paho

In the Lego robot subproject of the MoDeS3 it was important to integrate the sensor information from the Lego sensors, the control and also the logic responsible for the safety. For this purpose, we have implemented an advanced control protocol in Python. This script runs on an embedded Linux distribution, called EV3dev. With this operating system we can utilize the Lego devices connected to the EV3 brick, while have access to all generic Linux packages, like the mosquitto broker.

The simplified overview of depdendecies is depicted on the next picture:

Overview

Our logic is able to detect when the motors are overdriven, or the robot is getting to a twisted and dangerous position, and prevents it from further attacking its limits by stopping them. By this protocol other components can control the crane by MQTT messages, or stop it if any other sensor detects something dangerous.

The code snippet below runs in a cycle and if it notices chage in the state of the touch-sensor, sends a message through MQTT (Paho). If needed it also executes some safety routines.

using MQTT

Sensors of the hardware are depicted on the following figures: these sensors provide the information:

Sensor Sensor2

The solution is built modularly, each part is responsible for certain movements and sensor information. The control software enables the user to control any of the motors individually, and get back raw sensor data through MQTT. A safety modul is observes the behaviours and available information and intervenes if something goes wrong.

As part of the Eclipse IoT Challange 2016, we shall use as many open source implementations of IoT standards, and Eclipse based technologies as we can. For communication we chose MQTT and its open source broker (Mosquitto) and client (Paho) implementation.

MQTT

Here is a short description about MQTT from its homepage:

MQTT stands for MQ Telemetry Transport. It is a publish/subscribe, extremely simple and lightweight messaging protocol, designed for constrained devices and low-bandwidth, high-latency or unreliable networks. The design principles are to minimise network bandwidth and device resource requirements whilst also attempting to ensure reliability and some degree of assurance of delivery. These principles also turn out to make the protocol ideal of the emerging “machine-to-machine” (M2M) or “Internet of Things” world of connected devices, and for mobile applications where bandwidth and battery power are at a premium.

As you can find out, MQTT architecture consists of brokers and clients. Brokers interconnect clients through different topics. All clients receive the messages published to the topic they are subscribed for. A message can be anything that is convertible to a byte array. This way an N:N connection cardinality can be easily achieved as depicted on the next figure.

Different open source and proprietary implementations of MQTT brokers and clients exist in most programming languages (C, C++, Java, .NET, Python, JavaScript, etc) . We use Mosquitto as a broker, and Paho as a client implementation.

Mosquitto

The Mosquitto broker is the focus of the project and aims to be a lightweight and function MQTT broker that can run on relatively constrained systems, but still be powerful enough for a wide range of applications.

To get started with Mosquitto visit its website at http://www.eclipse.org/mosquitto/. Download and install it on your computer. We use the binary compiled for Ubuntu as it works seamless.

The default address of the running Mosquitto service is tcp://localhost:1883, on localhost over TCP protocol at port 1883.

However, if you would not like to install Mosquitto yourself, there are two Mosquitto brokers publicly available online:

One that is operated by Mosquitto website itself at http://test.mosquitto.org/
One that is operated by Eclipse at tcp://iot.eclipse.org:1883

The Paho project provides open-source client implementations of MQTT and MQTT-SN messaging protocols aimed at new, existing, and emerging applications for Machine‑to‑Machine (M2M) and Internet of Things (IoT).

To get started with Paho visit its website at http://www.eclipse.org/paho/ and look for the client sample codes. We use Java Client through Maven.

Demo time

Now we will provide some sample code snippets in Java which demostrate a “Hello World” message exchange between two clients. We use Maven for dependency management so the pom.xml is provided as well.

The code snippets are available at https://www.eclipse.org/paho/clients/java/.

Publisher (sender) client sample:

https://gist.github.com/923823d29bea1f5c2d0c

Subscriber (receiver) client sample:

https://gist.github.com/697b3507e0b3f890f105

Last but not least, the pom.xml that is necessary for Maven:

https://gist.github.com/c5a981cc480eea96ea98