Saturday, September 02, 2006
EZ8 RepRap Schematics
In any case, I indicated I might post some schematics for the controller boards I'm shooting for. I am about 30% finished writing the microcontroller code for this particular set of schematics. My plan initially is to wire everything up for one motor only, and see how well the spline stuff works. These schematics do not have the power control circuitry, but I am just using a LM317T adjustable voltage regulator with a 220ohm / 390ohm resister circuit to give ~3.3V output for the Z8F082A microcontroller and NAND gate (This is the schmitt version, but a standard TTL 7400 should work too, assuming you get the correct voltage to it.) I'll probably use something similar for the 5V required on the SN754410, and I haven't yet figured out what I'm going to do for the motors.
This is where I think I am going with the first EZ8 board I am building -- I'm trying to get control of 2 Extruder drives and the X/Y axis in one device. I think this will be necessary for the accuracy I'll need for the spline curve tracing/fitting algorithms. It assumes at most 2 extruder heads. If there are more, then I think I'll just drive one, and send the signals to a slave controller that can switch among the different heads (assuming only one head needs to print at any one time. Multi-printing would be much more difficult.)
The ZUP signal is to a separate Z-Axis controller, and is used to ask it to lift up temporarily while the head moves to a new printing location (I don't want it to drag across thread that has already been printed.) It will lift it up for as long as the signal remains high. The Z-Axis controller will probably also have the responsibility for maintaining the extruder head temperatures. Haven't figured things out that far yet.
I also have the option to wire in the CTS signal on PA3, but I'm going to leave this off for now. Maybe I'll use this as GPIO for some other function.
Two of these daughterboards would be needed to be able to control all 4 motors - one for the extruders, and one for the X/Y axis motors. This board assumes the motors are 1A max. Each board can easily be converted into a 2A board simply by tying the appropriate inputs and outputs together (and removing one of the rectifiers -- only one is needed per motor). Two of these boards can be converted to a 4A board, again, by tying together appropriate inputs and outputs, and removing the spare rectifiers. These rectifiers are rated at 400V @ 4A, and are $0.95 each at www.mouser.com. They theoretically should be able to handle a larger load, simply because they are off 95% of the time (they only switched on when the motor is generating more back EMF power than is being supplied to them by the H-Bridge.)
This is the motor circuitry itself. I have left the shaft encoder hand-wavy. I might use the design that Forrest has in his schematic, or I might see if I can get an optical encoder to work (I have some from some broken printers I've taken apart).