A rover is only a rover if it can move around. To do this, at the core of our Euro RPyRover is a Dagu Rover 5 chassis with tracks powered by two DC motors. In this article we explain how to use a Raspberry Pi, Python and a Qik controller to drive these motors.
- Raspberry Pi B+
- Dagu Rover 5
- Qik 2s9v1
- USB battery pack (as used to charge mobile phones on the go)
- AA batteries
- Breadboard and wires
|Qik 1 GND||RPi 6 GND|
|Qik 2 Vcc||RPi 1 3V3|
|Qik 3 UART RX||RPi 8 TXD|
|Qik 4 UART TX||RPi 10 RXD|
|Qik 7 and 8 M1||Motor 1|
|Qik 9 and 10 M0||Motor 0|
|Qik 11 GND||Motor battery pack Negative|
|Qik 12 VMOT||Motor battery pack Positive|
Using the serial port
By default RPi outputs operating system logs to the serial port, so to use the serial port to communicate with a component we must first disable this feature.
You can see here how to do it in more detail, but basically you edit the codlin.txt file with the command:
sudo nano /boot/cmdline.txt
It should contain a line similar to:
dwc_otg.lpm_enable=0 rpitestmode=1 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 rootwait
You need to delete the two parameters in red to get the following:
dwc_otg.lpm_enable=0 rpitestmode=1 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 rootwait
Save (Ctrl-X and then OK) and then reboot with the command
Commanding the Qik
The function of the Qik 2s9v1 is to prevent the electrical noise and power demand of the motors from disturbing the microcontroller (the Raspberry Pi). In this circuit the RPi is powered by a USB battery pack and sends commands to the Qik through the serial bus, and the Qik drives the motors using a separate power source (the AA batteries).
The serial bus has just two wires (and a third for ground, naturally), so we need the RPi pins named TXD (transmit) and RXD (receive). The TX of the RPi connect to the RX of the Qik and vice-versa.
To command the two DC motors, named M0 and M1, we need to send commands, through the serial bus. Each command is made of one command byte, followed by a parameter byte that represents the speed. Actually the speed is only 7 bits, so it’s value is between 0 (stopped) and 127 (maximum speed).
The Qik commands we’ll use are:
- 0x88 (hexadecimal) = Motor 0 forward
- 0x8A = Motor 0 reverse
- 0x8C = Motor 1 forward
- 0x8E = Motor 1 reverse
Example: to drive the motor M0 forward at half speed we would send (0x88, 0x40), because 0x40 (hexadecimal) is 64 in decimal (approximately half of 127).
To use the serial bus of the RPi we will use the PySerial library. On OS X or Linux computer it will already be installed. If you have a Windows machine you can learn to install it here.
In our Python program we start by importing the necessary libraries and opening the serial port at 9600 baud (no need for higher baud rate).
import serial import time import math def open_serial_port(): ser = serial.Serial('/dev/ttyAMA0', 9600) ser.write(bytearray())# exit autodetect mode
We then define four functions to move the rover forward (both motors moving forward), backward (both motors reversed), rotate left (one motor reversed and the other forward) and right.
As changing the speed of the motors instantly from zero to the maximum speed shook the chassis we decided to increase the speed gradually over two seconds, varying the parameter 20 times at tenth of a second intervals. This the use of the “Math” and “Time” libraries, that come with Python, and additional functions to decelerate the speed gradually back to zero.
def forward(): for t in range(0,21): ser.write(bytearray([0x88,math.floor(t*127/20),0x8c,math.floor(t*127/20)])) time.sleep(0.1) def break_forward(): for t in range(20,-1):
You can download the R_Motors.py motor library on the Euro RPy-Rover’s GitHub Python repository.
Next tutorial: Capturing images from the rover