Building a Robotic Arm — Part 2

Building the mechanical assembly

In the first part of this series I listed the parts required to build the mechanical assembly for a robotic arm. You might remember my delight is discovering that no instructions were included! Yes, I’m one of those silly people who assemble IKEA furniture and then look at the instructions to see if I did it correctly. (Surely having parts left over just means I have assembled it more efficiently, right…) Realising that most people won’t want to spend an hour staring at a pixelated image of the assembled arm on the amazon product page to determine the best order in which to assemble it I decided to create some instructions.

Assembled robotic arm

There’s a few ways you can put these arms together. I opted to copy how the arm was assembled according to the pictures on the amazon product page.

A quick note on fasteners

Before we begin there are 4 types of fasteners used in this robot arm kit. To avoid any confusion in this guide I will refer to them by a number as shown in the image below.

Nuts and bolts
  • Machine screw #1 is used to attach the metal servo horn to the servo and also to attach the servo horn to the metal parts of the robotic arm. These screws came with the metal servo horns and were not part of the robotic arm kit. Some equivalent screws were included with the arm kit but I decided not to use them as they were shorter.
  • Bolt and nut #2 are used to connect all the non-moving metal parts together.
  • Bolt and nut #3 are used with a bearing (not pictured) to connect the 4 main moving joints of the robotic arm.
  • Bolt and nut #4 are used to attach the body of the servos to the robotic arm.

#1, #2, and #3 all have the same thread which I believe is an M2 thread. #4 appears to have an M4 thread.

Right, with that out of the way let’s get going! Read More

Building a Robotic Arm — Part 1

Introduction

Apparently a quarter life crisis is a real thing. Not wanting to miss out on this phenomenon I decided to build myself a robotic arm. Not the sort of arm that replaces a missing limb, but the sort that are used in factories around the world to pick, place, and construct products.

Unfortunately living in an city quickly shrunk my dream of building a giant robot to a more apartment friendly size. Probably for the best!

I have absolutely no idea what I will use a robotic arm for. I suspect the desire to make one came from watching Simone Giertz and her “Shitty Robots”. These robots are designed to complete a single task, but instead fail spectacularly often resulting is a huge mess.

Since I am mainly interested in electronics and software I opted to buy a kit for the mechanical parts. There are several options out there; searching ebay, amazon, and Alibaba for “robot arm” yields 1000’s of sensible results. Most variants are built around standard hobby servo motors that are typically used in remote control cars and planes. While these motors are not exactly known for their repeatability, smoothness, or accuracy they are very cheap!

I opted for a “Mirocle Aluminium Robot 6 DOF Arm” from amazon which was £22.50. Unfortunately it has subsequently gone out of stock, but searching ebay for “6 DOF robot arm” gives many identical kits.

My kit, like most of the ones I saw, did not come with servos. I ended up buying six super cheap “Tower Pro MG996R” clone servos off ebay for £21.50 delivered. Given the price the servos are actually of reasonable quality, and have metal gears. However, if you are not a cheapskate like me, buy better servos!

Tower Pro MG996R servo with top removed

The servo horns supplied with the servos were plastic. I believe my robot arm kit was supposed to come with some replacement metal ones, but they weren’t in the box. Instead, I purchased some metal ones off ebay relatively inexpensively.

Now I had everything I needed to build the mechanical assembly of the arm. I was very happy to find that no instructions were included with kit. Figuring it out myself is much more fun 🙂

All the parts ready for assembly (Knolling)

Click below to read the next post:

Part 1 — Introduction (You are here)
Part 2 — Building the Mechanical Assembly
Part 3 — Designing the Electronics System (Pending)
Part 4 — Developing the Microcontroller Software (Pending)
Part 5 — Developing the PC control software (Pending)

Strandbeest Model

Strandbeest are an invention by Dutch artist Theo Jansen. The “creatures” are designed to live on their own on beaches in the Netherlands, taking power only from the wind. They are constructed almost entirely from yellow plastic tubes and scrap materials. Some Strandbeest are even able to detect when they are entering the sea, and switch direction to go back up the beach.

I saw a Strandbeest model on eBay and couldn’t resist buying it and building one. Below are some pictures and videos from the assembly process.

All the parts

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Cheap LED Torch Repair

I recently purchased a torch from Amazon for a grand total of £2.29 including delivery from China. The torch had very good reviews, one even claiming that the torch would change my life! I was very sceptical about how good the torch would be; with such a low retail price cost the BOM cost would have been pennies.

After waiting 3 weeks the torch finally arrived. The torch appeared to be mechanically well made, however after inserting the battery it became clear that it did not work. Considering the cost this was hardly surprising. I decided to take the torch apart to see how it worked and what was wrong with it.

The torch came apart very easily; the main mechanical components can be seen below.

Inside the torch head the electronics for the torch can be seen. Given the price I was expecting a simple series resistor or just I direct connection to the 1.5V AA battery. Instead an IC was used to drive the LED. The markings on the IC indicate it is a CX2859. A quick search shows that this is likely to be a 3 level PWM dimmer IC. Unfortunately I could not find a datasheet or any more information about the chip.

The soldering on the board wasn’t great, but everything seemed to be connected OK.

Using the continuity tester on a multimeter I identified the torch’s circuit layout. The schematic is shown on the below.

I removed the LED and connected it to a 3V coin cell battery. The LED worked fine, meaning the issue was with the driver circuitry.

With a battery connected I checked the voltages at various points on the circuit board. Unfortunately everything seemed fine indicating that it was likely to be the driver IC that was faulty.

I noticed on the torch’s Amazon page that it would also work with a 3.7V 14500 lithium ion cell. Since I didn’t have one to hand, I tested it at 3.7V using a bench power supply. Unfortunately that didn’t work either.

I will probably just bodge a constant current driver circuit into the torch as I doubt I will be able to source a replacement CX2859 IC. As always, you get what you pay for!

Bluetooth controlled LED flashing Frisbee

A few friends and I decided it would be a good idea to make a light-up Frisbee so we could continue playing with it after it became dark. The first version featured 8 LEDs located around the edge of the Frisbee. Each LED was connected in parallel via a resistor to a small LiPo battery. This worked well and looked really good in the dark.

I decided to make a second version using a microcontroller so that the LEDs could be flashed in various patterns. Since I had several BLE112 Bluetooth Low Energy (BLE) modules left over from a previous project, I decided to make a Bluetooth controlled flashing Frisbee! Why…? because “Everything is better with Bluetooth” (Sheldon Cooper).

The BLE112 modules include a 8051 microcontroller which can be programmed using a propitiatory scripting language called BGScript. BGScript was created by Bluegiga, the manufactures of the BLE112 modules. BGScript works great for simple applications, so it was perfect for this project.

As part of a previous project I had designed a small PCB to breakout the pins on the BLE112 module to through hole 0.1” headers. The PCB was 1.3” x 1.6” and manufactured by OSH Park. In the interest of hacking the Frisbee together quickly I decided to reuse this PCB as well.

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Kindle 3 (Keyboard) Screen Replacement

My Kindle’s screen recently broke and unfortunately it was out of warranty (and it was probably my fault anyway!). While amazon did offer to sell me a new Kindle at a reduced price, I wanted to keep my old kindle due to its worldwide free 3G web browsing capability. I ordered a new screen from eBay for around £40. Changing the screen was pretty simple and took about 30 minutes.

I made video of the process in case instructions would be useful for anyone. I meant to take lots of photos during the process, but I was a little pushed for time, hopefully what I did take will suffice. Read More