title: Discrete to continuous time transfer function
body: <p>I implemented a class to identify ARX models in Python. The next step is the calculation of optimal PID parameters based on LQR. Apparently a continuous time model is required and I have the following possibilites:</p>
  
  <ul>
  <li>transform the discrete time model to a continuous time model,</li>
  <li>identify a continuous time model,</li>
  <li>adapt the LQR approach to determine optimal PID parameters to the discrete time domain.</li>
  </ul>
  
  <p>In Matlab the first two approaches are easily done, but I need them in Python. Does anybody know how Matlab implemented <code>d2c</code> and has a reference?</p>