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This overview provides an introduction to control systems, focusing on closed loop control and feedback mechanisms. Topics include control symbols, block diagrams, transfer functions, and the advantages of closed loop control. Examples of control systems in different domains are discussed, and the use of feedback to solve problems in open loop systems is explored. The concept of transfer functions and control diagram elements are also covered.
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EET273 Electronic Control Systems Week 1
Overview • Syllabus • Homework/Quizzes/Labs • Class website • Lab Overview • Lab syllabus • Lab 0 overview • Intro to Control • Control symbols • Block diagrams • Transfer functions
Lab Overview • Building control circuits using CLICK PLC, and other control elements • Ok if you’ve never used a PLC, we’re just going to run basic programs • Lab 0 will include creating and running a basic PLC program and getting familiar with the Control System Trainers • Need to make your own wires! Purchase a box of spade terminals • Details are in Lab Syllabus
Intro to Control • This class: • Closed loop controls • Advantages of closed loop control • How to design/tune a controller in a (mostly) non-mathematical way • Sensors, actuators, relays, etc. • Other control classes: • Very mathematical • Few real world examples • Little discussion of how control systems are actually implemented
Intro to Control • Control is all about feedback • A system: takes an input, produces an output • A control system: takes an input, produces an output, but feeds back the output to influence the system behavior in a positive way. • Closed loop systems almost always have better performance than open loop systems • Control theory applies to wide range of different types of systems: • Electrical • Mechanical • Software algorithms • Financial models • Economics/psychology/sociology, etc. • Anything that can be considered a “system” with an input, output, and feedback mechanism
Intro to Control • Examples of control systems: • Cruise controller in your car • Sprinkler system w/moisture sensor • Various Manufacturing processes • Water buffer tank, sensor to keep the level consistent • Robotics/Self-driving Cars/Obstacle Avoidance robots, etc. • Stock market prediction algorithm – very complex system! • Presidential Election: • 19 republicans / 16 democrats • Each election cycle can be considered “feedback” to the electoral process, which parties adjust to, in order to stay relevant, and have a chance at winning the next election.
Intro to Control • Using feedback/control can solve many problems of an open-loop control system: • Performance: • Improve system performance by making system adjustments under varying load conditions • Stability: • Make a less stable process more stable (water buffer tank), or stabilize a process that is otherwise unstable to the point that it is useless (inverted pendulum) • Automatization: • Automatize a process that would otherwise require user monitoring/intervention
Control Terminology • Process Variable (PV) – output variable to be controlled • Setpoint(SP) – input to the system, desired output • Controller – module that processes output and feeds back to input • Final Control Element (FCE) – actuator that is acting on the process • Manipulated Variable (MV) or Output – Controller output variable • Open Loop – no feedback from output to input • Closed Loop – with feedback from output to input
Transfer Functions • Transfer functions are a mathematical representation of a linear system • TF = Output / Input of the system include units! • For an amplifier/op-amp: TF = Vo/Vi the gain of the op-amp • Transfer functions can be used to represent any linear system • In electronics: resistors, capacitors, inductors, linear gain elements • Some TF’s are frequency dependent, we won’t get into frequency dependent TF’s too much in the class, but know they exist! Ex: temp sensor 1mV / 5°C 1V/1mV 20mA / 1V Temp Sensor Amplifier Heater Controller
Control Diagram Elements • TF Block • Blocks can be linked in series and multiplied together • Adder (summer) • Used to sum input and feedback signals • Basic feedback system • G / (1 + GH)
Control diagram • e = r – yH • y = eG • y = (r – yH)G substitute Eq. 1 into Eq. 2 to eliminate e • y = rG – yGH • y + yGH = rG • y(1 + GH) = rG • y/r = G / (1 + GH) The transfer function of a closed loop system
Control diagram • In an op-amp, G is very large • If G = 100,000, and H = 1/10 G / (1 + GH) = 100k / (1 + 10k) ~ 9.9990 • If G = 1,000,000, and H = 1/10 G / (1 + GH) = 1M / (1+ 100k) ~ 9.99990 • So long as G is very large, gain is dominated by the value of H • Good implementation of feedback can make the properties of the system much less of a factor • In the case of an opamp, the accuracy of the gain is much more dependent on the value of the resistors, not on the OL gain of the op-amp itself. • Since resistors are easier (and cheaper) to control the value of, this makes for a much better amplifier design
Summary • Closed-loop control can improve the performance of a system by making the output more closely match the desired output (setpoint). • Closed-loop control can make a system that is otherwise unstable and thus unusable perform in a way that is useable. • Links: • Inverted Pendulum: https://www.youtube.com/watch?v=a4c7AwHFkT8 • Ball and plate: https://www.youtube.com/watch?v=j4OmVLc_oDw • PID Control: https://www.youtube.com/watch?v=fusr9eTceEo • TED Talk: https://www.youtube.com/watch?v=w2itwFJCgFQ • Balancing Cube: https://www.youtube.com/watch?v=n_6p-1J551Y
