Ingeniero

EQ - Temperature control with electronic controller
LOGITRON - model THP 482
Aim of laboratory work
The students will carry out the analysis of simple control loop (circuit) and they will verify static and dynamic properties of controlled system (plant). They will meet with microprocessor controlled electronic controller modern conception and its application possibilities. In
the practicalapplication of temperature control of laboratory model they will verify action of
many types of controllers (P, PI, PID) with the activity of disturbance variable and changes of
reference variable. They can observe influence of set up controller parameters on the running of control process.

Theoretical backgrounds
Basic terms
The general scheme of control loop and signification of variablesis introduced in Fig. 1.
disturbance
variables

z1
controlled
system

z2

V

y
controlled
variable

manipulated
variable

controller

e=w-y

w

deviation

reference
variable

Fig. 1 Scheme of controlled loop

Control loop is composed of controlled system and controller. Controlled system represent of technological apparatus (or its certain part), in which run thetechnological process
under certain, pre-arrange conditions. The variable, whose value is stand on pre-set conditions we name controlled variable. Controlled variable is output variable of controlled system.
Reference variable determine desired value of controlled variable and after its comparison
with instant value of controlled value we will get deviation. Deviation is input variable intocontroller. The task of controller is to remove (eventually minimize) deviation in the time as
short as possible, so that transient deviations from desired value have been smallest as possible. Control is accomplished by working of manipulated variable on controlled system.
Variable, which cause the change of controlled variable from the desired value we name disturbance variable. The basic claim isso that controller will act by means of manipulated variable against influences of disturbance variables. Difficulties of solution control task consist in,
that influence of disturbance, which acts on controlled system, didn't show immediately in the
output, but till in time sequence, which is caused by concrete dynamic properties of controlled system.

1

The variables in control loop inFig. 1 are brought out in so-called deviation ratio (dimensionless) expression. This way of expression is more general description, which
simplify of control task solution. Output variable from our controlled system is really temperature θ. For introduction of zero initial conditions we will think over it’s deviation from any steady value, e.g. θ0, so Δθ = θ - θ0 and dimensionless expression ofvariable y (dimensionless temperature) we get dividing this deviation with the steady value, then
y = Δθ /θ0 = (θ -θ0)/θ0. Similarly are expressed the other variables of control loop. At nonlinear controlled systems we use as a reference the value in steady state, at linear systems
we can use other reference values than in steady state.

Controlled system
Static properties of controlled systemare most often described by characteristic
curve, which give dependence of output variable on input variable in steady state. At the controlled system we relatively often meet with nonlinear shape of characteristic curve. As far as
that reality will complicate mathematical description and solution, we can in any cases approach to linearization of characteristic curve. As we think over control onconstant value, so
we can expect small deviations from working point. Than we can make linearization of characteristic curve namely either by graphical means with tangent in working point or by mathematical means with using of Taylor's expansion of function in working point. Figure 2 shows
the way of determination of characteristic curve and its linearization. Left in the picture is step...