3 System Design

The requirements for digital temperature displays and setpoint entry alone are enough to dictate that a microcontrollerbased design is likely the most appropriate。 Figure 2 shows a block diagram of the students’ design。 

The microcontroller, a MotorolaMC68HC705B16 (6805 for short), is the heart of the system。 It accepts inputs from a simple four-key keypad which allow specification of the set-point temperature, and it displays both set-point and measured chamber temperatures using two-digit seven-segment LED displays controlled by a display driver。 All these inputs and outputs are accommodated by parallel ports on the 6805。 Chamber temperature is sensed using a pre-calibrated thermistor and input via one of the 6805’s analog-to-digital inputs。 Finally, a pulse-width modulation (PWM) output on the 6805 is used to drive a relay which switches line power to the resistive heater off and on。

Figure 3 shows a more detailed schematic of the electronics and their interfacing to the 6805。 The keypad, a Storm 3K041103, has four keys which are interfaced to pins PA0{ PA3 of Port A, configured as inputs。 One key functions as a mode switch。 Two modes are supported: set mode and run mode。 In set mode two of the other keys are used to specify the set-point temperature: one increments it and one decrements。 The fourth key is unused at present。 The LED displays are driven by a Harris Semiconductor ICM7212 display driver interfaced to pins PB0{PB6 of Port B, configured as outputs。 The temperature-sensing thermistor drives, through a voltage pider, pin AN0 (one of eight analog inputs)。 Finally, pin PLMA (one of two PWM outputs) drives the heater relay。

Software on the 6805 implements the temperature control algorithm, maintains the temperature displays, and alters the set-point in response to keypad inputs。 Because it is not complete at this writing, software will not be discussed in detail in this paper。 The control algorithm in particular has not been determined, but it is likely to be a simple proportional controller and certainly not more complex than a PID。 Some control design issues will be discussed in Section 4, however。

4 The Design Process

Although essentially the project is just to build a thermostat, it presents many nice pedagogical opportunities。 The knowledge and experience base of a senior engineering undergraduate are just enough to bring him or her to the brink of a solution to various aspects of the problem。 Yet, in each case, realworld considerations complicate the situation significantly。

Fortunately these complications are not insurmountable, and the result is a very beneficial design experience。 The remainder of this section looks at a few aspects of the problem which present the type of learning opportunity just described。 Section 4。1 discusses some of the features of a simplified mathematical model of the thermal properties of the system and how it can be easily validated experimentally。 Section 4。2 describes how realistic control algorithm designs can be arrived at using introductory concepts in control design。 Section 4。3 points out some important deficiencies of such a simplified modeling/control design process and how they can be overcome through simulation。 Finally, Section 4。4 gives an overview of some of the microcontroller-related design issues which arise and learning opportunities offered。

4。1 MathematicalModel

Lumped-element thermal systems are described in almost any introductory linear control systems text, and just this sort of model is applicable to the slide dryer problem。 Figure 4 shows a second-order lumped-element thermal model of the slide dryer。 The state variables are the temperatures Ta of the air in the box and Tb of the box itself。 The inputs to the system are the power output q(t) of the heater and the ambient temperature T¥。 ma and mb are the masses of the air and the box, respectively, and Ca and Cb their specific heats。 μ1 and μ2 are heat transfer coefficients from the air to the box and from the box to the external world, respectively。