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Rotating and Lifting Head Pt2
Thursday, 23 August 2007

This is the second part of my 914 rotating AND lifting head mod. I'm writing about it as I build it so I can talk about issues I have resolved and show progress as it goes, but it's still looking pretty good so far!. Check out part one if you missed it before.


I've started on the electronics this week, they are based around two PICAXE 18 'high power' project boards which have four power output devices each capable of sinking 1.5A which is fine for my 1A per phase stepper motors. This will be interfaced to the serial port of the mini-itx board so that I can send commands like 'head up/down' etc.

I also built an SMA extension lead for my wireless bridge so it can be mounted in the drive bay area of the Bot - while the aerial is kept in the head where it is not surrounded by metal.

The eye assembly still fits in the head shell along side the lifting mechanism. I've made a small aluminium bracket to hold it in place - both this and the aerial are mounted on the studs/screw threads that originally were the mounting points for the head plastic to the chassis.

The rotating stepper motor is coupled to the turntable part as originally planned. However, since the motors are 1.8 degrees per step, even with stepper motor interleaving for half steps, the motion is a little jerky. I'll be building a custom gear box over the next week or so to bring this down by a ratio of at least 3:1. I also found that the original coupling between the lifting stepper motor and it's threaded drive shaft was not straight. This made the lifting motion kind of move in a 'wave motion'.  I've made a new longer tighter coupling out of a bit of HDPE that holds the two shafts perfectly end to end.

The long and short of it is that it works ok to lift the head up. It’s a little slow, but this is mainly because the stepper motor cannot be driven at more than 880 pulses per second. I’ve been starting at 200 pps right at the bottom of the lifting range to give more torque, but I’m planning to ramp up the speed over the 3800 total steps for the full range of the lift.

There's a picture below of the view up inside the head shell - you can see there is quite a bit of space there to store extra sensors etc which was the original intention of the project.

Next steps are to polish off the electronics and rotating gear train - then do a little cosmetic tidy up. I've still not decided if I will try to hide the lifting mechanism from view when the head is raised, but I think I'll try flooding the area with lots of blue LEDs first.

You can also find all these pictures in the Image Gallery. Watch this space for part three!

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