Glossary

Piecewise Polynomial

A piecewise function is function of the form

f(t) = \begin{cases}
  f_1(t) :& 0 \leq t < t_1 \\
  f_2(t) :& t_1 \leq t < t_2 \\
  \hfill \vdots \hfill & \hfill \vdots \hfill \hfill \\
  f_n(t) :& t_{n - 1} \leq t \leq t_n.
\end{cases}

where 0 < t_1 < \cdots < t_n. It is naturally possible to define piecewise functions on unbounded domains too, but in typical applications the domain is bounded.

A piecewise polynomial is a piecewise function where each of the f_i(t) is a polynomial, possibly vector-valued. With suitable number of pieces and polynomial degree, piecewise polynomials make a highly expressive function class that is still simple and numerically stable.

_images/piecewise_poly.png

A piecewise polynomial with four pieces. At the point \alpha = 2, this function is continuous but not differentiable. Such a nonsmooth polynomial would not be used for quadrotor trajectory planning.

In Crazyswarm, we use degree-7 polynomials with 4-dimensional vector-valued output: (x, y, z) position and yaw angle. The quadrotor’s differential flatness property makes it possible to compute other states (attitude, acceleration, angular velocity) from these four values.

Setpoint
A collection of desired values for some or all of the quadrotor’s state that the feedback controller should try to achieve. For example, a setpoint may specify position, velocity, and acceleration – or just position.