change, count.
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Fig. 7-3.
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Fig. 7-4.
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CHAPTER 7 Sequencing and Programs
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Fig. 7-5.
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Switches.
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To wrap up this section, let s look at some more switches. A switch is de ned by how many circuits, or poles, it controls and how many states or throws it has. The switches we have looked at so far are single-pole and singlethrow, or SPST. Figure 7-5 shows the four common switch con gurations. The SPST switch you should already recognize. Note that this is shown as a normally open (N.O.) switch. SPST switches also come in normally closed (N.C.), which starts closed and opens the circuit when it is switched. The next switch is double-throw, making it an SPDT switch. The pole is the switching part and there is still just one of these. However, the switch makes a di erent connection in each of its two positions. One of the connections may be N.O. and then the other is N.C. In practice there may not be a distinction unless the switch is spring-loaded to return to a standard normal position. If you put two SPST switches into one package so they share a switching mechanism, you get a DPST switch. Likewise, a pair of SPDT switches work together to make a DPDT switch. Switches may toggle, latching onto their new state after they are adjusted, or they may be spring-loaded so they return to a default position. Switches that return to their default are called momentary contact switches. They perform their switching action while being pushed on but return to a default state when the in uence passes. The switching mechanism may be a button or lever, converting mechanical motion into an electrical connection. However, there are other ways of operating a switch. A vial of mercury with two wires poking into it, for example, makes a simple tilt-powered switch. Air pressure, electricity, and vibration can all be used to control switches.
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Cam Control
Control information can be encoded on a cam. A cam is a wheel with one or more projections. In the simplest form, a cam is a round wheel mounted o -center, like the eccentric cam shown in Fig. 7-6. Eccentric doesn t mean
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