X-Git-Url: http://git.rohieb.name/bachelor-thesis/written-stuff.git/blobdiff_plain/df75c351fb7b1deeb49fa444b1bef90e13c76290..31c576ac200aa443984d141a393893acff4787e5:/Ausarbeitung/preliminaries.tex

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@@ -1,4 +1,5 @@
 \chapter{Preliminaries}
+This chapter describes the preliminary topics. \todo .
 
 \section{Dead reckoning}
 The process of \definition{dead reckoning} describes an inexpensive method for
@@ -17,25 +18,232 @@ these techniques are rather expensive to deploy, cannot (yet) be used in real
 time, or are even impreciser than relative approaches\cite{umbmark}, so dead
 reckoning can still be useful for the time being.
 
+In the following, the iRobot Roomba serves as an example of an autonomous,
+mobile agent, which can be used to implement dead reckoning for lack of either
+built-in absolute positioning and other relative approaches.
+
 \section{iRobot Roomba 500}
-Originally, the \definition{Roomba 500} is an autonomous vacuum cleaning robot,
-manufactured by the US-based company \definition{iRobot}. It has the size of a
-disc of about 34~cm in diameter and 9~cm in height; and normally follows its
-own, non-customizable logic to detect dirt and clean rooms. However, it is also
-easily controllable over a serial port, which provides a two-way
-communication at 5~V TTL levels over a Mini-DIN connector, with a speed of
-either 19,200 or 115,200 Baud. Over this serial port, the Roomba speaks a
-specified protocol, called the iRobot Roomba Open Interface \cite{irobot-oi},
+Originally, the \definition{Roomba} is an autonomous vacuum cleaning robot,
+manufactured by the US-based company \definition{iRobot}. The 500 series
+currently represents the third generation of iRobot's cleaning robots, and the
+first generation of robots controllable over an external interface.
+
+\subsection{Hardware design}
+\todo{diagram?}
+\paragraph{Wheels}
+The Roomba lives in a cylindrical case with diameter of about 34~cm and height
+of about 7~cm. It has two main wheels which are positioned slightly behind the
+centerline, so the Roomba leans forward due to gravity, and a small caster on
+the front to prevent it from sliding on the floor. The main wheels can be
+controlled over two independent motors, each one allowing to turn the connected
+wheel with a minimum of 10~mm/s and a maximum of 500~mm/s in each direction.
+One of the main wheel motors consumes about 300~mA in their slowest rotation
+speed, and about 1000~mA when driving with normal speed. Each wheel is also
+equipped with a drop sensor that tells if the respective wheel has dropped into
+a hole or similar, and does not reach the ground anymore. These sensors are
+realized with a spring pushing the wheel towards the ground, with the spring
+force adjusted to the Roomba's weight, and a micro switch which triggers if the
+wheel drops below a specified level. Furthermore, both wheels feature rotating,
+toothed discs, which in conjunction with an LED and a photo-electric resistor
+act as an optical interrupter. This system can be used to measure the wheel's
+current speed by counting the optical interruptions the wheel causes while
+moving.
+
+\paragraph{Brushes}
+In addition to the wheel motors, the Roomba has a motor which operates the
+vacuum brush, and a small motor on the front connected to a side brush, to allow
+cleaning of room corners.
+
+\paragraph{Bumper shield}
+Since the main movement direction in normal operation is forward, the front of
+the Roomba consists of a crecent-shaped bumper shield which contains several
+sensors. This bumper\index{Roomba!bumper} is spring-loaded and on the one
+hand absorbs shock to reduce damage, on the other hand, it allows the Roomba to
+detect obstacles in front of it, both via infrared sensors as well as by
+mechanical means. There are two sensors for mechanical bump detection, located
+30° to the left and to the right of the bumper's center, each implemented as
+photo-electrical interruptors. Additionally, six infrared sensors are unevenly
+distributed over the bumper, facing away from it in a star-like manner. Each one
+of them allows the Roomba to recognize objects in a maxmimum distance of 10~cm.
+Finally, the bumper shield contains four infrared sensors facing downwards,
+acting as cliff sensors \index{Roomba!cliff sensor} to recognize steps or
+similar chasms which could be dangerous for the Roomba to drive towards.
+
+The back part of the Roomba contains the main vacuum brush\index{Roomba!vacuum
+brush}, and the reservoir for holding dirt. Both of them can be removed, though
+the removal of the main brush reduces the Roomba's weight and slightly
+unbalances the Roomba so the springs used for the wheel drop sensors are not in
+balance anymore and push the Roomba upwards, so it tilts more to the front when
+accelerating forwards. There is also a sensor on the underside
+\index{Roomba!dirt sensor} for detecting particularly dirty regions of the
+floor, which is implemented as a capacitive touch sensor.
+
+\paragraph{Battery}
+The battery\index{Roomba!battery} is placed in the front part behind the bumper,
+it is a rechargeable NiMH battery and holds a capacity of 3300~mAh which lasts
+for about 90 to 120 minutes under normal operation. The Roomba can also find its
+home base and charge itself when it has finished cleaning or runs out of energy
+by using a special infrared sensor mounted on top of the Roomba. This sensor can
+see in all directions and is able to detect the home base by looking for a
+special infrared signal the home base\index{Roomba!home base} emits. The same
+principle is used for so-called "`virtual walls"'\index{virtual wall} which can
+be placed by the user in regions the Roomba should not move into.
+
+\subsection{Behaviour}
+\paragraph{Intended Behaviour}
+The Roomba normally follows its own, non-customizable algorithm to detect dirt
+and clean rooms. It is kind of a random walk\index{random walk}, controlled by
+the internal logic, which tries to keep the Roomba away from dangers like
+stairs and walls (by evaluating the cliff and bump sensors), and direct it to
+the more dusty regions of the room (by using the dirt sensor). The random walk
+concept allows a more or less complete coverage of the room, given the time for
+cleaning is large enough, while at the same only needing very little information
+about the environment. Of course, that concept is not very efficient when it
+comes to cleaning rooms, but cleaning time is not neccessarily the constraining
+factor, and the robot still saves the human some time.
+
+\paragraph{Roomba Open Interface}
+However, robots of the Roomba 500 series are also easily controllable over a
+serial port, which provides a two-way communication at 5~V TTL levels over a
+Mini-DIN connector, with a speed of either 19,200 or 115,200 Baud. Over this
+serial port, the Roomba speaks a specified protocol, called the
+\ignoreoutput{\ac{ROI}}\definition{\acl{ROI}} (\acs{ROI})~\cite{irobot-oi},
 which allows the user to interact with the robot's internal logic, reading its
 sensor values, and control its movements and cleaning behaviour.
 
-In our setup, the iRobot Roomba 500 is used as an instance of an autonomous,
+After starting the communication with the Roomba by sending the \cmd{Start}
+command, the robot is in a state called \definition{Passive mode}. In this mode,
+the user cannot control the robot by himself, but the internal logic defines
+icants behaviour. However, the user is able to read the internal sensors. The
+\ac{ROI} then allows the user to set the Roomba into two different modes:
+\begin{itemize}
+  \item In \definition{Safe mode}, the Roomba monitors the wheel drop, cliff
+    and internal charger sensors, and reverts into Passive mode if safety
+    conditions occur, so the Roomba is not harmed.
+  \item In \definition{Full mode}, the user has full control over the Roomba,
+    and has to take care not to harm the Roomba by evaluating the wheel drop,
+    cliff and internal charger sensors by himself.
+\end{itemize}
+
+In particular, every command is assigned an \ac{opcode} of one byte length,
+followed by a fixed amount of bytes as parameters which depend on the opcode.
+For example, to start the communication with the Roomba, the \cmd{Start}
+command has to be sent, which has the \opcode{0x80} and takes no parameters. The
+\cmd{Safe} command to put the Roomba into safe mode has \opcode{0x83}, and like
+the \cmd{Full} command with \opcode{0x84}, it takes no parameters.
+
+For example, to start the communication with the Roomba and set it into Safe
+mode, one would send the following bytes over the serial interface:
+\begin{verbatim}
+0x80,             // Start command
+0x83              // Safe command
+\end{verbatim}
+The, additional commands can be sent over the \ac{ROI}, like actuator commands
+for controlling the Roomba's driving behaviour.
+
+\paragraph{Actuator commands}
+The \ac{ROI} specifies various actuator commands to control the Roomba's wheels,
+brushes and \ac{LED} displays, and let the Roomba play tunes. However, the
+central command needed for the experiments in thie thesis is the \cmd{Drive}
+command, \opcode{0x89}, which takes 4 additional bytes as parameter: the first
+two bytes specify the velocity that the Roomba's centerpoint should travel with
+while driving, and the third and fourth bytes specify the radius of the arc the
+Roomba's centerpoint should describe. The Roomba then calculates the required
+right and left wheel velocities internally without further interference of the
+user.
+
+The velocity is interpreted in mm/s, the value can range from -500~mm/s to
+500~mm/s, with negative values implying backwards movement. The radius is
+interpreted in mm, ranging from -2000~mm to 2000~mm. Negative values make the
+Roomba turn toward the right, whereas positive values make it turn toward the
+left. There are also four special values for the radius: \magicnumber{1} makes
+the Roomba turn on the spot in counter-clockwise direction, \magicnumber{-1}
+makes the Roomba turn on the spot in clockwise direction, and
+\magicnumber{0x7fff} and \magicnumber{0x8000} make him drive straight.
+
+For example, to drive straight with a velocity of 1000~mm, one would send the
+following bytes over the serial interface:
+\begin{verbatim}
+0x89,             // Drive command
+0x03, 0xe8        // parameter velocity: 0x03e8 == 1000
+0x80, 0x00        // parameter radius: special value "straight"
+\end{verbatim}
+
+A little disadvantage of the \ac{ROI} \cmd{Drive} command is that the robot is
+modeled as a state machine. In the previous example, the Roomba would keep on
+driving until it runs out of energy, or a safety condition occurs which causes
+the Roomba to revert into Passive mode, or a new \cmd{Drive} command with the
+velocity parameter set to zero is sent. Thus, if the user wants to drive a
+specific distance, he has to calculate the time the robot needs to travel that
+distance, measure the time, and stop the robot after that time interval has
+passed. When using incorrect clocks, or when using inaccurate timers, this can
+lead to errors in movement. Because of that, it is appropriate to monitor the
+Roomba's movement, for example with its internal sensors.
+
+\paragraph{Input commands}
+The Roomba~500 series features a total of 49 different sensor values. Among the
+sensors mentioned above, there are also some internal values concerning battery
+charge, capacity, and temperature, motor currents, and even some more (or less)
+useful variables like the characters read from the infrared remote control, the
+current \ac{ROI} mode or the currently playing song. Nevertheless, there is
+also the possibility to query the travelled distance, the turned angle and the
+internal encoder counts ("`ticks"') for the left and right wheel. Each sensor
+value is 1 or 2 bytes long and is assigned a specific \definition{packet ID}.
+Some packet IDs also describe groups of multiple sensor values sent together.
+
+Sensor values can be retrieved either by explicit polling or by enabling a
+stream of values that is sent every 15~ms. Explicit polling works through the
+\cmd{Sensors} command (\opcode{0x8e}), which takes the packet ID of a single
+sensor as parameter, or through the \cmd{Query List} (\opcode{0x95}) command,
+which takes multiple packet IDs headed by the total number of requested packets
+as parameter. Both of these functions send back the requested values directly.
+
+By using the \cmd{Stream} command (\opcode{0x94}), it is possible to receive
+the requested sensor values every 15~ms. This is very convenient for real-time
+behaviour, when the sensor values have to be evaluated very often. As the
+\cmd{Query List} command, the \cmd{Stream} command takes the total number of
+packet IDs followed by the requested packet IDs as parameter. It sends back the
+sensor values in packets using the following format:\\
+\verb|0x13|, $n, p_1, v(p_1), p_2, v(p_2), \ldots, p_n, v(p_n), c$\\
+where:
+\begin{description}
+  \item[$n$] is the number of bytes sent back, excluding $n$ and $c$,
+  \item[$p_i$] is a requested packet ID, $i = 1, \ldots, n$
+  \item[$v(p_i)$] is the value of the packet with the packet ID $p_i$
+  \item[$c$] is a checksum, with
+    $\sum_{i=1}^n\left(p_1 + v(p_1)\right) + c + n \equiv 0 \mod 256$
+\end{description}
+
+Example: The following byte sequence requests data from the left cliff
+signal (packet~ID \magicnumber{0x1d}) and virtual wall sensor (packet~ID
+\magicnumber{0x0d}):
+\begin{verbatim}
+0x94,         // Stream command
+0x02,         // parameter: 2 packets following
+0x1d, 0x0d    // parameter: request packets 0x1d and 0x0d
+\end{verbatim}
+
+The Roomba then returns the following bytes every 15~ms:
+\begin{verbatim}
+0x13,         // Header byte
+0x05,         // 5 bytes following, except checksum
+0x1d,         // Packet ID 0x1d following
+0x02, 0x19,   // Data for Packet ID 0x1d (2 byte)
+0x0d,         // Packet ID 0x1d following
+0x00,         // Data for Packet ID 0x0d (1 byte)
+0xb6          // checksum: 0x5 + 0x1d + 0x2 + 0x19 + 0xd + 0x0 + 0xb6 = 256
+\end{verbatim}
+
+In our setup, an iRobot Roomba~530 is used as an instance of an autonomous,
 mobile robot to conduct the experiments described afterwards. For that, the
-Open Interface is used to control the Roomba's movements from a netbook which is
-running Wiselib code.
+Roomba's movements are controlled over a netbook mounted on top of the Roomba
+(cf.~Figure~\ref{fig:roombasetup}), which is running Wiselib code. The Wiselib
+code in turn uses the \ac{ROI} and especially the \cmd{Stream} and
+\cmd{Drive} command to control the Roomba.
 
 \section{Wiselib}
-The \definition{Wiselib}\cite{wiselib} is a C++ algorithm library for sensor
+The \definition{Wiselib}\cite{wiselib} is a C++\index{C++} algorithm library for
+sensor
 networks, containing for example algorithms for routing, localization and time
 synchronization, and is strongly focused on portability and cross-platform
 development. In particular, it allows the user to develop applications that run
@@ -45,8 +253,42 @@ platforms are diverse sensor node platforms, like iSense, Contiki and TinyOS,
 but there are as well implementations for the diverse x86-compatible Personal
 Computer platforms, and the Shawn sensor network simulator.
 
-Moreover, the Wiselib includes code to control the iRobot Roomba over a
+\subsection{Architecture}
+\paragraph{Concepts and Models}
+Wiselib makes strong uses of \definition{concepts} and \definition{models} as
+central design objects. Concepts serve as an informal description of interfaces,
+only existent in documentation, defining expected parameters and types. Models
+however implement these interfaces in C++ code while fulfilling their
+specification. The Wiselib algorithms can in turn rely on the concepts as a
+generic specification, and take models as template parameters to use their
+functionality, so a function call will be immediately resolved to a specific
+model at compile time without the need for an additional function call as it is
+the case with virtual inheritance.
+
+This makes cross-platform development easily possible. For example, to implement
+a routing algorithm, one can rely on the concept of a Radio to send and receive
+data packets, without needing to implement code specific to the used radio
+hardware. The users of that routing algorithm can now choose which radio model
+they want to use, according to their needs and the underlying hardware, provided
+that their radio model also implements the same Radio concept that the routing
+algorithm uses.
+
+\begin{figure}
+  \centering
+  \includegraphics[width=.8\textwidth]{images/Wiselib-Arch.pdf}
+  \caption{Wiselib architecture\label{fig:wiselib-arch}}
+\end{figure}
+Besides algorithms, the Wiselib also consists of two other main parts: the
+internal interface and the external interface (see Figure
+\ref{fig:wiselib-arch}).
+
+\paragraph{}
+
+\subsection{Roomba}
+Moreover, the Wiselib includes code to control the iRobot
+Roomba\index{Roomba} over a
 serial interface, and getting access to its internal sensor data, using the
-iRobot Roomba Open Interface mentioned earlier. \todo{cite Wisebed book
-chapter on Roomba code}
+iRobot Roomba Open Interface mentioned earlier.
 
+\todo{cite Wisebed book chapter on Roomba code}
+\todo{which roomba sensors were used?}
\ No newline at end of file