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4.5. CLIENT/SERVER SEPARATION 35
value, the frequency f = 1 T
at which the TF Data has to be captured can be calculated as
P k+1 = AP k A T
[ σ
2
] [ ] [
s,k+1
0 1 T σ
2
] [ ]
0 σv,k+1
2 =
s,k
0 1 0
0 1 0 σv,k
2 T 1
From this it is possible to write
σ 2 s,k+1 = σ 2 s,k + T 2 σ 2 v,k
T 2 = σ2 s,k+1 − σ2 s,k
σ 2 v,k
This equation allows to calculate the frequency, at which the TF data must be captured, for an arbitrary
worst case position quality of σ 2 sk+1 = 0.12 rad. To do this also the values for σs,k 2 and σ2 v,k
have to
be known. An assumption for this example is σs,k 2 = 0.012 and σv,k 2 = ( π 4 )2 . From these values the
frequency can be calculated as
T 2 = σ2 sk+1 − σ2 sk
σ 2 vk
= 0.12 − 0.01 2
( π 4 )2
= 0.016
⇒ T = 0.1267 sec
f = 1 T
= 7.89 Hz
From this calculation follows, that the TF Data must be captured with a frequency of at least 7.89 Hz
to achieve the desired quality. In reality and with some safety this normally results in a frequency of
8 to 10 Hz.
4.5 Client/Server Separation
As known from component-based systems and also stated in the requirements in Section 4.2, it is
important to separate components in a clear way. This separation allows to divide the overall system
complexity into small parts, which can be implemented by components. Out of these components
complex systems can be built.
The separation of components is done by defining the messages exchanged between the components.
These messages are called Communication Objects or in short CommObjects. CommObjects
can either be plain data structures like they are in ROS or they can specify additional access methods
like it is in SMARTSOFT. These additional methods allow operations on the contained data like conversion
or filtering. In contrast thereto in ROS additional methods in special libraries are provided.
Both approaches have pros and cons, but in general they provide the same capabilities and thus it is
not distinguished between them in this work.