Building a home robot: Part 5 - arms and hands

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I wanted Roobert to get two identical hands with separate moveable fingers.

Because of this (and the small size) I decided to use a commercial construction kit instead of designing and constructing the hands on my own.

Although it was a construction kit it was fun for hours to assemble the hands:

Each arm is constructed from the hand construction kit, 3 servos, 3d printed servo brackets and an I2C servo controller. Because the servo controller seemed to be unable to shut down the servo power, I attached a relays for each arm to turn the servo power on/on.

The servo holder for the upper arm parts are printed in 3D:

The complete arms:

The right arm:

A roobert-hand-assembling-workplace :-)

Building a home robot: Part 4 - moving the head up and down

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Because the monitor and raspberry pi are placed in the front of the head most of weight is there. So the motors for up/down  movement have to be powerful enough to handle this.

I decided to use two stepper motors with built in gearboxes. They also are the axis for the head movement. I mounted them back to back into a 3d printed box ,so they are moving in contrariwise directions.

Putting together neck and front head:

Assembling including cables, Raspberry Pi and electronics:

The two stepper motors have much power - but still not enough to lift the head. To fix this problem I placed a counterbalance made from lead in the back of the head.

The first test with provisional counterbalance:

After some hours of testing another problem occurred. The mechanic connection between the two motors and the 3d printed part was very small.

Only 4mm PLA to connect  to the whole head wight and movement:

And the PLA material was not strong enough to handle this for a long time – so the freedom of movement increased.

To solve this I used two small parts of aluminum with holes matching to the axis of both stepper motors.

Building a home robot: Part 3 - the chassis

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If you want to build an autonomous robot there are at least two things to solve: Battery power and automatic charging.
Nobody wants to build a robot which needs to be charged by hand.  But building and programming a chassis that can find its docking station and charge automatic is complex and/or expensive.

So it is great that there is an existing consumer product you can use: The roomba vacuum robot from iRobot. You can get a used one for round about 150 €. If its vacuum or brush unit is broken it may cost less - and you don’t need these units.

The Roomba also supports an official and well documented serial port to control it and read its sensors.

So for my robot project I used an old iRobot Roomba.It was originally white color – but after some years of usage and sunlight some sort of ugly yellow instead.

To refurbish it I sanded it, painted it in my favorite color and bought a new battery.


In my case I wanted to use MakerBeans to build the main body of the home robot. So I mounted a small base of MakerBeans to the iRobot chassis. 

To find Roombas serial port you have to remove the robots cover. Next to the serial port there are some screws you can use to attach your own robot parts to the chassis:

 

 

Building a home robot: Part 2 - Neck design and movement

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My first approach to move the neck and head was to use servos. Because the rotation has to move a large weight to rotate the head left/right, I bought a servo driven ball-bearing base.

The first try was a servo driven ball-bearing base.



Mechanical everything worked fine – the base had : enough power solve the movement.
But it was very loud. It made the typical plastic server “scrieeeee” sound and the base amplified the sound by acoustic resonance.
It all sounded like a big cheap RC toy from the 80s :-/ Nothing you want to hear from a futuristic robot with a cute and modern body design.

The next idea was to create laser cut gears on my own and mount them on a big ball-bearing:

As engine I wanted to use a stepper motor

Everything seems fine – even a first manual test moving the motor and gears by hand.
But the first electronic driven test was a little bit annoying: It was much more quiet and strong than the RC servo but still loud.

The next idea to solve this was to use a fan belt to connect the gears to prevent the plastic-on-plastic sound. But for this the complete construction would have to be changed.

So I tried another idea: Creating the motor gear  with 3d printing from rubber instead of laser cut acryl:

This worked very fine and now the gears are working very silent.

The endstop to home the rotation: