private formula
The rise time is the time required for the response to rise from 10%
to 90%, 5% to 95%, or 0% to 100% of its final value
| Tr | rise time (dimensionless) |
| π | pi |
| β | angle between "natural frequency" and "x axis" (dimensionless) |
| ωd | damped natural frequency (dimensionless) |