A robotic leg

This post was more of a note to self than something of actual value. Anyway there were some advances, some tabs open in the browser and I felt it was better to try to share them here for future reference than waiting for an (eventual) finishing point of the project.
Maybe it will be useful for somebody.

In a mobile camera we anticipated the idea of making something that can move. In fact, there was an inspiration in Stubby. Full featured, miniature hexapod. There is more info in Stubby the (Teaching) Hexapod.

My only (and not small) problem with that design were the manual abilities and the tools needed: wooden cutting, mechanization,… I was wandering (physically and mentally) about my possibilities and one of the solutions was to use wood sticks; I’d need to cut and make perforations but that was not too scaring for me.

Internet is plenty of projects such as A spider called “Chopsticks” that is using chopsticks for the legs and Popsicle Stick Hexapod R.I.P.. Their ideas were similar to my own ones and they gave me some encouragement. I also had dicovered Build a 12-Servo Hexapod. It has some limitations but shows some interesting ideas.
Just to comply with my initial statement (more tabs!), we can see some more proyects like
Hexpider with a different design (it can even write!) and 6-legged robot project. All of them have helped me providing insight and ideas about the movement and articulations (at a very basic level, some elaboration is needed that will be shown in further posts).

With these ideas I visited a DIY store in order to get inspiration. I forgot quickly the idea of wooden sticks because I discovered some plastic tubes that seemed to me more convenient: they should be easier to cut and they should be lighter. You can find also alluminun sticks that would have a nicer look, but at this stage of the project the plastic tubes seemed easier to use.

Palo

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My supposition was correct and this material is easy to manage: we can make holes and fix the servo with a screw, as it can be seen in the following image:

La pata #raspi #servo

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The picture is not very good, but it should be enough to get the idea about joining the different parts. I’m very grateful for similar pictures from other projects that provided hints about how to proceed. As you can see I’ve chosen a design wit three servos for each leg.

We have used cable ties for joining some parts, maybe we’ll need some better methds to improve these unions. It should be easy to make more ‘agressive’ operations if needed.

It was quite surprising to see how fast I could configure the leg with these tools, we will see if I can go so fast in the future (hint: no).

For the movement of the legs we had some experience with servos (Adding movement: servos). The whole code was rewritten following the ideas of PiCam.

Rápido-lento-rápido #raspi #err #errbot

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On the software side, I will only show a couple of small programs that can be found at servo.

The first one can move each joint in and independent way (we wanted to be able to test them from the command line legOneMove.py.

We have the three joints associated to three GPIO ports:

servoGPIO=[17, 23,15]

and we will use a function for the transormation of an angle in the needed pulse:

def angleMap(angle):
   return int((round((1950.0/180.0),0)*angle)/10)*10+550

The movement function is very simple:

def movePos(art, pos):
    servo = PWM.Servo()
    print art
    servo.set_servo(art, angleMap(pos))
    time.sleep(VEL)

Shame on me, I discovered that I was needing the last delay because when the program finishes it stops sending the needed pulses and the movement is not completed.

Finally, in

movePos(servoGPIO[int(sys.argv[1])], int(sys.argv[2]))

we are passing as the first argument the joint we are moving (mapped to the adequate GPIO). The second argument is the angle. Notice that no bound nor limit checking is done so, some bad things can happen if the parameters are not adequate.

The second program is legServo.py. It is a simulation of the movements needed for the leg in order to walk: raise the leg, move forward, lower it and move it backwards, and so on…
Some better movements will be needed in the future but do not forget that this is just a proof of concept.

Now we can see a video with a sequence of these movements repeated several times that I recorded with my son’s help.

En movimiento #servo #raspi

A post shared by Fernando Tricas García (@ftricas) on

We can now see another video with some previous tests, taking advantage of the wonderful YouTube video editor, with two joints and with three joints:

The next steps will be to construct the other legs (four or six) and we’ll need to see if we need some more hardware (may be we will need some more input/ouputs in order to control all the servos for the legs and maybe something more). We will need also something for the ‘body’.

This post was published originally in Spanish, at: Una pata robótica.

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