Masters Theses

Date of Award

3-1984

Degree Type

Thesis

Degree Name

Master of Science

Major

Electrical Engineering

Major Professor

James G. Barrett

Abstract

This thesis presents the design of a temperature controller which controls the firing of a kiln. The standard industrial temperature controller is designed to maintain a constant temperature close to a set point selected by the operator. A kiln cannot be fired independently with such a control. During the firing of earth materials, water of combination, carbon dioxide and other gasses are released at about 500 °C to 550 °C (the exact range depending on the materials). This process requires not a rapid seeking of temperature but a controlled rate of increase in temperature to the point of maturity after which the heat is decreased. In some specialized firings one or more soak periods may be required during which the temperature is held constant to achieve more complete vitrification. In most firings the decrease in temperature is not controlled because the kiln heat capacity is such that the decrease is not of excessive rate. Too great a rate in firing or cooling can produce a defective product. From empirical data the optimum rate for firing seems to be about 150 °C per hour.

The kilns used were refractory lined and heated by nichrome elements. These elements changed kiln temperature slowly enough that a controller with a time proportional output with a cycle rate of about twenty seconds was short enough that no thermal shock was transferred to the contents.

Firing speed was determined by a counter connected to line frequency which generated the controller ramp point. The ramp point was a voltage increasing linearly with time.

The controller adjusted output to maintain the temperature signal equal to the ramp point. The controller amplifier gain needed to be fairly high to keep the kiln temperature tracking closely to the ramp point during various stages of firing. An auto reset integrator was included to minimize accumulated tracking errors.

The first step in this design was to mathematically model the kiln response. Then a controller was designed to operate the kiln properly. The controller was built and tested. The controller has been operating satisfactorily.

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