Agriculture Reference
In-Depth Information
A regulator can be judged by the percentage of time it keeps the output within an
acceptable level of error or by the average or maximum error of the output compared
to the desired output for particular disturbances. The steady-state error is the error
when the system is presented with a constant input and disturbance. The time it takes
to successfully reject a disturbance can be another performance measure.
Measuring the performance of a servosystem can be more complex as multiple
measures are often used. First of all, the steady-state error may be determined based
on a constant input and disturbance. In addition, the tracking error in following a
prescribed path should be determined.
In some servo situations, the final position or a limited number of discrete positions
is important and the intermediate positions are of limited importance. Therefore, the
response of the system can be judged using classical control theory step response
performance measures, such as delay time, rise time, and settling time. The previ-
ously mentioned milking and fruit picking robots are examples of applications where
the time to move to the various required positions is important.
In other cases, the path and speed along the path is important. For example,
consider a pesticide-spraying robot. It must follow the correct path along the entire
path to ensure complete pesticide coverage. In addition, it likely needs to maintain
a near-constant velocity to maintain a constant application rate. In such a case, the
performance might be measured by a performance index that mathematically time
integrates the absolute value of the error or the square of the error (taking the abso-
lute value or squaring is necessary to ensure that errors are always positively accu-
mulated) along the path to generate an index of how well the servo followed the path.
Depending on whether the position at a particular point in time is important or not,
the error should either be measured from the desired point on the path at that particu-
lar point in time or just the nearest point on the path. Note that “path” here does not
necessarily just refer to a position in space, but any time-varying variable, such as the
time-varying temperature during a specified heating/cooling cycle.
1.8 MOREINFORMATION
It is again important to remember the great diversity in agricultural automation sys-
tems. The preceding discussion must be viewed only as illustrative, as there are
many ways of analyzing and designing agricultural automation systems. Whole cat-
egories must be neglected because of space considerations. The following chapters
discuss many different systems and provide informative examples of the variety of
approaches, techniques, and components that can be used in agricultural automation
systems. Adoption of these factors to other applications often leads to significant and
rapid advances.
Much information about agricultural automation can be found in the published
literature. Technical journals often have articles about such systems. Some journals
that frequently publish in this area include:
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