Control Systems in FE Electrical Exam
Wasim Asghar 
September 18, 2023Control Systems in the FE Electrical exam is a very important exam topic per the NCEES® guidelines. Apart from exam preparation, these systems have a lot of significance in modern electrical and industrial systems.
This study guide will help you cover all the important topics included in Control Systems topics in the FE Handbook.
Fundamentals of Control Systems
Control systems are essential components in various engineering and industrial applications. They play a pivotal role in regulating and maintaining desired outcomes in complex electrical and industrial processes. Let’s begin with some basics.
Definition and Characteristics of Control Systems
A control system is a collection of interconnected components designed to manage or regulate the behavior of a dynamic system. It aims to maintain specific outputs or performance criteria within desired limits.
Characteristics of Control Systems
It is necessary to begin with the key characteristics of Control Systems in the FE Electrical Exam.
- Objective – Control systems are established to achieve specific goals or objectives by maintaining desired output values, whether temperature, speed, pressure, or any other parameter.
- Feedback – Most control systems utilize feedback mechanisms to monitor the system’s output and make necessary adjustments continuously.
- Dynamic Nature – Control systems deal with dynamic systems that change over time, requiring real-time adjustments to maintain stability.
- Error Correction – Control systems detect errors between the desired and actual system outputs and take corrective actions.
- Efficiency – They optimize the use of resources to ensure the system operates efficiently and economically.
- Stability – Control systems aim to stabilize the system, preventing resonance or extreme deviations from set points.
Read our detailed guide on resonance in the FE Electrical exam.
Types of Control Systems – Open-loop and Closed-loop
There are two types of Control Systems for the Electrical FE Exam preparation that you must know.
- Open Loop System
- Closed Loop System
1. Open-loop Control Systems
Open-loop or non-feedback control systems operate without considering the actual output’s impact on the control action. They apply a predetermined input to the system and assume it will produce the desired result. These systems lack the ability to correct errors automatically.
Example: A washing machine with a timer setting is an open-loop control system. You set the washing time, but it doesn’t adjust based on the cleanliness of the clothes.
2. Closed-loop Control Systems
Closed-loop control systems, also known as feedback control systems, continuously monitor the system’s output through sensors and compare it to the desired setpoint. If a difference (error) exists, the controller adjusts the input to correct it. This feedback loop enhances precision and accuracy.
Example: A thermostat in a heating system is a closed-loop control system. It measures the current temperature, compares it to the desired temperature, and adjusts the heater accordingly.
Components of Control Systems
A control system comprises several essential components that work in tandem to achieve the desired outcome. Let’s explore these components in sequence:
Sensors and Transducers in Control Systems
Sensors and transducers are critical components in control systems, responsible for gathering data about the system’s current state. They convert physical parameters like temperature, pressure, or flow rate into electrical signals.
These signals serve as inputs to the control system, allowing it to make informed decisions and maintain desired conditions.
Working Hierarchy
- Sensing – Sensors are the first line of interaction with the controlled process. They detect and measure the physical parameter of interest, generating an analog signal (e.g., voltage or current) corresponding to the parameter’s value.
- Transduction – Transducers take the analog signal from sensors and convert it into a form suitable for further processing. This conversion can involve amplification, digitization, or conditioning of the signal.
- Data Transmission – The transducer’s output is then transmitted to the controller for analysis and decision-making. In an open-loop system, this is where the data acquisition ends.
Functionality in Open-loop vs. Closed-loop System
Sensors and transducers provide initial input data in an open-loop system. However, since open-loop systems lack feedback, the control action is predetermined and doesn’t rely on real-time sensor data for adjustments.
In a closed-loop system, sensors and transducers continuously provide feedback on the system’s actual state. The controller uses this feedback to detect errors and make dynamic adjustments to maintain the desired setpoint, ensuring precise control.
Actuators and Control Valves in Control Systems
Actuators and control valves are responsible for executing the control commands generated by the controller. They translate electrical control signals into physical actions within the controlled process, allowing the control system to influence and modify the system’s behavior.
Working Hierarchy
- Control Signal Reception – Actuators and control valves receive control signals from the controller based on sensor data analysis.
- Physical Action – Actuators, such as electric motors, pneumatic cylinders, or hydraulic pumps, convert electrical signals into mechanical movement or energy transfer. Control valves regulate the flow of fluids or gases in a process, adjusting the system’s operation.
- System Response – The physical actions initiated by actuators and control valves directly impact the controlled process, bringing it closer to the desired state or setpoint.
Functionality in Open-loop vs. Closed-loop System
In open-loop systems, actuators and control valves execute predetermined actions without considering the system’s real-time state. Their actions are solely based on the control inputs received from the controller.
In closed-loop systems, actuators and control valves are crucial components that respond dynamically to the controller’s continuous adjustments. They play an integral role in achieving precise control by aligning the system’s actual state with the desired setpoint.
Controllers and Compensators in Control Systems
Controllers and compensators are the decision-making (If-Else) components of control systems. They receive sensor input data, compare it to the desired setpoint, and generate control signals that direct actuators and control valves to adjust the system.
Working Hierarchy
- Error Calculation – Controllers calculate the error, which is the difference between the desired setpoint and the actual system output, based on the sensor data.
- Control Algorithm – Controllers apply a control algorithm to the error signal. The algorithm determines the magnitude and direction of the control signal to minimize the error and maintain the desired conditions.
- Control Signal Generation – The controller generates control signals sent to actuators or control valves. These signals dictate the actions needed to return the system to the desired setpoint.
- Compensation (in some particular cases) – Compensators may be used in closed-loop systems to improve system performance, stability, and transient response. They modify the control algorithm to account for specific characteristics of the controlled process.
Functionality in Open-loop vs. Closed-loop System
Controllers execute control actions in open-loop systems solely based on the predetermined control algorithm. They don’t consider real-time feedback, which can lead to inaccuracies and deviations from the desired setpoint.
In closed-loop systems, controllers continuously receive feedback from sensors, allowing them to adjust the control signals dynamically. This ensures that the system maintains precise control and minimizes errors.
Feedback Systems for Error Detection in Control Systems
Feedback systems for error detection mechanisms are integral to closed-loop control systems. They provide the means to monitor the system’s actual state, detect errors (deviations from the setpoint), and initiate corrective actions.
Working Hierarchy
- Sensor Feedback – Sensors continuously monitor the system’s output and provide feedback to the controller.
- Error Calculation – The controller calculates the error by comparing the output (feedback) to the desired setpoint.
- Control Action – Based on the error signal, the controller generates control signals to actuators or control valves, which adjust the system to minimize the error.
- Continuous Monitoring – The feedback loop remains active, with sensors continuously providing updated data. The controller iteratively adjusts the control signals to maintain stability and accuracy.
Functionality in Open-loop vs. Closed-loop System
Feedback systems and error detection mechanisms are unique to closed-loop control systems. They are absent in open-loop systems, which do not utilize real-time feedback for error correction. This absence can lead to reduced accuracy and stability in open-loop systems.
Mathematical Modeling of Control Systems
Below are some crucial mathematical models and methodologies instrumental in studying and analyzing control systems. Don’t forget to read dedicated guides on each topic suggested at the end of each section for in-depth study.
1. Transfer Function Representation
What is a Transfer Function?
A transfer function is a mathematical representation of a dynamic system’s behavior in the frequency domain. It relates the input and output signals of a system using Laplace transforms.
Transfer functions are essential for analyzing and designing control systems because they simplify complex systems into manageable components.
Read our detailed guide on Transfer Functions on the FE Electrical exam.
Why Use Transfer Functions?
Transfer functions provide a concise and intuitive way to describe a system’s response to different inputs. They allow engineers to understand a system’s behavior at various frequencies and help design controllers that meet specific performance requirements.
How to Obtain a Transfer Function?
- Laplace Transform – Start with the system’s differential equations that describe its dynamics. Use the Laplace transform in the FE Electrical exam to convert these equations from the time domain to the Laplace domain.
- Algebraic Manipulation – Manipulate the transformed equations to isolate the output in terms of the Laplace-transformed input and system parameters.
- Transfer Function Form – The result is a transfer function in the form of the output Laplace transform divided by the input Laplace transform.
2. State-Space Representation
What is State-Space Representation?
State-space representation is an alternative mathematical model used to describe dynamic systems. It represents the system’s behavior using a set of first-order differential equations. This representation is beneficial for multivariable and nonlinear systems.
Read more about State-Space Analysis.
Why Use State-Space Representation?
State-space models comprehensively describe a system’s internal dynamics and relationships between inputs, outputs, and state variables. They are especially valuable for control system design and analysis, including state feedback and observer-based control.
How to Obtain a State-Space Model?
- Define State Variables – Identify the minimum number of state variables required to describe the system’s dynamics. These variables represent the system’s internal state.
- Write State Equations – Express the system’s dynamics as a set of first-order differential equations. These equations relate the state variables and their derivatives to inputs and outputs.
- Output Equation – Write an output equation that relates the system’s output(s) to the state variables and inputs.
- Matrix Form – Organize the state equations and output equation into matrix form. The resulting state-space representation consists of matrices A, B, C, and D.
3. Modeling Electrical Systems in Control Systems
Modeling Electrical Systems
Electrical systems are commonly encountered in control systems, especially in applications like motor control and power electronics. Modeling these systems accurately is crucial for control design and analysis.
Key Concepts
- Kirchhoff’s Laws – Use Kirchhoff’s voltage and current laws to derive equations that describe the relationships between voltages, currents, and components in an electrical circuit.
- Component Modeling – Accurately model electrical components (resistors, capacitors, inductors, etc.) using differential equations or transfer functions based on their electrical properties.
- Network Analysis – Apply network analysis techniques, such as nodal or mesh analysis, to solve complex electrical circuit equations.
- Laplace Transforms – Transform the differential equations into the Laplace domain to obtain transfer functions or state-space models suitable for control system analysis and design.
Why Model Electrical Systems?
Accurate models of electrical systems allow control engineers to design controllers that regulate voltage, current, or power, ensuring the safe and efficient operation of electrical devices. They are also crucial for simulating system behavior before implementing physical control systems.
4. Linearization Techniques
Linearization of Nonlinear Systems
Many real-world systems exhibit nonlinear behavior, making them challenging to analyze and control directly. Linearization is a technique used to approximate the behavior of nonlinear systems around an operating point, simplifying control system design.
Learn our detailed guide on Linear Systems in the FE Electrical exam.
Key Concepts
Operating Point Selection – Choose an operating point (equilibrium point) around which the system will be linearized. This point represents the desired or typical operating conditions.
Taylor Series Expansion – Apply the Taylor series expansion to the nonlinear equations that describe the system’s dynamics. The expansion provides a linear approximation of the system’s behavior near the operating point.
Linearized Model – After the expansion, you obtain linearized differential equations that describe the system’s behavior as linear. These equations can be represented as transfer functions or in state-space form.
Why Linearize Nonlinear Systems?
Linearization simplifies the analysis and control design for nonlinear systems. Linear control theory, such as the design of proportional-integral-derivative (PID) controllers, is well-established and widely used. Engineers can apply these familiar tools to control complex processes by linearizing a nonlinear system.
From Analysis to Design – Journey of Engineers with Control Systems
In control systems, engineers follow a systematic path to analyze and design systems effectively. Let’s embark on this journey step by step.
Control System Analysis Techniques
1. Time Response Analysis
Time response analysis is where our journey begins. Engineers use this technique to understand how systems react to changes over time. It’s divided into two key aspects:
- Transient Response is observing a system’s behavior during its initial response to a sudden change. Engineers focus on parameters like rise time, settling time, and peak overshoot to assess the system’s dynamic behavior.
- Steady-state Response – Engineers analyze the system’s long-term behavior after the initial response settles. They calculate the steady-state error to assess how accurately the system tracks its desired output.
Read our detailed study guide on Transient Response in the FE Electrical exam.
2. Frequency Response Analysis
Engineers delve into frequency response analysis once they grasp the system’s time behavior. This technique helps them understand how systems respond to different frequencies of input signals. It involves two critical tools:
- Bode Plots – Engineers create Bode plots to visualize a system’s magnitude and phase response across various frequencies. These plots are invaluable for assessing system stability, gain, and phase margin.
- Nyquist Plots – Nyquist plots are another tool for analyzing system stability, especially in feedback control systems. They display the system’s frequency response in a way that helps engineers evaluate stability based on the system’s open-loop transfer function.
3. Stability Analysis
Stability is a paramount concern in control systems. Engineers use mathematical techniques to ensure systems remain stable under various conditions:
- Routh-Hurwitz Criterion – This criterion examines the coefficients of a system’s characteristic equation to determine if all roots have negative real parts, ensuring stability.
- Root Locus Method – Engineers employ the root locus method to visualize how the closed-loop poles of a system change as a control parameter varies. It helps them understand how changes in controller gain affect system stability.
4. Controller Design
Armed with knowledge about system behavior and stability, engineers proceed to design controllers:
Proportional, Integral, and Derivative (PID) Controllers: PID controllers are a cornerstone of control system design. They combine three control actions – proportional, integral, and derivative – to regulate systems efficiently.
These controllers are mathematically represented by equations that adjust control inputs based on error, integral of error, and derivative of the error.
Read more about Control System Analysis in detail.
Control System Design and Tuning
With the understanding of analysis techniques and controller design, engineers move on to the next phase:
Specifications and Objectives in Control System Design
Before diving into controller tuning, engineers set clear objectives and specifications. These define the desired system performance, transient response, steady-state error tolerance, and bandwidth.
Controller Tuning Methods
With objectives in mind, engineers employ various tuning methods:
- Ziegler-Nichols Method – Involving initial settings for PID controllers followed by gradual adjustments based on the system’s response characteristics.
- Cohen-Coon Method – Cohen-Coon focuses on step changes in setpoints and calculates PID parameters from the system’s response.
- Internal Model Control (IMC) – IMC is a model-based approach where engineers create a process model and design controllers to match the model’s dynamics, ensuring desired performance while maintaining stability.
And there you have it—a journey through control system analysis and design, from understanding system behavior to crafting controllers that ensure stability, performance, and robustness in the face of complexity.
Conclusion
You now have a rich understanding of the critical concepts covering all the depths of control systems in the FE Electrical exam per NCEES ® exam guidelines.
To cover all the Control System CBT Exam Specifications in detail, check out further reading guidelines mentioned throughout this study guide.
For FE Electrical exam preparation, check out the resources and tailored courses offered by Study for FE.
Licensed Professional Engineer in Texas (PE), Florida (PE) and Ontario (P. Eng) with consulting experience in design, commissioning and plant engineering for clients in Energy, Mining and Infrastructure.