Agriculture Reference
In-Depth Information
and therefore relatively static, situations may not be optimal for automation situa-
tions in which you have dynamic considerations. Usually automation systems can
have superior resolution and dynamic capabilities to those of humans. These supe-
riorities should be considered in the algorithm development. Conversely, humans
usually have superior adaptability compared to automation systems. Automation
systems will not be able to respond as well to unexpected conditions and situations.
Algorithms should be designed to be fault tolerant and to safely handle unexpected
situations.
1.5 ACTUATORS
Once the controller makes a decision, it must be converted to the proper action by
one or more actuators. It is often erroneously assumed that the actuators will imme-
diately produce the exact required action. However, in practice there are often inac-
curacies or problems.
Given the environments (such as outdoor weather or hostile interiors) that agri-
cultural automation systems are exposed to and the physical/chemical/biological
complexity of agricultural systems, it is not surprising that there can be external
disturbances that affect actuator performance. This is especially a problem in open-
loop systems where the system output is not sensed and is not fed back to the con-
troller. The actuator must be able to overpower any disturbance, especially in those
open-loop systems.
Unlike sensors, which can be designed to optimize static and dynamic accuracy
without many other concerns, actuators often have to supply substantial physical
outputs, such as forces, torques, speeds, accelerations, and flows. The need to pro-
vide substantial physical outputs often means that the actuators must compromise on
accuracy, resolution, linearity, dynamic response, and similar performance aspects.
Hence, these characteristics should be studied in the design of automation systems.
Also, being the last of the three components (after sensors and controllers) in the sys-
tem, actuators are sometimes unfortunately relatively neglected because of deadline
considerations in agricultural automation projects. Although actuator performance
issues may sometimes be partially compensated for by good sensor and controller
design, sufficient design time, testing, and investment with regard to actuators is
necessary to optimize overall system performance.
When controllers were discussed above they were broken down into three categories:
“on-off,” “three-position,” and “continuous” (where the last category included those
with multiple discrete levels). Obviously, the actuators should be selected to be com-
patible with the controller. The actuator must match its action with the decision made
by the controller.
Another consideration is where to perform the control on the system. “Primary”
control attempts to control the system close to its output. For example, in a rotating
output hydraulic system, primary control might involve valving the flow just before
the output motor. “Secondary” control would be a control removed farther away
from the system output. For this example, it could be the control of the displacement
of the hydraulic pump to which the motor is connected. Because the secondary con-
trol would not have the valve throttling losses of the primary system, the secondary
