Using The midTbot Controller

The midTbot Controller is a controller for midTbot pen drawing machines. It is sold on my Tindie store. When fully populated, it has the following features.

  • (2) Stepper Motor Drivers for the X and Y axes with microstepping selector jumpers
  • Servo Connector for Pen Lift
  • Arduino Nano Controller
  • 5V power supply
  • (2) Homing switches
  • Bluetooth connector for HC-05 or HC-06 module.
  • Aux power connector for easy access to 5V and 12V power.

Power Supply

You will need a 12V DC power supply with at least 3A of power. The DC plug should be a coaxial 5.5mm x 2.1mm type with center positive. This type of plug is the most common type and should be pretty easy to find.

Microcontroller.

You should use an Arduino Nano compatible controller. The controller bootloader will need to be changed to the smaller Arduino UNO bootloader in order to fit the firmware. I have instructions on how to do this here. If you would like to buy one already modified, I sell them on my Tindie store. Be sure to install the controller in the correct orientation. The USB connector should face the edge of the controller as noted on the silkscreen of the controller PCB.

Stepper Drivers

You will need 2 stepper driver modules. I recommend ones based on the Allegro A4988 controller or the TI DRV 8825 controller. The drivers often come with heatsinks. The current will typically be set low enough that heatsinks are not needed.

You will need to set the microstepping level. I recommend using 1/8 microstepping. That gives a good balance of accuracy, smoothness and speed. You set this using the jumper blocks. Each stepper driver type has its own configuration. Here are the jumper setting for A4988 and DRV8825

Microstep ResolutionA4988 JumpersDRV 8825 Jumpers
FullNoneNone
1/2MS1MS1
1/4MS2MS2
1/8MS1 + MS2MS1+MS2
1/16MS1+MS2+MS3MS3
1/32Not AvailableMS1+MS2+MS3

Note the MS1, MS2 & MS3 labels next to the jumpers.

The orientation of the drivers is very important. You will break the drivers and likely other parts if you insert them wrong. Most drivers have the pins labeled and the the controller PCB has the corner pins labeled. Be sure the labels match. Here are the correct orientations for the A4988 and DRV8825.

You should set the stepper driver output current to the level required by your stepper motors. The midTbot does not need a lot of power. If you set the power too high, the motor could get hot and damage the 3D printed part it mounts to. You set the current by adjusting a potentiometer and reading a voltage. The drivers need to be powered when you do this. You can install them and power the board to do this. Do not connect the motors when adjusting the potentiometer. Each driver will have a small hole with exposed metal. This is the measurement point. The metal adjustment screw of the potentiometer is also a place you can measure. The reference voltage to current formulea are here.

DRV8825 Current = VREF * 2   (Example 0.4V = 0.8A current)

A4988 is a little more complicated because there are different versions. See this page.

Stepper motors:

The controller is designed for NEMA14 motors. Be sure to get 4 wire motors. Just about any size should work, but I prefer to use smaller ones to keep the size and weight down. You will need to attach connectors to them.  Trim the wires to the correct length so they reach the board connectors when installed. Wires colors should be installed as shown.

The wiring legend on the board that shows where to plug in the motors might not be right. It depends on the way the motor manufacturer wired the coils and which stepper drivers you use. You may need to swap them or rotate the connectors 180° if your axes are moving the wrong way. Here is a photo of how mine are wired. Try this first. Note the wire colors and which motor goes to which connector.

Bluetooth

You can add an HC-05 or HC-06 Bluetooth module. They have different connectors, but just match the pin names on both sides. You will need to setup the module to have a baud rate of 115200, N,8,1. I have detail on how to do that here (HC-05, HC-06).

 

 


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