Basic Motor Control

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 battery (charged, of course)
• A motor board
• 2 large (7.5mm) green CamCon connectors to plug the power board and motor board together
• 2 lengths of a suitable gauge of wire for powering the motor board from the power board (one should be black/grey)
• A motor (see specification below)
• A small (5mm) green CamCon connector to plug the motor and motor board together
• 2 lengths of a suitable gauge of wire for your motor
• A Micro USB cable
• A USB hub
• A standard USB cable
• 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, so it’s just the motor-related parts that need explaining.

Motor Specification

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

• 12V motor
• A stall current of less than 10A (this is the current limit for the motor boards)

Connecting a Motor

To plug the motor into the kit, you’ll need to solder an appropriate gauge of wire to the terminals on the motor, and connect the other ends to the CamCon connector. Like so:

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

• Your motor into the motor 0 socket on the motor board
• The micro USB cable from the motor board to the USB hub
• The standard USB cable from the USB hub to the power board

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 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 with, 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.

Here’s the code:

from sr.robot3 import *
import time

R = Robot()

while True:

    R.motor_board.motors[0].power = 0.5
    time.sleep(3.0)

    R.motor_board.motors[0].power = 0
    time.sleep(1.4)

    R.motor_board.motors[0].power = -0.5
    time.sleep(1)

    R.motor_board.motors[0].power = 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 the sr.robot3 module’s Motors page. But, to summarise:

So, R.motor_board.motors[0].power = 0.5 sets the target power of the motor connected to output 0 on the first motor board plugged in to a USB hub to 50% forwards (i.e. a duty-cycle of 0.5 forwards).

As you would expect, then, R.motor_board.motors[0].power = -0.5 will put the this motor into reverse at 50% power. R.motor_board.motors[0].power = 0 will output no power to 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…

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 sr.robot3 import *
import time

R = Robot()

while True:

    R.motor_board.motors[0].power = 0.5
    time.sleep(3.0)

    R.motor_board.motors[0].power = 0
    time.sleep(1.4)

    R.motor_board.motors[0].power = -0.5
    time.sleep(1)

    R.motor_board.motors[0].power = 0
    time.sleep(4)

    # ^^ code from before ^^

    # power up to 70% (from 10%)
    for pwr in range(10, 80, 10):
        R.motor_board.motors[0].power = pwr / 100  # Convert from percentage
        time.sleep(0.1)

    # power down from 70% (to 10%)
    for pwr in range(70, 0, -10):
        R.motor_board.motors[0].power = pwr / 100  # Convert from percentage
        time.sleep(0.1)

    # set power to 0 for a second
    R.motor_board.motors[0].power
    time.sleep(1)

    # power "up" to -70% (from -10%)
    for pwr in range(-10, -80, -10):
        R.motor_board.motors[0].power = pwr / 100  # Convert from percentage
        time.sleep(0.1)

    # power "down" to -10% (from -70%)
    for pwr in range(-70, 0, 10):
        R.motor_board.motors[0].power = pwr / 100  # Convert from percentage
        time.sleep(0.1)

    # set power to 0 for a second
    R.motor_board.motors[0].power = 0
    time.sleep(1)

Again, as you’ve seen most of that before, it shouldn’t be too difficult to get your head around. The for loop may be new, however.

The for loop accepts anything that Python can iterate over to get multiple values, such as a list (a list, when printed, appears in square brackets like so: [1, 2, 3]).

For a comprehensive introduction to to lists, have a look at this WikiBooks article. The for loop will iterate over the list (i.e. take each element in turn) and make it available in the loop’s body as the variable after the the for keyword. Here’s an example:

for i in [1, 2, 3]:
    print(i)

The above would output:

1
2
3

Then there’s the range() object.

range() objects represent a sequence of numbers. They can be constructed with one, two or three arguments to describe what the sequence of numbers should be.

If only one argument is provided then the sequence starts at 0 and stops just before the given number. Otherwise the first argument is the number to start at, the second argument is the number to stop before and the (optional) third argument is how big the steps should be (by default steps are 1).

So, based on this, range(3) would represent the sequence 0, 1, 2 because it is shorthand for range(0, 3, 1).

So, taking range(10, 80, 10), for example, would output 10 as the first element, then 20, 30, ... up until x=10+i*10 for some i where i ensures x < stop (which, in this case, is 80). So, the 80 we’ve used could equally have been 77 or even 71 and the outputted sequence would still be 10, 20, 30, 40, 50, 60, 70.

Putting all of that together should mean you understand the above code. You might want to run the code on your kit to see if it does what you expect it to.

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.

You might also like to see if you can make the larger code example above more concise by creating functions for the repetitive parts. This tutorial seems good if you’re interested.