tracked vehicle with color camera and PC onboard, 2002
Barbora was designed primarily to participate in the Eurobot 2002 competition. Another even more important motivation to build the robot was the desire to build an universal bogie to be used in the following competitions and to have a reliable platform for further research and experiments. Unfortunately we have not finished the robot in time to qualify to the competition. However it has shown to be an excellent drive while building the robot and it has certainly helped us to achieve what we have.
The robot is built around a PC-like motherboard running a customised version of the Linux kernel. Two brushless DC motors provide the power to move and 4 optical USB mice the required odometry feedback. Additional sensors include onboard and offboard (wireless) CMOS cameras and optical reflective infra red sensors to detect the color of the floor bellow the robot. Unique helix storage system is the base for ball manipulation. The software side includes MCL (Monte Carlo Localization) position tracking, RRTs (Rapidly-Exploring Random Trees) path planner and fast image analysis based on color tresholding.
The boggie is built from the off-the-shelf components for the most part, namely ones used for the aircraft and automobile models, as well as some used in the process automation. The usage of the off-the-shelf components was motivated by overall lower production costs of the robot, as well as by the decrease of the developement time. Having done the preliminary design of the boggie construction and functional parts, the manufacturing details were tuned up by the company Vakuum Praha, a company collaborating closely with the Faculty of Mathematics and Physics, Charles University in Prague where most of the team members come from. Vakuum Praha specializes in vacuum science and and precise machinery and has machined the boggie for us.
The boggie is made of aluminium to be light and durable. Viewed from above the platform is roughly a squared, 30x30 centimeters (12 by 12 inches). The height of the boggie containing motors and bateries is around 6 centimeters (2.36 inches).
Locomotion of the robot is provided by two brushless DC motors with neodymium cores produced by a Czech company Mega Motor. These motors are lightweight (160 grams, 0.4 pounds) and provide excellent mechanic power — more than 600W each, giving the robot 1.2 kWatt (1.6 horsepower) peak thrust, resulting in peak acceleration of 20ms-2 (2G). The maximum speed of the boggie is 4ms-1 (9 mph). The robot should be able to reach the maximum speed in less than a quarter of a second (expecting perfect adhesion, no sliping). To achieve good stability (even in high speeds and obscure manoeuvres) and high traction the boggie is equipped with tracks made from industry standard HTD synchronous belt. Similar belts are used for a gearbox between the motors and the tracks because we have not been able find a more suitable gearbox fulfilling such a high requirements such as the ability to handle the power and high rpms and size small enough to fit inside the boggie.
The drawback of the motors is that they require a quite complex electronics to drive. Since they have no mechanical commutator they require three phase power with variable frequency at low voltage (12 Volt) and high current (50 Amperes peak). On the other hand due to the the lack of mechanical commutator the motors offer unprecedented reliability and very high power/weight and power/size ratios.
Fortunately the TMM 40e-3ph motor controller produced by a Czech company MGM-Compro is exactly what we needed and with some necessary modifications to its software (kindly made on our request by the manufacturer) are easy to use with our main computer. The modifications consisted mainly from adding the serial line communication instead of the regular PWM control signal used for radio controlled models.
As a power source 20 NiCd cells are used, with aggregate voltage of 12V, and capacity of 3400mAh. This allows the robot to run for 2 minutes (a match in the Eurobot 2002 competition lasts 90 seconds) at maximum power consumption. Because the real power consumption is approximately ten times less, this is more than enough (for the competition). However the test runs usually lasted much longer and the drawback of the construction was that the bateries had to be recharged while mounted on the robot. That forbided simultaneous testing and recharging which in turn has made the testing unconfortable and less effective. For further usage the batteries has to be made replacable to remove this limitation.
A double copper helix is the base of the ball manipulation system. The helix can store up to 7 balls inside the body of the robot. This allows the robot to collect as many balls as possible and to distribute them in to the appropriate baskets later on. The system is mounted on the top of the boggie in order to be easily replaceable by other manipulation devices for future competitions and/or research.
The balls are first collected by the opened part of the U shaped boggie. When in the front part the ball is lifted by a pair of little manipulation arms. These arms are powered by regular servo motors and driven by custom board based on the 555 chip. The board is connected to the LPT port of the mainboard.
The arms were the single most troublesome part of our robot. We expected them to be an easy fix and that's why we have postponed theirs fabrication having more important things to build. However it has proved to be a real headache, not working all the time.
When lifted 8 cm above the floor the ball is taken by the helix storage system. The helix itself stays fixed and the movement of the balls up or down is achieved using a rotating cylinder in the center of the helix. The cylinder is made of an aluminum pipe with a DC motor and two planet gearboxes inside. The cylinder is fastened to the boggie with a smaller pipe coming out of it in the center. The stator of the motor inside is attached to this pipe. The output of the double gearbox (and in turn the rotor of the motor) is attached to the outer housing made of the larger pipe. The overall effect is a cylinder with two wires coming out of it with a rotating outer housing when power is applied to the wires.
The main controll unit of the robot is a single board computer manufactured by Advantech, PCM-5864, 20×12 cm (8×5 in), equipped with an AMD K6®-2 processor running at 500 MHz, 128 MB of RAM, audio and video inputs, a 100MBit ethernet connection and an USB bus.
The motor controllers return the distance traveled. This information can be in principle used to calculate odometry. However, treaded vehicles suffer from the fact that the very principle of their operation is skidding and sliding offering very poor odometry data. To supplement the information about relative movement four USB IntelliMouse Explorer optical mice were used, offering resolution of 400 dpi and maximum tracking speed of 1.5 ms-1, by scanning the underlying surface at a refresh rate of 6000 frames per second. As optical sensors don't skid or slide over the surface, the robot can do manoeuvres (ie. controlled skiding and sliding) which classic odometry navigation using IRC (Incremental Rotating Counter) is not able to cope with gracefully.
The mouse based sensors provide relative movement information only. Therefore, the error of the estimated position on the playfield can grow out of bounds with time. To avoid this, the robot has five Vishay CNY-70 optical reflective infra red sensors to detect the color of the floor underneath itself. In the competition the sensors were used to detect a line grid drawn on the table. Using this data, the cumulative effect of the relative position tracking can be eliminated using the MCL (Monte Carlo Localization) scheme.
The power to the electronics is delivered by a switching power supply based on Maxim MAX787 components. This gives the computer relatively stable input voltage, while the voltage of the batteries varies under the load.
In order to locate the balls on the playing field the robot is equipped with color CMOS camera. The camera is connected to the main computer through a CVBS interface. Since the main board has video input, the picture from the camera is immediately available for processing. In addition to the onboard camera the robot can receive TV signal through microwave connection from an offboard camera located in the corner of the plaing filed (in a beacon).
Since we use industrial PC we are able to use the Linux operating system. Linux gives us us maximum freedom in customizing the kernel and drivers for maximum performance of the robot. The robot control software is partially implemented in kernel (realtime critical parts) and as userspace processes (CPU intensive computation — image processing, navigation, path planning).
Going with the description in a bottom up manner we start with the kernel drivers. The kernel drivers are composed of modules: tmm.o, mailbox.o, fifo.o, mice.o, ct69k.o, cvideo.o, serio.o, serport.o, elevator.o. Modules mailbox.o and fifo.o implement our own interprocess communication primitives. Both require special inode to be created and from the user perspective act like a file, meaning that it can be opened and closed, data can be read from it or written to it and process can wait for a data to arrive. What differentiates our IPC from the ones already provided by the kernel is the transparent support of one-to-many and many-to-many communications.
Mailbox is a storage for a single data structure. Typically one process writes the structure and other n can read it. Once a process reads the contents of a mailbox the mailbox appears to that process empty (ie. a process cannot read the same value more than once). If the mailbox is opened as blocking, next read will stop the reading process until new version of the data is available (ie. until the producing process writes into the mailbox again).
Fifo implements a queue-like access. When opened for reading it is initially empty. When opened for writing and no process has opened it for reading yet, it acts like a black hole throwing away all data written to it. When a process opens the fifo for reading it starts receiving all the data written to the fifo after the opening.
Other kernel modules are responsible for the various hardware parts of the robot. The userspace programms can communicate with the hardware through mailboxes, fifos and special devices created by these modules. The hardware includes the TMM 40e-3ph motor controllers (tmm.o), the IntelliMouse Explorer optical mice (mice.o), the video input (ct69k.o, cvideo.o) and the helix storage device (elevator.o) connected to parallel port.
Localization (position tracking)
A position information is calculated using the MCL (Monte Carlo Localization) algorithm. Relative movement information is obtained from the optical mice. Each pair of mice provides information about the change in position and orientation. We have 4 mice because the mice has proved to be very unreliable in our context. The mouse requires precise alignment with the surface and very precise height above the surface (this height is also quite small). Because the optical sensor uses magnifying lenses to detect even small irregularities on the underlaying surface it has short focus range. If the mouse is too high or too low it reports invalid data. Also the white lines present on the Eurobot 2002 playing field gave the mice real headache. Using 4 mice instead of the minimum 2 gave us the advantage of receiving 6 times (each pair) supposedly the same information allowing us to filter out some of the problems.
The information about the relative displacement and the change in orientation was used in the MCL algorithm. The MCL approach applies sample-based representation of the three-dimensional state space of the robot. When the robot moves or senses, sampling / importance re-sampling is applied to propagate the belief over time integrating relative displacement information with other sensors supplying information that can be compared with a known map thus reducing the otherwise unbound growth of the integration error.
Our path planning algorithm is an implementation of RRTs (Rapidly-Exploring Random Trees). A Rapidly-exploring Random Tree is a data structure and algorithm that is designed to efficiently search in nonconvex high-dimensional spaces. RRTs are constructed incrementally in a way that quickly reduces the expected distance of a randomly-chosen point to the tree. RRTs are particularly suited for path planning problems that involve obstacles and differential constraints (nonholonomic or kinodynamic). RRTs can be considered as a technique for generating open-loop trajectories for nonlinear systems with state constraints. An RRT can be intuitively considered as a Monte-Carlo version of a Voronoi diagram, and certain variations can also be considered as stochastic fractals.
The path planning takes commands from the strategy module and outputs a path. In addition it uses the information about the current location of the robot and the current locations of the balls. Output of the planning algorithm is a path specified as a sequence of waypoints. Each waypoint contains displacement relative to the previous waypoint in the sequence, the expected translational and rotational speed and the time when to reach it.
Input of the driving module (the so-called wheelman) is the path generated by the path planner. The wheelman's role is then reduced to a linear interpolation of the current translational and rotational speed from one waypoint to the other. The speeds are set by a feedback control loop based on the PID algorithm. The required feedback is supplied by filtering the data from the 6 pairs of mice.
The heart of our image processing module is YUV threshold based classification. The camera geometry is calibrated and the positions of the balls relative to the robot are calculated based on the fixed position of the camera with respect to the robot and the pixel coordinates of the respective color cluster.
The purpose of the object tracking module is to filter the output of the image processing module using some information about past positions of the reported balls. A Monte Carlo algorithm similar to the one used for the localization has been used for the object tracking. Since all the balls of the same color are from the point of image processing indistinguishable a single distribution is used for equally colored balls. However this algorithm was computationally expensive and it's performace was not as good as expected. The conducted experiments suggested that regular averaging over last couple of frames is less computationally expensive and more reliable. According to the results a decision has been made to abandon the Monte Carlo tracking in favor of the simple averaging.
The strategy planning module is based on the assumption that the robot is able to perform certain elementary actions. The task of this module is to generate a sequence of these actions in order to maximize the chance of winning a match in the Eurobot 2002 competition.
Our first approach was based on hand-coded winning strategy using a FSM (finite state machine). The approach had two primary flaws as we have discovered later on during the developement:
- the offline design of the optimal strategy (ie. before the match) appeared harder and harder during the developement to that extent that we all agreed that it is nearly impossible
- even if it was possible the robot would never have enough information (ie. complete) about the state of the match to act according to it
The second approach is based on discretization of the game state space. For each state there is an evaluation function which returns the worst possible score that could be reached. This score is based on the assumption that the robot can choose a “best” basket and decides to “defend” it — stand in front of it not allowing the oponent to take balls out of it and expecting that the oponent is as good as possible and has collected all the other balls.
The worst score is used to determine if the execution of a selected action can lead to a worse final score or if even after the execution of the action it is possible to maintain the current worst score. The safety of the execution is calculated for some constant time interval into the future.
The selection of the next action is based on two assumptions: the action is safe and the action is a part of a sequence that leads to an improvement in current worst score.
The elementary actions the robot was able to perform were to move to a point [x,y,α] or to command some of the actuators onboard. The elementary actions could be hand-combined to form compound actions. The planner then can build the plan using the compound actions. For example to take a ball out of the basket is a quite long sequence of movement and actuator commands that can be reused many times. By using the compound actions to build the plan the plan stays compact and relatively short (less branching). The fact that the compound actions can be decomposed to the elementary actions allows uniform determination of the level of the safety of the plan and also the executive responsible for executing them works with a well defined interface.
Conclusion and Further Work
A mobile robotic plaform Barbora was introduced. The platform is ready for deployment in our research facility.
Unfortunatelly the hardware proved to be more complicated than any of us expected, leaving virtually no time to debug and tune the software in time to compete at the Eurobot 2002 competition. However the competition has proved to be an exceptional motivation while building the robot. Further work aticipates polishing the software level and providing more experimental results about its performace.
|Vojtěch Pavlík||mechanics, electronics, navigation, Linux kernel|
|Kamil Řezáč||mechanics, electronics, motor control|
|Zbyněk Winkler||strategy, high-level control|
|Martin Mareš||image processing, strategy, Linux kernel|
|Petr Daněček||image processing, object recognition|
|Jan Kára||path planning, RRT|
|Jaroslav Sládek||object tracking, finances|
|Markéta Kyloušková||French interpreter, moral support, T-shirts|
This project could not be realized without the funding and support of following people and organizations:
- Smrcek, American University of Guttenberg
- Faculty of Mathematics and Physics, Charles University in Prague
- Physics on Stage
- MGM Compro
- and all the team members that filled whatever there was missing
If you have found this project interesting or you have some comments about it, please feel free to use our feedback form.