Agriculture Reference
In-Depth Information
Mechanical controllers often work by balancing forces or by using an unbalanced
force to actuate some output. Such forces can be generated by pressures acting on
areas, masses being accelerated, shear from flowing fluids, deflections of springs or
other elastic members, or other methods. For example, when a float balances grav-
ity against buoyancy, the float opens a valve to automatically maintain the level of a
fluid in a container.
It should be noted that many of the early automation systems were “machinery-
centered.” They were designed to solve problems of machines into which they were
incorporated or to improve machine productivity. For example, the Ferguson system
was designed to maintain a constant draft force on the tractor rather than to solve
an agronomic tillage problem. More of the recent systems are “plant-centered” or
“animal-centered,” thereby attempting to improve agricultural production quantity
or quality, to minimize resource consumption, or to minimize environmental impact.
Because most recent sensors produce an electrical output of voltage, current, or
a digital signal according to the quantity being measured, most controllers became
electrical or electronic in the late twentieth century. A common configuration in
continuous controllers compares the desired system output to the current actual sys-
tem output through their sensor signals being input to a differential amplifier circuit.
Electrical and electronic controllers are compact and linear, although sometimes
difficult to service by the farm operator.
The current trend in agricultural automation is to replace electrical and electronic
controllers with computer controllers. Computerized systems inherently easily han-
dle the outputs from digital sensors and switches. Through the use of multiplexed
analog-to-digital converters, computerized systems can also determine the values
reported by analog sensors.
The controllers must take all the sensor signals and resolve them into coherent
and reliable information. The signals received may vary widely for such character-
istics as signal level, frequency content, and noise. The controller front-ends must
properly process the signals to obtain reliable information. But the most important
task of the controller is to decide the proper action. The controller must weigh all the
information it has received, decide what the proper action should be, and communi-
cate the action to the actuators.
In mechanical or analog electrical or electronic systems, this decision was usually
structured as issuing an output that depended on a mathematical function (usually
linear) of the error between the desired output and the current output. An example
is the PID control discussed above. Much more complex algorithms can be used in
computer-controlled systems. This gives a tremendous amount of design freedom to
the agricultural automation system creator or user. Another advantage of computer
controlled systems is that no hardware changes are necessary to change the control
algorithm. Improvements and other changes can easily be made in software.
The determination of the proper algorithm and any of its parameters should be
made by someone who understands the local agricultural situation. Many times
these algorithms are devised by agronomists, horticulturalists, or animal scientists
with input from other scientists and economists. Usually they are just computer
implementations of human decision-making practices. However, the programmers
should account for the fact that guidelines and best practices developed for human,
