Basic Movement

The aim of this tutorial is to get a motor turning with your kit. To complete this tutorial, you’ll need the following:

  • The power board
  • A motor board
  • The Raspberry Pi
  • A battery (charged, of course)
  • Two large (7.5mm) green CamCon connectors to plug the power board and motor board together
  • Two lengths of a suitable gauge of wire for powering the motor board from the power board; we recommend that one be black and the other be red
  • A motor (see specification below)
  • A small (5mm) green CamCon connector to plug the motor and motor board together
  • Two lengths of a suitable thickness of wire for your motor; we recommend that they be blue (or any colour other than black)
  • Two Micro USB cables
  • The Raspberry Pi power cable

You’ll also need these, which aren’t provided with your kit:

  • The memory stick
  • A soldering iron
  • Some solder wire
  • Wire strippers
  • A small slotted/flat blade screwdriver (for the CamCon screws)

You should be familiar with the setup for most of the above now from the kit assembly tutorial, so it’s just the motor-related parts that need explaining.

Motor specification

There is a certain specification that the motor(s) you use must meet. The criteria are as follows:

  • Brushed DC type
  • Rated for at least 12V
  • A stall current of less than 10A (this is the current limit for the motor boards

When designing your robot you should bear in mind that while each motor board can deliver 10A on each output, all the power needs to go through the power board, which has its own limiting per output.

Connecting a motor

To plug the motor into the kit, you’ll need to solder the two lengths of blue wire to the motor terminals, and connect the other ends to the CamCon connector. Like so:

Motor connected to CamCon

You may want to insulate the motor’s terminals with some insulation tape (or heat shrink tubing if you’ve got it) like in the image above.

Now you need to connect the motor to one of your motor boards. You’ll need to connect the following:

  • Your motor into the M0 socket on the motor board
  • The micro USB cable from the motor board to the USB hub

This is almost ready, but the motor board also needs the power that it will be delivering to the motor. This is done by connecting together the two larger CamCon connectors, using the other two lengths of wire. Be sure that the cable connects the positive motor rail output (“+”) of the power board to the positive power input of the motor board, and likewise for the ground (“-”) output. See the power board and motor board documentation to see which is which.

You must always use black or grey for ground (0V) connections (and only for these), so that it’s clear where these are.

You can now connect this into the power board on one end, and the motor board power connection on the other.

Some code

The example program we’ll write will do a number of things with the motor: forwards and backwards, and different power settings, for example. Let’s begin. To start off, we’ll just make a motor move forwards, then backwards, and then repeat.

Forwards & backwards

Doing this is actually very easy; the only thing you need to realise is that a positive number is forwards and a negative number is backwards.

The actual direction of travel of a motor, when mounted on a robot, will depend on its orientation and the way in which the wires are connected to the motor board. If the motor appears to be going in the wrong direction, just swap the motor’s positive and negative wires over.

Here’s the code:

from robot import Robot
import time

r = Robot()

while True:
    r.motor_board.m0 = 0.5
    time.sleep(3)

    r.motor_board.m0 = 0
    time.sleep(1.4)

    r.motor_board.m0 = -0.5
    time.sleep(1)

    r.motor_board.m0 = 0
    time.sleep(4)

You’re familiar with the first few lines; in fact, the only lines you may not be familiar with are the r.motor_board... lines. For a comprehensive reference to the motor object, see motor API page. But, to summarise:

r.motor_board.m0 = 0.5 sets the target power of the motor connected to output 0 on the first motor board to half speed forwards (i.e. a duty-cycle of 0.5 forwards). As you would expect, then, -0.5 will put the this motor into reverse at half power. r.motor_board.m0 = 0 will brake the motor and stop it.

So, if you put the above code on your robot, you should be able to see a motor spin forwards, stop, spin backwards, stop, and then repeat…

If you find that the motor doesn’t turn when you run the above code, check that you’ve got all the cables connected to the right places, in particular note that the motor board has two outputs.

Changing the speed

Now we’re going to modify the program to vary the speed of the motor. Our aim is to do the forwards and backwards bit (as above), but, before we loop round again, ramp the power up to 70%, then down to -70%, and then back to 0 (all in steps of 10%). Here’s the code:

from robot import Robot
import time

r = Robot()

while True:

    r.motor_board.m0 = 0.5
    time.sleep(3)

    r.motor_board.m0 = 0
    time.sleep(1.4)

    r.motor_board.m0 = -0.5
    time.sleep(1)

    r.motor_board.m0 = 0
    time.sleep(4)

    # ^^ code from before ^^

    # power up to 0.7 (from 0.1)
    for pwr in range(10, 80, 10):
        r.motor_board.m0 = pwr / 100.0
        time.sleep(0.1)

    # power down from 0.7 (to 0.1)
    for pwr in range(70, 0, -10):
        r.motor_board.m0 = pwr / 100.0
        time.sleep(0.1)

    # set power to 0 for a second
    r.motor_board.m0 = 0
    time.sleep(1)

    # power up to -0.7 (from -0.1)
    for pwr in range(-10, -80, -10):
        r.motor_board.m0 = pwr / 100.0
        time.sleep(0.1)

    # power down to -0.1 (from -0.7)
    for pwr in range(-70, 0, 10):
        r.motor_board.m0 = pwr / 100.0
        time.sleep(0.1)

    # set power to 0 for a second
    r.motor_board.m0 = 0
    time.sleep(1)

Next steps

From here, you should be able to make your robot move about in a controlled manner. See if you can make your robot drive forwards to a given point, stop, turn around and then return to its starting point.