Omni-Biped. A lpha Rex is certainly cool. However, the turning mechanism is missing something, don t CHAPTER 5

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1 CHAPTER 5 Omni-Biped A lpha Rex is certainly cool. However, the turning mechanism is missing something, don t you think? In this chapter you ll meet the Omni-Biped (see Figure 5-1), a smooth COG-shifting biped that can walk and also turn. It is omni-directional, hence the name. It shares the leg frame shape with Alpha Rex, but uses motors in a whole different way: each motor drives the leg to which it is attached. This feature allows the robot to turn quickly left and right within its own footprint. Figure 5-1. Omni-Biped with arms an aesthetic addition 143

2 144 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.

3 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...

4 CHAPTER 5 OMNI-BIPED 163 Building Instructions I ll stop chattering it s time to build! You can build Omni-Biped using only the NXT retail set parts. If you stop before building the arms assembly (Step 70), you ll have a walking base to be freely customized. For example, you could follow the MINDSTORMS retail set instructions to build the Alpha Rex upper body to attach to these legs; otherwise, you can embellish the model with many additions, such as arms, hands, a tail, or whatever you desire. You must pay attention not to upset the equilibrium of the robot and avoid excessively protruding or asymmetric features. To start with, build the body as illustrated in Steps

5 164 CHAPTER 5 OMNI-BIPED Figure 5-6. Omni-Biped bill of materials

6 CHAPTER 5 OMNI-BIPED 165 Table 5-1. Omni-Biped Bill of Materials Quantity Color Part Number Part Name DAT Electric MINDSTORMS NXT Ultrasonic Sensor 4 White DAT TECHNIC Beam 9 6 Dark gray DAT TECHNIC Beam 11.5 Liftarm Bent 45 Double 1 Black DAT Electric Cable NXT 20cm 1 Black DAT Electric Cable NXT 35cm 1 Black DAT Electric Cable NXT 50cm 6 White DAT TECHNIC Beam DAT Electric MINDSTORMS NXT Motor DAT Electric MINDSTORMS NXT 2 Light gray DAT TECHNIC Axle Joiner Perpendicular with 4 Pins 2 Black DAT TECHNIC Pin Long with Stop Bush 6 Light gray 3648.DAT TECHNIC Gear 24 Tooth 2 Black DAT TECHNIC Axle Joiner Perpendicular 3L 6 Dark gray DAT TECHNIC Beam 3 2 Black DAT TECHNIC Angle Connector #2 2 Light gray DAT TECHNIC Beam 5 Bent 90 (3:3) with 4 Pins 8 Dark gray DAT TECHNIC Beam 5 Liftarm Bent 90 (4:2) 2 Light gray DAT TECHNIC Axle 5 5 Dark gray DAT TECHNIC Beam 5 8 Dark gray DAT TECHNIC Beam 7 Bent 90 (5:3) 2 Black 3706.DAT TECHNIC Axle 6 6 Dark gray DAT TECHNIC Beam 7 Liftarm Bent 53.5 (4:4) 4 White DAT TECHNIC Beam 7 2 Black 3707.DAT TECHNIC Axle 8 2 Light gray 3713.DAT TECHNIC Bush 2 Light gray 3647.DAT TECHNIC Gear 8 Tooth 4 Black DAT TECHNIC Angle Connector #1 28 Blue DAT TECHNIC Axle Pin with Friction 6 Light gray 6536.DAT TECHNIC Axle Joiner Perpendicular 2 Black DAT TECHNIC Axle 2 Notched 53 Black 2780.DAT TECHNIC Pin with Friction and Slots 2 Light gray 3673.DAT TECHNIC Pin 2 Black DAT TECHNIC Angle Connector #4 (135 degree) 6 Black DAT TECHNIC Angle Connector #6 (90 degree) Continued

7 166 CHAPTER 5 OMNI-BIPED Table 5-1. Continued Quantity Color Part Number Part Name 8 Orange DAT TECHNIC Bionicle 1 3 Tooth with Axlehole 2 Light gray DAT TECHNIC Gear 20 Tooth Double Bevel 18 Black 6558.DAT TECHNIC Pin Long with Friction and Slot 2 Dark gray DAT TECHNIC Axle Joiner Perpendicular with 2 Holes 4 Dark gray DAT TECHNIC Axle Joiner Perpendicular Double Split 14 Light gray 4519.DAT TECHNIC Axle parts total (all included in the NXT retail set)

8 CHAPTER 5 OMNI-BIPED 167 Start building the right foot. In Step 2, insert the blue axle pins at the end of the bent beams.

9 168 CHAPTER 5 OMNI-BIPED Add a 15-long beam with 8 black pins, then join the foot parts with the dark gray bent liftarms. Finally, add the ankle hinge.

10 CHAPTER 5 OMNI-BIPED 169 Add the other two ankle hinges that allow the biped to bend the ankle to shift the weight smoothly. The right foot is completed.

11 170 CHAPTER 5 OMNI-BIPED Now build the right leg. Here you must use two 15-long beams and a 9-long beam to join them.

12 CHAPTER 5 OMNI-BIPED 171 Reinforce the leg using 9-long and 7-long beams. Add the bent beam where the legs cams will be attached and add the black pins.

13 172 CHAPTER 5 OMNI-BIPED The right leg is done.

14 CHAPTER 5 OMNI-BIPED 173 Start building the left foot. In Step 27, insert the blue axle pins at the end of the bent beams.

15 174 CHAPTER 5 OMNI-BIPED Add a 15-long beam with eight black pins, then join the foot parts with the dark gray bent liftarms. Finally, add the ankle hinge.

16 Add the other two ankle hinges. The left foot is completed. CHAPTER 5 OMNI-BIPED 175

17 176 CHAPTER 5 OMNI-BIPED Now build the left leg. Here you must use two 15-long beams and a 9-long beam to join them.

18 CHAPTER 5 OMNI-BIPED 177 Reinforce the leg using 9-long and 7-long beams. Add the bent beam where the legs cams will be attached, and the left leg is finished.

19 178 CHAPTER 5 OMNI-BIPED Attach the two legs together, using the central dark gray bent beams as a reference. Place the right leg forward and the left leg backward, as shown.

20 Now you re building the left motor subassembly. CHAPTER 5 OMNI-BIPED 179

21 180 CHAPTER 5 OMNI-BIPED Attach a black pin in a hole of the 24-tooth gear; this gear must be rotated so that two of its holes are aligned with the 5-long beam holes. Use Figure 5-2 as reference.

22 CHAPTER 5 OMNI-BIPED 181 Insert the left motor subassembly in place. The cam pin goes in the free hole of the central bent beams of the leg assembly, the last hole of the 5-long beam goes in the gray pin of the leg, and the external ankle hinge pin goes in the first round hole of the motor assembly s bent beam.

23 182 CHAPTER 5 OMNI-BIPED Now you re building the right motor subassembly.

24 CHAPTER 5 OMNI-BIPED 183 Attach the black pin in a hole of the 24-tooth gear, so that it is the opposite hole with respect to where you placed the pin in the other leg cam. This is not crucial now, but the correct alignment of cams and legs is essential later.

25 184 CHAPTER 5 OMNI-BIPED Insert the right motor assembly onto the robot structure as before. The walking base is completed.

26 CHAPTER 5 OMNI-BIPED 185 From now on, you are building the robot s upper body. You can get creative or continue building as illustrated. Rotate the model and add the perpendicular joiners, blocking them with two 5-long axles.

27 186 CHAPTER 5 OMNI-BIPED Add four black long pins.

28 Add the 7-long beams. CHAPTER 5 OMNI-BIPED 187

29 188 CHAPTER 5 OMNI-BIPED Place the NXT on the legs and put three pins where shown. In the picture you see a flying NXT because the instructions are meant for both those who will use normal batteries or the Li-Ion battery pack, which makes the NXT one unit taller than normal.

30 Place two bent beams to lock the NXT on this side. CHAPTER 5 OMNI-BIPED 189

31 190 CHAPTER 5 OMNI-BIPED Turn the model to see the robot s back. Add three pins as before.

32 Add two bent beams again and the NXT is now completely locked onto the legs. CHAPTER 5 OMNI-BIPED 191

33 192 CHAPTER 5 OMNI-BIPED Build the robot s head.

34 CHAPTER 5 OMNI-BIPED 193 Attach the right motor to NXT output port B using a 50cm (20 inch) cable. See the next step (85) to see where to pass the cable.

35 194 CHAPTER 5 OMNI-BIPED The cable must pass tightly in the space between the motor s white beams, and you must block it there with a long pin with the stop bush. The cable turn in the bottom of the foot must clear the ground or the robot won t walk correctly.

36 CHAPTER 5 OMNI-BIPED 195 Attach the left motor to NXT output port A using a 35cm (14 inch) cable. Pass and block the cable as shown. Check the previous caption as a guide.

37 196 CHAPTER 5 OMNI-BIPED Attach the Ultrasonic Sensor to NXT input port 1 using a 20cm (8 inch) cable.

38 CHAPTER 5 OMNI-BIPED 197 Start building the arms decorative assembly. This submodel is optional and can be replaced or customized as you want, just paying attention not to compromise the robot s balance.

39 198 CHAPTER 5 OMNI-BIPED Build the arms themselves and attach them to the rest of the assembly.

40 Build the hands. CHAPTER 5 OMNI-BIPED 199

41 200 CHAPTER 5 OMNI-BIPED Attach the arms subassembly to the robot.

42 CHAPTER 5 OMNI-BIPED 201 The Omni-Biped is completed. Summary In this chapter, you ve been introduced to a small and quick biped robot the walker alternative to the LEGO wheeled Tribot. This is a simple project, with respect to other ones you ll find later in the book. Still, it offers many ideas, and an occasion to show some techniques that could be useful in other situations. Alpha Rex inspired the Omni-Biped legs shape, but notice how small but fundamental modifications have notably improved the gait, especially regarding turning. The motor is a mobile part of the leg itself, while in Alpha Rex, motors are hung inside the leg frame and don t move during the gait. Here, every motor drives a whole leg, both stepping and leaning, while in Alpha Rex the motors control different movements of both legs together; that is, one motor controls robot leaning, the other controls the stepping. Furthermore, the turning mechanism is much more elegant in Omni-Biped than in Alpha Rex, whose solution (with rubber grippers) is not realistic. On the software side, you saw the difference between single-task and multitask programs. We also touched the tip of the iceberg with regards to concurrent programming difficulties and their solution: the mutual exclusion semaphores. Also, the modulo operator (%) could

43 202 CHAPTER 5 OMNI-BIPED be useful in your future projects. Finally, we analyzed the operation, the uses, and the implementation of the hysteresis cycle. Having three sensor ports left, there s plenty of space for add-ons and new features. Don t forget the third motor in your box that is waiting for action! The following exercises might also inspire you to build some new type of robot. If your creativity needs to be sparked some more, keep on reading the next chapters. Exercise 5-1. Hardware Ideas Rebuild the leg frames to shape a chicken-like leg, with a reverse bent knee. Modify the motor placement accordingly, to keep using the motors as structural support. Add a tail and a head (like a dinosaur) that would follow the legs movement, helping the COG shifting. For example, when the robot is leaning left, the tail would be bent left, and the same for the right side, balancing the robot. You might drive the tail with the third motor or use the leg motors themselves. After having read the AT-ST instructions in Chapter 4, try to add sensors and modify the software to let your robot reset its leg position automatically at program startup. Don t let that third motor go to waste! After having read Chapter 7, you can use its fetching arm as a starting idea to develop a grabber for Omni-Biped. Could you give it the ability to find objects autonomously? The world is full of line-following robots on wheels. There aren t that many line-following walkers. There s no need to say a word more. Do it! Exercise 5-2. Software Ideas Using the multitask version of the software, add new tasks to play a melody while walking, and display animation. Be careful when using the mutex variables to synchronize display access for animation and string messages, and voice announcements with music. After having read Chapter 3 about FSMs and Chapter 6 about the NXT Turtle, try to give the Omni-Biped an autonomous behavior. You can use as state names (and corresponding functionality) Lazy, Normal, Worried, and Dancing; transition events among states could be an incoming obstacle, a sharp sound, a sound pattern, or a timer elapsing. The robot in Lazy state could stand still, performing some random movement; when it senses a sharp sound, its state could become Worried and it would walk a bit. If the sounds continue around it, the Dancing state could be triggered, and the robot would eventually start to dance. In Normal state, finally, it could walk, avoiding obstacles. These are just a few ideas, but you can customize the robot s behavior as you prefer.