The Robotics & control field focuses on Electromechanical control systems and their application to industry, military, aerospace, and research. Robotics & control brings many engineering disciplines together requiring knowledge pertaining to electronics, mechanics, and software.

Section Overview

This lab modifies the DC motor control circuit built in the previous lab to create a servo motor. The Mbed is implemented in a simple circuit that uses a feedback potentiometer and a position potentiometer to control the position of the motor.


  • Modify a DC motor into a servo motor.
  • Learn about feedback and control systems.
  • Learn control systems terminology.


  • KL-46z.
  • USB to Mini USB Cable.
  • ECE111 Kit
    • Small white potentiometer.
    • PVC tubing.
    • Small Square Protoboard.


Task 1: Understanding What's a Control System

Control Systems

A control system is a device, or set of devices, that manages, commands, directs or regulates the behavior of other devices or systems. Industrial control systems are used in industrial production for controlling equipment or machines. There are two common classes of control systems, open loop control systems and closed loop control systems. In open loop control systems output is generated based on inputs. In closed loop control systems current output is taken into consideration and corrections are made based on feedback. A closed loop system is also called a feedback control system.

Block Diagram of a Typical Open And Closed Loop Systems.


There are a wide variety of control systems. Since many systems require continuous feedback, closed loop control systems are very popular. It is this type of system being implemented on the servo circuit. A closed loop system uses the concept of feedback to control the output of a system. Feedback is when an output signal loops back and affects the input to a system. 

Task 2: Understanding The Design Process


The control system design process begins by defining the output performance requirements. This performance is often measured by determining a set point and then comparing it to the output. The output waveform is measured using specific characteristics. These terms are defined below.

  • The desired output set via user input (in the form of a potentiometer in this case).
  • Rising Time - The time the system takes to go from 10-90\% of the steady-state, or final value.}
  • Overshoot - The amount that the actual output overshoots the final value (the point we want to reach).
  • Settling Time - The time required for the output to settle within a certain percentage of the final value.
  • Steady-State Error - The final difference between the actual output and the setpoint.

Using the terms defined above, label the graph in Figure above. The graph shows the response of a typical PID closed loop system.

Watch This Video.

Watch This Video



Graph of Closed Loop System Response


Task 3: Making a Control System.

Making the Servo

Servos are made up of an electric motor mechanically linked to a potentiometer for feedback. In order to control them, specific code is needed. This code is provided for you on the ECE 111 page on the Mbed website. In the circuit being built in this section, a position potentiometer enables the user to define what position they want the motor to move to. The takes the user input, and powers the motor until it reaches the defined position.


Schematic of a Motor as a Servo Circuit


For the KL-46z to know when the motor has reached the correct position, a feedback potentiometer is used. It is attached to the shaft of the motor, so that the potentiometer turns with the motor. While the KL-46z is powering the motor, it is also checking the feedback value. When the feedback matches the user input, the KL-46z knows to stop the motor, because the defined position has been reached.

Using the motor control circuit built in the previous lab, follow the schematic in Figure above to make the necessary additions to create a servo.

A smaller white potentiometer needs to be added to the hub of the motor, as shown in the next Figure. Following are the instructions for attaching a feedback potentiometer to the motor.


A motor with the attached feedback potentiometer.


  • Push the piece of plastic tubing around the inner shaft of the motor The green shaft next to the motor body.
  • Push the potentiometer into the the open end of the plastic tubing.
  • Mount the potentiometer on the protoboard so the drilled holes line up with the mounting holes on the motor and the potentiometer is in line with the motor shaft.
  • Place the bolts through the protoboard.
  • Add four spacers to each bolt.
  • Tighten the bolts into the motor, but make sure they are not too tight otherwise the potentiometer might not be able to move.
  • The plastic tubing may need to be cut for it to fit without putting too much pressure between the motor and the board with the feedback potentiometer. Shorten the tubing if necessary.
  • Solder the potentiometer onto the protoboard.
  • Get Lab5_basic from Mbed ECE111 website. Program the Mbed.
  • Assemble the circuit following the schematic. The connection between the 4.7kOhm and the potentiometer forms a voltage divider.
  • All three legs of the the potentiometer will be used. If on MacOS, be sure to use the terminal to program the Mbed.
    • As the Mbed is searching in one direction for a position, swapping the connections on the motor will allow the Mbed to default to the opposite direction.

Study Questions

Type answers to the study questions below. Please keep answers clear and concise. Turn in the questions at the beginning of lab next week. You will be required to type all study questions for the future labs as well.

  1. What does the ``P" in PID control stand for? How does it work?
  2. What does the ``I" in PID control stand for? How does it work?
  3. What does the ``D" in PID control stand for? How does it work?
  4. List 3 examples of specific systems in which PID control is implemented.
  5. Fill in the Graph of Closed Loop System Response at the beginning, using the correct terminology(Rising Time, Settling Time,Steady-State Error)


Lab5_basic will set the servo to different positions, change it so it will be controlled using the potentiometer similar to how Lab4 works.