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Rotating and Lifting Head Pt1
Friday, 10 August 2007

ImageThis is the first part of my 914 rotating AND lifting head mod. I'm showing it as I build it, but I'm pretty confident it will work so look out for part two!. All designed and built by James - 914pcbots.com Admin.

Having been quite inspired by Dave Evan's 914 rotating head mod - as I think most people were, I decided to have a go myself. But, I also wanted a place to fit extra sensors so they could scan around, as well as a place to mount my IP web cams without removing the USB web cam in the 914's eye.

I decided it would be quite cool if the head could lift up as well as rotate. This would enable sensors and accessories to be housed under the head and for them to look outwards when the head was raised, if the whole lot rotated too then this would fill all my requirements. This would also allow many possibilities for future expansion of the 914 - and I wouldn't have to cut or drill the plastics. Sounds like a good plan right?... but then it was time to work out how to do it...

Firstly I ordered some professionally cut plastic parts made from 10mm thick High Density Polythene (HDPE). This is great stuff because it machines easily, but it's very sturdy which means that you can screw it together with machine screws in un-tapped holes and they stay there. I ordered two discs with holes in the middle (two in case I made a mistake with one), and then a whole lot of different width strips all one metre long to make the rest of the parts from.

I used eSheet in the UK for this - they were very helpful and very reasonably priced. I just described what I wanted to them and they cut and delivered it within a few days. I've used a 6" Lazy Susan bearing - the hole in my HDPE disc matches the hole on the bearing so cables can run through the middle. I've removed the top head plate from the chassis which will be permanent in order for the lifting mechanism to be fitted in it's place.

I bought some surplus stepper motors on eBay which are 5V/1A per phase and quite large looking, these will be used for both the rotating and lifting. I decided to use a 'direct drive' method for the rotating part as it was the easiest way to make it work - the steps of the motor are 1.8 degrees so this may be too jerky and I'll have to implement some gear reduction at a later stage - but there's plenty of space in the Bot's top two drive bays for this.

Now it was time to build the lifting part. Obviously the lifting mechanism has to stay parallel and raise vertically, but also collapse to a small space to fit under the head - at the same lifting high enough to be useful. I decided that it should raise up at least 4"/10cm or higher. I used the strips of HDPE to build a scissor-lift style mechanism, all the joints have 6mm bearings and all the cross braces / axis are 6mm threaded steel rod with locking nuts. This sits on top of the rotating disc so everything will rotate as one. The stepper motor is hinged and drives the mechanism up/along with a screw thread which runs inside a long threaded hex nut.

Inside the head shell there is another piece of HDPE fitted to it's very inside-top. This will be the new mounting point for the head plastic, the eye / web cam assembly will also be attached to this instead of mounting to the original head plate. As you can see, the head lifts up just over 4"/10cm and the whole thing rotates. There is more than enough space for the IP web cam and probably a small laser scanner and a lot more. Stuff can be mounted right out to the square corners of the Bot because the head shell rotates too, so no need to stick to the inside of the disc - provided whatever is mounted on there is above the top of the body plastics so it doesn't get caught on the corners.

When the head is down it is still free to rotate, and when the eye assembly is back in there (yes it does fit) it will look just like a standard 914.

The next steps are to check out the rotating stepper motor as it's not yet attached to the rotating top part, sort out the driver electronics and test some software, and generally finish it off cosmetically. I'd like to somehow cover the mechanism with an expanding concertina type of housing so you can't see the lifter when it's raised - although this may prove impractical so alternatively I may just flood the entire underneath with blue LED light.

You can also find all these pictures in the Image Gallery. and watch this space for part two!

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