Mobile robot platform

Mobile robot is one of the most popular robot for construction. Very common are sumo robots, sports robots (football, volleyball etc.), robots simulating rescue operations (firefighting, person or object finding etc.) and many other. For these kinds of robots there are many different competitions in the world and in Estonia, even standard classes have been developed (eg sumo robots). The common feature for these types of robots is mobile platform, which may have different construction and capabilities, but its main functionality remains the same. It is controlling the motors and basic navigation, which includes avoiding objects and travelling to the desired destination. Usually, a specific functionality is added to the main functionality, which is planned according to the requirements and opportunities set for the project.

Here we look at the documentation of a typical mobile robot platform project and its different phases.

Plan and construct a multifunctional mobile robot platform with basic navigation functionality using HomeLab components. Robot platform must have an easy-to-change operational functionality, when equipped with different gadgets:

• Manipulator
• Radar
• Camera

Robot must be able to move on a flat surface in indoors.

• Maximum dimensions: 20 cm x 20 cm x 20 cm
• Maximum weight 2 kg
• Speed max. 0,2 m/s
• Full autonomy

• Must be constructed mainly from HomeLab components
• Cannot exceed the cost limit 10 000 EEK

The overall model of the system is presented as a block diagram. It describes the structure, behaviour and other important aspects of the system. As an example, an hierarchical model of the overall system is depicted below.

Model of system structure

For this task the team used a brainstorming method and generated 3 conceptually different solutions. Evaluation matrix was compiled and the most optimum construction was found. The main differences of the solutions lay in the movement schemes.

Design solutions

Simplified evaluation matrix was as following:

Evaluation scale was 1-10 points and weight factor 0-1. Weight factors were chosen according to the requirements and restrictions set for this system. Eg, although solution 2 was significantly more capable moving on rough ground, it wasn't required in the preliminary task and therefore the weight factor was low.

Based on the assessment the optimum solution for given task was proved to be a platform moving on two wheels with two separate motors. Further work continued developing the chosen solution into a real system.

Mechanics was tried to make as simple as possible, while following the principle of modularity. The front and the rear bumper are identical modules. Electronics have three modules, which are placed on top of each other, allowing simple ribbon cable connections, while ensuring relatively simple changeability of modules. Motors have been selected from HomeLab kit: motors with integrated reducer and coder, which are connected directly to the actuator of the motors. Model aircraft wheels have been used, because they are very light and strong enough for the robot. To simplify the construction the bottom and the top plate are identical. Plates are equipped with holes, allowing different devices to be attached on the top plate. Besides the electronic modules a battery fits between the plates as well.

Primary 3D model of the robot and location of its components.

The bumper of the robot is projected separately and it is integrated with touch sensors and line following sensors. The bumper is made from PCBs and therefore has electricity in addition to construction. Line following sensors are soldered directly to the bumper of the bottom plate. Touch sensors (micro switches) are placed between the bumper plates and are covered by a single rubber piece at front. The rubber piece absorbs the hit and at the same time enables to identify where the hit came from.

Bumper plate drawing

The electronics of the system is described as a principle scheme and electronic scheme with PCB assembly scheme.

Block diagram of electronic components

As an example, Line following sensors electric scheme and respective PCB assembly scheme of the robot's bumper is shown.

Electric scheme of the bumper sensors

Assembly scheme of the bumper

The control system of the robot derives from behavioral model and is set by the functionality, requirements and restrictions of the initial task. From the behavioral model of the system a specified control program is created, which in turn is the basis for software program code. All three levels (behavioral model-algorithm-source code) must be consistent with each other.

Algorithm describes the control logic of the system and is depicted as a block diagram. A few elements and description of their relations is enough to create a simple algorithm. If the algorithm of the robot is composed correctly, then it is relatively easy to compose a control program for this robot. Mainly two different objects are used in the algorithm: a rectangle with rounded corners, which marks an activity and a small diamond for controlling a condition, followed by a startup of further activities in accordance with the results of the inspection.

Meanings of the symbols used in the algorithm:

Algorithm state diagram

Simple navigation

#include <homelab/module/motors.h>
#include <homelab/pin.h>
#include <homelab/delay.h>
 
// Defining bumper pins
pin front      = PIN(C, 0);
pin frontleft  = PIN(C, 1);
pin frontright = PIN(C, 2);
 
//
// Main program
//
int main(void)
{
	// Initiating motors 0 and 1 
	dcmotor_init(0);
	dcmotor_init(1);
 
	// Sensor pins as inputs
	pin_setup_input_with_pullup(front);
	pin_setup_input_with_pullup(frontleft);
	pin_setup_input_with_pullup(frontright);
 
	// Endless cycle
	while (true)
	{
		// Clockwise motor startup 
		dcmotor_drive(0, 1);
		dcmotor_drive(1, 1);
 
		// Controlling the middle sensor signal
		if (pin_get_value(front))
		{
			// Reversal of the motors
			dcmotor_drive(0, -1);
			dcmotor_drive(1, -1);
 
			// Paus 1 second
			sw_delay_ms(1000);
 
			// Left motor clockwise startup  
			dcmotor_drive(0, 1);
 
			// Paus 2 seconds
			sw_delay_ms(2000);
		}
 
		// Controlling the left sensor signal
		else if (pin_get_value(frontleft))
		{
			// Reversal of right motor
			dcmotor_drive(1, -1);
 
			// Paus 2 seconds
			sw_delay_ms(2000);
		}
 
		// Controlling the right sensor signal
		else if (pin_get_value(frontright))
		{
			// Reversal of left motor 
			dcmotor_drive(0, -1);
 
			// Paus 2 seconds
			sw_delay_ms(2000);
		}
	}
}

Robot platform completed under this project is largely made from plastic, except from motor mountings, which are made from aluminum. Electronic modules are placed on top of each other and the battery is loose between the plates. Bumpers are made from PCB and painted black. The top plate of the robot is completely flat, allowing to attach different desired devices. A simple radar was installed on the robot, which consisted of a small RC servo motor and an infra red sensor. As a second solution, intelligent camera module was installed on the platform for solving machine vision problems. Both solutions are brought out on the following pictures. Standard manipulator was tested as a third device, which components are controlled with standard servo motors as well, using serial interface for controlling their actuator.

Robot with infrared radar

Robot with intelligent camera module (CMUcam3)

Economic calculation includes the cost of components and robot production costs. The currency used in following tables is Estonian Kroon (EEK).

Table of components cost

Estimated labor and production cost for a single copy.

Estimated cost of the robot 7845.-

The cost calculation of the robot is estimated, since it is an educational project, where most of the work and construction is done in significantly larger volumes, but without direct charge. Therefore, the work and the approximate time spent does not reflect the real situation.

Mechatronic system (Robot) is created as a team work with a firm timetable and budget, thus having most of the significant features of a project. The key activities of the project management were: time planning, team work planning and management, budget monitoring and obtaining supplies, current reporting to the supervisor, presentation and documentation of the outcome. The project report includes working groups minutes of meetings, project plan (preferably in a Gantt diagram), resource allocation (including human resources), planned and actual budget. For example a simple action plan is given as a Gantt diagram.

Action diagram of a project

Economic calculation showed that the production cost of the robot is quite high, especially when dealing with a single original, but remains within a predetermined initial task. Production costs could certainly be substantially reduced through the optimization of materials and components, and producing a larger quantity of robots at the same time. During this project we learned how to project a mechatronic system, how to construct and test it, which gave us the first time experience of this kind.

At the end of the work a fact revealed: in order for the robot to function properly significantly more time should be planned for testing, especially for the software testing. Different modules may not always work properly together, although as a separate experiment it works. This shows that the integration of the modules of the system is a serious challenge, and therefore more time and resources should be planned for this.

In conclusion, we believe that the project was very interesting and instructive, it gave an indication how to design and construct integrated systems.

1. HomeLab generic manual http://home.roboticlab.eu
2. ATmega2561 datasheet
3. Dudziak, R., Köhn, C., Sell, R., Integrated Systems & Design, TUT Press, 2008
4. Friendenthal, S., Moore, A., Steiner, A., A Practical Guide to SysML, Elsevier, 2008
5. Perens, A. Project Management, Külim, 1999
6. Bräunl, T. Embedded Robotics, Springer-Verlag, 2003