Intérpretes musicales destacados

One of the easiest ways to sense the environment is with aswitch. We already implied the use of an electrical switch in Chapter 6, with the car’s bumper. We also saw a nonelectrical switch in Fig. 6-9.

A switch, in electrical terms, is a mechanical device that can make or break an electrical connection. There are many different types of switch, but the one we look at here is mechanically actuated, that is, you change it by pushing on a lever or button.

Figure 7-1 shows switches that can sense the position of their respective machines. The top machine rotates. As it rotates, a lever spins around and pushes on a lever on either the left-hand switch or the right-hand switch.

The rotating machine has a flaw in the way the switches are placed. If the machineovershoots, or goes too far, it may bend the lever or break the switch. The machine at the bottom of Fig. 7-1 has a sliding part. If this machine overshoots, it simply passes underneath the switch with no risk of breaking it. Note how the levers on the switches are pointing in opposite directions. Why are they doing that?

Fig. 7-1. Limit switches.

CHAPTER 7

Sequencing and Programs

A schematic for both machines in Fig. 7-1 is given in Fig. 7-2. There are a few new symbols in this schematic. Note, for example, the alternative ground symbol in the bottom-right corner. You can’t let subtle changes in symbology fluster you, since there are many different ways of drawing the same thing. If it looks like a duck and quacks like a duck, but smells just a bit fishy, it’s still probably a duck.

The switches are along the top. These arenormally openswitches, meaning that when no other forces are acting on them they create an open circuit. No electricity can flow. Once you push the button (or lever, or what have you), the circuit closes and electricity flows through the switch. A normally closed switch is the opposite. Electricity can flow through it until the button is pushed.

Switches that are used to mark the extremes edges of a motion are known as limit switches, since they guard the limits, or ends, of the motion. In this schematic, the left-hand switch closes when the machine is in the LEFT position, and the right-hand switch is for the RIGHT position. The little numbers on the switches refer to the terminal numbers, which may be printed on the switch itself. They have no other meaning.

The big blank boxes labeled load can be anything. A load is something that puts a load on the circuit, using some of its power. It could be a resistor, a motor, or some other device. All we can tell from this circuit is that when the LEFTswitch closes it powers the left-hand load, and when the RIGHT switch closes it powers the right-hand load.

If the left-hand load is a motor that makes the machine move to the right, and the right-hand load is a motor that makes the machine move to the left, what would happen? Not much. But assume that a motor stays on until another motor starts. What then? This requires a more complex circuit than the one shown, but we can gloss over these issue for these examples.

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 off.

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 significant states, where there is a change, count.

Fig. 7-3. Grabbing arm.

Fig. 7-4. Grabbing schematic.

CHAPTER 7

Sequencing and Programs

To wrap up this section, let’s look at some more switches. A switch is defined by how many circuits, or poles, it controls and how many states or throwsit has. The switches we have looked at so far aresingle-poleandsingle- throw, orSPST. Figure 7-5 shows the four common switch configurations.

The SPST switch you should already recognize. Note that this is shown as anormally open(N.O.) switch. SPST switches also come innormally closed (N.C.), which starts closed and opens the circuit when it is switched.

The next switch isdouble-throw, making it anSPDTswitch. The pole is the switching part and there is still just one of these. However, the switch makes a different 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 SPSTswitches into one package so they share a switching mechanism, you get aDPSTswitch. Likewise, a pair ofSPDTswitches 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 influence 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.