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+--- Thread: Some History of Loop Control, and What it Teaches (/showthread.php?tid=29)
Some History of Loop Control, and What it Teaches - Tom - 06-24-2021
A ship's wheel was traditionally linked to the ship's rudder by cables. The helmsman needed strength as well as skill to keep the ship on course, especially in bad weather. The industrial revolution brought the possibility of using machinery to make the job easier. The first such commercial use is said to have been fitted on SS Great Eastern in 1866. The Great Eastern was exceptionally large, so larger forces were needed to turn the rudder. A steam powered servo mechanism was built to rotate the rudder to match the position of the ship's wheel. Instead of making it easier to pilot the ship in a straight line, the new servo powered steering caused a tendency to wander off course, first to one side and then to the other. No matter how practiced the helmsman, the oscillation problem persisted.
A helmsman takes pride in holding a steady course, and that skill is immediately evident when looking at the wake of the ship. Unlike a car on a road, the path of a ship through the water leaves a disturbance visible for an extended time and distance that serves as a record of the helmsman's success. A weaving wake was not acceptable.
Engineers thought their systems needed to be more powerful to make the rudder movements faster. That way the rudder position would more closely match the wheel position. In fact, increasing the power assist made the problem worse, but that unexpected result pointed the way to the solution. When the engineers reduced the amount of power assist available, the rudder responded more slowly, and the helmsman could again steer a straight course. The lesson for us here today is that a control system needs to respond at the correct rate. Faster is not automatically better.
Once a helmsman could steer an arrow-straight course using a servo-controlled system, then it became reasonable to contemplate an autopilot that could hold a ship on course. That feat was first accomplished in the 1920s on an oil tanker owned by The Standard Oil Company. A hundred years later, automatic control loops are so numerous that we take them for granted.
Still, the underlying lesson from those first marine servo systems is largely forgotten. Modern controls often involve a computer. These are called computer-in-the-loop controls. When the loop doesn't behave as desired, the first instinct is to conclude that the computer isn't keeping up, just as the first marine engineers concluded that their servo wasn't powerful enough. The need for slower, less powerful, loop response is, and has always been, counter-intuitive.
If you have never steered a boat with a tiller or wheel, the helmsman analogy may not contribute to your intuitive understanding. Take a more everyday example. Think of a heavy frying pan on an electric stove. You turn up the heat setting when the pan isn't hot enough. If you wait until the pan reaches the desired temperature before you turn the dial setting back down, the pan will overheat. Then, if you turn the setting down in response, the pan will cool off too much. The temperature will then oscillate around the desired set point, much as the Great Eastern was weaving around the desired bearing.
If you have prior experience with the particular stove and pan, you may know in advance where to set the dial. That advance knowledge is often not available in a computer-in-the-loop control, so the loop needs a way to achieve equilibrium. For starters, the best way is to slow it down. Speeding the loop up will only result in larger oscillations. Smart systems can learn, as a practiced cook learns, how to better adjust the pan temperature, but you may be surprised how easy it is to fool a “smart” system. The control doesn't know that you poured a cup of room temperature broth into the pan, or that you have just put a lid on.
The takeaway is that if you are having trouble stabilizing your loop, resist the urge to speed it up, and begin by slowing it down.
Tom Lawson
June, 2021