A Low-Cost Control System Experiment for Engineering Technology Students

A low-cost control system experiment is presented. The plant for the control system is a modified Radio Control (RC) servo. The servo is modified by removing the controller, then bringing the two motor leads, and the three potentiometer (pot) leads outside the servo case. The motor and pot are connected to an Arduino Uno R3. Inputs for auto manual selection, manual inputs, and additional pots are connected for reference and gain adjustments. This experiment is part of an electronics course in Engineering Technology and Management (ETM). The electronics course uses the Arduino Uno R3 to teach electronics, real time programing, and with this experiment, PID control. There is no time allotted in this course for traditional frequency based or time based control system design, but this simple set up allows the student to experience many of the situations that arise in industrial control systems. The student learns about input, output, feedback, and control. The first step for the student is to move the servo by hand and observe the feedback from the pot. The student then runs the motor and observes the direction. The student can then determine the sign of the feedback, and implement manual control. The motor is connected to two Arduino outputs. This creates an H-bridge. Dead-band control is implemented by setting the outputs high and low for one direction, low and high for the reverse, and low and low for stop. The student sets the dead-bands to allow the servo output to follow a reference pot. At this point the student has performed his/her first control system startup and likely experienced many of the challenges of commissioning a control system. The Arduino outputs connected to the motor are capable of Pulse Width Modulation (PWM). Proportional output is achieved by setting one output pin low, while the other is controlled using PWM. This allows proportional control of the motor in either direction. The student can now implement proportional only control. The student can turn the pot by hand and feel the control system response. This gives the student a direct kinesthetic learning experience. Advanced learners can now add integral and derivative control. The addition of integral control forces the student to understand initialization and integral windup. The student writes the Proportional Integral Derivative (PID) control algorithm such that auto/manual transitions and gain changes are bumpless. The addition of the control system experiment cost less than $5.00 per student, but provides the student with a realistic control system problem complete with all of the vagaries of an in-plant control system commissioning. Introduction Engineering Technology and Management (ETM) students fill many roles in the work force. ETM students are generalist and most will become involved with control systems at some time during their career. The control system experiment described here is designed to give the students a working understanding of a control system so they are prepared for positions in industry. Typical roles for ETM graduates include operations, maintenance, sales, supervision, and quality. Each of these roles may include some involvement with control systems. Students employed as operators will use equipment with feedback control mechanisms. Maintenance employees will be required to diagnose and repair control equipment. Supervisors will have operational, or maintenance duties over controls systems. Sales personnel will represent automated machinery, controls components, or control equipment. Quality engineers will use manual control systems where quality data is used to modify the process in some way to maintain quality. Not every student will be a controls engineer but all are likely to benefit from the experience. ETM students at our university are required to take an electronics course. The course is taught using the Arduino [1]. The focus of the course is on electronics and real time programming. The course teaches students electronics, schematics, circuit analysis, programing, and how to use a breadboard. This lab experiment leverages the existing course materials and implements a Proportional Integral Derivative (PID) controller using software in the Arduino. Existing methods of teaching control systems particularly in electrical engineering involve too much math for our ETM students. The students take physics, chemistry, and a first course in calculus, but have no application specific knowledge of integration, differentiation, phasors, or frequency domain techniques. Thus, we must provide the students with application specific training. The lab experiment described here focuses on working knowledge such as vocabulary, block diagrams, startup, and tuning. The lab experiment seeks to teach the students what a control system is, how it works, and how to use it. The students write code in the Arduino to implement a PID controller. The focus is on heuristics rather than theory. This lab was inspired by Ray’s paper [2] “An Inexpensive Control System Experiment: Modeling, Simulation, and Laboratory Implementation of a PID Controller-Based System”. This one to two week lab uses an electronic model of a mass spring damper system as the plant, and a PID controller to teach controls. Both the plant and the PID are constructed on a bread board with operational amplifiers (op amps) and discrete components. Matlab© SIMULINK is also used to simulate the control system. An oscilloscope is used to show the system response. Our students have no familiarity with op amps or Matlab© SIMULINK. The focus of our course is real time software so we would like to use the software to implement the PID. Again because the students have no familiarity with op amps we would like to have a different plant. In this case a modified RC servo as will be described. Kiefer [3] describes a similar challenge with the mathematics of controls systems and teaches many of the items needed for Ray’s approach. He describes a six part lab for mechanical engineering students in which he covers electrical components, op amps bread boards, and circuits. Our students cover all of this material with the exception of op amps. The course by Kiefer is self-paced and there is no mention of how long it takes a student to complete the course. Content and Format The content and format of this paper may be unfamiliar because a great deal of the technical detail is contained in the lab document. Rather than repeat or rewrite the lab content it is included in its entirety Appendix 1. The Arduino start code for the course is included in Appendix 2. Readers interested in the technical details are encouraged to read the lab in addition to the paper. Those wishing to duplicate the lab can start with the code in Appendix 2. The goals, or student outcomes are presented first. This is followed by as description of the lab including a few technical details that are explained in the lectures but not in the body of the lab. This is followed by a description of a trial run through the lab with a group of three students. The results of the trial are assessed based on an assignment given after the first hour of the lab, and a survey given at the end of the class. This is followed by a summary and conclusions. Student Outcomes The desired student outcomes are as follows: 1. Explain what a control system is and give examples 2. Analyze a control system and identify Plant, Input, Output, Feedback, Setpoint, Controller 3. Design and implement controls including Manual, Dead-band, Proportional, Proportional / integral and PID 4. Startup and trouble shoot a control system 5. Recognize States and know how to initialize them First we would like the students to know what a control system is and give some examples. As instructors we may take for granted what a control system is but for a student with no prior experience of what a control system is there will be questions. Once the student can identify a control system, the questions are then, what are the parts, how do they interact? Figure 1 shows a student response to the question: Give an example of a control system and label the following: plant, controller, feedback, setpoint, error. This question was given as an exercise after the first lecture. The single loop PID is normally what engineers think of when you say the word “controller” but there are many other types of controllers. The PID is just a good solution to a large number of problems. Other types include manual control, Dead-band control, and Proportional control. The students will be starting at the beginning so simpler controls are an ideal step before trying to program the PID. Having worked on many control systems in industry the first step of any controls system is startup, checkout. Once the system is operational then maintenance and troubleshooting are required. The author’s recommendation for startup is included in the lab. Nothing is ever perfect so the student will invariably run into trouble and have to troubleshoot the system. Finally, there is the concept of a state. An integral is a state. The derivative requires a state. The meaning of integral and derivative may be unclear to the student at this point but they can appreciate that these values that are passed from one iteration of the control to the next. It is important for students as future control engineers to recognize a state and insure it is properly initialized. Figure 1 A student response to the question: Give an example of a control system and name the parts. A list of the parts underlined by the student is given. Lab Technical Description An annotated photograph of the experimental setup is give in Figure 2. The schematic is given in the lab document Appendix 1. The plant is an RC servo, the controller is the Arduino. Potentiometers (pots) are used for the reference, the gains, and the feedback from the servo. There is an auto/manual switch and manual increase and decrease pushbuttons (PBs). There