Control Systems Intro - ON/OFF Control Systems

       In many applications, it is not enough to be able to measure a quantity.  Often you want to control a quantity.  Either you want the quantity (for example, temperature) to stay constant at some fixed value (like you expect when you set the wall thermostat) or you want to make the quantity vary in some predetermined fashion (for example, making an antenna point to a satellite as it crosses from horizon to horizon).  Consider what you need to do to control a quantity.  We'll use temperature as an example, but it could be any physical quantity.

        Some of the ways that you can control a variable include the following.         There is one fly in the ointment.  In the schemes above, the control effort should really take into account the dynamics of the system you are trying to control - but that's a more advanced topic.

        When you build a control system it is helpful to have a picture of how the signals are produced and affect things in your system.  Control system designers usually use a block diagram to show how signals flow in a control system.  Here is a block diagram of a typical control system.

Interpreting this block diagram, we have the following.


Simulating an ON-OFF System.

        We have simulators available for ON-OFF control.  The first simulator controls a second order system.  You can use those simulators to gain some insight into the behavior of ON-OFF control systems.


Second Order System
Third Order System