Agriculture Reference
In-Depth Information
Indirect measurement of radiant temperature does not accurately assess the radiant
load placed on the animals, and overheating may result. Hoff (2009) demonstrated
that simple air temperature measurements were not suitable for assessing, and thus
accurately controlling, radiant heaters. Variation in heating patterns from radiant
heaters is exploited to prevent overheating by allowing animals to regulate the inci-
dent radiant load through behavior (movement) to maintain thermal comfort.
8.6.4 B
ACKUP
C
ONTROL
As HVC systems have evolved, early adopters of microprocessor controllers identi-
fied the need for robust mechanical backup for life-support function. Standardized
approaches to transient overvoltage testing were developed (Gates et al., 1992b) that
have resulted in robust systems. Many backup systems have been developed and are
in use ranging from simple thermostats wired in parallel with key stages of ventila-
tion and heating (Gates et al., 1992c, 1992d) to various automated means of opening
curtains during a power failure. Over the past decade or so, producers have included
automated backup power generation on larger facilities so that if grid power is lost
the electricity to barns can be quickly and automatically restored.
8.7 APPLICATIONS OF CONTROL THEORY
A substantial literature exists on development of livestock and greenhouse models
for design and management purposes. Environmental control systems in animal
housing are inherently nonlinear (Gates et al., 2001; Chao et al., 1995, 2000). In the
context of control theory, facilities are typically considered the plant, whereas the
interaction of the occupant animals with the surrounding environment is treated
as a disturbance (Cole, 1980). Application of control theory is limited in housing
control system design, arising from the difficulty in specifying a dynamic model of
the system that is sufficiently scalable for the sizes and variety of facilities encoun-
tered (Chao et al., 2000). Dynamic models of livestock housing are complex given
the coupled nature of moist air relationships and animal-environment interaction
(Daskalov et al., 2005), the dynamic changes in both occupant loading from grow-
ing animals and external weather events and nonlinear relation with the control
system.
A mixture of feedback and open loop control exists in animal housing. Feedback
control is used for control of temperature, inlet opening adjustment, and feed dis-
tribution, whereas parameters such as moisture and air quality are generally con-
trolled in open loop fashion. Control of ventilation rate is of particular interest as it
is considered the primary parameter of interest in animal housing control systems.
Ventilation rate is controlled differently depending on the secondary parameter of
interest: feedback control for temperature, and open loop control for moisture or air
quality control (Berckmans and Vranken, 2006). In practice, operation of exhaust
fans is controlled with temperature feedback, but flow rate control for single speed
exhaust fans is implemented through variable area inlets adjusted to a meet a pre-
scribed static pressure operating point (Gates et al., 1991a). This method is typi-
cally used in U.S. poultry facilities and contrasts with the variable fan systems with
