基于单片机的温度控制外文文献及中文翻译

Temperature Control Using a Microcontroller:

An Interdisciplinary Undergraduate Engineering Design Project

James S. McDonald

Department of Engineering Science

Trinity University

San Antonio, TX 78212

Abstract

：

This

paper

describes

an

interdisciplinary

design

project

which

was

done

under

the

author’s supervision by a group of four senior students in the Department of Engineering Science

at Trinity University. The objective of the project was to develop a temperature control system

for an air- filled chamber. The system was to allow entry of a desired chamber temperature in a

prescribed range and to exhibit overshoot and steady-state temperature error of less than 1 degree

Kelvin in the actual chamber temperature step response. The details of the design developed by

this group of students, based on a Motorola MC68HC05 family microcontroller, are described.

The pedagogical value of the problem is also discussed through a description of some of the key

steps in the design process. It is shown that the solution requires broad knowledge drawn from

several

engineering

disciplines

including

electrical,

mechanical,

and

control

systems

engineering.

1 Introduction

The design project which is the subject of this paper originated from a real-world application.

A prototype of a microscope slide dryer had been developed around an OmegaTM model

CN-390 temperature controller, and the objective was to develop a custom temperature control

system to replace the Omega system. The motivation was that a custom controller targeted

specifically for the application should be able to achieve the same functionality at a much lower

cost, as the Omega system is unnecessarily versatile and equipped to handle a wide variety of

applications.

The mechanical layout of the slide dryer prototype is shown in Figure 1. The main element

of the dryer is a large, insulated, air-filled chamber in which microscope slides, each with a

tissue sample encased in paraffin, can be set on caddies. In order that the paraffin maintain the

proper consistency, the temperature in the slide chamber must be maintained at a desired

(constant) temperature. A second chamber (the electronics enclosure) houses a resistive heater

and the temperature controller, and a fan mounted on the end of the dryer blows air across the

heater, carrying heat into the slide chamber. This design project was carried out during academic

year 1996

–97 by four students under the author’s supervision as a Senior Design project in the

Department of Engineering Science at Trinity University. The purpose of this paper is

to describe the problem and the students’ solution in some detail, and to discuss some of the

pedagogical opportunities offered by an interdisciplinary design project of this type. The

students’ own report was presented at the 1997 Nat

ional Conference on Undergraduate Research

[1]. Section 2 gives a more detailed statement of the problem, including performance

specifications, and Section 3 describes the students’ design. Section 4 makes up the bulk of the

paper, and discusses in some detail several aspects of the design process which offer unique

pedagogical opportunities. Finally, Section 5 offers some conclusions.

2 Problem Statement

The basic idea of the project is to replace the relevant parts of the functionality of an Omega

CN-390 temperature controller using a custom-designed system. The application dictates that

temperature settings are usually kept constant for long periods of time, but it’s nonetheless

important that step changes be tracked in a “reasonable” manner. Thus the mai

n requirements

boil down to

·

allowing a chamber temperature set-point to be entered,

·

displaying both set-point and actual temperatures, and

·

tracking step changes in set-point temperature with acceptable rise time, steady-state error,

and overshoot.

Although not explicitly a part of the specifications in Table 1, it was clear that the customer

desired digital displays of set-point and actual temperatures, and that set-point temperature entry

should be digital as well (as opposed to, say, through a potentiometer setting).

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 an

alog- 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 divider, 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.

μ

1 and

μ

2 are heat transfer

coefficients from the air to the box and from the box to the external world, respectively.

It’s

not hard to show that the (linearized) state equationscorresponding to Figure 4 are

Taking Laplace transforms of (1) and (2) and solving for Ta(s), which is the output of

interest, gives the following open-loop model of the thermal system:

where K is a constant and D(s) is a second-order polynomial.K, tz, and the coefficients of

D(s) are functions of the variousparameters appearing in (1) and (2).Of course the various

parameters in (1) and (2) are completely unknown, but it’s not hard to show that, reg

ardless of

their values, D(s) has two real zeros. Therefore the main transfer function of interest (which is

the one from Q(s

), since we’ll assume constant ambient temperature) can be written

Moreover, it’s not

too hard to show that 1=tp1 <1=tz <1=tp2, i.e., that the zero lies between

the two poles. Both of these are excellent exercises for the student, and the result is the openloop

pole- zero diagram of Figure 5.

Obtaining a complete thermal model, then, is reduced to identifying the constant K and the

three unknown time constants in (3). Four unknown parameters is quite a few, but simple

experiments show that 1=tp1 _ 1=tz;1=tp2 so that tz;tp2 _ 0 are good approximations. Thus the

open-loop system is essentially first-order and can therefore be written

(where the subscript p1 has been dropped).

Simple open-loop step response experiments show that,for a wide range of initial

temperatures and heat inputs, K _0:14 _=W and t _ 295 s.1

4.2 Control System Design

Using the first-order model of (4) for the open-loop transfer function Gaq(s) and assuming

for the moment that linear control of the heater power output q(t) is possible, the block diagram

of Figure 6 represents the closed-loop system. Td(s) is the desired, or set-point, temperature,C(s)

is the compensator transfer function, and Q(s) is the heater output in watts.

Given this simple situation, introductory linear control design tools such as the root locus

method can be used to arrive at a C(s) which meets the step response requirements on rise time,

steady-state error, and overshoot specified in Table 1. The upshot, of course, is that a

proportional controller with sufficient gain can meet all specifications. Overshoot is impossible,

and increasing gains decreases both steady-state error and rise time.

Unfortunately, sufficient gain to meet the specifications may require larger heat outputs than

the heater is capable of producing. This was indeed the case for this system, and the result is that

the rise time specification cannot be met. It is quite revealing to the student how useful such an

oversimplified model, carefully arrived at, can be in determining overall performance

limitations.

4.3 Simulation Model

Gross performance and its limitations can be determined using the simplified model of

Figure 6, but there are a number of other aspects of the closed-loop system whose effects on

performance are not so simply modeled. Chief among these are

·

quantization error in analog-to-digital conversion of the measured temperature and

·

the use of PWM to control the heater.

Both of these are nonlinear and time-varying effects, and the only practical way to study

them is through simulation (or experiment, of course).

Figure 7 shows a SimulinkTM block diagram of the closed-loop system which incorporates these

effects. A/D converter quantization and saturation are modeled using standard Simulink

quantizer and saturation blocks. Modeling PWM is more complicated and requires a custom

S-function to represent it.

This simulation model has proven particularly useful in gauging the effects of varying the

basic PWM parameters and hence selecting them appropriately. (I.e., the longer the period, the

larger the temperature error PWM introduces. On the other hand, a long period is desirable to

avoid excessive relay “chatter,” among other things.) PWM is often difficult for students to grasp,

and the simulation model allows an exploration of its operation and effects which is quite

revealing.

4.4 The Microcontroller

Simple closed-loop control, keypad reading, and display control are some of the classic

applications of microcontrollers, and this project incorporates all three. It is therefore an

excellent all-around exercise in microcontroller applications. In addition, because the project is

to produce an actual packaged pro

totype, it won’t do to use a simple evaluation board with the

I/O pins jumpered to the target system. Instead, it’s necessary to develop a complete embedded

application. This entails the choice of an appropriate part from the broad range offered in a

typical microcontroller family and learning to use a fairly sophisticated development

environment. Finally, a custom printed- circuit board for the microcontroller and peripherals must

be designed and fabricated.

Microcontroller Selection.

In view of existing local expertise, the Motorola line of

microcontrollers was chosen for this project. Still, this does not narrow the choice down much. A

fairly disciplined study of system requirements is necessary to specify which microcontroller, out

of scores of variants, is required for the job. This is difficult for students, as they generally lack

the experience and intuition needed as well as the perseverance to wade through manufacturers’

selection guides.

Part of the problem is in choosing methods for interfacing the various peripherals (e.g., what

kind of display driver should be used?). A study of relevant Motorola application notes [2, 3, 4]

proved very helpful in understandingwhat basic approaches are available, and what

microcontroller/peripheral combinations should be considered.

The MC68HC705B16 was finally chosen on the basis of its availableA/D inputs and

PWMoutputs as well as 24 digital I/O lines. In retrospect this is probably overkill, as only one

A/D channel, one PWM channel, and 11 I/O pins are actually required (see Figure 3). The

decision was made to err on the safe side because a complete development system specific to the

chosen part was necessary, and the project budget did not permit a second such system to be

purchased should the first

prove inadequate.

Microcontroller Application Development.

Breadboarding of the peripheral hardware,

development of microcontroller software, and final debugging and testing of a custom

printed-circuit board for the microcontroller and peripherals all require a development

environment of some kind. The choice of a development environment, like that of the