Hello Torsten,

If you consider the complete control loop in discrete time (the system
has to be discretized), then you do not need any transfer function for
your hold device. Instead, it is considered in the calculation of the
discrete time system representation. If, e.g., the used D/A has a zero-
order hold, than the discretization method makes use of the fact that
the input u is constant during a sampling interval.

Yes, if I use the ZOH method when discretizing, the ZOH will be included.
What if there are two feedbacks, i.e a voltage and a current feedback (see
diagram below)? Should the ZOH be placed in both loops as shown in the
second diagram? My system is sampling all feedbacks at the same time, so
there is essentially only one delay; however, I think two zero order holds
would add two delays.
Stephen
<https://octave.1599824.n4.nabble.com/file/t372348/TwoLoopControlDiagram.jpg>
<https://octave.1599824.n4.nabble.com/file/t372348/TwoLoopControlDiagramWithZOH.jpg>
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