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Control System Design - Index | Book Contents |
| Section 2.9
2. Introduction to the Principles of Feedback
We have seen that one of the key issues in feedback control is that
there must exist suitable measurements to feed back. Indeed, if one
can measure a variable, then there is a good chance that one can
design a controller to bring it to a desired reference value.
A more accurate description of the feedback control loop,
including sensors, is shown in Figure 2.11. From this
figure, it can be seen that what we actually control is the
measured value rather than the true output. These can be quite
Closed-loop control with sensors
Hence the measurement system should ideally satisfy requirements
such as the following:
- Reliability. It should operate within the necessary range.
- Accuracy. For a variable with a constant value, the measurement
should settle to the correct value.
- Responsiveness. If the variable changes, the measurement
should be able to follow the changes. Slow responding measurements
can not only affect the quality of control but can actually make
the feedback loop unstable. Loop instability can arise even though
the loop has been designed to be stable for exact measurement of the
- Noise immunity. The measurement
system, including the transmission path, should not be significantly
affected by exogenous signals, such as measurement noise.
- Linearity. If the measurement system is not linear, then at
least the nonlinearity should be known, so that
it can be compensated for.
- Nonintrusive measurement. The measuring device should not significantly
affect the behavior of the plant.
In most of the sequel, we will assume that the measurement system
is sufficiently good, so that only measurement noise needs to be
accounted for. This ideal measurement loop will be known as a
unity feedback loop .