.Modelica.Clocked.UsersGuide.Clocks .Modelica.Clocked.UsersGuide.ClocksA central element of the Modelica.Clocked library is a clock. Below, the most important information for clocks is summarized. For more details, see the Modelica Language Specification, Chapter 16 (for Modelica Language Version ≥ 3.3).
The Clock type is a base data type introduced in Modelica 3.3 (additionally to Real, Boolean…) which can be understood as a specialized Boolean type: Clock variables are either true (also called "active") or false ("inactive"). It is specialized in the sense that each Clock variable generates a partition consisting of a set of equations which gets activated when the clock is active. In fact, every variable and every equation of a Modelica model (starting with Modelica Language Version 3.3) is assigned to a partition:
This feature is visualized in the figure below where c(ti) is a clock that is active at particular time instants ti and r(ti) is a variable that is associated to this clock. Notice that a clocked variable has only a value when the corresponding clock is active:
The Clock type introduced above is a base type of the Modelica language and it is mostly intended for textual model description. The Clocked library encapsulates this base type and allows for graphical modeling thanks to the clock blocks and connectors it provides.
Similarly to RealInput, RealOutput etc., clock input and output connectors, called ClockInput and ClockOutput, are defined in sublibrary ClockSignal.Interfaces in order to propagate clocks via connections.
A clock signal can be generated with one of the blocks of sublibrary ClockSignals.Clocks:
The output signals of the blocks in the above figure are clock signals, by default visualized with dotted grey lines.
With the blocks of sublibrary ClockSignals.Sampler a clock signal can be sub-sampled, super-sampled, or shift-sampled to generate a new clock signal. For example, with the following model, a periodic clock signal of 0.1 s is sub-sampled with a factor 3 and therefore a clock signal with a period of 0.3 s is generated:
The relationship between such derived clocks is exact, so it is guaranteed that at every 3rd tick of clock "periodicRealClock.y", the clock "subSample.y" is active.
A clocked partition is a set of equations that depend on each other and where the boundary variables are marked with sample and hold operators (boundary with other clocked partitions or with the global continuous-time partition). A sub-clock partition is a part of clock partition that use a specific derived clock, separated from other sub-clock partitions by operators like subSample and superSample. All equations within a sub-clock partition are active at the same time.
There are two types of sub-clock partitions:
Notice that it is an error if a partition doesn't fall into one of these two categories, e.g., if operators previous and der are both used in the same partition. This means it is not allowed to mix continuous time and discrete-time operators in a clocked partition (unlike in the global continuous time partition).
Also notice that in a clocked discrete-time partition all event-generating mechanisms do no longer apply. Especially neither relations, nor one of the built-in event triggering operators will trigger an event.
If a clock is associated to a clocked continuous-time partition, then an integrator has to be defined that is used to integrate the partition from the previous to the current clock tick. This is performed by setting, for the corresponding clock signal generating block, parameter useSolver = true and defining the integration method as String with parameter solver. Both parameters are to be found in tab Advanced of clock signal generating blocks. The possible integration methods are tool dependent. It is expected that at least the solvers "External" (= use the integrator selected in the simulation environment) and "ExplicitEuler" (= explicit Euler method) are supported by every tool. For an example, see Examples.Systems.ControlledMixingUnit.