\chapter {Interfacing a Servomotor} \thispagestyle{empty} \label{sec:servo} \newcommand{\LocSERfig}{\Origin/user-code/servo/figures} \newcommand{\LocSERscicode}{\Origin/user-code/servo/scilab} \newcommand{\LocSERscibrief}[1]{{\tt \seqsplit{% Origin/user-code/servo/scilab/#1}}, see \fnrefp{fn:file-loc}} \newcommand{\LocSERardcode}{\Origin/user-code/servo/arduino} \newcommand{\LocSERardbrief}[1]{{\tt \seqsplit{% Origin/user-code/servo/arduino/#1}}, see \fnrefp{fn:file-loc}} %%%%%%%%%%%%%%%python starts \newcommand{\LocSERpycode}{\Origin/user-code/servo/python} \newcommand{\LocSERpybrief}[1]{{\tt \seqsplit{% Origin/user-code/servo/python/#1}}, see \fnrefp{fn:file-loc}} %%%%%%%%%%%%%%%python ends %%%%%%%%%%%%%%%julia starts \newcommand{\LocSERjuliacode}{\Origin/user-code/servo/julia} \newcommand{\LocSERjuliabrief}[1]{{\tt \seqsplit{% Origin/user-code/servo/julia/#1}}, see \fnrefp{fn:file-loc}} %%%%%%%%%%%%%%%julia ends %%%%%%%%%%%%%%% OpenModelica starts \newcommand{\LocSEROpenModelicacode}{\Origin/user-code/servo/OpenModelica} \newcommand{\LocSEROpenModelicabrief}[1]{{\tt \seqsplit{% Origin/user-code/servo/OpenModelica/#1}}, see \fnrefp{fn:file-loc}} %%%%%%%%%%%%%%% OpenModelica ends A servomotor is a very useful industrial control mechanism. Learning to control it will be extremely useful for practitioners. In this chapter, we will explain how to control a servomotor using the \arduino\ board. We will begin with preliminaries of servomotors and explain how to connect a typical servomotor to the \arduino\ board and Shield. We will then explain how to control it through the Arduino IDE and other open source software tools. % Scilab scripts, Scilab Xcos, Python, Julia, and OpenModelica. We will provide code for all the experiments. \section{Preliminaries} \label{sec:servo-pril} A servomotor is a rotary control mechanism. It can be commanded to rotate to a specified angle. It can rotate in positive or negative direction. Using servomotors, one can control angular position, velocity and acceleration. Servomotors are useful in many applications. Some examples are robotics, industrial motors and printers. Typical servomotors have a maximum range of $180^\circ$, although some have different ranges\footnote{All the angles in a servomotor are absolute angles, with respect to a fixed reference point, which can be taken as $0^\circ$.}. Servomotors typically have a position sensor, using which, rotate to the commanded angle. The minimum angle to which a servomotor can be rotated is its least count, which varies from one model to another. Low cost servomotors have a large least count, say, of the order of $10^\circ$. A servomotor typically comes with three terminals for the following three signals: position signal, Vcc and ground. Position signal means that this terminal should be connected to one of the PWM (Pulse Width Modulation) pins \cite{arduino-pwm} on \arduino. This book uses PWM pin 5 for this purpose. Rest two terminals (Vcc and ground) need to be connected to 5V and GND on \arduino. \tabref{tab:servo-connect} summarizes these connections. We now explain how to connect a typical servomotor to the Shield attached on the \arduino\ board. On the Shield, there is a three-pin header at one of the ends. The pins of this header have been marked as $1$, $2$, and $3$. These pins: $1$, $2$, and $3$ are internally connected to 5V, PWM pin 5, and GND on \arduino\, respectively. As discussed before, a typical servomotor has three terminals. Thus, the readers need to connect these three terminals with the three-pin header, as shown in \figref{fig:servo-shield} before running the experiments given in this chapter. \begin{figure} \centering \includegraphics[width=\lgfig]{\LocSERfig/servo-uno-shield.jpg} \caption{Connecting servomotor to the Shield attached on \arduino} \label{fig:servo-shield} \end{figure} \begin{table} \centering \caption{Connecting a typical servomotor to \arduino\ board} \label{tab:servo-connect} \begin{tabular}{lc}\hline Servomotor terminal & Arduino board \\ \hline Position signal (orange or yellow) & 5 \\ Vcc (red or orange) & 5V \\ Ground (black or brown) & GND \\ \hline \end{tabular} \end{table} \section{Connecting a servomotor using breadboard} This section is useful for those who either don't have a Shield or don't want to use the Shield for performing the experiments given in this chapter. A breadboard is a device for holding the components of a circuit and connecting them together. We can build an electronic circuit on a breadboard without doing any soldering. To know more about the breadboard and other electronic components, one should watch the Spoken Tutorials on Arduino as published on {\tt https://spoken-tutorial.org/}. Ideally, one should go through all the tutorials labeled as Basic. However, we strongly recommend the readers should watch the fifth and sixth tutorials, i.e., {\tt First Arduino Program} and {\tt Arduino with Tricolor LED and Push button}. In case you have a servomotor and want to connect it with \arduino\ on a breadboard, please refer to \figref{fig:servo-bread}. \begin{figure} \centering \includegraphics[width=\hgfig]{\LocSERfig/servo-bb-dark-color-wires.jpg} \caption{A servomotor with \arduino\ using a breadboard} \label{fig:servo-bread} \end{figure} As shown in \figref{fig:servo-bread}, there is a servomotor with three terminals. These terminals are used for the same three signals, as that explained in \secref{sec:servo-pril}. The connections shown in \figref{fig:servo-pot-bread} can be used to control the position of the servomotor, depending on the values coming from a potentiometer. As shown in \figref{fig:servo-pot-bread}, analog pin 2 on \arduino\ is connected to the middle leg of the potentiometer. Rest of the connections are same as that in \figref{fig:servo-bread}. \begin{figure} \centering \includegraphics[width=\hgfig]{\LocSERfig/servo-pot-bb-dark-color-wires.jpg} \caption{A servomotor and a potentiometer with \arduino\ using a breadboard} \label{fig:servo-pot-bread} \end{figure} \section{Controlling servomotor through Arduino IDE} \subsection{Controlling the servomotor} \label{sec:servo-ard} In this section, we will describe some experiments that will help rotate the servomotor based on the command given from Arduino IDE. We will also give the necessary code. We will present four experiments in this section. The Shield has to be attached to the \arduino\ board before doing these experiments and the \arduino\ needs to be connected to the computer with a USB cable, as shown in \figref{arduino}. The reader should go through the instructions given in \secref{sec:ard-start} before getting started. \begin{enumerate} %\setcounter{enumi}{-1} \item In the first experiment, we will move the servomotor by $30^\circ$. \ardref{ard:servo-init} has the required code for this. This code makes use of a library named {\tt Servo} \cite{servo-lib}. Thus, we include its header file at the top of \ardref{ard:servo-init}: \lstinputlisting[firstline=1,lastline=1] {\LocSERardcode/servo-init/servo-init.ino} Next, we create a {\tt Servo} object and call it {\tt myservo}, as shown below: \lstinputlisting[firstline=2,lastline=2] {\LocSERardcode/servo-init/servo-init.ino} Most Arduino boards allow the creation of 12 servo objects. Next, we initialize the port for serial communication at data rate of 115200 bits per second. Following to this, we mention the pin to which the servo is attached, as shown below: \lstinputlisting[firstline=5,lastline=5] {\LocSERardcode/servo-init/servo-init.ino} With this, we issue the command to rotate the servomotor by $30^\circ$ followed by a delay of 1000 milliseconds: \lstinputlisting[firstline=6,lastline=7] {\LocSERardcode/servo-init/servo-init.ino} At last, we detach the servomotor. Once this code is executed, the servomotor would move by $30^\circ$, as commanded. What happens if this code is executed once again? The motor will not move at all. What is the reason? Recall that what we assign to the motor are absolute positions, with respect to a fixed origin. As a result, there will be no change at all. \item In the second experiment, we move the motor by $90^\circ$ in the forward direction and $45^\circ$ in the reverse direction. This code is given in \ardref{ard:servo-reverse}. In this code, we have added a delay of 1000 milliseconds between the two instances of rotating the servomotor: \lstinputlisting[firstline=6,lastline=8] {\LocSERardcode/servo-reverse/servo-reverse.ino} What is the reason behind this delay? If the delay were not there, the motor will move only by the net angle of $90-45 = 45$ degrees. The reader should verify this by commenting on the delay command. \item In the third experiment, we move the motor in increments of $20^\circ$. This is achieved by the {\tt for} loop, as in \ardref{ard:servo-loop}. Both {\tt i}, the loop variable and {\tt angle}, the variable to store angle, are declared as {\tt int} in this code. The code helps the motor move in steps of $20^\circ$ all the way to $180^\circ$. \item Finally, in the last experiment, we read the potentiometer value from the Shield and use it to drive the servomotor, see \ardref{ard:servo-pot}. The resistance of the potentiometer is represented in 10 bits. As a result, the resistance value could be any one of 1024 values, from 0 to 1023. This entire range is mapped to $180^\circ$, as shown below: \lstinputlisting[firstline=10,lastline=12] {\LocSERardcode/servo-pot/servo-pot.ino} By rotating the potentiometer, one can make the motor move by different amounts. As mentioned in \chapref{potmeter}, the potentiometer on the Shield is connected to analog pin 2 on \arduino. Through this pin, the resistance of the potentiometer, in the range of 0 to 1023, depending on its position, is read. Thus, by rotating the potentiometer, we make different values appear on pin 2. This value is used to move the servo. For example, if the resistance is half of the total, the servomotor will go to $90^\circ$ and so on. The servomotor stops for 500 milliseconds after every move. The loop is executed for 50 iterations. During this period, the servomotor keeps moving as dictated by the resistance of the potentiometer. While running this experiment, the readers must rotate the knob of the potentiometer and observe the change in the position (or angle) of the servomotor. \end{enumerate} \begin{exercise} Let us carry out this exercise: \begin{enumerate} \item In \ardref{ard:servo-loop}, the loop parameter {\tt i} starts from 1. From what angle will the motor start? If one wants the motor to start from $0^\circ$, what should one do? \item How does one find the least count of the servomotor? If the variable {\tt angle} is chosen to be less than this least count in \ardref{ard:servo-loop}, what happens? \item What happens if 180 in Line 10 of \ardref{ard:servo-pot} is changed to 90? What does the change 180 to 90 mean? \end{enumerate} \end{exercise} \subsection{Arduino Code} \lstset{style=mystyle} \label{sec:servo-arduino-code} \addtocontents{ard}{\protect\addvspace{\codclr}} \begin{ardcode} \acaption{Rotating the servomotor to a specified degree} {Rotating the servomotor to a specified degree. Available at \LocSERardbrief{servo-init/servo-init.ino}.} \label{ard:servo-init} \lstinputlisting{\LocSERardcode/servo-init/servo-init.ino} \end{ardcode} \begin{ardcode} \acaption{Rotating the servomotor to a specified degree and reversing} {Rotating the servomotor to a specified degree and reversing. Available at \LocSERardbrief{servo-reverse/servo-reverse.ino}.} \label{ard:servo-reverse} \lstinputlisting{\LocSERardcode/servo-reverse/servo-reverse.ino} \end{ardcode} \begin{ardcode} \acaption{Rotating the servomotor in increments} {Rotating the servomotor in increments. Available at \LocSERardbrief{servo-loop/servo-loop.ino}.} \label{ard:servo-loop} \lstinputlisting{\LocSERardcode/servo-loop/servo-loop.ino} \end{ardcode} \begin{ardcode} \acaption{Rotating the servomotor through the potentiometer} {Rotating the servomotor through the potentiometer. Available at \LocSERardbrief{servo-pot/servo-pot.ino}.} \label{ard:servo-pot} \lstinputlisting{\LocSERardcode/servo-pot/servo-pot.ino} \end{ardcode}