Omni-Biped

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 

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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 . . .


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 74–111.

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Figure 5-6. Omni-Biped bill of materials


Table 5-1. Omni-Biped Bill of Materials 

Quantity Color Part Number Part Name 

1 56467.DAT Electric MINDSTORMS NXT Ultrasonic Sensor 

4 White 40490.DAT TECHNIC Beam 9 

6 Dark gray 32009.DAT TECHNIC Beam 11.5 Liftarm Bent 45 Double 

1 Black 55804.DAT Electric Cable NXT 20cm 




2 53787.DAT Electric MINDSTORMS NXT Motor 

1 53788.DAT Electric MINDSTORMS NXT 

2 Light gray 48989.DAT TECHNIC Axle Joiner Perpendicular 1×3×3 with 

4 Pins 

2 Black 32054.DAT TECHNIC Pin Long with Stop Bush 

6 Light gray 3648.DAT TECHNIC Gear 24 Tooth 

2 Black 32184.DAT TECHNIC Axle Joiner Perpendicular 3L 

6 Dark gray 32523.DAT TECHNIC Beam 3 

2 Black 32034.DAT TECHNIC Angle Connector #2 

2 Light gray 55615.DAT TECHNIC Beam 5 Bent 90 (3:3) with 4 Pins 

8 Dark gray 32140.DAT TECHNIC Beam 5 Liftarm Bent 90 (4:2) 

2 Light gray 32073.DAT TECHNIC Axle 5 

5 Dark gray 32316.DAT TECHNIC Beam 5 

8 Dark gray 32526.DAT TECHNIC Beam 7 Bent 90 (5:3) 

2 Black 3706.DAT TECHNIC Axle 6 

6 Dark gray 32348.DAT TECHNIC Beam 7 Liftarm Bent 53.5 (4:4) 


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 32013.DAT TECHNIC Angle Connector #1 

28 Blue 43093.DAT TECHNIC Axle Pin with Friction 

6 Light gray 6536.DAT TECHNIC Axle Joiner Perpendicular 

2 Black 32062.DAT TECHNIC Axle 2 Notched 

53 Black 2780.DAT TECHNIC Pin with Friction and Slots 

2 Light gray 3673.DAT TECHNIC Pin 

2 Black 32192.DAT TECHNIC Angle Connector #4 (135 degree) 

6 Black 32014.DAT TECHNIC Angle Connector #6 (90 degree) 

Continued

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Table 5-1. Continued 

Quantity Color Part Number Part Name 

8 Orange 41669.DAT TECHNIC Bionicle 1 × 3 Tooth with Axlehole 

2 Light gray 32269.DAT TECHNIC Gear 20 Tooth Double Bevel 

18 Black 6558.DAT TECHNIC Pin Long with Friction and Slot 

2 Dark gray 42003.DAT TECHNIC Axle Joiner Perpendicular with 2 Holes 

4 Dark gray 41678.DAT TECHNIC Axle Joiner Perpendicular Double Split 

14 Light gray 4519.DAT TECHNIC Axle 3 

237 parts total (all included in the NXT retail set)


Start building the right foot. In Step 2, insert the blue axle pins at the end of the bent beams.

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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.


Add the other two ankle hinges that allow the biped to bend the ankle to shift the weight 

smoothly. The right foot is completed.

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Now build the right leg. Here you must use two 15-long beams and a 9-long beam to join them.


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.

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The right leg is done.


Start building the left foot. In Step 27, insert the blue axle pins at the end of the bent beams.

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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.

Add the other two ankle hinges. The left foot is completed. 

CHAPTER 5 ■ OMNI-BIPED 175

176 


Now build the left leg. Here you must use two 15-long beams and a 9-long beam to join them.


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.

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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.

Now you’re building the left motor subassembly. 


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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.


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.

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Now you’re building the right motor subassembly.


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.

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Insert the right motor assembly onto the robot structure as before. The walking base is completed.


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.

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Add four black long pins.

Add the 7-long beams. 


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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.

Place two bent beams to lock the NXT on this side. 


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Turn the model to see the robot’s back. Add three pins as before.

Add two bent beams again and the NXT is now completely locked onto the legs. 


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Build the robot’s head.


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.

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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.


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.

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Attach the Ultrasonic Sensor to NXT input port 1 using a 20cm (8 inch) cable.


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.

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Build the arms themselves and attach them to the rest of the assembly.

Build the hands. 


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Attach the arms’ subassembly to the robot.


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

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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.

Omni-Biped

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