[From Bill Powers (2009.12.02.0920 MDT)]
Arthur Dijkstra 2 dec 2009 --
AD: Is the choice for control system design not dependent on the nature of the system to be controlled ?
I think in technical systems error control is often sufficient, if the sensors are precise and the controlling dynamics are relatively high. E.g. a temperature control in a hospital might not get the desired accuracy when it is controlled only by feedback because the effect of the heaters to counteract the measured temperature drop is too slow.
I can't imagine that any hospital would rely on open-loop control, if the accuracy of control were critical. In general, a closed loop system will react faster, will correct temperature variations faster, and will keep the final temperature more exactly at the intended level, if properly designed. Moreover, the closed-loop system will counteract the effects of opened and closed windows, heat sources like operating-theater lights going on and off, changes in air circulation when people go in and out through doors a lot, under or over voltage in the electrical supply, changes in the quality of heating fuel, increases or decreases of the number of people present, and any other disturbances whether from known or unknown sources, occurring at unpredictable times, and undetectable except for their effects on inside air temperature. No open-loop system with any practicable design could do those things.
The closed-loop hospital temperature control system can react quickly to temperature changes because it can crank the heater power up not to what is anticipated as the amount needed to achieve the final temperature, but far above that level, causing the temperature to rise very rapidly. But as the temperature approaches the final level, the output power is reduce more and more, so the heater power is reduced to the final level just as the temperature levels out at the set point. This does not require any complex calculations or arrays of sensors distributed all over the outside walls, or computers stuffed with thermodynamic equations and properties of materials, or accurately adjustable heaters with absolutely unchanging characteristics, or sealed windows and airlock doors. Your own home thermostat works this way by adjusting the duty cycle of the furnace. When you first turn up the reference temperature in the morning, the duty cycle starts out at 100%, with the furnace on continuously. If it stayed on that way, the house would drastically overheat. But as the temperature rises, a simple "anticipation" circuit in the wall box measures the rate of change of temperature and lies to the comparator, telling it that the final temperature has been reached. That turns the furnace off, and after a while the rate of change slows down a little and the anticipation circuit say "oh, my goodness, it's freezing in here" and the furnace turns back on. So the duty cycle gradually changes, reaching the final duty cycle just as the room air comes to the desired temperature. This happens automatically and needs no fancy calculations.
Of course you could design an open-loop controller that would reproduce the same changes in duty cycle, but you'd probably have to recalibrate it every day and watch out for unexpected north winds or excessive sunshine, not to mention all the other undetectable causes of errors mentioned above. This controller would probably cost hundreds if not thousands of dollars, compared with the $30 for a digital programmable home thermostat. And it wouldn't work as well.
In a management system (a socio technical system) controlling on feedback only (thus without a model) is often not effective and might even be dangerous. Safety management in a an organization based only on feedback would be dangerous. The sensors to detect unsafety as input for feedback control are problematic.
I agree, a little planning or attention to design details certainly helps avert disasters at times. But there are severe limits to how much can be accomplished without continuous monitoring of the actual, as opposed to hoped-for or intended, results. A management that simply issues orders to subordinates and waits for the intended results to be achieved is doomed to failure. The real world is not that repeatable, reliable, or disturbance-free, and that includes the people receiving the orders.
In fact, it's possible to create closed-loop systems in which the controlled variable is some continuously-computed future state of the world. An example is an automatic aircraft-landing system. In that case there is a perceptual input function that measures distance to the runway, altitude, speed, flap settings, and engine thrust, and displays to the pilot not where the airplane is, but where on the runway it will touch down if all else remains equal. The pilot, a closed-loop control system, operates the controls to keep the displayed touchdown point stationary at the proper place. The "model", in this case, is built into the perceptual input function, and is not used to calculate the required output, but simply to predict the landing place wherever it is. the calculations get more and more accurate as the touchdown point nears, because the computation errors make less difference in the path of the airplane. The pilot controls the display as if it is any present-time variable, because of course the prediction is occurring, over and over, in present time. An automatic negative feedback control system given the same input information could also land the airplane, but you will notice that the FAA hestitates to approve any such device. That device, though a closed-loop system, still could not detect wind shear or traffic in the air or approaching the runway.
Watch NASA TV when a shuttle docks to the space station. Exactly this kind of predictive control is used to show the shuttle pilot where the shuttle will be in half an hour or an hour, after going through the loops that the orbital dynamics generate. The pilot, in present time, simply operates the controls to move the displayed future point to the place on the screen where he wants it, and keeps it there.
Is the combination of feedback and feedforward not preferable in most situations. I imagine a continuum from 100% feedback based via feedback and feedforward to 100% feedforward. The combination of feedback and feedforward depends on the nature of the system and its controller. Is this an acceptable line of reasoning ?
Sure, if feedforward can speed things up a little, by all means use it. But it's no way to achieve accuracy and speed. All it can do is fill in the fraction of a second before the controlled variable starts to change, and it can do that only approximately, and at considerable expense for the required sensors and computers.
I'm quite aware of the tradition that has grown up in some areas of control engineering, following the lines first set up by Ashby. I think it's a bad mistake, but who am I to tell them how to spend their money?