Control Systems: The Hidden Science
A couple of questions before we start.
1- Have you ever heard phrases like “automated temperature and humidity control” in some air-conditioner advertisements, or “cruise control in modern vehicles” in some car review?
2- Did the words “automated” or “control” mean anything other than just a common English word?
If your answers are Yes and No respectively, these articles will help you understand these common day-to-day terms with some engineering insight.
I too had those same answers for many years. It was during my formative years when I took courses on control systems, did I realize the amount of research and engineering that goes into designing an automated control system.
But, what is Control System?
A control system consists of subsystems and processes (or plants) assembled to obtain a desired output with desired performance, given a specified input. Too formal? Basically, a control system helps you to make your system perform in a desired way. Without it, your system will not follow your command.
Take for example the automated ventilator block diagram with the lung model used extensively to design cheap ventilators in these days of crisis¹ is shown below. Don’t fret, this is fairly advanced model. We will come up with simpler models later. You can see how multiple subsystems like Check Valve Subsystems, Control Signal Subsystem, Humidifier are interconnected to obtain a desired output.
Why is it used?
We build control systems for four primary reasons:
- Power amplification: To get higher output power given a low input power.
- Remote control: Almost all of Robotics.
- Convenience of input form: In fan regulator, input is position, output is fan speed.
- Compensation for disturbances: You drive on a bad road but still you feels bumps less, thereby balancing ride comfort and safety.
But more importantly, it is successful. Take for example, the air travel. The safest mode to travel fast. Whenever we get on to an airplane, billion components have to function in a complex coordinate way to keep the aircraft airborne and maintain its trajectory.
Where is it used?
It finds important applications everywhere. Regulating the blood sugar level in the diabetic model in biological sciences² (in our body, the amount of sugar should remain within a certain boundary. Pancreas helps us in that. In diabetes it does not work properly. Hence, we develop a model to mimic it.) to advanced assisted driving systems like anti-lock braking system [nerd ref. 1] in a newer vehicle. Control engineering plays an essential role in a wide range of control systems, from simple household washing machines to high-performance F-35 [nerd ref. 2] fighter aircraft.
Since when is it being used?
This study goes back almost two thousand years ago, the first feedback control device on record is thought to be the ancient Ktesibios’s water clock in Alexandria, Egypt around the third-century B.C.E.
Many centuries later James Clerk Maxwell wrote his seminal paper in 1868 “On Governors” where he demonstrated the importance and usefulness of mathematical models and methods in understanding complex phenomena signalling a new beginning in the design of control systems.
In 1903, the first motored flight by Wright brothers took place. They were the first to realize the use of “control surface” [nerd ref. 3] and that an uncontrolled flight is not feasible. Within 4 decades, we were flying aircraft in world wars, a couple of decades later reaching the moon and developing highly sophisticated aircraft like SR-71 Blackbird [nerd ref. 4].
Being present everywhere, since ages, and having a successful track record, I believe are the markers for a famous field of study. But, sadly it is not. It lacks a distinct identity. People don’t think it in the same way as they do about Computer science or Biotechnology. It is a hidden technology. It is part of everything that we get involved within terms of technology in our life. But it is seldom talked about.
Modern Control
Modern-day control engineering is a relatively new field of study that gained significant attention during the 20th century with the advancements of technology. It can be broadly defined as the practical application of control theory.
Early developments
The first wave in the control theory
At the beginning of the 20th century, most of the important work in control theory was done in the field of telecommunication. Many seminal papers came out of Bell Labs. Nyquist (1932) and Bode (1940) were the stars of what we now call the classical control techniques. Both world wars were fought with these technologies in hand and led to the development of iconic war machines like the Japanese zero aeroplane, the German V-2 rockets [nerd ref. 5], and the B-17 and B-29 bombers of USA.
The second wave
The second wave happened in the 1960s. This also, as many other engineering marvels of the 20th century, had a connection to the Bell Labs. With the development of transistors by Shockley in 1947 and later the MOSFET by Atalla and Kahng in 1959 at Bell Labs, led to the development of smaller, cheaper and powerful computers. This lead to the newer approach to the control theory which included optimization and recursive approach. These help us choose the best among the many available options based upon a set of constraints. For example, you have to buy a car. You compare different options based on constraints like time, money, space, comfort, safety etc. Faster computers helped us decide on these choices and improve technologies like flight trajectories of rockets and aircraft.
Current state, the third wave
We currently are in the third wave. Control has found new ways to apply abstraction to technological problems. We can focus on specific parts of the system, then model and test it separately thereby reducing complexity. This independence was not present earlier. With the increase in the use of sensors and actuators in fields like robotics, automation, self-driving cars many new fields of control theory have come up. For example, currently, a strong focus is on the research of unmanned fighter jets. But you cannot remove the human part altogether (when to pull the trigger…. or not. — James Bond). For this co-operative and decentralized control is used. Here, co-operative control is used to analyze the control of multiple cooperative fighter jets. To include the human in the loop, decentralized control is used. These studies combined with a dynamic model of the physical system plays a vital role in the development of current technologies.
The image below concisely covers some common control strategies.
As you can see above, this is a vast field. It is many a times difficult to go through different types of control strategies. There are many resources present but they all are distributed and their problem statements are different.
Introducing a problem:
In the next series of articles, I will try to explain intuitively, many of these strategies through the same system/problem statement. Unlike the complicated ventilator design shown in the beginning, I will discuss one of the most classical and well-researched topic of “control of an inverted pendulum”. The design and modelling of this system is simple to understand and is the benchmark problem in many engineering projects.
Outline of the next articles
- Development and analysis of the inverted pendulum’s mathematical model
- Discussing controllability and observability
- Designing controller based on various techniques
- Designing observers based on various techniques
- Introducing Principle of Duality and Separation Principle
- Development of non-linear controllers
It is said that every time you put up an equation, you lose half of your readers. Although the articles contain equations, I will explain all the topics with intuitive examples as well.
References:
[1]: https://in.mathworks.com/help/physmod/simscape/examples/medical-ventilator-with-lung-model.html
[2]: https://link.springer.com/article/10.1007/s00422-018-00791-5
Nerd section
[1]: Anti-lock braking system (ABS)
Have you ever seen a vehicle losing control and skidding erratically especially on a slippery surface like an icy road? Well, ABS helps you prevent that. This is present in almost all modern land vehicles and aeroplanes.
Ideally, the speed of the vehicle should match that with the wheels. But if the wheel is rotating significantly slower than the speed of the vehicle, it means that the vehicle is skidding. This can be prevented by reducing the force on the brakes in a specific controlled manner. Even experienced drivers find it difficult to do that. ABS actuates the valves to reduce hydraulic pressure to the brake at the affected wheel, thus reducing the braking force on that wheel; the wheel then turns faster. Wheels can also turn faster than the speed of the car like a car stuck in a puddle. ABS helps in increasing the brake pressure and in turn, reduces the wheel speed.
For more information, check out this video below.
Understanding Anti-lock Braking System (ABS): https://www.youtube.com/watch?v=98DXe3uKwfc
[2]: F-35 fighter jet
The Lockheed Martin F-35 Lightning II is an American family of single-seat, single-engine, all-weather stealth multi-role combat aircraft. This is an all-in-one aircraft. It is intended to perform both air superiority and strike missions while also providing electronic warfare and intelligence, surveillance, and reconnaissance capabilities. Regarded as the best predator in the sky, it comes in 3 variants. The costliest on them, the F-35B costs a whopping $115 million. It is capable of vertical landing and vertical take-off.
For more information, check out this video below.
First F-35B Vertical Take-off Test: https://www.youtube.com/watch?v=zW28Mb1YvwY
Is The F-35 Worth $115 Million?: https://www.youtube.com/watch?v=zDujFhvgUzI
[3]: Control Surfaces
Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft’s flight attitude. The Wright brothers are credited with developing the first practical control surfaces. It is the main part of their patent on flying. Ailerons, elevators, and rudders help in controlling the three rotational axes: roll, pitch and yaw respectively. These are some of the common control surfaces. Without these, an aircraft is similar to the toy planes we make with the paper. And trust me, no one would like to fly in that.
For more information, check out this video below.
Control surfaces: https://www.youtube.com/watch?v=ovU3ziWkZWU
[4]: The SR-71 Blackbird
The Lockheed SR-71 “Blackbird” is a long-range, high-altitude, Mach 3+ (north of 3700kmph) strategic reconnaissance aircraft. There is so much to talk about this magnificent aircraft. Most of the credit goes to the legendary American aerospace engineer Clarence “Kelly” Johnson who was responsible for many of the aircraft’s innovative concepts. Equipped with the first turbo-ram jet engine, the Pratt & Whitney J58, it could cruise at Mach 3.3. During aerial reconnaissance missions, the SR-71 operated at high speeds and altitudes (ceiling altitude of 85,000) feet to allow it to out-race threats. Developed to collect important information about enemy territory during cold wars, it was not a fighter aircraft. At that altitude, almost all of the Surface-to-air missiles (SAMs) lost their fuel to catch it. None of the fighter jets could reach this altitude. Even when the missiles fired from the fighter jet, the Blackbird just had to speed up to Mach 3+ to outrun them. During its service life, no SR-71 was ever shot down.
For more information, check out this video below.
The Insane Engineering of the SR-71 Blackbird: https://www.youtube.com/watch?v=3hYSnyVLmGE
Kelly Johnson talks about his greatest creation the SR-71, Uncut interview: https://www.youtube.com/watch?v=n8kBiy6RkOs
[5]: German V-2 rocket
It was the world’s first long-range guided ballistic missile. The missile was developed during the Second World War in Germany as a “vengeance weapon”, assigned to attack Allied cities as retaliation for the Allied bombings against German cities. The V-2 rocket also became the first artificial object to travel into space by crossing the Kármán line (the boundary between Earth’s atmosphere and outer space) with the vertical launch on 20 June 1944, thereby starting the space race. Research into military use of long-range rockets began when the studies of graduate student Wernher von Braun attracted the attention of the German Army. Does the name Wernher von Braun click anything? Well, he later became an American aerospace engineer through the Operation Paperclip. He was recruited along with 1600 other Nazi engineers and technicians to help the US during the cold wars. He was the chief architect of the Apollo Saturn V rocket. The rocket which took first men to the moon.
For more information, check out this video below.
Wernher von Braun talks about V-2 rocket: “ Hitler’s Secret Weapon “, (1977): https://www.youtube.com/watch?v=gSlGxlAusSE
Original Footage of German V-2 Rocket Development Tests [HD]: https://www.youtube.com/watch?v=7YFU4KaJSSc
