Humidity Controller by D928wS

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									  Humidity Controller
ECE 345 Final Project Proposal

 Submitted by: Gary Stoedter

      TA: Lee Rumsey

     September 15,1999
Introduction

        I propose to design a simple humidity controller for use in the Air Conditioning
Research Center (ACRC) environmental test chambers. The current humidification
systems consist of a sensor for temperature or humidity, a commercial PID controller, a
solid state relay, and a heating element. To better understand the problems to be
addressed in my design, a short description of the operation of the current system is
needed.

        The operation of the system is as follows: The signal generated by the sensor is
fed into the PID controller. The sensor's signal either represents the actual relative
humidity, or the dew point temperature in the chamber. The PID controller sends a signal
to the solid state relay. In most cases this is an ON/OFF signal. Some of the controllers
have the ability to generate a 4-20mA signal to send to the relay. The solid state relay
"closes" or "opens" in response to the PID controller's signal. The action of the relay
causes current to flow to a heating element. This heating element is immersed in a
container of water. The water will boil and produce steam that is then piped into the
environmental chamber. Adding or removing power from the heating element allows
indirect control of the humidity in the environmental chamber. The power into the
heating element is measured using a watts-transducer and recorded by a data acquisition
system. This power is then used in energy balance calculations used by the
experimenters. As you would expect, a reasonable amount of precision is required.

        These commercial controllers, although they allow operation of the facilities for
experiments, do not perform the task of humidification as well as the experimenters
would like them to. There are three main issues to be considered in improving upon the
existing systems that I will address in my design.

        First, the ability of the commercial controllers to associate a specific power rating
to the heating element with a specific relative humidity is limited. This is because these
controllers are designed to control temperature-not humidity. Because the humidity is
controlled indirectly through a temperature measurement, the commercial controller has a
difficult time finding the correct time constant needed for precision operation.

        Second, in certain systems, the ability of the data acquisition system to acquire
data exceeds the ability of the controller to effect a change. For example, one system
uses a solid state relay that "skips" cycles of the 60 Hertz power signal in order to effect
power control of the heating element. When the set point and the actual humidity are
nearly equal, the number of cycles "skipped" grows. In some cases the relay may be on
only 1% of the time, meaning 99 out of 100 cycles would be skipped on the 60 Hz line.
If the data acquisition system polls the watts-transducer for the humidifier during one of
the skipped cycles, it will register either zero watts or some nonsensical number (i.e. a
negative number). Thus the data is in error and the experimental results become
unusable.
        Third, the cost of the current systems is excessive for the performance they
provide. The system mentioned in the previous paragraph cost over five hundred dollars
for controller and solid state relay. Implementation costs drove this system to well over a
thousand dollars. Clearly a cheaper alternative needs to be found.

Design Considerations

      Referring to the block diagram on the last page, you can see that the design is
made up of several parts.

       Input will be selectable by the user. The choices will be 4-20mA or 0-5 VDC.
This accurately reflects the signal capabilities of the sensors currently in use by the
ACRC.

       Signal conditioning will be used (if necessary) to make the signal input from the
sensor useable by the comparator segment.

        Comparator-this section will compare the set point signal to the actual signal of
the sensor.

          Control-this section will send the control signal to the control element for the
heater.

        Reference-this section will consist of a power supply and a potentiometer to set
the desired setpoint.

        Element control-this section will basically consist of an SCR to limit power flow
to the heating element.

Specifications

       The specifications used to evaluate this design will be fairly subjective. It is
intended to prove compliance by using a current laboratory setup at the ACRC to

								
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