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Rotating and Lifting Head Pt3
Tuesday, 04 September 2007
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This is the last part of the 914 rotating and lifting head mod. You can check out part one and part two if you missed them. All the photos can be found in the image gallery.

This week I have been mostly working on the rotating mechanism which I've rebuilt twice. The main issue is that there is quite a lot of weight to move around due to the lifting mechanism and it's stepper motor. The original plan was to use a direct drive method from the stepper motor which was mounted under the top plate of the chassis in the top drive bay. This worked ok, but even using half-stepping the movement was still quite jerky.

I decided that I needed some gearing to make the movement a little smoother. Firstly I built a gear box using some plastic gears and mounted that in the chassis. This worked ok, but the resulting motion appeared to be a little like some sort of Victorian era steam powered invention. I think this is because quite a lot of momentum builds up in the head as it turns, and because there are gaps between the gears this causes a sort of 'buffeting' as the head gear bounces off the motor gear. Of course this is magnified to the outer radius of the head so it is quite noticeable.

As the 914 itself uses stepper motors to drive it's wheels, I took a hint from the way the drive train has been designed and next I used a toothed timing pulley and belt combination. This came out a little larger as the two pulleys are further apart than the gears were, which unfortunately means that the assembly occupies most of the top two drive bays at the rear of the Bot. I relocated my Buffalo Airstation wireless bridge to a slot behind my Alphacool LCD display at the front of the Bot, which is largely wasted space, to make space for the larger size of the gearbox. The results from this drive train is much better - it feels much more solid and there is much less slack when the head is stationary.

I've also sorted out the stepper motor drivers which are each made from Picaxe-18 high power project boards programmed to pulse the right stepper motor phases and the correct time. I've also added a piggy-back board to one of the project boards to take a spare output from the Picaxe to turn on some blue LEDs mounted up in the head shell. Both drivers share an RS232 bus and are currently connected to the mini-itx board's serial port. I can therefore command the rotating motion for speed, direction and distance, as well as turn on and off stepper motor holding power. The lifting stepper motor driver takes commands for 'up' and 'down' and deals with the blue LEDs which can be turned on and off at will.

Watch this space in the next two days for a video of the mod in action!

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