Electronics

Controller

I am using the Arduino board, which is “just” the bare necessities around the Amtel ATmega328 chip. More importantly the Arduino also supplies the infrastructure to get the firmware written and uploaded and a USB connection to the pc.  For an initimate description of the controller see its own pages, this goes down to circuit board drawings.

There are 20 pins of various types. Pin 13 is kind of special (it blinks during boot and such like) so I want to avoid that one. Pin 0 and 1 are used by the serial interface, so they can not be used. I need 4 pins for each stepper motor control, 3 for a DC motor (2 out 1 in), and 2 pins for the heater (1 out, 1 analoge in).  One input pin will be used as error input (stop!).  The analog pins can be configured to be digital. If I use two expensive Stepper motors for the XY and some (cheaper) DC motors for Z and plastic feed that is 11 (4+4+2+2+1) digital outputs, 3 digital input, 1 analog input. Hmmm…  pin shortage can be solved by using som external chips (intelligent steppoermotor controllers) , but I want to minimize that, on principle.

(Alternate controller PIC. )

Motor Interface

This is a H-bridge. I have found several very tiny boards for motor control, but they cost (around 200DKK). But it seem easy to build your own for next-to-nothing by just buying a simple L298N chip at DKK 30/piece plus the same again for the surrounding stuff. The point is that the whole logic and timing is coming from the controller. A single H-bridge is used for a DC motor, two H-bridges for a Unipolar Stepper motor. For bipolar steppers it is enough with four drivers.

Alternate chips to L298N seen: TB6612FNG, L6203, L6225N, TLE4207, MC33926. Chips more suited to Steppermotors (build-in control circuitry and microstepping) A4983, A3967, SLA7042, SN75440N. Also anything with drivers would do, like L6202 and A6800. I found datasheets for all of these. Yes, you need to have some understanding of electronics to understand them. My problem was what was available at which electronics store. Usually the Post&package cost as much as the order, so I want to avoid buying in too many places.

L298N chip has been used in the prototype XYplotter driving small DC motors (8V, 200mA)

Power supplies

Again, easier to buy a standard supply, but it might be possible to salvage from an old pc or such like. There will need be a several amperes for the motors possibly at 12-24V. The controller at 5V comes from the USB connection, as the final machine will only run when connected with a pc, to send the large amount of commands.Only the motors need a supply, and any old transformer should do, it need not be an accurate voltage.

Failsafe

Endstop switches can be mounted. They would be or’ed together and (via a relay?) cut power to all motors, and signal to the controller. Not to be confused with a second end-stop switch (mounted slightly before the failsafe) to be used for automatic calibration.

Front panel

Should or should not? A virtual front panel could run on the pc. This requires more software to be written, both in the controller and on the pc and they need to communicate. A front panel would be useful for setup, debugging, or if/when something else than printing is done. On the other hand it requires a keypad which need to be scanned. It adds cost and no essential functionality. At the moment the front panel is “nice to have”.

Practical Problems

I am not that good with my hands. The ProofOfConcept model showed some weaknesses with using the breadboard. My first soldered prototype board contained a few (but fatal) errors, and a very stupid and large misunderstandinbg on the chip orientation.


3 Responses to “Electronics”

  1. Hi, brilliant information. Do you by any chance have a list of parts with links to buy from?

  2. As you can see on the dates … the project has been shelved. Never got around to buying more parts.

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