Jack, H., “Discrete Controls Systems in the Curriculum”, ASEE North Central Section Conference Proceedings, Lansing, April, 2000.
Logical Control Systems in the Curriculum
by Hugh Jack, Padnos School of Engineering, Grand Valley State University
Grand Valley State University has a clear mission to support the local community. For the School of Engineering this often means meeting the needs of local manufacturers. All of our undergraduates participate in a mandatory co-op program and over 90% of our graduates take jobs in local manufacturing companies. To ensure that we are satisfying our mission we frequently survey company needs (both formal and informal). Both companies and local professionals identify controls as a significant need. As would be expected, the curriculum had a traditional linear controls course in the early 1990s. This course was not satisfying our mission, so a new controls course was designed. An important component of this course is the use of PLCs (Programmable Logic Controllers) as the supporting technology. The result of adding this course has been i) increased interest in our graduates, ii) numerous donations from local companies, iii) higher levels of student satisfaction, and iv) very successful senior projects done for local industry.
This paper includes a detailed description of the course EGR 450: Manufacturing Control Systems. The course contains lecture, laboratory and project components. This course is required for junior manufacturing engineers, and is a popular elective for mechanical and electrical engineering students. The course covers i) basic programming principles, ii) sensors and interfacing, iii) sequential programming, iv) data functions, v) serial and network interfacing and vi) analog I/O and PID control. PLCs are the implementation platform throughout to support the pedagogy, and bring practical relevance to the theory and structured methods presented.
Web pages for the course, including on-line notes, are available at http://claymore.engineer.gvsu.edu.
What is the purpose of your program? Is it to produce high quality, effective graduates? Where do most of your graduates go? What do most of your graduates do? At Grand Valley State University almost all of our alumni work with local manufacturers. They work in areas such as manufacturing engineering, production and design engineering. We often discuss the program with employers and graduates. They were repeatedly commenting that ‘control systems are very important but are not in our program’. This was puzzling at first because there was a controls course (EGR 455) that covered traditional linear control theory. Further examination made it clear that logical control theory was needed, not linear control theory. Most companies were using PLCs (Programmable Logic Controllers) to implement logical control systems. A PLC oriented controls course was developed to address this need.
The importance of PLCs and logical controls becomes clear when considering the entire creation of a product. For example, consider a new automobile. The vehicle design may contain a dozen linear and non-linear control systems such as the cruise control, fuel injection system, automatic gain control in the radio, etc. The automobile design also contains numerous logical control systems such as the radio mode controls, the maintenance monitoring and warning system, automatic transmission controls, etc. The final car may require the design of a couple of dozen control systems for a vehicle that contains 20,000 parts. To manufacture each of these parts some sort of controlled system will be required. Even the most basic manufacturing station, such as a press, requires that some logical control be used for safety interlocks. Engineers employed in manufacturing engineering are much more likely to design logical controls for one of the thousands of manufacturing machines.
Linear controls courses became popular decades ago. They taught how to use common components in negative feedback loops to improve system performance. The mathematical rigor constrained the topics to engineering programs. By default, simple logical control implemented with relays was relegated to technology and trade programs. Since that time logical control systems have matured and now include digital systems such as PLCs. Table 1 compares the modern linear controls course with a logical control course. A linear controls course must appear later in the curriculum because of the mathematical prerequisites. This is often too late to be reviewed in other courses or used in a capstone project. Linear control courses also prepare students for graduate school research topics, although PLCs are beginning to make a showing in research publications4.
Table 1: A Comparison of Linear and Logical Controls Courses
Contemporary PLCs still support many of the simpler modes of logical control. But, they also include complex functions that require engineering design skills. Published texts support PLC use at the technology level and lack the rigor required for engineered designs. The course EGR450 uses additional notes and materials to provide the engineering topics required. The course was first offered in 1997, and it has been offered four times to date. The course materials and laboratories were reviewed during previous ABET visits without concern. Many of our graduates are working as controls engineers, and we have begun to gain a reputation for graduating high quality controls engineers.
2.0 Linear Controls
An example of a negative feedback linear controller is shown in Figure 1. The control loop includes a computer running Labview software that will position a DC motor. The computer contains a data acquisition card that allows analog outputs and inputs. The analog output goes to a high power op-amp that amplifies and inverts the voltage from a -5V to +5V range, to a +12V to -12V range. This circuit then drives a permanent magnet DC motor. The shaft of the motor turns a potentiometer shaft. The potentiometer provides a voltage to the computer that is sampled by the data acquisition card. The Labview program compares a desired and actual position for the potentiometer. The difference between these (the system error) is amplified by a gain Kg, and then used to set the output voltage that drives the motor.
Figure 1: A Motor Positioning System
A block diagram for this system is shown at the top of Figure 2. The contents of the blocks are then replaced with transfer functions, and then combined to derive a transfer function for the entire system.
Figure 2: A Block Diagram for a Linear Control System
Here the resulting equation is second order, so the system can be designed using the damping coefficient, or response time/natural frequency to select a value for ‘Kg’. A critical damping ratio is used so that the system will have the fastest response without overshoot. The other system coefficients are properties of the physical components. The transfer function for the system could then be analyzed to estimate responses to different disturbances, and estimate the effects of parameter variations, system stability, etc. The drawback of these basic techniques are that they ignore phenomenon such as friction in the motor, and saturation of the op-amp when the output voltage approaches the rail (power supply) voltages.
A typical list of topics found in a linear controls course is given below. As mentioned before these topics are mathematically intensive and require significant prerequisites. The courses often begin with a focus on analysis techniques and reach control system design at the end of the course. And, because control design is not discussed until the end of the course, the labs tend to focus on the modeling and analysis of control systems.
Laplace transforms and their use in problem solving
bode plots, initial/final value theorems
responses to step, ramp and other functions
standard controller forms (PID, lead-lag, etc.)
root locus plots to evaluate stability
advanced topics include Nichols charts, etc.
In some cases these courses will proceed on to more advanced courses in digital control systems, non-linear control systems or applications courses.
3.0 Logical Controls
The state diagram for a system to open and close a door is shown in figure 3. When button ‘C’ is pushed, the door will open, until the open sensor is tripped, it will then become idle again. The door closed button will have a similar effect. The first scan function ensures that the controller will start in an idle state. The state diagram is then converted to three Boolean equations, one for each state. These equations take care of turning states on/off.
Figure 3: A State Diagram for a Door Opener and Closer
The equations are then converted to the ladder logic shown in Figure 4. Each rung of the ladder logic corresponds to an equation. The ladder logic will then be entered into a PLC.
Figure 4: Ladder Logic for the State Diagram and Equations
Programmable Logic Controllers are commonly available from local and national suppliers. Costs for the equipment begin at about $200 for a very simple model that could execute the ladder logic in Figure 4. Most modern controllers have core sets of functions and capabilities. The size and cost of a PLC is determined by the interface options. Some basic input/output types are listed in general order of importance below.
basic AC/DC inputs and outputs at voltages from 10VDC up to 220VAC
connection to data devices (RS-232, RS-422, ethernet, etc.)
direct control of motors (servo motors, stepper motors)
direct reading of sensors (such as thermocouples)
direct human interfaces (HMIs, etc.)
The major benefit of a PLC is the ability to write programs. The following programming capabilities are supported by most PLCs.
Advanced programming functions
math (floating point and integers)
ability to work in a variety of number systems (decimal, binary, octal, hexadecimal, BCD)
FIFO/LIFO stacks, shift registers, sequencers
program control (subroutines, branching, looping)
•Advanced computational features
interrupt driven processes (a program only runs when an event occurs)
fault driven processes (eg. a program only run when there is a divide by 0)
processor status (such as overflow, last scan time, etc.)
There are six standard programming approaches available for PLCs. Of these ladder logic is the most common, but others are gaining popularity.
Instruction List (IL): looks like assembly language
Ladder Diagram (LD): looks like traditional electrical ladder diagrams
Structured Text (ST): looks like traditional programming languages (eg. BASIC)
Sequential Function Charts (SFC): a diagramming method that allows concurrent processes
Function Block (FB): this allows dataflow programming methods (similar to Labview)
Flowchart: a flowchart is used to determine operation sequences
With the right hardware and software more advanced topics can be supported with PLCs. This can make it possible to use the PLC for linear controls, non-linear controls, networking, etc. For example, a PID controller can be developed using an encoder input card on the PLC and an analog output to a motor amplifier. The PID calculation is built into most PLCs.
A reasonable controls course is able to cover all of the basics and half of the advanced topics listed.
4.0 A Logical Controls Course
At Grand Valley State University we designed a logical controls course entitled EGR450: Manufacturing Control Systems. This course was created to provide skills that would make the students knowledgeable and capable with industrial control systems. The desired outcome is that students become capable of conducting a good engineering design of a logical control system. At present many of our graduates are doing this work at local companies. EGR450 is required for students in Manufacturing Engineering (typically 5 to 10 students). It is an elective for mechanical and electrical students (about 45 total), and it is typically taken by 40 of the 45.
PLC based controls courses are offered in many community colleges. High quality texts are available to support these technology courses2,5,6,8,9. At universities, PLCs are sometimes taught as a topic within an automated/integrated manufacturing course. Textbooks that support this approach are also available1,3. None of the books support an engineering approach to PLC based control. To overcome these problems I have used a technology level PLC book5 with supplemental materials. The supplemental materials are available at http://claymore.engineer.gvsu.edu/eod/egr450.html. The topics for the 14 week course are listed below.
4 Combinatorial Logic with Boolean Algebra
6 Sequential Logic with State Diagrams and Petri Nets
The controls concepts in the course are reinforced with laboratory work. The first half of the laboratory sequence builds basic understanding and programming skills. The last half of the sequence introduces advanced topics and the control of complex systems.
1. Introduction to micro PLCs: tutorial
2. Basic ladder logic design and PLC interfacing: combinatorial stamping press control
3. Intermediate ladder logic design and PLC interfacing: encoder controlled motor
4. Advanced ladder logic design and PLC interfacing: sequential traffic lights
5. Introduction to PLC-5s: tutorial
6. Analog I/O and PID Control of a DC motor
7. PLC Networking and Serial Communications: with DH+ and RS-232C
8. Control of a multistation keytag maker
9. Control of a multistation keytag maker (cont’d)
10. Introduction to embedded controllers
11. Programming embedded controllers
The course concludes with a major project that includes some form of controller, most commonly a PLC. The student projects and the course notes can be viewed at http://claymore.engineer.gvsu.edu/eod/egr450.html
EGR 450 replaced a previous linear controls course, EGR 455: Automatic Controls, that was never highly successful. The success of EGR 450 was overwhelming, and it continues to be the most popular senior elective for all disciplines. EGR 450 also serves as an excellent prerequisite for EGR 474: Integrated Manufacturing System which discusses the design and implementation of fully integrated manufacturing facilities.
Indicators of the success of this course are clear. Of the 12 senior capstone projects conducted during the 1998-99 year, only three did not use a PLC for some form of control. Many prospective employers visit Grand Valley specifically to hire students with PLC design skills.
1. Bollinger, J.G., Duffie, N.A., “Computer Control of Machines and Processes”, Addison-Wesley, 1989.
2. Bryan, L.A., Bryan, E.A., Programmable Controllers, Industrial Text and Video Company, 1997.
3. Chang, T-C, Wysk, R.A., Wang, H-P, “Computer-Aided Manufacturing”, second edition, Prentice Hall, 1998.
4. Feldmann, K., Colombo, A.W., Schnur, C., Stokel, T., “Specifications, Design, and Implementation of Logic Controllers Based on Colored Petri Net Models and the Standard IEC 1131 Part I: Specifications and Design”, IEEE Transactions on Controls Systems Technology, November 1999.
5. Filer, R., Leinonen, G., “Programmable Controllers and Designing Sequential Logic“, Saunders College Publishing, 1992.
6. Petruzella, F., Programmable Logic Controllers, Second Edition, McGraw-Hill Publishing Co., 1998.
7. Society of Manufacturing Engineers and SME Education Foundation, “Manufacturing Education Plan: Phase I Report”, available at www.sme.org, 1997.
8. Stenerson, J., “Fundamentals of Programmable Logic Controllers, Sensors and Communications”, Prentice Hall, 1998.
9. Webb, J.W., Reis, R.A., “Programmable Logic Controllers, Principles and Applications”, Prentice Hall, 1995.
Hugh Jack is an Assistant Professor in the Padnos School of Engineering at Grand Valley State University. He has been teaching there since 1996 in the areas of manufacturing and controls. His research areas include, process planning, robotics and rapids prototyping. He previously taught at Ryerson Polytechnic University for 3 years. He holds a Bachelors in Electrical Engineering, and Masters and Doctorate in Mechanical Engineering from the University of Western Ontario.