How the electrons flow
If it is one thing that tends to overwhelm people , it is understanding electrical circuitry . And there is no shortage of it in a machine like this , so let me explain a couple of the basics . I have the machine computer in a standalone cabinet with the monitor , keyboard and mouse . From this cabinet we run a standard 25 pin printer cable to the machine . These 25 pin ports are a little rare these days as most printers are operated by USB , hence the reason I used an older computer with one of these ports already installed . It ’ s no big problem if you don ’ t have one though , as a card with one can be fitted to your computer and plugged into one of the PCI slots on the motherboard . They sell for about $ 30.00 . I understand there are now adapters out there to allow you to run the machine via USB , but I have not had the opportunity to use one of these yet .
The Breakout board
This printer cable carries all of the information required to run the machine to an electronic board called the “ breakout board ”. This breakout board becomes the central interface for all of the machine systems . The drivers for the stepper motors , limit switches , emergency stop buttons and other input and output devices all run through this breakout board . It also provides protection to the computer of any return voltages , through the use of opto isolators . And it all operates at a very safe 5 volts provided from a DC plug pack type adapter . I use a plug pack which used to be a battery drill charger . From this board we run three wires to each motor driver unit , two wires to an emergency stop button and if you run limit switches , they all get connected together in series and run two wires back to the breakout board . There is an additional 48 volt power supply which connects directly to the stepper drivers to provide power to the motors . When you connect it all up it looks like a bit of a birds nest , but really it is pretty straight forward . The Homann designs website I mentioned earlier has some schematic diagrams of the hook up in their documents section which gives a pretty good pictorial presentation .
The Stepper motor drivers
These driver units are a very clever piece of equipment , but before we get into it , let ’ s have a look at what we need it to do . The most common stepper motors will step at 1.8 degrees per step , or if you like , 200 steps per full revolution . If we have the motor connected to a rack and pinion drive of say 50mm pitch per revolution we could expect our axis to move a lineal distance of 50mm per turn or if we divide this by 200 it will move 0.25mm per step . Now although we may be happy with this tolerance , these driver units have the ability to do what is called micro stepping . Through the selection of small switches on the driver it can actually step and hold the motor at intervals between each of its normal steps up to around 25,000 steps per revolution . As you could imagine this really starts to tighten up the tolerance of travel . But you get nothing for nothing . If you start running the steppers at this rate , everything slows down because the computer can only provide the pulses at a certain speed . It becomes a balancing act of how accurate you want versus how fast you want it to run .
Top : Retractable undercarriage mounting flex plates cut from 3mm G10 Above : Flight control horns cut and shaped to the exact size for the jet . Saw toothing and small holes ensure maximum gluing strength .
Next edition
That has probably given you enough to think about for now . In part two of this article I will go through the construction process of the machine itself . I live in Auckland and am happy to discuss or give any help I can to others . Feel free to contact me at mvfiasco @ xtra . co . nz or 09 5330076 or 021734854 .