ROBOTICS TXT 4.0 Base Set
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Computers have become an integral part of our modern world. They surround us not only in the form of PCs, laptops or tablets: we carry them in our pockets as smartphones, they are in our cars, in dishwashers and washing machines, in heating controls, in e-bikes and in coffee machines. 

Year after year, they are becoming more powerful and at the same time smaller and more compact. As a result, their fields of application are also increasing, as they can now be tailored to the respective application and integrated into it as "embedded systems". They use sensors to detect and "measure" their environment and control motors, LEDs or other actuators. Using modern wireless communication technologies such as WLAN or Bluetooth, they can be networked with each other and exchange data with each other or with a control center. 
But how do such embedded systems and their respective "heart", the microprocessor, work? What is a microcontroller and how is it used to record and evaluate sensor data? How do you control mobile systems, e.g. robots or vehicles? How can the systems communicate with each other and exchange data? 

This learning unit focuses on understanding microcontrollers and their use. The central basics are explained and their practical implementation is deepened in the exercises (tasks). Various important areas of application for microcontrollers are presented: Measurement, communication, coding and encryption, control and regulation, robotics and autonomous driving.

 

Definition 

A microcontroller is a control system consisting of a microprocessor and a number of inputs and outputs. Sensors or memories are connected to the inputs, while actuators such as motors, LEDs or displays are connected to the outputs. Microcontrollers have a volatile working memory (RAM), which is deleted when switched off, and usually also have an EEPROM in which programs and data can be permanently stored. In contrast to laptops and PCs, there is neither a keyboard as an input unit nor a screen as an output unit; these must be controlled separately.  
Well-known and widespread representatives of such microcontrollers are, for example, the Arduino, the BBC micro:bit, the Calliope Mini and the TXT controller from fischertechnik.

The heart of the microcontroller is a microprocessor. In itself, a microprocessor is an integrated circuit (IC) that loads instructions and data from a (temporary) memory - called random access memory (RAM) - into the IC's registers, processes them in a computing unit and stores the result back in RAM. A microprocessor essentially consists of transistors - electronic "switches" that know exactly two states: "on" and "off". These transistors can be used to construct logical circuits such as "and", "or" and "not" links. The results are stored in a "flip-flop", a stable toggle switch (for more details, see the "Electronics" education box).  
A quartz crystal sets the clock for the microprocessor, i.e. the speed at which the microprocessor processes the instructions and internal calculation steps. The instructions are specified in a machine code (which varies depending on the type of microprocessor) and consist of a command (e.g. "add two values") and, depending on the instruction, several parameters (e.g. the two values to be added or the registers in which the values are stored).  

The performance of the microprocessor is determined by the clock speed, the data width in bits (which indicates how large the addresses and values may be that the microprocessor instructions can process in one step) and the machine code itself: The more complex the instructions that the IC can execute, the faster, as a rule, the data processing. To speed up access to the RAM, many microprocessors today contain their own cache memory in which they temporarily store instructions and data during processing.  
The microprocessors used today in laptops and PCs can process 64-bit long data (values, RAM addresses), contain several billion transistors and are clocked at several GHz (billions of oscillations per second). They can share the work ("multi-core") and have many megabytes of cache memory. In microcontrollers today, microprocessors with a word width of 32 bits and a clock frequency of less than one GHz are usually used - this is by far sufficient for the tasks of a microcontroller.

History 

Since ancient times, people have been experimenting with automata - constructions which, driven by air or water power, perform work independently, execute predefined motion sequences on command or even produce music. In the 15th century AD, Leonardo da Vinci (1452-1519) constructed numerous machines that impressed the people and nobility in theaters and royal courts. Wolfgang von Kempelen (1734-1804) built a functional speech synthesizer in 1791. The punch card control system for mechanical looms developed by Joseph-Marie Jacquard (1752-1834) in 1805 is considered the first "programmable automaton". 

However, the breakthrough for the development of flexible control systems only came with the development of the computer. The invention of the integrated circuit (IC) in 1958/1959 by Jack Kilby (1923-2005) and Robert Noyce (1927-1990) and the development of the microprocessor in 1971, invented and patented by engineers at Texas Instruments, made very flexible, lightweight, fast and wear-free, but above all programmable controls possible. The first microprocessor, the TMS1000, contained 8,000 transistors. Since then, the number of transistors in a microprocessor has doubled approximately every 18 months. This development is also known as "Moore's Law" after Gordon Moore (*1929), who derived this correlation in 1965 from the further development of ICs in the preceding years. Today's microprocessors consist of several billion transistors - and are barely larger than the TMS1000.

In the following, some basic connections and backgrounds are explained, which are required in the corresponding education kits and are deepened in the corresponding tasks.

 

How do you determine the temperature with a thermistor? 

A thermistor (also known as a thermistor or NTC resistor) is a resistor whose resistance value changes depending on the temperature. Thermistors have a negative temperature coefficient (NTC), i.e. the resistance value decreases as the temperature rises. The following diagram shows the resistance characteristic of the fischertechnik thermistor.

The fischertechnik thermistor has a resistance value of 1.5 kOhm at 25°C. As can be seen from the graph, the characteristic curve of the thermistor is not linear. However, you can approximate the temperature value reasonably accurately using a linear curve. To do this, you must determine a second measured value and calculate the equation of the linear function from it. 

The fischertechnik thermistor has a resistance value of 1.5 kOhm at 25°C. As can be seen from the graph, the characteristic curve of the thermistor is not linear. However, you can approximate the temperature value reasonably accurately using a linear curve. To do this, you must determine a second measured value and calculate the equation of the linear function from it. 

For a more precise calculation of the temperature value T (in Kelvin) from the resistance value R of an NTC resistor, the following Steinhart-Hart equation is usually used:

1/T=a+b(lnR)+c(lnR)³

The temperature in °C is obtained from T by subtracting 273.15.

With two additional measured values (in addition to the resistance value of 1.5 kOhm at 25°C), we can set up a system of equations with the three unknowns a, b and c. We do not have to solve this system of equations ourselves. We do not have to solve this system of equations ourselves: On the Stanford Research Systems website there is a "Thermistor Calculator" that calculates the parameters of the Steinhart-Hart equation we are looking for from the three measured values. 

What is redundancy? 

In information theory, redundancy refers to data that can be omitted without loss of information. A code is redundant if it requires more bits than the minimum required to represent the various characters. If all characters of the code are the same length, the number of additional bits of the code that are not actually required for the representation of information indicates its redundancy. For example, if we want to encode a simple 1/0 signal, we only need one bit. However, if we encode the signal with the digits "1" and "0" (as ASCII characters), our code requires eight bits. The coding therefore has a redundancy of seven (bits):

Redundancy = log₂ (number of possible code words/number of characters to be encoded)

In our example, the redundancy is therefore log₂ (2⁸/2) = log₂ (2⁷) = 7. 

 

How does encryption work?

Data encryption (also known as ciphering) is used to keep the content of a message secret between the sender and recipient and to effectively prevent unauthorized persons from "reading" it. To this end, encryption methods are used that "change" the message in a way agreed between the sender and recipient. As far as possible, it is not the encryption method itself that is kept secret, but only a key known only to the sender and recipient ("Kerkhoff principle"). In most cases, the characters of the message are replaced (substituted) or swapped (permuted).  

In the past, simple substitution ciphers were mainly used. One of the best-known methods is the so-called "Caesar cipher". This involves "shifting" the letters of the message to be transmitted in the alphabet by a fixed number of digits (the key). For example, with the key "3", an "A" becomes a "D", a "B" becomes an "E" and so on, until a "Z" finally becomes a "C". Decryption at the recipient's end is exactly the opposite. 

Caesar encryption is easy to crack because the frequency distribution of the letters in a natural language is not changed by the encryption: In a longer text, the most frequent letter by far is "E". If you know the most frequent letter of the encrypted text, you know how many places you have to "shift" the alphabet in order to decrypt the overheard text.  

In addition, there are only 25 different ways to shift the letters in the alphabet: That is a very small number of possible keys. You could therefore try out different shifts until a meaningful text emerges during decryption. 

A cipher published by Blaise de Vigenère (1523-1596) in 1586, in which the characters of the message are not all shifted by the same number of places in the alphabet, but according to a predetermined scheme, is much more difficult to crack. For example, the key "1 5 12 4 8 7 11" means that the first character of the message is shifted by one place, the second by five, the third by 12 and so on, and finally the seventh by 11 places in the alphabet. The eighth character is shifted again by one, the ninth by five and so on. The Vigenère encryption method was considered unbreakable for almost three hundred years.

The key is often not represented by a sequence of numbers, but by the corresponding letters of the alphabet - in the example above by "AELDHGK". This also makes it much easier to program the replacement of the characters (by "adding" the positions in the alphabet).