A Harsh Lesson

    Turns out power resistors have a nasty side to them, I had heard long ago that they could be dangerous if overloaded but until today I didn't know exactly how bad this could be. I had connected the 12V supply of an ATX power supply to test the 12V voltage sense wire, I wanted to see if I could potentially use this wire to sense the voltage at the load rather than just the termination inside the PSU, to acquire a reasonable voltage drop to test this idea I connected the 12V supply to a 1Ω 50W power resistor attached to a heatsink thinking this would be good enough for the short period needed to test it. I knew very well I would be send 12A into the little fellow, had I done the maths at the time I would of realised I was dumping 144W of power into a resistor designed for 50W and most certainly would of abandoned the idea or at least connected several more in series. I had 4 resistors/heatsinks at my disposal which connected in series would of given me 4Ω and 3A each, a very safe 36W to have carried out my experiment.

    But I didn't do this and proceeded with using just the one, I was aware after a few minutes that it was getting hot but decided that it would be safe for a little longer, I would of avoided the resulting explosion a minute or so later if I had stopped there when I though about it.

    Without warning there was a loud pop and the end of the power resistor shot across my desk. The resistor was not flat on my desk but instead about head height when it shot past my field of view dragging a red hot resistance wire behind it, Had I been leaning forwards at the time I would of taken that in the bloody ear burns and all. I did not avoid all injury though, the ceramic core shattered and spewed sand like fragments across my desk and into my right eye which is still sore even now several hours after the incident.

    What did I learn about this little incident and can pass onto others? Don't abuse power resistors thinking that they can take the punishment, they wont simply burn up like their smaller counterparts. even with heat sinking you can easily overload them so always do the maths before using one as a dummy load to make certain it can be done safely.

Been a while.... Again....

Times have changed

     Allot has changed since i last wrote something here, I won't bother explaining why so I can get right down to business. 

     Allot has changed since I last wrote something. Firstly the arm has now been mounted on a chipboard base along with all of its supporting electronics and master arm, this was so I could easily transport it and set it up for use away from my room.

Rough around the edges, 

     Secondly I have long since finished the power supply for it, It looks a little rough around the edges but it more than meets the requirements for the Arm and proved to be an excellent learning experience for myself, I've no doubt ill rebuild it in the future so I can correct some design flaws as well as improve it overall functionality.
    I designed it to do simple DC to DC conversion and provide two internally adjustable outputs with a maximum output of 5A per channel. There is also a selectable 5V, 6V, 8V & 9V output with a a maximum output of 1A. Both of the 5A channels use a LM338K in a TO-3 package and the selectable outputs use the LM78XX in a TO-220 package. Anyway I expect to write more in a later post.

     Thirdly I made a start on building a 20A dummy load but this project has since stalled, I'll return to it in due course but for now It will wait. There have been a few smaller projects like my decade resistance box and lots of miscellaneous bits and pieces. I have lots of time for new new projects now though and some upcoming ones are - 

• A modified ATX power supply (I want the -5V & -12V rails)
• An experiment to see if I can modify an off the shelf function generator kit into something more powerful, 
• A reconditioned 1-12VAC power supply
• A re build of my Pulse Motor

Platform Overview

Please click to enlarge and view the colored sections more easily

     For now though I'm going to write an overview of the Arms platform and the functional blocks it is made up from and their basic function, below you can see a birds eye view of the arms platform broken up into sections, the function of each will be explained below in varying detail.

Slave Arm

    In the above picture (click to enlarge) you will see the slave arm at the top of the picture, This has

now had its elbow servo upgraded from a HS-645MG to use the same HS-805BB Giant scale servo as the shoulder, the HS-645MG was moved up the arm to replace the weak HS-422 used in the wrist.

     Overall I would say the Arm has slowed down a little but has instead gained a huge boost to its overall strength, I'm not too bothered about the slow down as it is still plenty fast enough to keep up with the movements of the Master Arm. The small grey blocks on the servo cables to the bottom right of the picture are ferrites, These were an attempt to filter out noise caused by the fluorescent lighting caused by the lights in the kitchen downstairs, the jury is still out on whether its effective or not or if it's even the kitchen lights causing the problems,

Master Arm

     On the far left you will see the Master Arm, This consists of four HS-422 servo's that have had their control chips removed, the servo wires are then directly soldered to the potentiometer inside the servo to provide a analogue reading. I have kept the Servo motors inside to provide moving resistance otherwise the servo and arm tend to flop around a little and do not hold their position very well. The servo's present control the Base, Shoulder, Elbow and Wrist.

     The small block to the bottom right of the Master Arm is a strip of prototype board used to gather together all the analogue wires (yellow) & their power supplies so it can easily be transferred down a strip of ribbon cable to the Arduino.

Auxiliary controls - Blue

     The Auxiliary controls consist of two Xbox triggers (or so I believe) and a 10K Ohm potentiometer, The trigger marked R provides control over the Grip, the spring loaded nature of the trigger means it has to be held down to maintain a hold on an object, once released the grip returns to its open position. This seems to work quite well compared to using a standard potentiometer as I find it gives you a much faster grip and release time over turning a pot, something the grip servo has no trouble keeping up with.

     The potentiometer provides a fixed control over the rotation of the grip head, I did try experimenting with controlling the rotation with a trigger but it proved to be far too temperamental and twitchy to control. As you can see in the picture I have designed it to fit into the palm of the left hand, Its not terribly comfortable but it does its job nicely.

Auxiliary Panel - Green

    Not a terrible amount to say about the auxiliary panel really, I designed and built the PCB to provide me with a few extra controls to prototype and play with, the only control in permanent use is the switch which I use to toggle between Master Arm control and the bank of potentiometers (Pink) used for precise manual control.

Arduino Mega 2560 - Red

   The Arduino Mega 2560 controls all the functions of the platform and arm such as

• Analogue input from Master arm and Potentiometer bank.
• Control of the 16x2 LCD including the RGB back light.
• Digital output to the Slave Arm.
• Inputs from Auxiliary Panel.

    At some point in the future I make the required improvements for the arduino to sense the input voltage and current from the power supply, should the current be too high or the voltage wander outside a desired range I will have it flash the LCD red and display a warning to alert myself of a undesirable situation.
    I can't really think of anything else to write about the Arduino, I didn't build it myself and explaining its function goes outside the realms of this post, onto the next one!

16x2 RGB LCD - Yellow

    There is nothing special about the LCD to speak off, It's your run of the mill 16x2 character screen with a RGB back light, the only special part about it is the little breakout board I designed to mount it on.

    On the right side of the board is a selectable power input, either an externally regulated 5V supply can be used to power the LCD or the jumper pin can be moved to make use of the onboard LM7805 (SMD caps on the underside), I do need to mark which way round the pins are though before i burn out the arse end of the LCD by accident.

   The three BC547 transistors in the center are used for PWM control of the LCDs RGB backlight, on the far left is a preset potentiometer used for contrast control of the LCD. the connector on the bottom is the input from the Arduino with the three RGB inputs clearly marked on the right side of the connector.

Power Distribution - Dark Red

An old picture from before mounting

    The Power distribution board does exactly what it says on the tin, It provides two completely separate points of distribution to power something, only it's a bit more specialised than that. You could split it down the middle and have two mirrored halves if you wanted with each half providing the following - 

• Reverse Polarity Protection
• A current Shunt (0.01 Ohms)
• A 3:1 voltage divider
• and of course the usual on/off switch, fuse, Smoothing & status LED

Layout and planning of the board

    Most importantly is the ability for the board to provide power to the servo's and redirect information form the Arduino to the servos, the only other option would be splitting up the servo cable so the Red/Black wires goes to the power supply and the Yellow goes straight to the Arduino, something I'm not willing to do.

    I am not sure as to what the maximum current carrying capacity of the board will be, with the tinned traces on the underside to increase capacity I would expect at least 2.5A and likely more than that form it, Maximum voltage is 16V as set by the Nichicon capacitors.

Potentiometer Board - Pink

    Once again there is not an awful lot to say about this board, As you can see in the picture each of he Potentiometers has been assigned to a specific joint on the Slave Arm. It's a PCB with six potentiometers screwed to it... what else do you expect.

As penance for such a crappy last item here is Al typing on a keyboard with a chopstick, Enjoy.