Let's Make Robots!

Robot arm design, my ideas and progress so far.


Building an arm for your robot can be the hardest part of robot design. There can be many different designs besides a humanoid design. For example you might have a CNC style XYZ axis or even a tentacle design where multiple joints are surrounded by cables. For BoozeBot I've decided to go for a humanoid design because I would like him to be able to pick things up like a person would. Reguardless of your design the first problem you are likely to face is strength vs weight.

As a rule, the stronger your arm, the heavier it will be. This means more battery power wasted just moving the arm before it even tries to pick anything up. For this reason it pays to have most of the weight in the shoulder or better still in the body of the robot.

Brake and gear change cables from pushbikes can be used to connect motors within the body to joints of the arm. Hobby shops sell a lightweight plastic version used for model planes so that servos withing the body can be linked to rudders, elevators, ailerons and flaps.

In my design I am mounting my elbow motor with my shoulder motor so the weight of both are supported by the chassis. In doing so I've come up with an idea that allows two small motors to do the job of two big motors thus saving weight and money. Below are the main parts for my shoulder and elbow joint.


I'm using some motors that I think are for powerwindows in cars (about 25W each) and some old grinder gearboxes. I have combined two in a custom housing for the shoulder joint. The Elbow joint is straight from the grinder.




The motors will bolt to the chassis allowing the custom box in the centre free movement. When both motors turn in the same direction at the same speed, the whole joint turns allowing both motors to work together to raise or lower the arm.

When both motors run in opposite directions at the same speed, the joint stays in position but the shaft you can see in the photo turns and drives the elbow joint. Once again the two small motors work together to opperate a single joint.

My motors are driven by a special circuit that opperates them like a servo. By having one pot connected to the shoulder and one to the elbow I can drive them like ordinary servos.

Even better. When my software tells one motor, say the elbow motor to move, that motor running by itself causes both joints and therefor both pots to move. This causes the shoulder motor to automatically turn on to maintain the shoulder at the correct position. This means no complicated calculations are needed to control the two motors. The servo controllers do the hard work for you :D

Below is a meccano mockup of the arm and a video to better explain the mechanics.




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genius man, putting all the power for it in the shoulder means less weight in the arm. Looks nothing like a toothbrush though. 

Hi OddBot!

Great design - excited to see it develop !  Does the original picture correspond at all to the design? 

That is a beautiful system. I am at a loss for words, that is such an awesome set-up.

I agree. Did you notice how the "fore arm" stays level when one motor is driven? When at 0:24 in the video the whole arm is almost horizontal, Oddbot turns one motor and holds the other one still. The arm lowers but the fore arm maintains its horizontal angle. It could hold a glass with milk and lower it without spilling!

I suppose equal gear ratios in both joints are imprtant for that.

Great music score Oddbot. Wanna share artist and title?

Hey Rik, after what happened to you when you gave credit for your sound tracks I decided to keep it to myself. Here is a hint. It was a great game in the 90's and it is suited to this site.
Sounds Mythical.
Welcome to Clan Wolf
It is good for big robots that need a strong arm. I am hopeing that it will lift at least a 2 litre milk container for making coffee. I found that I could buy a cheap nasty imported grinder and steal it's gearbox cheaper than what I could buy a gearbox for. Grinder gearboxes are perfect because they have good bearings and strong gears.

Yeah, I'm a little confused with the sholder joint as well. I understand differentials (i've rebuilt a few) but don't understand how that translates here. I see a total of 3 bevel gears (2) inputs attached to motors and one output. I guess I simply don't understand the need for the second motor.

Wait a sec... Is this like a locker? 2 motors together move the whole case up and down, while one fwd/ one rev rotates the shaft? 

G'day Chris, to answer your question. Yes, when both motors spin in one direction, the whole case rotates with the arm attached to the case. In opposite directions the shaft turns and opperates the elbow.