Why x not y

[SpencerGreene]

Why x not y

[SpencerGreene]

Maybe a dumb question but in the PID section:

controls-engineering-in-frc/fundamentals-of-control-theory/pid-controllers/derivative-term.tex

Line 62 in a76d9d7

the \gls{error} at timestep $k$. $e_k$ is defined as $e_k = r_k - x_k$ where

the expansion is shown as e_k = r_k - x_k. Why isn't it e_k = r_k - y_k? The spring equation uses x but the control system example and diagram use y, am I missing something?

[calcmogul]

The state and output of the PID controller are equivalent in this context, but I'll modify it to use y for consistency.