Omni-Biped

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144 CHAPTER 5 ■ OMNI-BIPED History of a Biped The biped robots that you see in these pages are not the first ones I’ve made, but they’re the result of years of trial and error, and much experience. In 2001, my quest to create the perfect biped started. With good old RCX, I achieved the results that were shown in Chapter 1. However, I was limited by the motors’ lack of precision and the everlasting lack of Rotation Sensors. In fact, in the good old Robotics Invention System set, such sensors were not included, but were available only in extension sets. Without Rotation Sensors, the robots’ movements could be driven only using precise timing; that is, to make a shaft turn for a small number of degrees, you had to start the motor, wait for an estimated number of milliseconds, and stop the motor. As you can guess, this method is based on the estimation of motor speed (how much will it travel in this lapse of time?), and therefore is rough. The new NXT servomotors integrate the Rotation Sensor (also called encoder) and so solve that old motors’ precision problem: now you can rotate a motor shaft by an exact amount of degrees. As soon as I got the new NXT set, in the early phase of the MINDSTORMS Developer Program (MDP), I didn’t even try to build the Tribot—the quick start guide rover bot. I immediately rushed on to the Alpha Rex, due to my curiosity to learn how LEGO designers had created a stable, smooth walking biped. It was a great-looking robot, indeed! It gave me a bright idea about how to integrate the bulky NXT servomotors inside the legs’ frame, and how to use the mass of the NXT brick as a counterweight, thus turning a possible cause of unsteadiness into an advantage. But I have to say that the steering mechanism, based on a rubber gripper, disappointed me. So, I thought up something different. In the Omni-Biped, each leg is entirely driven by a motor to obtain two coordinated movements: the ankle is bent to shift the robot’s center of gravity (COG), and the foot swings back and forth, to achieve stepping. With good reason, this biped belongs to the third of the categories introduced in Chapter 1, the “smooth COG shifting” one. The hardware configuration secures the stability, just as for the Alpha Rex. Furthermore, the software manages all leg-synchronization hassles, because this robot uses no sensor to know which side it’s leaning on or which foot is loaded. Synchronicity between legs is essential for this biped. When both motors turn in the same direction the robot walks straight; when the motors run in opposite directions, the Omni-Biped turns in place. For the biped to walk and turn correctly, you must make sure to align the legs correctly every time you start the Omni-Biped program. To do this, rotate the bevel gear to align the 5-long beam with the diagonal that connects the 24-tooth gear holes. Adjust the alignment of both legs, using Figure 5-2 as reference.
CHAPTER 5 ■ OMNI-BIPED 145 Figure 5-2. Correct alignment of the legs Single-Tasking vs. Multitasking Before introducing the programs for the Omni-Biped, let’s discuss single-task and multitask programs. In the simplest case, a program can be seen as a sequence of actions; for example: • Go forward for three seconds. • Turn left for two seconds. • Wait for an incoming object. • Go backward until the object goes out of sight. • And so on . . .
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