Power Source Finding Mobile Robot
Ryan Miller
MEng Cybernetics,
abstract
This paper outlines the design and construction of a small mobile robot that can be deployed into an environment and monitor its battery status while performing a pre-programmed task. When it is deemed that the battery power has reached a sufficiently low level the robot seeks out and docks with a recharging station, through the use of infrared communication, in order to replenish its battery power and continue with the task until a power source is no longer available. This method of power source location is a continuation of a previous years triumph, which allowed a mobile robot to successfully find a standard wall socket and accurately insert a plug from which to draw power.
1.Introduction
Mobile robots are being used increasingly in many hostile situations, from bomb disposal to fire fighting and even space exploration. The cost of deploying a mobile robot into a given situation is greatly out weighed by the cost of potential loss of life, and so is becoming a popular choice. Many mobile robots require a remote human operator to control their actions. This in itself poses problems with loss of communication and bandwidth of the signals required to perform certain tasks, the ultimate goal however is to achieve full automation. Examples of such robots are the NASA Mars Landers, most recently Spirit. Despite the complexity any of the mobile robots all have one basic need, power, and the ability to replenish their power is one of the first steps required to achieving total automation. Spirit [1] utilises a multi-panel solar array that collects energy from the sun during the periods of the day when the robot is found on the light side on the planet and charges a set of batteries onboard, enabling it to continue operating throughout the day when solar energy may not be available. Spirit has an effective life span of 90 Martian days, as after this point the solar panels collect too much dust and reduces the power producing potential.
Last year a student successfully produced a mobile robot that could locate a standard wall socket and insert a 3 pin connector to draw power [2]. This was proved to be a very effective method of drawing power in order to potentially recharge the batteries. However, during the demonstration of the project, the robot took a significant amount of time to make the connection and the robot essentially becomes immobile.
Continuing from the successes of last year, the power source finding method, adopted in this paper assumes that a recharging station has already located a wall socket, and is drawing power.
The robot to be introduced in this paper is an adaptation of the University of Reading’s dwarf robots, in which ultrasonic sensors are used to achieve simple obstacle avoidance. In order to recharge the robot’s batteries, the robots docks with the recharging base station. The base broadcasts a modulated infrared (IR) signal that the robot can lock onto through the use of an array of infrared phototransistors, when it is decided the battery power is running low. The robot is programmed to follow the IR based on the signal strength received and locate the base.
A robot such as this would be best suited to smaller indoor environments, such as an office due to the attenuation of the IR signal and the power required to transmit it. However, this technique could relatively easily be adapted to accommodate other mediums such as radio signals for larger environments and function in the same way.
Figure 1. Ring of IR Receivers
2.Design and OPERATION
2.1.Base Design
The base structure contains the necessary circuitry required to recharge the batteries of the robot and transmit the IR signal the robot needs to find its way back.
The base actually broadcasts two IR signals to allow the robot to successfully dock. The first of the signals is transmitted at a wide angle and allows the robot to determine the position of the base within the room. The second signal is a much narrower beam that determines the point on the base onto which the robot must reverse to make the physical connection and charge the batteries. Figure 2 shows a basic diagram of the base and illustrates the importance of the second signal in locating the connector upon the base.
As the connection between the robot and the base must be accurate a series of grooves were added to the floor directly opposite the connector that assists in guiding the wheels back to a specified position directly centred over the connector. Should the robot reverse towards the connector at an angle that is not perfectly in line with the connector, the grooves force the wheels to follow the tread and brings the robot back square with the connector.
The circuitry used to charge the batteries of the robot was a simple off the shelf, variable voltage and current charger for Nickel Cadmium batteries that has been set up for the required purpose.
Figure 2. Signals Emitted From Base
2.2.Robot Design and Operation
The robot has a very simple design, much like the seven dwarfs only with added sensors and circuitry in order to receive the IR signals broadcast from the base.
A PIC 16F873 Microcontroller controls all actions carried out by the robot. The PIC monitors the input signals from the IR phototransistors as well as the ultrasound and sends control pulses to the motors to achieve the desired effects. While the batteries are running at a sufficient power level, the PIC ignores the input signals from the IR receivers as they are considered irrelevant and do not affect the basic operation of the robot. When it is deemed the available power from the batteries is inadequate to maintain normal operation for a significant period of time, the signals from the IR receivers takes precedence over those from the ultrasound and guides the robot back to the charging stations location while still avoiding incoming obstacles.
The robot uses four ultrasound transducers mounted on the front of the robot for obstacle avoidance, two for transmitting ultrasound pulses and two to receive. The PIC uses the echoes from the ultrasound to detect objects on the front left and front right of the robot and sends a signal to the motor on the appropriate side to turn the robot away.
Using the ring of 16 IR phototransistors mounted on the top of the robot, the orientation of the base in relation to the robot can be determined by observing which of the transistors the signals is strongest on. By adjusting the signals sent to the motors, the robot can position itself to ensure the strongest signal is being received on the transistor located directly on the front of the robot, allowing the robot to drive directly towards the source of the signal. Once the robot reaches a certain distance from the base structure, indicated by a particular signal strength, the robot maintains that distance while patrolling the perimeter of the structure until the narrow angle signal from the base is detected. As before the robot alters its position to receive the signal, this time, on the transistors located on the rear and begins to reverse. As mentioned earlier, the base utilises grooves in the floor to guide the robot backwards to the connector, greatly improving the chances of making a successful connection. Once the robot detects that power is being drawn from the base, the power to the PIC is cut off allowing the battery to charge without interruption, once the battery charging is complete, power is redirected to the PIC and the robot moves off and continues with its standard program.
2.3.Infrared Communication
The IR signals broadcast from the base form the integral part of the project. The signal that the robot homes into is a simple repeating sinusoidal wave, with the peak-to-peak voltage indicating the signal strength and distance from the base. As this signal is being transmitted through the air, which introduces a lot of ambient noise, this signal is modulated over a carrier signal of higher frequency at the base.
Figure 3. Emitted Amplitude Modulated Signal
There are several forms of modulation that can be used for this task and it was decided that amplitude modulation would be the method of choice. The amplitude modulator constructed imprints a 1 KHz modulating signal onto a 600 KHz carrier signal, this allows the original 1 KHz signal to be recovered by the robot using a demodulation circuit with less noise than if transmitted directly through the air. However, as two signals are required to be transmitted in order for the robot to dock as discussed above, a second modulator was constructed using the same carrier frequency as before and utilising a 100 KHz modulating signal. These are transmitted through separate emitters on the base but received through the same receiver ring on the robot. When this signal is demodulated on the robot the output is found to be a combination of both the 1 KHz and 100 KHz modulating sinusoids, the output is then fed into two separate band-pass filters with a Q factor high enough to isolate the two frequencies. These are then fed into two different I/O pins of the PIC allowing the robot to monitor the two signal strengths independently. Figure 3 shows the wide angle Amplitude Modulated signal emitted from the base.
2.4.Electronics
Both the robot and the base require circuitry to transmit and receive the IR signals. The base required two modulating circuits based on the MC1496 Balanced Modulator/Demodulator [3] to be constructed that combined the 1 KHz and 100 KHz modulating signals with a 600 KHz carrier signal for transmission through the environment. The output from the modulating circuitry was fed through a transistor amplifier in order to boost the signal to a level that would allow for the signal to be transmitted at its maximum strength.
Feeding the two modulators were three oscillating circuits, constructed using MAX038 High Frequency Waveform Generators [4]. These were configured to produce stable sine waves of 1 KHz, 100 KHz and 600 KHz, with the 600 KHz generator feeding both modulating circuits. Figure 4 shows the board containing the modulation circuitry and sine wave generators.
Figure 4. Transmitted Signal Generation Circuitry
The robot utilises a 4067 16-Channel Analogue Multiplexer [5] to address each of the 16 IR phototransistors in the receiver ring. The PIC Microcontroller uses the 4 address lines of the multiplexor to select each of the phototransistors in turn and output received signal to the demodulation circuitry.
The demodulation circuitry uses the same MC1496 Balanced/Demodulator as that for the modulation only configured slightly differently. The output of the demodulator will contain both the transmitted signals from the base when the robot is in close enough vicinity to the docking point, and so the output from the demodulator is channelled into two separate band-pass filters.
The band-pass filters were constructed by cascading two second order operational amplifier filters configured to act as high and low pass filters. By doing so, a band-pass region was created where by the upper and lower cut off frequencies could be specified.
Figure 5 shows the flow of data from the base to the PIC on the robot.
Figure 5. Path of Signal Through Circuitry
3.Testing
The testing phase for the development of such a robot can be very time-consuming and difficult to debug should anything go wrong. As this is the case, during the development of the software, each task the robot must perform to result in successful operation was broken down and tested individually.
The most basic operation the robot must be able to perform is simple object avoidance. The robot is programmed to only produce and receive signals from the ultrasound transducers, listening to the echoes and determining the distance from objects. It must be ensured that the robot can see any potential obstacles far enough in advance to take the appropriate action and turn in the opposite direction.
The next task is to ensure the robot can receive and understand the two IR signals broadcast from the base. The robot is programmed to head towards the strongest occurrence of the transmitted signal and follow it, stopping at a predetermined distance from the source.
Once both of these tasks can be performed successfully, they are combined to produce a robot that can follow an IR source while still avoiding objects. It must be ensured that the precedence of one signal does not hinder the robots ability to use the other, for instance the robot must not purposely drive in the opposite direction to the source while there is no obstruction between the robot and its target.
It should be stated that at the time for preparing for this paper, not all the tests outlined in the text had yet been implemented, nor the results obtained
4.conclusion
This paper has outlined a potential solution to the continuing problem of supplying mobile robots with an uninterrupted power source for use in situations where automation is the preferred method of control. It has shown how a low budget, low bandwidth broadcasting system can be used for basic position location and guiding.
Robots are becoming increasingly complex and can perform a wider variety of tasks with each new generation created, however the most impressive of which are stationary units with a constant supply of voltage from a mains source. By allowing the robot to find its own source of power from that available, greater mobility can be achieved and their abilities used in a wider environment.
5.Further Developments
As the robot is required to return to the base at relatively regular intervals during the course of its operation to charge it batteries, the robot could be used to perform other tasks that require returning to a set position with little extra effort. Such a task could involve object placement and retrieval whereby the robot may rescue another stranded robot, deploy shorter range robots deeper into the environment than they could navigate by themselves or even retrieve items of value or importance from hostile situations.
The robot could also be used to transfer data from one location to another where it is impractical or undesirable to do so. By storing it within the robots’ internal memory and downloading it to the base structure during a charging cycle the data could be removed from the environment using other medium, perhaps power line transducers or telephone lines. The robot could be used as an agent to retrieve data from the environment allowing a terrain map to be created of the surrounding area, the base structure could then process the data using potentially greater resources than are available on the robot itself and plot routes for which the robot can follow.
Acknowledgements: The Author would like to acknowledge Mr Chris Vanstone, Mr Les Comley, Prof. Paul Sharkey and Mr Iain Goodhew for their help and input into the creation of this project.
6.References
[1]NASA Jet Propulsion Lab (2004, Feb), “The Rovers Energy”. [Online]. Available:
[2]S. Cook, “Power Source Finding Mini-Robot” SCARP Paper (2004, Feb).
[3]Motorola, MC1496 Balanced Modulator/ Demodulator Datasheet.
[4]Maxim, MAX038 High Frequency Waveform Generator Datasheet.
[5]Phillips, HEF4067 16-Channel Analogue Multiplexer/Demultiplexer Datasheet