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Cheap Reprap Electronics

Posted by wayland 
Cheap Reprap Electronics
March 21, 2010 01:34AM
Building a reprap seems fun! but I have one issue: I'm pretty much broke all the time. Getting stuff for the mechanical parts for a repstrap is easy, I pretty much have everything and am starting to build it. Electronics are harder to scavenge though...

I think I'm going to be using sanguino electronics. I already found cheap electronics surplus bipolar stepper motors (95 cents a piece). I'm pretty sure I can shell out the $3 for the optoendstops and the DC motor driver and PWM, but I guess my biggest issue is the $35 stepper drivers. (By the way, do you need 4 stepper motors and the DC extruder motor? or do you chose one? I'm a little confused). So, any cheap ways to get around the need for expensive stepper motor drivers?
Re: Cheap Reprap Electronics
March 21, 2010 02:58AM
You need three stepper motors for the x,y and z axes, then either another stepper motor or a dc motor for the extruder. You get the best results with 4 steppers, but they do need to be powerful - are you sure the 95 cent ones have enough torque? Post any information they have on them, or take a picture and someone can give you an idea. For stepper drivers, you /might/ be able to get them from the inside of a printer/scanner, but the chance of finding high-current,bipolar,easy to solder ones is minuscule. Its almost certainly easier, whatever your method of income, to spend time making some money then buying them.
Re: Cheap Reprap Electronics
March 21, 2010 04:23AM
check out the electronics I made for mine on the cheap in my blog smiling smiley

basically, anything that accepts gcode and controls 4 lots of stepper signals and a high current heater will run a reprap.

There's the official reprap electronics, many of us make our own, some are repurposing existing CNC equipment, and still others are handing an old computer over to EMC.

Every robotics project (and reprap is a robot) exists on a sliding scale between innovation and expense. if you can't afford expensive, innovate!


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Wooden Mendel
Teacup Firmware
Re: Cheap Reprap Electronics
March 21, 2010 06:36AM
The Pololu stepper driver costs $13 instead of the $35 for the stepper driver at MakerBot. The downsides are that you have to add your own connectors and it needs to be cooled with a heatsink or possibly a fan.



Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]
Re: Cheap Reprap Electronics
March 21, 2010 01:02PM
james: The 95 cent ones seem fine. They were on sale from $10 and run on 12v, I'm not sure about the current off the top of my head but it was somewhere between .5 and 1 amp. A step is 18 degrees, but that should be fine if I get a small enough microstepping driver. Those were all the specs they had, but they aren't small. And even if they're not enough, I'm out $3.

Also, it's not really easier to spend time making money for me; I'm 16 and have been applying for work everywhere I go, but no luck so far.

triffid_hunter: Your blog looks pretty cool. If you don't mind my asking, though, why did you put rotary encoders on your steppers? Is it just a failsafe?

mccoyn: Thanks for the tips on pololu boards. I will look into those.

Edited 2 time(s). Last edit at 03/21/2010 01:09PM by wayland.
Re: Cheap Reprap Electronics
March 21, 2010 09:47PM
wayland Wrote:
-------------------------------------------------------
> triffid_hunter: Your blog looks pretty cool. If
> you don't mind my asking, though, why did you put
> rotary encoders on your steppers? Is it just a
> failsafe?

my steppers have no encoders- where did you get that idea?


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Wooden Mendel
Teacup Firmware
Re: Cheap Reprap Electronics
March 22, 2010 06:20AM
wayland Wrote:
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>A step is 18 degrees, but that should be fine if I get a small
>enough microstepping driver.

Most people use motors that are 1.8 degrees. It does not mean that you can't use them, just that they may have trouble with the print resolution. gears can always be added to alter the linear motion, or if you are using a threaded rod as your drive they may be fine.

What torque to they provide since that is more important that voltage and current.


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[mfsamuel.blogspot.com]
Re: Cheap Reprap Electronics
March 22, 2010 09:44AM
Wayland,
If the steppers go a full 18 degrees per step, and only pull half an amp, you will be low on both torque and resolution. I have been reading about micro stepping. It is great smooth, quite movement from a stepper motor, but the small the micro step size, the less accurate a fraction of one full step they are.

Both problems would be solved by using gears. However, I am still trying to track down an inexpensive source for stepper motor gears so that I can use cable drive for my McWIre and a lower powered stepper motor for the extruder. At the speeds and torque levels we are using in the McWire and Mendel, plastic gears should be fine.

The reason most people use a stepper for the extruder is that you need some way of controlling how fast the plastic comes out, even as there are slight changes in the temperature or consistency of the plastic. With the stepper you go so many steps per second and you know how many cubic millimeters of plastic came out. With a DC geared motor, you get less precision with PWM unless you have some kind of feed back system. An encoder on the output shaft (optical or magnetic), or a pressure sensor inside the melt zone of the extruder.

I hope this helps.

Mike


Team Open Air
Blog Team Open Air
rocket scientists think LIGHTYEARS outside the box!
Re: Cheap Reprap Electronics
March 23, 2010 10:44PM
@ triffid_hunter: my mistake, I think I saw the mouse encoders and assumed you used them as rotary encoders for your steppers - looking back I have no idea how I reached that conclusion.

@ mfssamuel: I think you misread - my motors are 18 degree, not 1.8 degree. I actually just looked back at the store, they have no current specs, so apparently my subconscious made up the "between .5 and 1 amp". It was a surplus store so they don't have the datasheets or anything, and I've tried searching online but no luck. I can toy around with them, though, and use different gears etc. I am using belt drives for the x and y axes and a threaded rod for the z axis - think darwin, but without the nice printed parts.

@ rocket_scientest: If I'm low on torque, no big deal - I can just buy new ones, they were only 95 cents. I'm under the impression, though, that the x and y axes don't really need a lot of torque. I am using belt drives and can just alter the size of the gear running the belt to change the resolution. Thanks for explaining the stepper motor extruder thing, though.
Re: Cheap Reprap Electronics
March 24, 2010 02:49PM
Yes, if you have access to lots of gears, it is easy to slow the motor down to the point where the torque is high enough. This will also help bring the distance moved per step down to something more reasonable for accurate part creation.

To test the steppers that you have, you will need something that creates the sequence of steps. Unlike a DC brushed motor, which you simply hook up to a battery and it runs, the stepper needs up and down pulses (bipolar) or pulses sequencing through all 4 coils (unipolar). The easiest and cheapest way to do that is with the Polulu stepper driver A4983 breakout board or A4983 with voltage regulators. But even with these, you still need a source of pulses to set the rate the motor steps (speed). This can be as simple as a 555 timer with RC timing and a variable resistor. Or an Arduino, Picaxe, Basic Stamp, Attiny, or PIC microcontroller chip if you are comfortable programing them. I would recommend the Arduino or ATtiny as that will get you started learning to program the controllers used on most repraps.

Once you have a source of stepper drive pulses, connect the motor to a small battery (unless you are positive that the are rated for 12V) with a multimeter setup for current measurement in the loop. Setup a stepping rate similar rate to what you expect to use on you machine (a few revolutions per second). Try to hold the shaft of the motor and stall it out. This will show you the max current draw for that voltage. If you started with a low voltage, increase until the stepper starts to get hot, or you exceed the 2 Amps the A4983 can handle.

To determine the best gear ration for your new reprap, the best way is to experiment. Keep adding more and more gear reduction until the stage moves smoothly, then go a little further. The problem is that if the stepper motor ever fails to make a single step while driving the stage, you loose the correct count of steps form the zero point, and you part starts to tilt. You want to have a margin of error in torque so that every step pulse sent out by the control electronics results in the X, Y, or Z axis moving on step, with NO missed steps!

I hope this helps, and have fun playing with the motors!

Mike


Team Open Air
Blog Team Open Air
rocket scientists think LIGHTYEARS outside the box!
Re: Cheap Reprap Electronics
March 24, 2010 04:47PM
Thanks, but I'm already pretty well-read on how stepper motors work. Right now I'm concerned mostly with building the actual cartesian robot and buying the electronics. The pololu drivers seem perfect for what I need, though. I will report back when I have everything set up.
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