# Controls Glossary¶

reference

The desired state. This value is used as the reference point for a controller’s error calculation.

input

An input to the (hence the name) that can be used to change the .

• Ex. A flywheel will have 1 input: the voltage of the motor driving it.

• Ex. A drivetrain might have 2 inputs: the voltages of the left and right motors.

Inputs are often represented by the variable $$\mathbf{u}$$, a column vector with one entry per to the .

output

Measurements from sensors. There can be more measurements then states. These outputs are used in the “correct” step of Kalman Filters.

• Ex. A flywheel might have 1 from a encoder that measures it’s velocity.

• Ex. A drivetrain might use solvePNP and V-SLAM to find it’s x/y/heading position on the field. It’s fine that there are 6 measurements (solvePNP x/y/heading and V-SLAM x/y/heading) and 3 states (robot x/y/heading).

Outputs of a are often represented using the variable $$\mathbf{y}$$, a column vector with one entry per (or thing we can measure). For example, if our had states for velocity and acceleration but our sensor could only measure velocity, our, our vector would only include the 's velocity.

process variable

The term used to describe the output of a in the context of PID control.

setpoint

The term used to describe the of a PID controller.

error

minus an or .

state

A characteristic of a (e.g., velocity) that can be used to determine the future behavior. In state-space notation, the state of a system is written as a column vector describing it’s position in state-space.

• Ex. A drivetrain system might have the states $$\begin{bmatrix}x \\ y \\ \theta \end{bmatrix}$$ to describe it’s position on the field.

• Ex. An elevator system might have the states $$\begin{bmatrix} \text{position} \\ \text{velocity} \end{bmatrix}$$ to describe its current height and velocity.

A state is often represented by the variable $$\mathbf{x}$$, a column vector with one entry per .

system

A term encompassing a and it’s interaction with a and , which is treated as a single entity. Mathematically speaking, a maps to through a linear combination of .

plant

The or collection of actuators being controlled.

controller

Used in position or negative feedback with a to bring about a desired by driving the difference between a signal and the to zero.

observer

In control theory, a system that provides an estimate of the internal of a given real from measurements of the and of the real . WPILib includes a Kalman Filter class for observing linear systems, and ExtendedKalmanFilter and UnscentedKalmanFilter classes for nonlinear systems.

moment of inertia

A measurement of a rotating body’s resistance to angular acceleration or deceleration. Angular moment of inertia can be thought of as angular mass. See also: Moment of inertia.

control input

The input of a used for the purpose of controlling it

control effort

A term describing how much force, pressure, etc. an actuator is exerting.

after reaches equilibrium.

step response

The response of a to a .

step input

A that is $$0$$ for $$t < 0$$ and a constant greater than $$0$$ for $$t \geq 0$$. A step input that is $$1$$ for $$t \geq 0$$ is called a unit step input.

rise time

The time a takes to initially reach the after applying a .

settling time

The time a takes to settle at the after a is applied.

system response

The behavior of a over time for a given .

model

A set of mathematical equations that reflects some aspect of a physical behavior.