Here is a block diagram of a typical control system. You will find these components in the vast majority of control systems. You can roll the mouse over each component in the block diagram to get a description of what that component does.

Here are those relationships in more detail.

• E(s) = U(s) - X(s)
• This relationship is for the summer/subtractor (shown with a blue circle)
• W(s) = G_C(s)E(s)
• This shows how W(s) - the control effort that drives the system, G_P(s) - is related to the error.  The controller is probably an amplifier - probably a power amplifier - that provides and output to drive the plant, G_P(s).
• Y(s) = G_P(s)W(s)
• This shows how the output, Y(s), is related to the control effort that drives the plant (system being controlled ) with a transfer function, G_P(s).
• X(s) = G_S(s)Y(s)
• This shows how the sensor output, X(s), is related to the output of the system, Y(s).  There are many sensors that have dynamics that need to be accounted for with a transfer function.  For example:
• Temperature sensors have thermal time constants and don't respond immediately to a sudden temperature change.
• Gyroscopes - used to measure attitude in aircraft, submarines, etc. - are second order systems with natural frequencies, damping ratios, etc.