The envelope system is an interface created specifically for the OSRD gait calculation. It allows you to manipulate different space/velocity curves, to slice them, to end them, to interpolate specific points, and to address many other needs necessary for the gait calculation.
A specific interface in the OSRD Core service
The envelope system is part of the core service of OSRD (see software architecture).
Its main components are :
1 - EnvelopePart: space/speed curve, defined as a sequence of points and having metadata indicating for example if it is an acceleration curve, a braking curve, a speed hold curve, etc.
2 - Envelope: a list of end-to-end EnvelopeParts on which it is possible to perform certain operations:
- check for continuity in space (mandatory) and speed (optional)
- look for the minimum and/or maximum speed of the envelope
- cut a part of the envelope between two points in space
- perform a velocity interpolation at a certain position
- calculate the elapsed time between two positions in the envelope
3 - Overlays : system for adding more constrained (i.e. lower speed) EnvelopeParts to an existing envelope.
Given envelopes vs. calculated envelopes
During the simulation, the train is supposed to follow certain speed instructions. These are modelled in OSRD by envelopes in the form of space/speed curves. Two types can be distinguished:
- Envelopes from infrastructure and rolling stock data, such as maximum line speed and maximum train speed. Being input data for our calculation, they do not correspond to curves with a physical meaning, as they are not derived from the results of a real integration of the physical equations of motion.
- The envelopes result from real integration of the physical equations of motion. They correspond to a curve that is physically tenable by the train and also contain time information.
A simple example to illustrate this difference: if we simulate a TER journey on a mountain line, one of the input data will be a maximum speed envelope of 160km/h, corresponding to the maximum speed of our TER. However, this envelope does not correspond to a physical reality, as it is possible that on certain sections the gradient is too steep for the train to be able to maintain this maximum speed of 160km/h. The calculated envelope will therefore show in this example a speed drop in the steepest areas, where the envelope given was perfectly flat.
Simulation of several trains
In the case of the simulation of many trains, the signalling system must ensure safety. The effect of signalling on the running calculation of a train is reproduced by superimposing dynamic envelopes on the static envelope. A new dynamic envelope is introduced for example when a signal closes. The train follows the static economic envelope superimposed on the dynamic envelopes, if any. In this simulation mode, a time check is performed against a theoretical time from the time information of the static economic envelope. If the train is late with respect to the scheduled time, it stops following the economic envelope and tries to go faster. Its space/speed curve will therefore be limited by the maximum effort envelope.