Control Theory


What do we mean by Control Theory?
       In everyday life we experience a wide range of controlled apparatus and environments. These range from the very simple to the highly complex and very often the complexity is not obvious.   Every time you drive a car, you, yourself are participating in a control system trying hard, sometimes very hard, to keep the car travelling along the middle of the road. When you press the button on a light controlled crossing, you initiate a sequence of light changes, which enable you to cross the road safely, and when you relax in the comfort of your centrally heated home, spare a thought for the thermostat, which works constantly for you maintaining a comfortable temperature. In all these examples and thousands more, that we meet in everyday life, there are some common principals operating in the background helping to achieve the level of control required. This is the theory of control.  To examine these principals in more detail lets consider the different kinds of control system, starting with the simplest...




The Sequencer
            Experts in the field of control theory would like us to refer to this as open loop control, for reasons that will become obvious later, but I prefer the more descriptive term of “sequencer”.  In simple terms a sequencer does exactly that, it performs its pre-defined sequence of operations, at pre-defined times, accurately and without fail. It can either start over again when it reaches the last operation, or, as in the case of the light controlled crossing, wait for an initiator (button press).   The power and benefits of this type of control are almost always underestimated. For such a simple principle, the degree of usefulness of sequencers cannot be overstated.  Many chores that are boring or repetitive can be handed off to a sequencer, or, if the sequence of operations occur too fast for humans to control, the sequencer is essential. Lets consider some examples:

The Washing Machine.
      When people are asked to think of something they have at home that uses sequences, the washing machine is the most common answer. Although there is (normally) no computer anywhere to be seen, it is the classic sequencer. Using electro-mechanical  sequencing units traditionally they rely on the user to rotate a dial to a defined position before starting the automatic operation. The rotation of the dial continues under automatic operation in small steps until it reaches a stop point, terminating the selected “program”.  At each new step there is the possibility for something new to happen. For example, the water inlet valve may open to fill the tub with water or the drum may start spinning to expel water etc…. But equally nothing new may happen. That is, some small steps may just be there to perform wait operations.  The important point here is that each of the steps in the operating cycle of the machine is of the same duration.  In order to achieve different durations for events they must be programmed to exist over an appropriate number of steps.  For example, the spin drying cycle must be 5 minutes long. This means that all of the steps during this time must have the same set of outputs active (i.e. spin on and outlet water valve open).   This example constitutes the simplest form of sequencer.
  The next step up in complexity takes us to the sequencer that uses variable step durations.

Security Lighting.
   It is more and more common now for people to set their home room lights on a “timer” so that when they are away on holiday their house appears occupied. Some security advisors also recommend having a radio on “timer” to create a noise within the house to further deter any would-be intruder. Usually these timers have a number of programmable on/off times that can be set from a small keypad or set of buttons and LCD display. In most cases the on and off times can be set according to the time of day and the day of the week. This allows for day to day variations. There would be nothing more obvious to a burglar than a house light, which turned on at exactly the same time every night. These devices are still simple sequencers, but it would be impractical for them to use fixed step durations. Imagine the scenario… you want the timer to turn on a light for about 15minutes every night. You would have to have at least 84 steps available per day and then program 83 of them to be off !!.
           One point to note here is that the light timer control illustrates the use of a sequencer based on events (on/off) linked to a particular time and day. The other type of sequencer in the same category would be linked to duration. i.e. any given step in the sequence would have a programmable duration, after which the next step would be activated, with no regard to actual time of day. Clearly this is not suitable for security lighting but may be appropriate for traffic light control (for example).


Next:  Directed Sequencer


Directed Sequencer   Feedback Control   Proportional Control   Derivative Control  Integral Control PID Control


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