# Robot's architecture

## Mechanics

### Assembly

Servomotors are provided with brackets, which are plastic parts allowing fixation on the side of the motor.

We will also provide a set of screws that you can use for the assembly.

Drawing the frame is your responsability!

## Electronics

### Power

The robot is powered using a 3S (about 12V) battery. This voltage is then stepped down to provide the 5V required for Raspberry Pi.

### Servomotor

MX-12W are servomotors that integrate different parts:

• A DC motor
• A convenient plastic box with numerous screw possibilities
• A gearbox (gears based reducer)
• An encoder (here magnetic)
• A power electronics, including:
• A H bridge
• An half-duplex uart communication bus

### Gearbox

Typical DC motors can provide high rotating speeds with low torque.

The gearbox ratio reduces the rotating speed and increases the torque.

### H bridge

We mentionned the H bridge before without exactly explaining what it is.

An H bridge is an electronic circuit that uses transitors for:

It allows:

• Providing power to the motors (getting amps passing, here 2.5A max)
• Switching the polarity, which changes the rotation direction

Indeed, microcontrollers inputs/outputs only allow for a few mA to go through them. It is therefore not possible to plug a motor on them. H bridges can be used to provide this power and invert direction.

### Communication bus

Dynamixel servomotors like MX-12W are plugged using 3 pins connectors: ground, power and serial data.

The motors are plugged in a serial fashion, therefore those 3 wires are common to every motor plugged (meaning the same power/data/groud wire goes through every motor)

Motors are identified by a software ID, a number that identifies them on the bus.

### Connection with USB2AX

Once you plug the USB2AX board to your computer, it will appear as an UART device. On Linux, you will discover a pseudo-file named typically /dev/ttyUSB0:

\$ ls /dev/ttyUSB*
/dev/ttyUSB0

On Windows, it will appear as a COMx port, where x is a number that you can find having a look in the devices manager.

You can test that the board works by checking devices appearing when you plug it.