Agriculture Reference
In-Depth Information
system would likely be more energy efficient. But this would likely come at the
expense of poorer dynamic response and less stability. It would also be less or more
expensive depending on the particulars of the system.
The actuator should be selected to have sufficient range and resolution, and, of
course, adequate static and dynamic accuracy. It needs to have sufficient power out-
put, yet minimal power consumption so that it is highly efficient. The actuators are
often the most expensive components in agricultural automation systems. The agri-
cultural automation system designer must carefully choose the right actuator from
the available/existing actuators or in designing a new actuator.
Actuators used in agricultural automation systems tend to be electrical, mechani-
cal, fluid (such as hydraulic or pneumatic), or some combination thereof. With
the increasing use of computer controls and the relatively large amounts of power
required in many agricultural automation applications, many actuators are elec-
tromechanical or hydromechanical. Actuators requiring large amounts of power to
operate can be multistage. For example, consider a computer-controlled system in
which high linear forces and powers are required. The output from a controller (itself
already including amplification from the computer's natural output) might be routed
to a valve that controls a low pilot pressure. The pilot pressure then actuates a larger
valve that controls high pressure/high flow to a hydraulic cylinder, which provides
the force and power.
1.6 REGULATORS AND SERVOS
It is sometimes useful to differentiate in agricultural automation whether the system
mostly functions as a regulator or a servo. A regulator typically attempts to maintain
some relatively fixed system state or output. Alternatively, a servo has a desired out-
put that dynamically varies with time.
An example of a regulator is an air temperature control system for a greenhouse or an
animal housing facility. The temperature setpoint will not change, or if it changes it will
do so infrequently or gradually. The automation system primarily regulates to maintain
the setpoint. It seeks to maintain a constant temperature in spite of the disturbances from
the outside weather, the inside activities, and the uncertain openings to the outside. This
type of system should be designed for fast and accurate disturbance rejection.
Examples of a servo would be the mechanism to attach a teat cup on a robotic
milking machine or a robot for picking fruit. These robots are not primarily main-
taining a constant position like a regulator would do. The primary task would be
accurately moving dynamically along a path. Such systems should be designed for
path following performance.
1.7 PERFORMANCE
There is a need for reliable quantitative measures of agricultural automation perfor-
mance. These should relate to the goals of the automation system. For example, a
fruit picking robot could be judged by the percentage of fruit that were successfully
picked, the percentage and magnitude of fruit damage, and the average time it took
to pick a fruit.
