Exploring The Prusa Mendel Electronics

Recap: I'm assembling a Prusa Mendel 3D RepRap printer from a kit I purchased from nwreprap.com.

With the physical assembly complete, the next step is to complete the assembly at the electronic level.  After the electronic assembly is complete, I believe the only things that remain are to install the software and perform the calibration and then I should be able to print.

Before I started the electronic assembly, I decided to learn more about the electronic components of my Prusa kit. 

The Big Picture

The Prusa Mendel connects to a PC via a USB cable.  The PC runs various software programs that control the Prusa.  The Prusa Mendel also connects to a 12 VDC power supply.  The USB and power connections terminate on the controller board on the Prusa Mendel.  The controller board is the brain of the machine.  It connects to all of the electronic components.  Designs are loaded from the PC into the controller board in the form of instructions over the USB cable.

Big Picture: Prusa Mendel is connected to computer and power supply

The controller board is actually made up of three components:

• Arduino Mega 2560
• Ultimachine RAMPS 1.4
• Pololu md09b A4988 Stepper Motor Driver (4)

The "base" is the Arduino Mega.  The RAMPS 1.4 plugs into the Arduino.  4 Pololu Drivers plug into the RAMPS 1.4.  The Prusa's stepper motors connect to the Pololu Stepper Motor Drivers and the rest of the electronic components such as the limit switches, thermistors, etc. on the Prusa connect to the RAMPS.

Stepper motors are plugged into Plolu Drivers
Pololu Drivers, 12 VDC power and many other devices are plugged into RAMPS
USB and RAMPS are plugged into Arduino

The following electronics components were included in my kit from NWRepRap.com :

• Controller Board assembly (including Arduino Mega 2560, Ultimachine RAMPS 1.4, Pololu md09b A4988 Stepper Motor Controllers (4) )
• Kysan 1124090 Stepper Motor (5)
• YXDY S-360W-12 Power Supply
• Limit Switches (3)
• MK1 heat bed (previously discussed in this blog entry)

Arduino Mega 2560 R3

This is a very popular controller board that has uses far beyond 3D printing.  It has numerous features that make it ideal for a wide variety of applications.  One novel aspect of Arduino products is the standard way in which connectors are exposed which allows a variety of interchangeable modules to be plugged into the Arduino.  These modules are known as shields.  One of the well-known shields is the RAMPS 1.4 which allows the Arduino to communicate with the Prusa Mendel devices and the stepper motor drivers.

Here is the product page for the Arduino Mega 2560.

Arduino Mega 2560 R3 with USB connector (red)

Ultimachine RAMPS 1.4

Here is the product page for the Ultimachine RAMPS 1.4.
Here is the RepRap.org page for the RAMPS 1.4

RAMPS 1.4 with power connector (yellow) and 4 Pololu Stepper Drivers (red)

Pololu md09b A4988 Stepper Motor Controller

Here is the product page for the Pololu md09b A4988.
Here is the RepRap.org page for the Pololu

According to Pololu's web site, this is basically a breakout board for the Allegro A4988 stepper motor controller which is the large component you see in the center.  The Allegro A4988 web site is here.

Now I'm not really sure why it wasn't possible to integrate the Allegro A4988 stepper motor controller directly onto the RAMPS and eliminate the need for the Pololu component.  Perhaps it was to give users the option to choose a different stepper motor driver.  Perhaps it was to allow easy replacement if it burned out.

Kysan 1124090 Stepper Motor

Here is the product page for the Kysan 1124090 Stepper Motor.

This is a good video on stepper motor basics.

A good motor FAQ appears here.

Stepper motors are used in the RepRap because when used properly (i.e. not overloaded) they are easier to control because the do not require a closed loop system.  A closed loop system is one with a motion sensor that detects the effect of the applying current to the motor and a controller which adjusts the current sent to the motor while monitoring motion sensor to get the desired result.  With a stepper motor, it is typical to operate in an open loop fashion which means that current is applied in pulses to the coils and it is assumed that the motor will "step" a pre-defined amount as a result of each pulse.  Of course if the motor is over-loaded or stuck or not connected and does not actually move, the controller will not detect this.

YXDY S-360W-12 Power Supply
The power supply included in my kit is somewhat of a mystery.  It is Chinese-made and quite "generic" looking.  It comes in a separate box within the kit, but no documentation is included.  I can't seem to find a web site for it, except for an ebay listing.   The ebay listing seems to contain some set up instructions.  I've yet to see an ebay listing used to disseminate technical info like this.  Here are the specs taken from the ebay listing:

• Input: AC 100-120V 60Hz / AC200-240V 50Hz Manual switch.
• Output: DC12V 30A .
  T.C.: ±0.02%℃
  Start time:<1S
• Auto on/off cooling fan .
• High Efficiency, Low Temperature.
• High reliability .
• LED indicator for power on
• Built-in EMI Filter.
• Over Load and Short Circuit Protection.
• Over Voltage Protection.
• Auto-recovery after protection.
• Overload protection:105%-150% Rated power
• Auto-recovery over voltageprotection:115%-135% Voltage nominal Value
• Working condition:0~45℃ 20%-90%RH
  Storage condition:-20℃ ~80℃ 10%~95%RH
• ASL <2000M
• Dimensions: 214mm x 114mm x 50mm.
• Weight: 920g

YXDY S-360W-12 Power Supply