Team:ETH Zurich/Mechanics

When thinking about the design of a outdoor field research robot we have to think about the requirements it has to fulfil. First it has to drive around autonomously, ideally also through rough terrain. Secondly it should house all the parts required for the sensor to work, including the microscope, bubbling mechanics and syringe pumps. In order to do this it has to be in the rough size range of 30x40cm. Ideally we need a multi layer structure in order to be able to fit everything in a confined space. Additionally to the minimum requirements, there are a few extras like building a modular platform such that future research projects can make use of it. With these guidelines in mind we evaluated different concepts. Using a RC Car as a base and building on top of that or using an electrical rectangular box and adding wheels to it were both options we thought of but quickly discarded. While these approaches seem easier to implement they are not suitable for our task due to their fixed rigid structure which makes it hard to add additional components. There are however professional solutions available for tasks very similar to ours. One example is HUSKY. It is a unmanned ground vehicle designed to carry research sensors and to drive around in the field. Unfortunately such a robot costs around 20‘000 EUR which makes it way too expensive for our project. Thus we decided to develop and build the whole robot from head to toe with flexibility and affordability in mind.

Our Idea

Barebones of the Robot design

We decided to use mainly 3d printed parts and laser cut plexiglass. This approach is common in prototyping and provides a lot of freedom during the design process. Additionally when having a 3d printer available the materials are quite inexpensive, reducing the total cost significantly.
Regarding the driving system we chose to have four electric motors with a wheel mounted directly on it. Inspired by other mobile robotic platforms like the HUSKY, we fixed all four motors and wheels without adding a car-like steering mechanism. This design is very robust and gives the robot the capability to turn in place around its own axis. This is very important for us as our moving pattern is to go straight and then turn an angle between 0 and 360 degrees. [ TRALALA LINK High Level Model] If we had to maneuver around multiple times in order to turn properly this would cost a lot of time.
Regarding the overall, we defined the position of the parts with the goal of optimizing weight distribution and construction flexibility. The battery in the bottom center, microscope in the back, syringe pumps on top and the electronics in the front. This gave us a good first starting point from which we designed the robot.

Implementation

To design the 3d objects we used Autocad, for which a free student version is available. By modeling the whole robot with all its parts we had a very good idea whether everything fits together and how it would look like before actually printing the parts. When the design was verified, we export the files for the 3d printer (.stl file) and the laser cutter (pdf file). We had the opportunity to use a 3D printer and a laser cutter from our department at ETH. We used the 3D printing material ABS, which is impact resistant, dimensionally stable and can handle high temperatures very well. This makes it a good choice for our mechanical parts.
The general design of our robot is based on using screws and nuts to hold the parts together. By using mainly standard m3 or m6 screws the robot can be fully disassembled into its single components. This also makes it easy to replace and redesign individual parts. Thanks to the precision of the 3D printer and laser cutter, it was easy to design the parts such that they fit neatly into each other.

Technical Specifications

Side View

Front View

Top View

Robot technical drawings

Robot Specifiations

Feature	Specification
Weight	6 kg
External Dimensions	450x380x400 mm
Speed	0.318 m/s
Power Supply	12V
Runtime	15 hours
Operating System	Linux/ ROS
Software Repository	https://github.com/aromaeth
Sensors	IMU, Ultrasonic

The Parts

Main Plate
The robot has one main base plate (blue), where the four electric motors with the respective wheels are fixed. We leave a cut out for the battery in the middle and designed respective holes to mount the majority of our electrical components.

Support Connection
As we only use 5mm plexiglass for our plates and have a very wide wheel base we need to support the robot. This is why along the front and back axle we 3d printed two long supporting parts (red) in order to increase the rigidity of the robot. Additionally the front one houses the two motor drivers.

Lower Plate
The support connection not only increases the stability of the robot but also connects the main plate with the lower one (blue). On this plate we mounted the battery (red) allowing us to get a very low center of mass. As the microscope (green) is very tall, we mounted it on the back of the lower plate decreasing our total height.

Wheels
With a corner bracket we mounted the four motors and wheels (blue), driving the robot around. The fixed wheels enable AROMA to turn around its own axis. We chose a long wheel base to reduce the friction significantly when turning.

The top Plate
The top plates’ (light blue) main purpose is to hold the three syringe pumps (dark blue). Additionally we fit the motor drivers on there too, which are necessary to drive each syringe properly.

Electronics

AROMA obviously comes with electronics too. A small motorcycle battery provides sufficient power. We added a On/Off Switch and a fuse to protect the components against potential high currents. With the help of a converter we not only get the direct 12V from the battery but also 5V, used by the stepper motors and the Raspberry Pi, the brain of the robot. The Raspberry Pi is essentially a small computer, where we installed Linux and ROS - a software framework for robotics. Learn more about software . A big advantage of having the raspberry pi are the Genral Purpose Input/Output (GPIO) pins. With these we are able to interface with all the external devices like motors and sensors directly. They can be configured as an input or an output providing 0 or 3.3V. Together with our motor drivers we can use this binary signal to steer the robot around and coordinate the syringe pumps and lights.

AROMA with the electronics highlighted in green

Driving Motor

DC motor schematic drawing

In order to move the robot we used DC motors. DC motors use brushes and a commutator to reverse the electrical current every half turn in order to keep the rotor - the rotating part of a motor - constantly turning in the same direction. Thanks to this approach, applying a reversed voltage results in a opposite turning direction. This allows us to both drive backwards and forward and additionally turn on our own axis. By adding a gear in front of the motor we transform the low-torque high-speed rotation of the motor into low-speed and high-torque. This allows the motor to apply enough torque to move the whole robot properly. We used the L298N, H-bridge to transform the control signal from the Raspberry Pi into a 12V high current signal driving the motors. You can see the schematic diagram on the left. Using the L298N additionally allows us to switch between on and off states very quickly. By doing so at specific rates we can reduce the voltage and thus control the speed of the motor.

Stepper Motor

Stepper motor schematic drawing

Stepper motors are the ideal solution for an automated syringe pump as they can do very precise movements with a very precise speed. Find out more about our self built syringe pump.
The basic principle of a stepper motor is fundamentally different from an ordinary DC motor. There is a inner magnet effectively divided up in different sections, which look like a gear wheel. The outer coils have corresponding teeth which provide magnetic impulses. The Change of an impulse makes the coil rotate by one small step, in our case 1.8 degrees. This allows us to turn them in arbitrary speed and we can hold the motor still in a certain position. By stepping very fast one gets one smooth motion turning the motor in the desired direction, but for us the interesting case is to step slowly and having a very precise slow movement with a constant turning and holding torque. In order to implement that, we again need a motor driver (L298N) amplifying the output signal of the Raspberry Pi. On the graphic on the right we provide the schematic diagram. We used the same L298N chip as a driver, and 4 Pin bipolar stepper motors to drive our syringes.

The Cost / Result

As mentioned above, there are similar systems existing on the market offering a mobile robotic platform. The big difference here are two main factors. One is flexibility: while some systems may be more durable they are not as customizable as our system. We provide the CAD files to download and build our system, such that future teams are free to build their own version of AROMA, either as we did or customized with additional or alternative parts for different applications.

Download CAD files or instructions

The second difference is obviously the price. When considering systems like the HUSKY we are looking at a price of roughly 20’000 Euros, whereas our system costs less than 300 Euros. This makes it very accessible to other research groups and we hope it will serve as a starting platform to future teams.

Assembly Cost of the Robot

Parts	Cost €
3D Parts Ca. 500 gram of Material	40
Acryl Glass Material	20
Motors x4	120
Battery	90
Cables/ Safety Switch	20
Total	290

The Result: AROMA fully functional and assembled