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EXPERIMENTING WITH TILT AND GRAVITY SENSORS
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35.1 Sensors to Measure Tilt
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One of the most common means for providing a robot with a sense of balance is to use a tilt sensor or tilt switch. The sensor or switch measures the relative angle of the robot with respect to the center of the earth. If the robot tips over, the angle of a weight inside the sensor changes, and this change produces an electrical signal that can be detected by electronics in the robot. Tilt sensors and switches come in various forms and packages, but the most common are the following:
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Mercury-filled glass ampoules that form a simple on/off switch. When the tilt switch is in one position (say, horizontal), the liquid-mercury metal touches contacts inside the ampoule, and the switch is closed. But when the switch is rotated to vertical, the mercury no longer touches the contacts, and the switch is open. The major disadvantage of mercury tilt switches is the mercury itself, which is a highly toxic metal. The ball-in-cage (see Fig. 35-1) is an all-mechanical switch popular in pinball machines and other devices where small changes in level are required. The switch is a square or round capsule with a metal ball inside. Inside the capsule are two or more electrical con-
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FIGURE 35-1 Typical ball-in-cage switches.
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35.1 SENSORS TO MEASURE TILT
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tacts. The weight of the ball makes it touch the electrical contacts, which forms a switch. The capsule may have multiple contacts so it can measure tilt in all directions. Electronic spirit-level sensors use the common fluid bubble you see on ordinary levels at the hardware store plus some interfacing electronics. A spirit level is merely a glass tube filled, though not to capacity, with water or some other fluid. A bubble forms at the top of the tube since it isn t completely filled. When you tilt the tube, gravity makes the bubble slosh back and forth. An optical sensor an infrared LED and detector, for example can be used to measure the relative size and position of the bubble. Electrolytic tilt sensors are like mercury switches but more complex and a lot more costly. In an electrolytic tilt sensor, a glass ampoule is filled with a special electrolyte liquid that is, a liquid that conducts electricity but in very measured amounts. As the switch tilts, the electrolyte in the ampoule sloshes around, changing the capacitance between two (or more) metal contacts.
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35.1.1 BUILDING A BALANCE SYSTEM WITH A MERCURY SWITCH
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You can construct a simple but practical balance system for your robot using two small mercury switches. You want mercury switches that will open (or close) at fairly minor angles, perhaps 30 to 35 degrees or so just enough to signal to the robot that it is in danger of tipping over. You may have to purchase the switch with these specifications through a specialty industrial parts store, unless you re lucky enough to find one on the used or surplus market. Mount the switch in an upright position. If the level of the robot becomes extreme, the switch will trigger. You can directly interface the switch to an I/O (input/output) line on a PC or microcontroller. However, you ll probably want to include a debounce circuit in line with the switch and I/O line since mercury switches can be fairly noisy electrically. A suitable debouncer circuit is shown in 29.
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35.1.2 BUILDING A BALANCE SYSTEM WITH A BALL-IN-CAGE SWITCH
The four-conductor ball-in-cage switch is a rather common find in the surplus market, and it s very inexpensive. If the switch is tilted in any direction by more than about 25 to 30 degrees, at least one of the four contacts in the switch will close, thus indicating that the robot is off level. You can use a debouncer circuit with the ball-in-cage tilt sensor. Because the ball-in-cage sensor has four contacts (plus a center common), you can either provide independent outputs of the switch or a common output. With independent outputs, a PC or microcontroller on your robot can determine in which direction the robot is tilting (if two contacts are closed, then the ball is straddling two contacts at the same time). However, unless you come up with some fancy interface circuitry, you ll need to dedicate four I/O lines on the PC or microcontroller, one for each switch contact. Conversely, with the common output approach you can wire all the outputs together in a serial chain. The switch will close if the ball touches any contact. This approach uses only one I/O line, but it deprives the robot of the ability to know in exactly which direction it is off level. A variation on this theme is to use resistors of specific values to form a voltage divider. When you connect the resistors to an I/O line capable of analog input, you can easily determine by the changing voltage at the input which contact switch has been closed.
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