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
- The battery (charged, of course)
- The cable to connect the battery to the power board
- 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.
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.
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.robot import * import time R = Robot() while True: R.motors.m0.power = 50 time.sleep(3.0) R.motors.m0.power = 0 time.sleep(1.4) R.motors.m0.power = -50 time.sleep(1) R.motors.m0.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.motors... lines. For a comprehensive reference to the
motor object, see the
motor page. But, to summarise:
R.motors.m0.power = 50 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.motors.m0.power = -50 will put the this motor into reverse at 50% power.
R.motors.m0.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.robot import * import time R = Robot() while True: R.motors.m0.power = 50 time.sleep(3.0) R.motors.m0.power = 0 time.sleep(1.4) R.motors.m0.power = -50 time.sleep(1) R.motors.m0.power = 0 time.sleep(4) # ^^ code from before ^^ # power up to 70 (from 10) for pwr in range(10, 80, 10): R.motors.m0.power = pwr time.sleep(0.1) # power down from 70 (to 10) for pwr in range(70, 0, -10): R.motors.m0.power = pwr time.sleep(0.1) # set power to 0 for a second R.motors.m0.power = 0 time.sleep(1) # power "up" to -70 (from -10) for pwr in range(-10, -80, -10): R.motors.m0.power = pwr time.sleep(0.1) # power "down" to -10 (from -70) for pwr in range(-70, 0, 10): R.motors.m0.power = pwr time.sleep(0.1) # set power to 0 for a second R.motors.m0.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 a Python
[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
forkeyword. Here’s an example:
for i in [1, 2, 3]: print i
The above would output:
1 2 3
Then there’s the
range() function. The
range() function returns a
list with its contents dependent on the arguments passed to it. The Python documentation explains it quite nicely:
range([start], stop[, step])(note: the square brackets here mean ‘optional’)
stepargument is omitted, it defaults to
1. If the
startargument is omitted, it defaults to
0. The full form returns a list of plain integers
[start, start + step, start + 2 * step, ...]. If
stepis positive, the last element is the largest
start + i * stepless than
stepis negative, the last element is the smallest
start + i * stepgreater than
So, based on that,
range(3) would return the list
[0, 1, 2] because it is shorthand for
range(0, 3, 1). From the quote, you can see that this would return a list starting from
0, and finishing with the integer one less than
3, hence the
[0, 1, 2].
range(10, 80, 10), for example, would output
10 as the first element, then
20, 30, ... up until
x=10+i*10 for some
x < stop (which, in this case, is
80). So, the
80 I’ve used could equally have been
77 or even
71 and the outputted
list 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.
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.