# What is the Reference in Control Theory?

I've started studying Control and I've come across some concepts that I'm finding a bit difficult to understand.

For example, in the system showed here, there is a signal called "reference" as well as a feedback block. As far as I know, the feedback block has the function of getting the output as close as possible to the reference. But here is what I don't understand: what if the reference is, for example, an impulse, and I want the system to show a step when an impulse is at its input?

I'm getting confused with the difference between reference signal and input signal. Why would someone connect a feedback block in order to get the output to follow the reference? What's the advantage of doing this?

## 1 Answer

Imagine that you're heating (or cooling) a home with a modern furnace (or air conditioner).

the reference or set point is the temperature that you set your thermostat to be. the feedback signal is the actual temperature that is measured with some kinda thermometer.

the actual value that you are trying to control, whether it's temperature or the position of a robot arm or the position of a pointer in an asynchronous sample rate converter, that actual value is compared to your reference value. the difference between those two signals is what drives your "controller".

in the case of heating your home, if the actual temperature is below the reference (what you want your temperature to be), the furnace heat is increased. if the actual temperature is above the reference, the furnace heat is decreased.

• Very clear. One more question: what's the relationship between the input and the reference? For example, in a system that controls the water level of a tank, the input signal would be the voltage that controls the pump, and the reference would be the level I want the tank to have. How are these two quantities related, in this example or in any other one? – Tendero Apr 22 '16 at 22:04
• @M.S. I guess that the pump would be binary. You turn it on when level is below some level and turn it off when it above some level, where level1 < level2 rather than level1=level2 to avoid thrashing. – Valentin Tihomirov Apr 22 '16 at 22:37
• what you're calling the "input" is not the input to the whole system. the reference is that input. what you call "input" is the input to the plant. which is the output of the controller. you might want to download this nice .pdf doc done by a friend of mine. – robert bristow-johnson Apr 22 '16 at 23:54
• Go Tim!!!!! ;-) – Peter K. Apr 23 '16 at 0:16