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Application of MC200 Series Programmable Logic Controller in Sterilizing Dryer

1    Overview

    The sterilization dryer is a kind of pharmaceutical machinery, which is used to heat, sterilize and dry glassware. In the past,
the conventional temperature controller was used to control the temperature, and the small-scale PLC was used to control the mechanical
operation. With the improvement of production automation level, the temperature measurement and control data also need to be uploaded to
the PLC, touch screen or host computer, so the overall control scheme using PLC is gradually promoted.

2    System configuration diagram

    The PLC uses MC200-3232BTA, the expansion module uses MC200-4TC, and two thermocouple signals for temperature measurement are connected.
 The touch screen uses MZ600-TT07S. The following diagram shows the structure of the control system:

3    Temperature control methods and strategies

    Temperature control is the main function of sterilization dryer. Through precise control, the temperature can be maintained at a
deviation of about (+1 degree) of the set value, so as to meet the requirements of sterilization processing of medical utensils.
At the same time, the working temperature should meet the requirements as soon as possible after the start-up, and the preheating
and fluctuation time should not be too long.

    The heating elements of the dryer are composed of seven groups of infrared heating tubes, each of which is controlled by an
 AC contactor. Since the power of seven groups of infrared heating tubes is very high, it is impossible to control the temperature
by adjusting all heating tubes all the time in operation, otherwise the temperature is difficult to control and the fluctuation time
is very long.

    The following point 1 describes the sectional quick heating control, and point 2 describes the precise temperature control strategy.

        1.  The design adopts the method of quick heating in groups to speed up the preheating process.

        When the difference between set temperature and actual temperature is greater than 50 degrees, the PLC will put all
seven groups of heating pipes into the heating process. When the temperature difference between the set temperature and the actual
temperature is less than 30 degrees, the PLC will close three groups of infrared heating tubes, and the remaining four groups of
heating tubes will continue to heat. When the temperature difference between the set temperature and the actual temperature is
less than 15 degrees, the PLC will close two more groups of infrared heating tubes, at the same time, the heating process of
the last two groups of heating pipes is adjusted by PID.

        In this way, the preheating time of the system is greatly shortened and the response to fluctuations is accelerated.
The multiple temperature switching points mentioned above are determined by finding the thermal inertia of the heating pipe of the
equipment during debugging process.

        2.  PLC uses fixed heating cycle and variable heating pulse width to control the steady heating process.

        When the last two groups of heating pipes are working, the control strategy of PID tuning is realized by changing
the proportion of heating time pulse width in the fixed heating cycle.

        3.  Proportional band and proportional coefficient;

        In some commonly used temperature control strategy designs, people often introduce the concept of "proportional band"
in the PID setting. That is to say, when determining the proportional coefficient, consider the proportional adjustment first, and
then determine the proportional value with a fixed precision difference of PV-SV.

        For example, only consider the proportional adjustment, when the desired temperature difference is (±10 degrees),
the precision data is 10 degrees. When the PV is less than SV and the temperature difference is above 10 degrees, the PID output is
the maximum (100%). When the temperature difference is less than 10 degrees, the output decreases linearly, and outputs 0% until
when there is no temperature difference.

        In this case, the "proportional band" is set to 10 degrees. The proportional value KP can be calculated from this ratio.
For MC200 PLC, since the output value of PID is generally set to 0-32767, KP = 32767/10, that is 3276.7;