# The Growing Importance of Control Theory for DIYers

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Control system theory is a branch of engineering that handles how to manipulate a dynamical system’s inputs to change the behavior or outcome of the system to something that is desired. The concept is simple enough to understand. In fact, humans do it regularly and intuitively when walking, driving, or playing video games—though many find it difficult to apply in practice when developing control systems for their projects. Often DIYers will purchase a controller then resort to the manufacturers’ recommended controller gains or they will tune the gains through a cumbersome trial-and-error process. In general, this method works fine for the patient engineer as long as the system is sufficiently simple. However, this method breaks down with so-called multiple-input multiple-output (MIMO) systems whose dynamics are coupled in such a way that renders this method impractical. But do DIYers need to worry about designing and building such complicated MIMO systems any time soon? Absolutely! And current trends suggest we’re already there.

Control systems have been around at least as far back as the float-regulated water clock developed by the Greeks in the third century BC; however, control theory as a branch of mathematics wasn’t explored until much later in the mid-19th century. Since that time, the theory has been used almost entirely by professional engineers and mathematicians rather than DIY builders, but that trend is beginning to change. I’m not saying professionals with accredited degrees aren’t still the majority of control theory users; however, with the rise of inexpensive open-source development platforms, free online software libraries, and the vast array of available sensors and actuators, control theory is becoming a major requirement for part-time DIYers, as well.

The simplest control systems are open loop. An open-loop control system is one in which the change in the input is not a function of the measured output. Open-loop controllers work best on systems that are predictable, repeatable, and robust to disturbances. A good example of an open-loop control system is a stepper motor where the user only needs to command a set number of steps and does not need to measure the motor’s final position to know with great confidence that it is where it is supposed to be.

On the other hand, closed-loop control systems vary the input based on the measured output. To accomplish position control with a brushless DC motor for example, the user would need to feed back the measured position and adjust the input voltage appropriately. This closed-loop system is more robust to changes in the environment, but consequently introduces a whole new set of problems including stability, overshoot, settling time, and other loop performance measures. If you build a system with multiple closed-loop paths that must work together, you can see how the complexity grows.

Kits for DIY projects (e.g., robotic sumo cars, maze-following robotic mice, six-axis stabilized quadcopters, and auto-piloted model aircraft) are now easily accessible to the individual. All of these projects rely heavily on control-system theory because they require multiple sensors and actuators working closed loop in conjunction with each other. These are MIMO closed-loop systems and they require more than clunky guess-and-try design methods.

So where will DIYers turn to gain the knowledge necessary to develop the controllers for these complex systems in the future? Luckily, along with the capability for individuals to build these projects comes the means with which to learn the skills. With the rise of YouTube and other information-sharing sites, people now have access to more educational content than ever before. In addition to open-source hardware and software, there is also this “open-source” library of free knowledge where a creator can learn and share just about anything.

The potential for video-based education is limitless, but it will probably won’t replace traditional education in classrooms any time soon, if ever. However, it is already proving an invaluable resource to many people who are looking to increase their knowledge base to tap into their full project-building potential.

Continuing advances in hardware and software mean home projects are going to become more capable. With this capability comes a necessary complexity in their control systems. This should be celebrated because these projects come with a certain amount of pride, a sense of accomplishment, and valuable knowledge gained. The knowledge is generated and shared within the community, feeding back to a new generation of DIYers who, in turn, share their new gained knowledge. It’s a positive feedback system that shows no sign of slowing down.

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##### Brian Douglas
control systems engineer | + posts

Brian Douglas is a control systems engineer based in Seattle, WA. He holds an MS in Aerospace and Mechanical Engineering (Dynamics and Controls) from the University of Southern California. Brian is the content creator of the Control System Lectures YouTube channel, which is dedicated to providing an intuitive and practical understanding of control system theory (www.youtube.com/user/ControlLectures). He has worked at The Boeing Company since 2003 developing satellite and aircraft guidance, navigation, and control systems. He is also married to a wonderful wife who supports him with all of his numerous hobbies.