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Mini Cube Robot

A small 5x5cm Cube Robot with differential two wheel drive. Modular design with a simple and cheap base.

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This is, as the name suggests, a Mini Cube Robot with dimensions 5x5cm. It is designed to be cheap, simple and easily expandable through additional stacked modules. The base of the robot is formed by a differential drive drive-train that uses two cheap M10 sized brushed motors with a planetary gear and encoders as well as 6 IR reflective and 4 contact sensors, 3 (2) for the front and 3 (2) for the back. All this is controlled by the Motor Drive PCB, which additionally also act as the power supply for the whole robot. Any additional features can easily be added by stacking additional modules/PCBs on top.

There are a few modules already planned, with some in early design phase:
- A main control and communication board (probably using a STM32F4 or STM32H7 with bluetooth and NRF24L01+ radios)
- A sensor board for the top with IMU and a couple of ToF sensors, forming a cheap low resolution LiDAR
- A dedicated processing board using either a FPGA or a SoC, or a combination of both

Background

The idea behind this robot is to create a cheap, small and simple robot platform that has all the basic features in a fixed basis and to which additional features can easily be added. Making the basis cheap, small and simple allows the robot to be built in large numbers, for use in cooperative and swarm robotics.

This is planned to be a long term project, over several years, to be expanded upon and adding features both through new hardware and software algorithms. The first phase is to develop the robot base, both the drive mechanics and control software as well as the basic collision sensor acquisition and tuning.

There are a few modules already planned, with some in early design phase:
- A main control and communication board (probably using a STM32F4 or STM32H7 with bluetooth and NRF24L01+ radios)
- A sensor board for the top with IMU and a couple of ToF sensors, forming a cheap low resolution LiDAR
- A dedicated processing board using either a FPGA or a SoC, or a combination of both

Besides the hardware there are many software algorithms to be implemented and tested, under others:
- Integration with PC software, including ROS (python library) and maybe Unity (C# library) with a costume environment
- Localization, and SLAM, with simple sensors, without high resolution LiDAR
- Path planning with different path finding algorithms (A*, RRT, RDT, FM)
- Cooperative and swarm robotics

Hardware

The robot basis is built around the drive-train which is powered by two M10 Brushed Motors with planetary gears and a plastic leaf for the encoder. The output of the planetary gear is a 11 tooth modulus 0.5 gear that drives a 24 tooth gear on a axle that is connected to a wheel. The wheels are made out of a 24mm diameter plastic pulley with a 26x3.1mm outer diameter O-Ring as a tire.

This whole drive assembly is hold in place by two 3D printed parts, the Drive Bottom Holder where all is mounted into and the Drive Top Cover which closes the mechanics off. The Drive Bottom holder also has place for Collision Sensor Boards, in the front and back, as well as a place for two Motor Encoder PCB, one for each motor. On the bottom of the Drive Bottom holder there are mounting places for plastics skids, used to make the robot stable by adding a third, and forth, contact point with the ground.

Each Collision Sensor Board holds two contact collision sensors, buttons, and three IR reflective collision sensors (TCRT500). The PCB slides into place and connects to the Motor Drive PCB for acquisition and control. The two Motor Encoder Boards each use a IR reflective collision sensor (ITR8307) together with a voltage comparator to output a pulse each time the motors plastic leaf passes in-front of it.

The Motor Drive PCB is the heart and brain of the Mini Cube Robot platform. It has at its core a STM32F103C8 MCU that controls the dual H-Bridge motor driver, STSPIN240, acquires all the sensors and does the interface with the rest of the robot, future modules, over an expansion header with I2C, UART and SPI interfaces. The Motor Drive PCB also is the power supply for the robot through a 1S LiPo that is connected to a battery charger and manager IC, the BQ24230. The Motor Drive Board provides both raw battery voltage rails as well as a dedicated 3V3 rail on the expansion header for other modules.

This project is in very early...

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  • 1 × STM32F103C8 Microprocessors, Microcontrollers, DSPs / ARM, RISC-Based Microcontrollers
  • 1 × STSPIN240 Low voltage dual brush DC motor driver
  • 6 × TCRT5000 IR reflective sensors
  • 1 × BQ24230 Lithium-Ion Battery Charger And Power-Path Management IC
  • 2 × M10 Motor w. Planetary Gearbox

View all 7 components

  • Reflective Collision Sensor Test

    NotBlackMagic11/25/2021 at 21:16 0 comments

    The robot drivetrain holds two collision sensor boards where each of them has three IR reflective sensors (TCRT5000). This sensor is composed of a IR LED and a phototransistor to sense the reflected IR light from an obstacle. The IR LED is driven through a 100 Ohm resistor, which gives a drive current of around 20mA, and the phototransistor is connected to a pull-up resistor of 10 kOhm that converts its current output to a voltage that is then converted by the MCUs ADC.

    To test the effective range of the IR reflective sensors, the distance that they can sense an obstacle, the setup shown in the figure bellow is used. The robot is set at a known distance from the target and the ADC value of the front facing sensor is registered.

    The test was performed with distances from 1 cm to 20 cm and with both a white and black paper target. All the tests were performed without any sunlight (window shades closed) and with very little indirect sunlight (window shades open but north facing window). This because sunlight has a very high IR content and any high indirect sunlight saturates the phototransistor and therefore obstacles can't be detected at any useful distance.

    Bellow is a figure showing the results for these tests.

    These results show that the IR reflective sensors can be used for distances bellow  around 5 cm, in this range the distance can be, more or less, estimated and without having to worry to much about the obstacle color. This is sufficient for the intended use, to detect an obstacle at a distance (> 1.5 cm) that allows the robot to turn around without having to reverse.

    As always there is some more information and results available on the Website. Also, more frequent progress updates on projects are published on Twitter.

  • Motor Encoder and Drive Results

    NotBlackMagic11/05/2021 at 10:26 0 comments

    Motor Encoder Tuning

    As mentioned in the previous project log, the motor encoders was tuned before gluing them in place. The first part of the tuning consists on setting the reflective sensors LED drive current, by changing the LED drive resistor, and the phototransistor output resistor, which sets the sensibility. The values used are 330 Ohm for the LED drive resistor, a drive current of 6mA, and 47 kOhm for the phototransistor resistor. These give a good output swing when the leaf passes in-front of the sensor, around 250mV, while keeping the current consumption relatively low.

    Next the voltage comparator threshold voltage is set to about the middle of the reflective sensor output swing, which is 2.75V. All these signals are shown in the figure bellow, in green the output of the reflective sensor, in blue the threshold voltage and in red the output of the comparator. The latter one had to be filtered by adding a 10nF capacitor to the output to remove false triggers/pulses caused by noise in the reflective sensor output. This is why it has a slow rise time and a high voltage level bellow 3.3V.

    The tuned motor encoder was then glued in place, the figure above is from the final assembly already, in more then one robot chassis and it always gave a reliable output confirming that with the used resistor values the motor encoder is reliable.

    Motor Drive Characterization

    With the motor encoders working, glued in place and the Motor Drive PCB added on top and soldered on it was time to test the motor drive. The motors are driven with two H-Bridges in a single IC, the STSPIN240. The motor RPM is controlled by using PWM signals to drive the H-Bridge. Bellow is a figure showing the voltage and current at one of the motors terminals. The current is obtained from the INA180 connected to the current sense resistor on the motor output of the STSPIN240.

    The figure shows that there is a slight voltage drop when the current increases, this can maybe be mitigated with higher capacitor values to bridge the current draw spikes but the drop is not large and so not considered a concern.

    Finally, with both the motor drive and encoders working it is possible to characterize the motor RPM vs PWM duty cycle. With that and using the gear ratio and wheel diameter, the expected robot drive speed with different PWM duty cycles values and supply voltages, battery voltage levels, is obtained. This is shown in the figure bellow.

    This figure shows that the maximum expected drive speed of the robot is around 90 mm/s with a full battery (4.2V) and decreases to around 75mm/s when the battery gets empty (3.5V). This means that the robot firmware will have a software limited maximum drive speed of 70mm/s so that the limited maximum drive speed can always reached, with any battery voltage.

    The next step is to characterize the reflective collision sensors.

    A more detailed description of the tuning of the motor encoder as well as some additional results of the motor drive are available on the Website.

    Updates and progress on this project, and other projects, are published on Twitter.

  • Hardware (PCBs)

    NotBlackMagic10/17/2021 at 19:08 0 comments

    This update is focused on the Hardware of the Mini Cube Robot drive-train (base). All necessary mechanical parts have arrived, screws, gears, motors, wheels etc... As well as all three PCBs (Motor Encoder, Collision Sensor and Motor Drive) and the needed components to populate them.

    First all PCBs were assembled and tested for functionality. Both the Motor Encoder and Collision Sensor Boards (in the figure bellow) are working, all sensors are returning a good signal and are working as expected.

    The Motor Drive PCB was also tested, the MCU is working as well as the Motor Drive and Battery Manager ICs. The peripherals are still being tested as well as the acquisition of all the sensors and the control of the motors.

    With the PCBs functionality tested, the first Mini Cube Robot base was assembled. The mechanical parts are fitted into the bottom drivetrain holder, together with the collision sensor boards and the motor encoder boards. The latter have to be glued in place but before that they were tuned so that the voltage comparator returns a clean signal whenever the motor's plastic leave passes in-front of it (results in a future update/log). Bellow is a view of the bottom drivetrain holder fully assembled.

    As can be seen all the parts fit very snug into there locations. The gears make good contact and the motors can spin the wheels perfectly. This is of course not the first version 3D printed, there where quite a few versions before arriving on these. The latest, the ones used here, 3D printed part files are linked to download.

    With this, the top cover was added and the Motor Drive PCB was mounted on top. The collision sensor and motor encoder boards have to be soldered to the Motor Drive PCB, this is something that is planned to be changed in a future version. The assembly can be seen in the figure bellow.

    Looking at the photo there are a few blue wires visible, those connect to the motor encoder IR reflective sensor outputs to monitor if the voltage output changed with the complete assembly. Nothing major was detected and the motor encoder output continued to work as expected.

    Finally the battery pack was added. It is composed of two 3D printed parts, the same part is used as the base and the top just flipped, holding a 1200mA Turnigy (HobbyKing) 1S LiPo in-between them, as well as a power switch. The fully assembled Mini Cube Robot base can be seen in the figure bellow.

    This is the current state of the Mini Cube Robot project. Next is the development of the software for it, to control the motors and read all the sensors.

    For some more details on each part and a first assembly guide are available on the website. The assembly instructions will be added to this project page later.

    Also, updates and progress on this project, and other projects, are published on twitter.

View all 3 project logs

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