Machine Design

In this week in which it was necessary to conclude the basket-robot I designed the electronics and configured the firmware with the help of Gianluca. I leave Alberto the part of the interface that controls the robot via GCODE.

Elettronic

The electronics required to control the robot are four NEMA 17 stepper motors connected in pairs but with opposite polarity, an IR sensor, a blue LED and a Bluetooth module.

Board

To control all the electronics I designed a board with EAGLE being careful that it was compatible with the firmware I chose to use: GRBL v1.1. This CNC firmware allows easy stepper control via GCODE.

Requirements

I used EAGLE to draw the board. I found in the Reprap.org wiki a library with all the fingerprints necessary to insert the drivers so I downloaded it and added it to the project. Unfortunately I noticed that some pins didn't match so I had to modify them.

download reprap 235 KB (.lbr)

Design

For the driver control scheme I was inspired by RAMPS 1.4.

Figure 01

Figure 1. Driver schematic on RAMPS 1.4

Because when I designed the board I still didn't know exactly what to use I put two connectors, one with the endstops and one with the spindle controls. For Bluetooth I used the FTDI connector.

download RoboTable 208 KB (.zip)

Figure 02

Figure 2. Schematic

Figure 03

Figure 3. PCB

Figure 04

Figure 4. Board

Drivers

To control the steppers I used external drivers connected to the board: the DRV8825. To limit the current supplied and avoid damaging the motors and/or the drivers themselves, they need to be adjusted by turning the small trimmer on the circuit. To know the current that will be delivered you have to measure with the tester the voltage between the GND and the head of the trimmer (Vref) and do the calculation as reported in the official documentation: ` I = Vref*2 ` .

However, since two motors are connected in parallel to each driver (with opposite polarity), the current that will go to each motor will be half of that supplied in total, that is : ` I = Vref ` .

I set them to 1v to avoid that the drivers, delivering too many amps, get damaged, but the motors hold up to 1.7A so they can not provide the full torque.
All this I did after mounting the drivers on the board and having powered it otherwise no value will be read.

Figure 05

Figure 5. Drivers

IR sensor

The sensor is used to detect the presence of the ball in the basket. It has three pins GND, VCC and SENS. I soldered an adapter cable to connect GND and SENS to an endstop (it is indifferent which one) and the vcc to the ISP connector.

Figure 06

Figure 6. IR sensor front.

Figure 07

Figure 7. IR sensor back.

led

The led does not have a particular function and I connected it to the DIR pin of the spindle to be able to control it via gcode.

Figure 08

Figure 8. Led in the red circle.

Bluetooth

Since I used the FTDI connector for Bluetooth, it was only necessary to solder a cable to be an adapter.

Figure 09

Figure 9. Bluetooth

Firmware

As already said before, the firmware I used is GRBL v1.1.

Edits

In order to use it, I had to make some changes.

In config.h:

In cpu_map.h I completely remapped the pins, now the ones connected are:

In default.h I set DEFAULT_a_STEP_PER_MM to 28 and increased DEFAULT_a_MAX_RATE, DEFAULT_a_ACCELERATION and DEFAULT_a_MAX_TRAVEL for each axes going to tries.

Lastly in protocol.c I have replaced line 74 with: 

bool s = false;
for (;;) {
  if (limits_get_state() && s) {
    printPgmString(PSTR("\r\n0\r\n"));
    s = false;
  }
  else if (!limits_get_state() && !s){
    printPgmString(PSTR("\r\n1\r\n"));
    s = true;
}

This allows you to receive a feedback by IR sensor via Serial.

Interface

Once switched on the robot will be in the position X0 Y0, by sending the command G1 X[posX] Y[posY] F[speed] you can tell it to move up to the coordinates indicated. He will normally use absolute coordinates, but it is also possible to switch to the relative coordinates with the G91 command (G90 to return to the absolute coordinates).
To control the led's turning on, instead, you have to use the commands M4 (turn on) and M5 (turn off) that normally are used to control the direction of rotation of the spindle. The feedback that is received by the sensor is a 0 when the ball enters the basket and a 1 when it exits.

Group Assignment

More info on the Opendot machine design page.