For those of you into robotics, and especially robotic arms, many of you have seen the cheap but impressive robotic arm that is currently on the market for less then $30 (£30 in the UK) These arms have 5 motors, and some models come equipped with a built in USB interface allowing the PC to control the Arm.
Whilst good value for money, these Robotic Arms do have certain limitations, and the biggest by far, is the lack of feed back, not that there is anything to feed back from!
These robotic arms dont actually have servo's as such, but a simple motor and worm drive gearbox with 4 cogs inside it. Anyone interested in Inverse Kinetics, wouldnt be impressed with the way this arm performs, and the supplied software for the USB version just simply relies on a timer, hoping to get the arm in a very near position to where it was programmed to go.
After reading a few very interesting topics on here, where people have interfaced these arms with an Arduino, the lack of positional feedback was very obvious, especially if you wanted any degree of accuracy, ie better then 2" resolution.
After playing around with small variable resistors to give feedback, I decided to strip down one of the "servo's" to see what room i had to play with. The following Tutorial, explains how these "servo's" can be simply and easily modified using 1 part, and a little time to give that much needed feedback, with surprising accuracy.
Parts needed:- 4 infra red slotted opto's.
3 or 4 core flat cable.
Tools needed:- Soldering iron and solder.
Endless supply of coffee!
Please note, the servo shown below is a finished one, hence the extra cable.
Step 1: Surgery
.The chances are, if you have one of these arms, you will have built it yourself and know how it goes together, so in which case, i will skip the obvious and get right down to the basics. There are 4 cogs or gear wheels inside the gearbox, driven by a worm drive. Take all of the parts out leaving the bare shell. Just to the right of the motor mount, and almost opposite the brown gear wheel, we need to cut some of the plastic away from the divide wall, this isnt critical, but a piece about 3mm X 10mm should be suffiicent, this is where the slotted opto will be glued.
Next modify the slotted opto, the mounting lug on the end opposite the chamfered LED needs to be cut off, next cut off about 1mm from the other lug. Next with care, cut off about 1mm of the chamfered housing for the LED, this is best done with the craft knife, be careful with your fingers, and make sure you dont crack the housing, as it is fragile, we have to remove enough plastic to expose the face of the LED, you may need to apply a drop of super glue afterwards, to hold the infra red LED in place. There is nothing special about these slotted opto's, i just happened to have a few in my junk box, practically any type will do.
Once the Slotted opto is cut and modified to fit, its time to prepare the connections, looking at the rear of the opto, and the LED with the chamfered edge nearest to you, cut the top left and bottom right connection short, to use as a terminal post. Next bend the bottom left diagonally to the top right and solder the two together, this forms the common connection or 0 volts.