After following many episodes of Robotica I was fascinated by one of the winners: Flexy Flyer. It is so simple and efficient on going everywhere and “over” everything. Starting on self-constructing an outdoor robot, this is the mechanical configuration I’ve thought about.

The most important thing on this kind of articulated chassis robot is the central hub. There are a lot of flanged bearings available, different quality, precision and, of course, costs. A very robust and cheap one can be found at Profibohrer on-line shop. My choosing process started from the central hole size, it must be large enough to allow all the connections between the two parts. With a 25mm diameter size this is the right bearing. A little bit heavy, but robust more than enough. A couple of them joined with a pipe, make the right joint for this robot.

Let me continue with the choosing of the right motors. Using the Drive Motor Sizing Tool on RobotShop, this is the result:

The motion will be managed by a couple of dsNav smart motor controller boards. One used as a master receives the commands from the main controller about speed, direction and distance, computes the speed to impose to each one of the front wheels and works to maintain it in every load condition. Then sends the computed speed to the slave motor controller in order to have each rear wheel at the same speed of the front wheel of the same site. In  this way we have a 4WD differential steering system. Controlling current and speed for each motor, we can control slippage, and stability of the robot in a some kind of ESP.

An Arduino board will manage the sensors, the batteries status, the actuators and every other low level service, receiving and sending data to the main board.

Following the experience I’ve made with mbed and GPS sensors I want to use this kind of high level controller to plan an autonomous navigation in an unknown environment like David P. Anderson has done with is Journey Robot.

Also with the telemetry I want to recycle the experience I’ve already done linking the robot and the console via WiFi protocol.

INPUT

Total mass of robot: 8 Kg

Number of drive motors: 4 [#]

Radius of drive wheel: 0.06 m

Velocity of robot: 1 m/s

Maximum incline: 20 [deg]

Supply voltage: 12 [V]

Desired acceleration: 0.1 m/s^2

Desired operating time: 30 min

Total efficiency: 65 [%]

OUTPUT (per drive motor)

Angular Velocity 159.24 rev/min

Torque* 6.5024 kgf-cm

Total Power 10.631 W

Maximum current 0.88595 [A]

Battery Pack 1.7719 [AH]

The Micromotors motor E192-12-25 fits the requirements:

Maximum torque 90 N cm (9.2 Kgf cm)

Speed 105-160 rpm (Max torque - No load)

Current 1.75-0.4 A (Max torque - No load)

A TANK-3A HP H-bridge is the perfect driver for this kind of motor.

The quadrature encoder AMT102-V and a Mounting Hub complete our needs for the motion.

A first bill-of-materials:

2 x flanged bearings, 4 x E192-12-25 motors, 4 x TANK-3A HP H-bridges, 4 x AMT102-V quadrature encoders, 4 x Mounting Hubs,

2 x dsNav motor controllers, 1 x Arduino board, 1 x mbed board, 1 x WiFly module.