DC motors and servo-motors controlled by Raspberry Pi 2B

Abstract

The expanding capabilities of today’s microcontrollers and other devices lead to an increased utilization of these technologies in diverse fields. The automation and issue of remote control of moving objects belong to these fields. In this project, a microcontroller Raspberry Pi 2B was chosen for controlling DC motors and servo-motors. This paper provides basic insight into issue of controlling DC motors and servo-motors, connection between Raspberry and other components on breadboard and programming syntaxes for controlling motors in Python programming language.

Full text

* Corresponding author: sustek@fai.utb.cz
DC motors and servo-motors controlled by Raspberry Pi 2B
Michal Šustek1,*, Miroslav Marcaník2,
Pavel Tomášek2 and Zdeněk Úředníček1
1Tomas Bata University, Faculty of Applied Informatics, Department of Automation and Control Engineering, 76005 Zlín, Czech
Republic
2Tomas Bata University, Faculty of Applied Informatics, Department of Informatics and Artificial Intelligence, 76005 Zlín, Czech
Republic
Abstract. The expanding capabilities of today’s microcontrollers and other devices lead to an increased
utilization of these technologies in diverse fields. The automation and issue of remote control of moving
objects belong to these fields. In this project
,
a microcontroller Raspberry Pi 2B was chosen for controlling
DC motors and servo-motors. This paper provides basic insight into issue of controlling DC motors and
servo-
motors, connection between Raspberry and other components on breadboard and programming
syntaxes for controlling motors in Python programming language.
Introduction
Nowadays, there is a high demand for wireless devices
controlled by Wi-Fi, GSM or Bluetooth [1]. These
technologies offer a simplification of our lives in home
automation or entertainment in the form of Radio
Controlled (RC) models. At the same time, the
possibilities of microcontrollers have risen in many
applications [2].
Current control systems are based on special
purpose devices. However, little attention is given to
universal microcontrollers that are able to perform a
wide variety of tasks [1, 2]. The remote control via
microcontrollers is more popular among researchers and
“RC fans” than among the public [3].
The performance growth of microcontrollers has
led to their ability to manage
complex
applications. At
the same time, there is a variety of manufacturers of
microcontrollers with many diverse types of processors
and performance. Raspberry and Arduino belong to the
most widely used microcontrollers [4]. Both of them
provide high performance, and they can be used in many
challenging automation applications.
The second generation of the Raspberry
microcontroller provides enough performance to replace
a standard PC in some audiovisual and automation
applications [5]. With the use of the wireless extension,
which can be achieved by a simple antenna, the device
becomes in a complex control station.
Section 1 describes a microcontroller Raspberry Pi
and its performance. Section 2 shows a basic
functionality of DC motors and servo-motors. In section
3 is insight into issue of connection between
components. In addition, section 4 describes basic
programming in language Python for one component of
each type (1 motor and 1 servo
-
motor).
1 Microcontroller Raspberry
A Raspberry Pi 2B is a small single-board device, which
supports Linux-based operating systems, USB
connections for mouse, keyboard, Ethernet adapter, and
other devices. On Raspberry board are HDMI connector
for attaching a monitor and general
-purpose inputs and
outputs [6]. A MicroSD card is used a storage device and
Python is used for programming.
In history Raspberry Pi was created in UK as a low
cost platform for teaching computer basics, in particular
Python [8]. The first generation of Raspberry was
introduced in February 2012. Since this time, a wide
variety of Raspberry microcontrollers has been created.
In additio
n, e
ach of these variants have different
performance and parameters.
Raspberry Pi 2B was chosen for the purpose of this
research. It contains 4-core 900 MHz processor, 1 GB of
RAM, 4 USB 2.0 ports, HDMI video output, 40-pin
GPIO (General Purpose Input/Output) header.
Fig. 1. Microcontroller Raspberry PI 2B [8].
Raspberry can be used as an unpretentious and
cheap replacement of a PC; h
owever, it has wide use in
home automation and remote control systems. This is
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© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative
Commons Attribution
License 4.0 (http://creativecommons.org/licenses/by/4.0/).

caused by an opportunity to use GPIO pins, which
function is programmable.
1.1 GPIO pins
General-Purpose Input/Output are generic pins on
integrated circuit of Raspberry. Behavior of these pins is
not defined and they can be programmed according to
users’ needs. Each pin can be configured as input or
output; it can be enabled and disabled; input value can be
readable and output value can be readable or writable
[8]. In some applications are GPIO pins used as
maskable interrupt (IRQ).
Ability of using the GPIO pins is provided by
external Python module RPi.GPIO. This module must be
imported into the main control program.
2DC motors and servo-motors
DC motors and servo-motors are main actuators part in
each robotic system. These components provide
movement and rotation around desired axis. In deeper
context, this component provide ability of movement.
DC motors and servo-motors are crucial in all robotic
projects. [12]
2.1 DC motors
Direct current motors are composed of three main parts
(rotor, stator, and commutator). The stator
circumferentially is provided with regularly spaced and
mutually oppositely oriented main magnetic poles and
commutation poles. The poles of the same polarity
follow the poles of the given polarity in the direction of
rotation of the anchor (rotor). The rotor has coils in the
grooves and these coils are connected to a mechanical
commutator. The commutator provides the supply of a
correctly oriented current to the coils of the rotating
anchor so that all currents of the flowing coil side form a
torque of the same direction in the magnetic field of the
main poles [12].
On magnetic neutral place between main poles of
commutator, are placed carbon brushes. The number of
the brushes is the same as the number of the main poles.
Current, which flows through anchor windings,
creates reactionary magnetic field that deforms and
weakens magnetic field around main poles and has effect
on commutator magnetic field. A compensating winding
is used to suppress the reactionary magnetic field. Today
a stator and a rotor are
created from isolated dynamo
-
sheet of metal in modern motors.
Fig. 2. Brushed DC motor [14].
Brushless DC motors are the second variant of DC
motors. Brushless motors provide electrical commutation
with permanent magnet rotor and stator with a sequence
of coils. A permanent magnet rotates
and current
carrying conductors are fixed in this type of motor.
Transistors or rectifiers at the correct rotor position
switch the armature coils in such a way that the armature
field is in space quadrature with the rotor fields [12].
Fig. 3. Brushless DC motor [15].
2.2 Servo-motors
A servo-motor (servo) is a motor
which uses
a feedback
to correct the output of the motor. The feedback is based
on information from a sensor or from a sensors and
external circuitry. The servo itself is consisted from a
DC motor, a potentiometer and a control circuit. They
are small but very energy-efficient devices, which are
controlled by electrical impulses. Gears for controlling a
shaft attach the motor. The power supply of the motor is
stopped, when the motor shaft is at the desired posi
tion.
If it is not in desired position, then it is turned in the
appropriate direction. The motor speed is proportional to
the difference between desired and actual position [12].
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Fig. 4. Servo-motors controlling pulses [8].
Signals are send as electrical pulses with variable
width, or a Pulse Width Modulation (PWM) is used. A
minimal and a maximal value of pulse and repetition rate
is usually distinguished. Usually servos can turn 90
⁰
in
each direction for total 180⁰.
The neutral position is defined as the position,
where the motor have amount of potential rotation equal
to direction in both side (clockwise and counter-
clockwise). The servo expects a signal every 20
milliseconds and the length of the electrical pulse
determines how far the motor turns. When the servo
moves on position, it will hold that position. If any
external force try to push against the servo, while it
holding a position, the servo will resist from moving out
of the position. The maximum amount of force which a
servo can resist is called the torque rating. The position
will not be held forever, the position pulse must be
repeated to instruct servo to stay in desired position.
There are two types of servos [13], AC and DC.
AC servos can work with higher current surges and are
used in industrial applications. On the other hand, DC
servos are not designed for high current surges and are
better usable for smaller applications. There are also
servos for continuous rotation.
In real application servos are used in RC models
(airplanes, walking robots), service robots and operating
grippers.
3 Motor connection
A small 6V DC motor PERMAX 280, a microcontroller
Raspberry Pi 2B, and an H-bridge L293D were used in
this project. The H-bridge is a simple circuit, which
contains switching elements. These elements are usually
Field-Effect Transistors (FET) transistors. All switches
can be turned off or on independently. The H-bridge can
be used to switch a direction of a motor depending on a
current flow. Table 1. presents how switch state can
effect the behavior of a motor.
Tab. 1. H-bridge switch combination [8].
S-1
S-2
S-3
S-4
Motor behavior
1
0
0
1
Clockwise turns
0
1
1
0
Counter-clockwise turns
0
0
0
0
Stop
1
1
-
-
Short circuit
-
-
1
1
Short circuit
1
0
1
0
Braking
0
1
0
1
Braking
Where 1 means the switch is closed, 0 means the
switch is opened, and – means it does not matter on the
state of a switch.
The H-bridge L293D contains 2 H-bridges, so it is
useful for 2
DC motors. The main advantages of this
chip are:
Thermal protection
Capable with Raspberry logic (3V)
Voltage range of 4.5 to 36V for motors
Motor’s peak current of 1.2A
Continuous motor current of 600 mA
In this work, all the components are connected on a
breadboard as depicted in Figure 5. The main advantage
of using L293D with Raspberry (it has 3V logic) is that
the control of pins need only a little current to control
motors.
Fig. 5. DC motor wiring with Raspberry Pi 2B [8].
4 Software solution
Python has been chosen as the programming language in
this work. Python is a higher programming language,
which does not need to have strictly defined variables,
unlike C++. In the deeper context, it is a hybrid
dynamically interpreted language from a group of
scripting languages. Graphic User Interface (GUI) IDLE,
which is well organized, was used to develop the
software itself. A module called Rpi.GPIO has been used
together with basic libraries.
4.1 Solution for DC motor
At the beginning, there is a need to import a module for
controlling GPIO pins, in particular RPi.GPIO, which
can be downloaded from www.python.org.
import RPi.GPIO as GPIO
import time
Then the mode for GPIO pins must be set up.
BOARD and BCM setup
can be chosen. BOARD option
specifies referring of the pins to the plug. BCM option
means referring of the pins by the Broadcom SOC
channel.
GPIO.setmode(GPIO.BCM)
There is also a need to select GPIO pins, which will
be used. The H-bridge L293D is used as a mo
tor driver
.
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Therefore, pin number 18 must be enabled to control the
speed of the motor.
enable_pin = 18
pin_1 = 23
pin_2 = 2
All used pins will be configured as outputs. These
pins help to control direction of the motor. We also
define PWM analog output, where
500
is PWM
frequency. The initial value of duty cycle is set to 0% of
the frequency.
GPIO.setup(enable_pin, GPIO.OUT)
GPIO.setup(pin_1, GPIO.OUT)
GPIO.setup(pin_2, GPIO.OUT)
pwm_motor=GPIO.PWM(enable_pin,500)
pwm_motor.start(0)
In the following block of code, the movement
function
for each
direction (forward, backward) and stop
function are defined. Pin 1 is responsible for forward
movement, on the other hand pin 2 is used for reverse
movement.
def forward(duty_cycle):
GPIO.output(pin_1, True)
GPIO.output(pin_2, False)
motor_pwm.ChangeDutyCycle(duty_cycle)
def reverse
(
duty_cycle):
GPIO.output(pin_1, False)
GPIO.output(pin_2, True)
motor_pwm.ChangeDutyCycle(duty_cycle)
def stop():
GPIO.output(in_1_pin, False)
GPIO.output(in_2_pin, False)
motor_pwm.ChangeDutyCycle(0)
Th
e following part of the program represents the
main loop which prompts the user for a command and
calls direction functions and the stop function.
try:
while True:
direction = raw_input('w – forward, x – reverse, t - stop')
if direction[0]=='t':
stop()
else:
duty_cycle= input('Duty cycle (0-100%)')
if direction [0]=='w':
forward(duty_cycle)
elif direction [0]=='s':
reverse(duty_cycle)
finally:
print("Cleaning up")
GPIO.cleanup()
This is elementary program for control DC motor
with the microcontroller Raspberry Pi 2B. This program
is written for one motor, which is wired to the
microcontroller by L293D motor driver.
4.2 Solution for servo-motor
The module RPi.GPIO must be used for GPIO control as
in the previous case.
import RPi.GPIO as GPIO
import time
The number of GPIO pin on Raspberry Pi 2B has to
be chosen. In addition, every servo needs a slightly
different length of pulse to maximize its range of angles,
two constant are used to set the pulse duration between
angles 0⁰ and 180⁰. Values of these variables represent
duration of pulse in milliseconds. Frequency is set up to
50 Hz, therefore it is giving a pulse every 20
milliseconds.
servo_pin=18
0_deg = 0.5
180_deg = 2.5
frequency= 50.0
Some calculations, which are related to the length of
pulse, were made for easier future modifications. Period
is 1000 milliseconds and is divided by frequency (50 Hz
in our case) so the result is 20 millisecond. In addition, if
there is a need to change a duty cycle, an
interval
between 0 and 100 must be used and the constant k can
be used to scale an angle to duty value. To convert the
pulse for 0⁰ to a corresponding value of duty between 0
and 100, is length of pulse multiplied by constant k. The
value of range of duty is calculated by multiplying the
span of pulse length by the constant k.
period = 1000/frequency
k = 100 / period
0_deg
_duty =
0_deg*k
pulse_range = 180_deg-0_deg
duty_range=pulse_range*k
The part, which initialize the GPIO pin, is similar to
the variant for DC control by Raspberry Pi 2B.
GPIO.setmode(GPIO.BCM)
GPIO.setup(servo_pin,GPIO.OUT)
pwm=GPIO.PWM(servo_pin, frequency)
pwm.start(0)
In angle definition, we convert angle into duty cycle
value, and then we call ChangeDutyCycle to set the new
pulse length.
def set_angle(angle):
duty_cycle=0_deg+(angle/180.0)*duty_range
pwm.ChangeDutyCycle(duty_cycle)
The main loop of control program is written below.
In the following part, a value of angle between 0⁰ and
180
⁰
is set. It could be used for steering, when 90⁰ is
forward direction, 0⁰ is turning left and 180⁰ is turning
right.
try:
while True:
angle = input("Angle 0⁰ to 180⁰")
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set_angle(angle)
finally:
print("Cleaning up")
GPIO.cleanup()
Conclusion
The high demand for wireless devices, which can
simultaneously perform a wide variety of tasks has led to
significant use of microcontrollers. This paper provides
elementary insight into the issue of microcontrollers and
the controlling of DC motors and servo-motors.
The microcontroller Raspberry Pi 2B has been
chosen as the brain of this project. This microcontroller
provides enough performance to run control system for
DC motors and servo-motors. 6V DC motor PERMAX
280, motor driver L293D and breadboard have been
chosen for creation of this system. This connection is
only experimental and it is not used in real robotic
platform yet. The controlling software is written in
programming language Python; however, we also need
to use GPIO pins. The module RPi.GPIO must be
imported into the program. The submitted syntaxes in
programs are the elementary way to control DC motors
and servo-motors. It can be modified for controlling
more motors and servo-motors.
The next work is planned to be focused on
extending the control system on a cell phone as a control
device, in particular, by using LTE (Long Term
Evolution) technology. The main difference in LTE
system will be in control software (Android, iOS
application). An LTE modem must be added to the entire
system. Another further work can be aimed at
implementation of this control system into a 4-wheeled
robotic chassis.
Acknowledgment
This work was supported by the Ministry of Education, Youth
and Sports of the Czech Republic within the National
Sustainability Programme project No. LO1303 (MSMT-
7778/2014) and also by the European Regional Development
Fund under the project CEBIA-Tech No. This work was also
supported by Internal Grant Agency of Tomas Bata University
under the project No. IGA/FAI/2017/004.
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