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1) Mechanics

A project log for Scribing Robot

Writing on a whiteboard with stepper motors

f4hdkf4hdk 04/22/2018 at 13:140 Comments

SHOULDER and ELBOW

The 2 main articulations (shoulder and elbow) are equipped with stepper motors. The transmission uses toothed belts and their pulleys.

These 2 articulations are very rigid. They are made with 3mm aluminium sheet, and braces made with cylindrical aluminium sections.

The articulations use ball bearing, exactly the same than these used on Lynxmotion articulations : 3mm inside and 8mm outside.

The gear ratio is 4.4 for both elbow and shoulder : 10 teeth on motor side, and 44 teeth on articulation side. The 2 motors are at fixed position, on the elbow. The goal is to limit inertia of the arm. Stepper motors are quite heavy compared to their torque.

To achieve this, the motor for the elbow is located on the axe of the shoulder. Therefore, the distance between shoulder and elbow articulations is determined by standard toothed belt lengths. All the dimensions are deduced from that.

Each of the 2 articulations (shoulder and elbow) allows more than 270° of travel. Therefore, we can reverse the shoulder like most SCARA robots. The goal is to use 2 different tools (pen and eraser) at each end of the pen and reach all the working area with these 2 tools. 

The forearm and the wrist is made with lighter pieces, from Lynxmotion.
We can tolerate more flexibility for these parts. And this part of the arm contributes a lot to inertia; therefore, we want to have lighter pieces.

The length of the forearm and the arm is exactly the same. For forearm, this length is the distance between elbow and tool. This greatly simplifies the reverse kinematics. Warning, the length of the forearm is measured between the elbow and the pen tip (not between elbow and wrist).

WRIST

The wrist is made with a standard RC servo. This articulation does not require good precision or high torque, unlike elbow and shoulder. I have just added a Lynxmotion parts to reinforce this articulation, and mainly a ball bearing at the back of the servo, in order to limit clearance.

The wrist has 2 functions :

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