DEVICES

Presentation of the Hardware

TekTrain device

The TekTrain device is one of the key elements of the system, as it determines the capabilities of the applications that can be developed in the context of educational robotics. In order for the TekTrain product to fulfill its educational purpose of assisting the teaching of robotics in the school environment, the TekTrain device is required to support a set of sensors and actuators for proper interaction with the environment.

Abstract device hardware interface diagram TekTrain

 

The hardware of the TekTrain device is divided into the following four categories: a) mobile platform, which provides support and mobility to the TekTrain device, b) central computation platform, c) peripheral sensors and d) peripheral actuators. In addition, a single platform (Expansion Board) provides support and connection of all sensors, actuators and battery to the computer unit.

 

Central Computer Unit

For the development of the device control software, it is necessary to have a central computer unit in which the peripheral devices will be connected (sensors and actuators). This unit will run both the communication software to and from the peripheral devices, as well as the device control software (eg motion control and remote access).

This unit consists of 2 subunits. An integrated system in which the device control software will run and an “expansion board” type board in which the power supply and interconnection circuits of the peripheral devices will be integrated. The processing power should meet the computational requirements of robotic algorithms, such as motion control, obstacle avoidance, mechanical vision and hearing, face identification, voice command recognition, etc.

Based on the aforementioned computing needs, the use of the Raspberry PI 3 Model B + was chosen, as it has quite high computing power and low power consumption, while at the same time it has built-in WiFi and BLE units. More specifically, important factors that led to the choice of this integrated system are the following: High processing power / Low power consumption / Linux Development Environment (Debian-based, Raspbian) / Peripheral units GPIO, I2C, PWM, SPI / Built-in WiFi unit / Built-in BLE unit / Built-in USB peripheral unit / Built-in audio-out peripheral unit for speaker connection

The motors are controlled via the DFRobot MDV 2x2A DC Motor Controller (L298N).

Motion platform base

Based on user requirements, the TekTrain device should be able to move independently in space. This requirement indicates the need for a mechanical platform with integrated wheels and engines, which will be the basis for the development of the TekTrain device. Also, the mechanical construction of the base should allow the installation on it of the central computer unit, peripheral sensors and actuators as well as the power distribution circuits.

The kinematic model of the base follows that of the differential drive (differential drive mobile base) and is based on the independent movement of the wheels located on each side of the body, thus allowing the execution of linear and on-site rotational movements. In case the base has four independently controlled wheels, the motors are tested in pairs per side (4WD Differential drive). Of course, in the case of four wheels, the kinematic model changes. However, further analysis of this differential drive model is considered outside the scope of this document, as it does not affect the hardware specifications of the TekTrain device base.

In addition, in order to control the movement of the base, it is necessary to have appropriate control circuits of the motors and the sensors encoding the motor trajectory. The motor control circuits to be integrated should allow speed commands to be sent to the motors via a digital protocol, such as Pulse Width Modulation (PWM), so that they can be controlled via the host unit. To correct the position estimation error (odometry), each engine must have a position / track encoder mounted on its axis. This allows speed control via an automatic closed loop control system. The following is a table of characteristics of the mobile platform base selected for the TekTrain device.

 

The device is based on a drive platform which consists mainly of a metal frame that carries the drive wheels, the wheel motors as well as suitable recesses and protrusions for the placement of the system sub-units. 

DFRobot Baron platform

As for the dimensions of the TekTrain device, it should not exceed 30 cm. This is related to the usability of the device and the safety of the students. Also, special importance is given to the weight of the final device, so that it does not exceed 1.5 kg. The use of the DFRobot Baron platform has been chosen. The platform is designed around a chassis made of aluminum, in which there are suitable recesses and protrusions for fixing the various modules. Includes 4 DC motors for wheel drive, 4 wheels and two wheel position / rotation speed encoders.

Platform
Material Description

Four independent drive axles (engines / wheels).

Specifies the installation of engines on each wheel of the vehicle, four in total. This supports the implementation of the aforementioned 4WD differential drive kinematic model.

Two or four engine controllers.

Support for the implementation of the 4WD differential drive kinematic model. To reduce costs the motors will be tested in pairs on each side of the base.

Level frame / floor for platform development.

Installation of integrated system, peripheral sensors, actuators, power circuits and motor control.

Ability to customize position coders in engines.

The mechanical construction of the base should support the adaptation of position / speed coding sensors to the axis of rotation of each motor. Encoders are necessary to correct the vehicle traffic error.

Sensors

The choice of sensors that will be supported in the project is important as they are the key elements that will be used in the context of the educational missions provided by the TekTrain product, both by students and teachers. It is also particularly important to support sensors of different types, in order to give more flexibility to the applications that can be developed. The sensors can be adapted at will, by the individual user, to different parts of the base of the TekTrain device, depending on the needs of the respective training mission, since it determines the necessary sensors / actuators that must exist in order to be processed correctly. For example, one exercise may use only the main actuators, while another may involve interaction with the user through a microphone or distance measurement. For this reason, at any time, it should be possible to switch sensors on the body of the platform, but also the automatic recognition of the connected sensors by the device control software. The following table lists the sensors that will be supported for customization on each TekTrain device. These sensors are based on user requirements.

Sensors
Sensor # of items Description
Ultrasonic distance sensor: HC-SR04 Ultrasonic Range Finder.       2 Two ultrasonic distance sensors for distance measurements of at least two meters.
Infrared technology distance sensor (InfraRed): Infrared Proximity Sensor Short Range – Sharp GP2Y0A41SK0F.        4 Two IR sensors for short and two for longer distances.
Distance sensor laser time-of-flight: VL53L1X Time of Flight (ToF) Sensor.        1 Laser time-of-flight laser technology distance sensor: VL53L1X Time of Flight (ToF) Sensor.
Ambient temperature sensor: BME680 Air Quality, Temperature, Pressure, Humidity Sensor Breakout Board.        1 An indoor temperature sensor.
Atmospheric pressure sensor: BME680 Air Quality, Temperature, Pressure, Humidity Sensor Breakout Board.        1 Atmospheric pressure sensor: BME680 Air Quality, Temperature, Pressure, Humidity Sensor Breakout Board.
Ambient humidity sensor: BME680 Air Quality, Temperature, Pressure, Humidity Sensor Breakout Board.        1 An indoor humidity sensor.
3D compass: SparkFun 9-DoF Sensor Stick.        1 A 3D compass for measuring the angle in 3D space. Using the 3D compass, the TekTrain device will be able to perceive changes in the orientation of the device in 3D space.
Three-axis accelerometer: SparkFun 9-DoF Sensor Stick.        1 A sensor for monitoring the acceleration of the TekTrain device on three axes (inside the inertia unit).
Gyroscope of three axes: SparkFun 9-DoF Sensor Stick.        1 Gyroscope for measuring angular velocity and maintaining orientation (within an inertial unit).
Microphone system: Adafruit Mini USB Microphone        1 A microphone system for receiving sound measurements / data from the environment (voice commands, audio directivity, etc.).
RGB camera: Raspberry Pi 8MP Camera board v2        1 An RGB camera capable of capturing images from the environment at a resolution that satisfies the machine learning algorithms that will be developed as part of the project.
Contact sensor: Tact Switch 12x12mm 7.3mm + Cap for Tact Button        10 Ten touch sensors that can be used either as status buttons or to identify the device’s contact with objects in space.
Line tracking sensor(line following): Cytron Line Sensor.        1 A line-following sensor that will be integrated at the bottom of the device, allowing the development of motion applications on tracks defined in white or black.
Engine position / rotation speed coding sensor: Included in the drive platform.        4 Four wheel position / speed coders. Applied to correct the position error. Produces odometry data

Actuators

Actuators are the elements that affect the environment by changing its state. Such parts can be motors, leds and a speaker. Both the sensors and the actuators will be sought to be able to adapt to different parts of the base, depending on the needs of each application.

Actuators
Actuator # of items Description
Base engines: DFRobot 6V,180rpm Micro DC Geared Motor with Back Shaft.        4 Four engines adapted to each wheel of the base.
RGB Leds: NeoPixel Digital RGB LED.       20 Programmable LEDs with the ability to adjust the color and intensity.
Speaker: Adafruit Mini External USB Stereo Speaker.        1 A speaker for playing audio messages and sounds in general.
Display: Pi Display 4″ HDMI 800×480 IPS Resistive        1 Used to view images / videos and emotion.
Pan-Tilt engine: Pan/Tilt Bracket Kit.        1 A PanTilt mechanism that is used to move the camera.

 

 

 

Ultrasonic distance sensors

6 HC-SR04 ultrasonic sensors with a measuring range of 2 cm to 4 m are placed on the platform. Each requires a + 5V DC power supply, a trigger input signal and an output signal that gives the acoustic return pulse to measure the distance. The sensors are connected to the expansion board via headers J3 to J8. Each header provides power supply (+ 5V, ground) and two terminals for input and output signals (DIN and DOUT).

 

LED strip

The LED strip requires, in addition to the power supply, an additional PWM signal to control the LEDs.