Designing a Micro Robot on a PCB
Micro robots are becoming popular in robotics and
hobby electronics fields rapidly since they have several positive factors with
comparison to the bulky designs. Generally, a design of a micro robot is
heavily limited by its size and obviously by its hardware components because of
they are designed to be light weight. Since these types of robots are designed
according to a specific application like swarm-robots, micro mouse or even for
educational purposes, we can design them with specific set of hardware
which won’t get affected by above limitations that much. Designing of a micro
robot can be bit difficult due to its highly compact nature. However, the same
compact nature can make the assembling process easier and the building cost
lower compared to a regular sized robot.
One of the key advantages of these type of robots is
that the whole hardware assembly can be done on a single circuit board or a
stack of circuit boards. It makes them compact and cheaper. Here in this post I
am going to discuss on designing a micro mobile robot with four wheels,
which is targeted to be a general purpose development robot platform. I will
explain the design considerations, how to design the modular schematics, PCB
drawing and printing PCBs with a PCB manufacturing Company..
Design
Considerations of the Micro Robot
Base
Basically my target was to design a four wheeled
mobile platform with individual motor for each wheel and all the peripherals are embedded in a single printed circuit board. Actually the four wheeled
design can also be achieved using only two motors with a simple gear plus bearing
set rather than using four motors. However, there is some mechanical part over
that implementation and therefore here I chose the simple individual motor
design for each wheel. In addition to above two four wheeled designs, this kind
of robots can be implemented with two driving wheels plus a free rolling caster
ball or a caster wheel. Following images show different design approaches for the same kind of robot platform.
Since we are interested in designing a micro robot, we
cannot use large wheels or bulky motors with big gear boxes. In that case we have
a huge advantage, because there’s no need of driving a heavy load due to the
light weight design of the robot and still we are using four motors to drive
that load. Speed of the motors is also a critical factor according to your
application. Since I am designing a general purpose robot, according to my
experience and the availability in the market, a considerably medium range RPM
value such as 200RPM or 300RPM will be enough for my application.
In this case I used a two cell Li-ion or Li-Po
battery (7.2V) as the power supply to the robot. Therefore, I selected 6V
motors for my purpose. With above specifications I found micro metal gear
motors with plastic brackets available in the online market. These motors are ideal for smaller robot designs
since they can provide a desirable torque with the metal gear box.I found a set of plastic wheels which can be fitted in
to the above motor shafts with the “D” shaped slot. Selecting the diameter is
depending on your application. I chose the following wheels for my project.
Controller
Obviously the heart of any kind of embedded system is
its controller. It can be a microprocessor, micro-controller, FPGA or any kind
of programmable logic device. For a micro robot, most of the time it will be a
micro-controller since it’s easy to program using a common programming language /IDE and interface all the required peripheral
devices to it through I/O pins. In the selection process of a matching
microcontroller, first of all we need to identify the required processing power
to handle all the processes.
In my design I have added an IR line detecting sensor
panel (QTR panel) with 8 sensors, 4 front facing IR emitting LEDs and photo
diodes to wall detecting task, 3 sonar sensors in three sides, 2 dual H bridge
motor drivers for 4 motors, encoder reading interfacing for two motors, an IMU
sensor, an I2C AMOLED display for monitoring or debugging purposes and
additional I2C and UART ports for extensional purposes. For all the above
mentioned peripheral devices, we need to have a considerable number of I/O pins
from the microcontroller. Since it is cheaper and easy to deal with the Arduino
IDE, I selected two Atmel Atmega328P-AU surface mounting microcontrollers which
can make the design more compact. Communication between two microcontrollers is
happening via I2C protocol. Motor controlling, IR sensor array reading, I2C
slaves handling and some other tasks are assigned to one chip and the rest of
are done by the other chip.
Motor Controller
As I discussed in the motor selection sub section, I
need to control four DC micro metal gear motors with the specifications of
6V operating voltage and 1.5A stall current. Since this is a light weight
application and need to control four motors, I selected MX1919 motor driver ICs
with dual H bridges inside each package. They are available in the market at a
very cheap cost and can operate in the 2V-10V supply voltage range. Each H
bridge is capable of driving a maximum current of 2.5A. I used two of these ICs
for my design. Main advantage of this model is, we can complete the motor
driver with lesser amount of extra components other than the IC itself.
Critical Components for the Design
ATMEL Atmega 328p-AU microcontrollers
MX1919 dual H-bridge motor drivers
Micro Metal Gear DC Motors (6V brushed)
Modular Design
Approach
The next step of our design process is implementation of the
CAD drawings of the whole robot’s circuit schematics. Since this is a
considerably complex circuit with a large number of components, the best
approach is to broke the whole schematic into small modules and connect them
through the labeled nets. In my design there are three circuits
as follows and used the Altium Designer 16.0 electronic design software package
for this design.
1.
Main controller
module and power distribution
2.
Sub controller
3.
Motor controller
We will discuss each of them briefly as following.
Main controller module
This schematic is based on the first Atmega 328P micro
controller chip and its peripheral interfaces. QTR panel, encoder reading for
one motor, ISP programming interface, I2C port, UART port for Bluetooth
communication or FTDI programmer interface are the features included in this
module. Other than that, we have the power distribution circuit as well within
this module. Since all the sensors we are using and the two Atmega328 chips are
working on 5V we need to regulate 5V from the Li-Po battery we are going to use
as the power source. Therefore, I placed several ports for plug a switching
buck converter and several other switches including main power switch and the
motor controller power switch in this module.
Sub controller Module
The second Atmega micro-controller and its peripherals
are the main parts of this module. Wall or obstacle detecting front 4 IR
sensors, 3 sonar sensors into three sides, Encoder readings for one motor, a
buzzer, I2C interface, UART interface for FTDI programming and
ISP programming interface are the peripherals which get connected to the second
micro-controller chip.
Motor Controller
As we discussed earlier, I have used two MX1919 dual H
bridge motor controlling ICs in my design. Since this IC package contains most
of the components needs to implement the dual H bridges, we have to place only
two zenar voltage regulators and two capacitors to complete the motor
controller.
Competed Circuit schematics can be found in the
following link:
PCB Design
After completing the schematic designs by connecting
each module on separate sheets using labels for each net, we need to do
the most difficult and time consuming task of the project which is the PCB
designing part. Here I used the Altium PCB designer to do the task. Since there
are lots of components to be integrated in a small circuit board, organization
of the component assembly is very critical. First of all, we need to define a
board shape according to our purpose and shape. I chose a simple shape as the
following images shows. In the front of the robot I placed the QTR panel, IR
sensors, Sonar and two micro-controller chips. Then four motors are placed in
the rear side of the board considering wheel and motor dimensions. Motor controlling
part is placed in the middle of the PCB and power management and switches are
placed in the far rear side of the PCB. You can customize this orientations
according to your need.
For the routing purpose I selected a two-layer PCB
design since we have to route a complex circuit in a small space. Thicknesses
of each copper trace is very important because they must be capable of carrying
different amount of currents according to the type of the trace. As an example,
a copper trace carrying current to a motor must be capable of carrying at least
the stall current of that motor. Micro-controller I/O traces may be thinner than
a power supply traces since they are carrying a less amount of current.
Finally, I added a Copper polygon pour to both top and
bottom copper layers and assigned labels and texts to each port and component
on the Top overlay layer (Silk layer) and competed the design by placing some
mounting holes for motor brackets and the PCB itself.
Following images shows that the both sides of the PCB from the design view and 3D view.
 |
| Legacy 3D view of the PCB |
 |
| printed Circuit boards from PCBWay.com |
You can find
the completed Altium PCB drawing from the following link:
Printing the PCB with PCBWay
After completing my PCB design, I chose a leading PCB
Manufacturing company in China called the PCBWay to get printed my Micro Robot’s
PCB since they are recognized over the world as one of the best quality PCB manufacturers
with so many additional services at a fair price. In my next post I will
discuss how to generate the Gerber files of this PCB design to send for
manufacturing to the manufacturers.
Services of PCBWay
1. PCB prototyping and manufacturing
They can not only produce FR-4 and Aluminum boards, but also advanced PCB like Rogers, HDI, Flexible and Rigid-Flex boards, with very reasonable price. Online instant quote page - https://www.pcbway.com/
Online Gerber viewer: https://www.pcbway.
PCB manufacturing process: https://www.youtube.
2. PCB assembly
SMT & THT assembly, starts from only $30 with free stencil and free worldwide shipping.
The components can be sourced and provided by us, or by clients themselves
Rough quote online - https://www.pcbway.com/pcb-
Factory video - https://www.youtube.com/
3. Layout and design
Partnering with quality service providers to offer design services - https://www.pcbway.com/
4. Open source community
Student sponsorship , shared PCB projects and so on. - https://www.pcbway.com/
More details:
https://www.pcbway.com/why.
https://www.pcbway.com/
https://www.pcbway.com/high-
They can not only produce FR-4 and Aluminum boards, but also advanced PCB like Rogers, HDI, Flexible and Rigid-Flex boards, with very reasonable price. Online instant quote page - https://www.pcbway.com/
Online Gerber viewer: https://www.pcbway.
PCB manufacturing process: https://www.youtube.
2. PCB assembly
SMT & THT assembly, starts from only $30 with free stencil and free worldwide shipping.
The components can be sourced and provided by us, or by clients themselves
Rough quote online - https://www.pcbway.com/pcb-
Factory video - https://www.youtube.com/
3. Layout and design
Partnering with quality service providers to offer design services - https://www.pcbway.com/
4. Open source community
Student sponsorship , shared PCB projects and so on. - https://www.pcbway.com/
More details:
https://www.pcbway.com/why.
https://www.pcbway.com/
https://www.pcbway.com/high-
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