Project Overview
This robot, like my previous Reach Robots, uses four servo motors. One servo for rotation, one for the jaws, and two for the reach movement. You control its movement using two PS2 joysticks and three push button switches at the rear. Mounted inside of the case are two RGB LEDs, used to indicate whether the robot is active or not. All part of the coding challenge.

The robot and case are two separate items, screwed together; allowing you to attach a different more advanced microcontroller if you wish, like an ESP32 used in my Mk1 version. Power is injected via the DC power socket on the NANO expansion shield, normally from a pack of 2x18650 lithium batteries, and the servo motors run off the shields regulated 5 volt rail..

The design of the mechanical linkages in the reach movement work in such a way that the movement range of the arm, that raises and lowers the head, needs to be constrained depending on the position of the vertical arm, otherwise lock-up could occur.

This potentially damaging limitation, is controlled within the code, with the vertical arm angle being set first, then the limits for the raising arm are derived from that. A calibration diagram is included to explain the angles I determined from my robot. As servo tolerances vary quite widely from one part to another, you will need to calibrate these ranges and enter new values into the code to get it to work correctly.

This critical procedure also applies to the robot if any of the servos fail and need replacing.

As with most servo projects, you will need to set up the servos when assembling the robot. This is achieved initially using a servo tester for course calibration, and two custom Windows apps, I have provided, created using the  ‘Processing’ IDE. The first app receives keyboard commands from the user and sends them to the NANO, which in turn moves the servos. So you can move the robot using keys and gain feedback on the actual servo values. The second provides sliders, which you can use to adjust the servo values, making it quick and easy to find values for specific locations of the robots arm.

The robot also uses code in the form of a ‘movement engine’, which causes the servos to sweep towards target positions, in a number of defined steps, and at a rate specified in the code. This can be used to create a sequence of interesting movements, including the jaws opening and closing.

The circuit diagram is shown here on the left, with the Arduino NANO expansion shield connected to the four servo motors on the right, with 5v DC power being fed to the servos from the on-board linear voltage regulator. The two PS2 joysticks plug directly into the analog pins, whilst the WS2812B RGB LEDs and button switches plug into the digital I/O pins. In addition to the X and Y potentiometers, the PS2 joysticks have built-in button switches too.

There are plenty of spare pins for additional items, should you wish to fit them, and the LED strip could easily be extended to many more LEDs if needed. The biggest challenge was getting all of the wiring to fi inside the case, so I have extended the depth of the case by 5mm to make this much easier. You essentially build the robot and control case as two separate items and then screws the two together.

All of the 3-D models are provided as STL files, zipped together into one file. They can therefore be used with a slicing application of your choice, to create the g-files for use with a 3-D printer. For my project I used PrusaSlicer, which is a free download from the internet.

My parts were printed in PLA, using 1.75mm diameter filament, fed through a 0.4mm nozzle. The first layer was printed at 0.3mm height, and subsequent layers at either 0.3 or 0.15mm, depending on the quality of print I desired. The weight of the printed parts was 185g, which means that you could get five sets from a 1kg real of filament.

The design relies on the joystick covers being printed separately, and then glued to the top plate. The rest of the components are held using nylon screws, or 2x10mm self tapping scrrews.