The SENTIN3L is a very interesting insect droid which walks on its two front legs and on a well disguised single rear wheel. This robot can be entirely built with the Lego Mindstorms Ev3 retail set and it is described in the book “The Lego Mindstorms Ev3 Laboratory” written by Daniele Benedettelli, where you can find also the building instructions. Keep reading to discover what makes this model so interesting.

This post has been just updated with a video demonstrating the abilities of this droid.

### The legs

This droid has each of the two front legs driven by an Ev3 Large Motor.

If you have a look at the picture on the left, you’ll notice that each leg is in turn made up of two smaller legs driven by the correspondent motor wheel. The smaller legs are assembled to the two motor wheels to transform the angular motion of the motor into steps: while the leg that touches the floors is pushed back to let the robot advance, the leg that is not touching the floor is moved forward to be ready for the next step.

This type of transmission, called cam, is simply obtained in this case by connecting the two technic beams at the diametrically opposed holes of the motor wheel.

### The main body

The SENTIN3L has two arms equipped with a deadly cannon and a terrible machine gun…. 🙂 The droid moves the two arms alternatively back and forth to fire. This alternative motion is smartly achieved transforming the angular motion of the Ev3 medium Motor located under the head into linear motion; in this case too, the two arms are phase-shifted of 180° to achieve the alternative motion. Very cool!

### Walking with a good control strategy

We have already said that the two Ev3 Large Motors drives the two front legs. However it’s not as straightforward at it might seem at first glance: if you turn the motors on at full speed, for instance, the droid will surely fall on its side after one or two unsteady, maybe comic, steps 🙂 !

What you really need to do, is to program the robot to synchronously move the legs, step after step, with the same martial cadence. The solution is to program two PID (Proportional Integrative Derivative) controllers whose goal is to regulate the motor angular position and to change the position reference of 180° (one step) once both motors reach the set point position. And you need to start the motion of both legs from a known motor angle.

More on this once the source code is ready!

Check out the video below and the photo gallery!

Have a look at the SENTIN3L gallery!

The SENTIN3L: a walking insect droid
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