Image Processing Reference
In-Depth Information
Out of control
Worst
Networked
control
Digital
control
Unacceptable
performance
A
B
C
Acceptable
performance
Continuous
control
Best
P A
P B
P C
Larger
Smaller
Sampling period
FIGURE . Performance comparison of continuous control, digital control, and networked control, as a function
of sampling frequency.
23.2.4 Network QoS vs. System Performance
When a network is used in the feedback loop of a control system, the performance of the system
depends not only on the QoS of the network, but also on how the network is used (e.g., sample time,
message scheduling, node prioritization, etc.) [,]. For example, consider a continuous-time con-
trol system that will be implemented with networked communication. Figure . shows how the
control performance varies vs. sampling period in the cases of continuous control, digital control,
and networked control. The performance of the continuous control system is independent of the
sample time (for a fixed control law). The performance of the digital control system approaches
the performance of the continuous-time system as the sampling frequency increases []. In an
NCS, the performance is worse than the digital control system at low frequencies, due to the extra
delay associated with the network (as described in Section ..). Also, as the sampling frequency
increases, the network starts to become saturated, data packets are lost or delayed, and the control per-
formance rapidly degrades. Between these two extremes lies a “sweet spot” where the sample period
is optimized to the control and networking environment. Note that, as discussed in Section ..,
the device delay can comprise a significant portion of the end-to-end delay; thus, the optimal sam-
pling period can often be more a function of the device speed than the network speed, especially for
faster networks with minimal collisions, such as switched Ethernet.
Typical performance criteria for feedback control systems include overshoot to a step reference,
steady-state tracking error, phase margin, or time-averaged tracking error []. The performance
criteria in Figure . can be one of these or a combination of them. Due to the interaction of the
 
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