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CHAPTER 5

DESIGN AND IMPLEMENTATION OF A PIC 16F877A

MICROCONTROLLER FOR PLASTIC EXTRUSION SYSTEM

5.1 REAL TIME HARDWARE MODEL

This chapter presents design and implementation of a complete

temperature control scheme of the plastic extruder incorporating the

embedded neuro fuzzy controller using a PIC 16F877A microcontroller.

The performance of the neuro fuzzy logic based temperature controller

for plastic extruder is investigated experimentally at different

temperature set point conditions. The experimental results show that the

embedded neuro fuzzy controller is more robust and, hence, found to be

a suitable replacement of the conventional controllers for the

temperature control of plastic extruder.

5.2 EMBEDDED SYSTEM

Embedded systems are highly specialized, often reactive sub

systems that provide information processing and control. Embedded

systems are omnipresent nowadays and make possible, the creation of

systems with a functionality that cannot be provided by human beings.

The term embedded system thus encompasses a broad class of systems,

ranging from simple microcontrollers to large and complex multi-

processor and distributed systems.

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The advent of intelligent programmable embedded design

provides the ability to implement any required hardware programmable

device for the design automation in industries and laboratories.

Laboratory and industrial automation use minimal hardware and

maximum support of software.

The process temperature is accurately controlled without

extensive operator involvement; with the use of a temperature control

relies upon a controller, which accepts a temperature sensor

thermocouple as the input. It compares the actual temperature to the

desired control temperature or set point, and provides an output to a

control element.

Microcontroller is a general purpose device, which integrates a

number of the components on to single chip. It has inbuilt central

processing unit, both read only memory and random access memory,

parallel digital I/O peripherals to make it as a mini computer.

Microcontrollers are available in different configurations, low cost and

compact size with power saving mode and high operating speeds. These

features encourages for implementing conventional and neural fuzzy

logic in an inexpensive microcontroller in the closed loop control

system.

The significant part of embedded system development is the

designing of hardware and software for the specific application. In the

present work PIC 16F877A 8-Bit microcontroller has been used to

implement the control algorithm and having special features like 32K

reprogramming flash memory, 512 bytes of internal RAM, 32

programmable I/O lines and eight interrupt sources. The microcontroller

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consists of a timer module and an analog to digital converter to accept

analogue input for data processing. To make the data flow between

controllers to other devices the serial I/O port is used.

5.3 INTRODUCTION TO PIC 16F877A CONTROLLER

PIC is a family of Harvard architecture microcontrollers made

by Microchip technology, derived from the PIC1640, originally

developed by the Microelectronics division of General Instrument. The

name PIC initially is referred to Peripheral Interface Controller. PICs are

popular with developers due to their low cost, wide availability, large

user base, extensive collection of application notes, availability of low

cost, free development tools and serial programming (and re-

programming with flash memory) capability.

The microcontroller is from PIC series. PIC microcontroller is

the first reduced instruction set computing based microcontroller

fabricated in complementary metal oxide semiconductor, that uses

separate bus for instruction and data allowing simultaneous access of

program and data memory. The main advantage of CMOS and RISC

combination is low power consumption resulting in a very small chip

size with a small pin count. The main advantage of CMOS is that, it has

immunity to noise than other fabrication techniques.

Microcontrollers offer different kinds of memories. EEPROM,

EPROM, FLASH etc. are some of the memories of which FLASH is the

most recently developed. The technology used in PIC 16F877 is FLASH

technology, so that data is retained even when the power is switched off.

Easy programming and erasing are the other features of PIC 16F877.

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The heart of the microcontroller is the CPU core. In the past, this has

traditionally been based on an 8-bit microprocessor unit. PIC micro

devices are grouped by the size of the instruction word. The three

current PIC micro families are

Base-line: 12-bit instruction word length

Mid-range:14-bit instruction word length

High-end: 16-bit instruction word length

5.3.1 Characteristics of PIC 16F877A

The PIC controller compared to other controllers is with low

cost. The clock speed of the controllers is high with the rate of 20MHz.

8Kx14 words of FLASH program memory, 368X8 bytes of data

memory (RAM), 256x8 bytes of EEPROM data memory and this is

enough for the temperature control application. At the maximum clock

rate, a PIC executes most of its instructions in 0.2 micro seconds or 5

instructions per microseconds. It has high speed in executing instruction.

The efficiency and accuracy is very high. The instruction set consists of

35 instructions. For executing a program it requires only small steps.

Power on reset and brown out protection ensure that the chip operates

only when the supply voltage is within sections. A watch timer resets

the PIC, if the chip malfunctions and deviates from its normal position.

Any one of the core clock options can be supported including a low cost

RC oscillator and a high accuracy crystal oscillator.

These versatile timers can be characterized by inputs; control

outputs and provide internal timing for program executions. The PIC

microcontroller has a number of inbuilt modules such as analog to

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digital converter, universal asynchronous transmitter and receiver that

increases versatility of microcontroller. The PIC IC (Integrated Chip) is

having wide operating voltage range from 2.5 to 6V, using power saving

devices with a less power loss.

The PIC start plus development system includes PIC start plus

development programmer and machine perception laboratory. The PIC

start plus programmer gives the product developer the ability to program

user software into any of the supported microcontrollers. The PIC start

plus software running under MP LAB provides full interactive control

over the programmer.

5.3.2 Core Architecture of PIC 16F877A

PIC controller architecture is simple. It is characterized by the

features like using separate code and data spaces. The PIC controller is

with small number of fixed length instructions and most of the

instructions are single cycle execution (4 clock cycles), with single

delay cycles upon branches and skips. The RAM location of PIC

controller function as registers, as both source and destination of math

and other functions. The data space mapped CPU, port, and peripheral

registers in PIC 16F877A microcontroller and the program counter is

mapped.

5.3.3 Data Space and Code Space of PIC 16F877A

PICs have a set of registers that function as general purpose

RAM. Special purpose control registers for on-chip hardware resources

are also mapped into the data space. The addressability of memory

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varies depending on device series, and all PIC devices have some

banking mechanism to extend the addressing to additional memory.

In other microcontrollers, the register movement is achieved

through the accumulator. External data memory is not directly

addressable. PIC code space is generally implemented as EPROM,

ROM, or FLASH ROM. In general, external code memory is not

directly addressable, due to the lack of an external memory interface.

5.3.4 Stacks of PIC 16F877A

PICs have a hardware call stack, which is used to save return

addresses. The hardware stack is not software accessible on earlier

devices, but this is changed with the updated series devices. Hardware

support for a general purpose parameter stack was lacking in early

series, but this is greatly improved in the updated series, making the

updated series architecture friendlier to high level language compilers.

5.3.5 Performance of PIC 16F877A

The clock speed is 20MHz. The TACQ or acquisition time is

19.2 µs. The architectural decisions are directed at the maximization of

top-end speed, or more precisely of speed to cost ratio. PIC architecture

was among the first scalar CPU designs, and is still among the simplest

and cheapest. The Harvard architecture in which instructions and data

come conveniently from separate sources simplifies timing and

microcircuit design greatly.

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PIC is particularly suited to the implementation of fast lookup

tables in the program space. The optimization is facilitated by the

relatively large program space of PIC and by the design of the

instruction set, which allows for embedded constants. The simplicity of

PIC, and its scalar nature, also serves to simplify significantly the

construction of real time code. It is typically possible to multiply the line

count of a PIC assembler listing by the instruction cycle time to

determine execution time. The delay is constant even though

instructions can take one or two instruction cycles, a dead cycle is

optionally inserted into the interrupt response sequence to make the

delay. External interrupts have to be synchronized with the four clock

instruction cycle. Internal interrupts are already synchronized.

5.4 NEURO FUZZY CONTROLLER IMPLEMENTATION

The conventional and neuro fuzzy controllers are simulated in

MATLAB/Simulink. The neuro fuzzy logic controllers have shown

excellent results in MATLAB/Simulink, especially when used with

nonlinear control systems. The Figure 5.1 shows the block diagram of

the temperature control of the plastic extruder. The temperature sensing

device senses the temperature and amplifier increases the signal. The

amplified signal is converted to digital signal given as the input to the

microcontroller and compared to the set point temperature. The error

signal drives the driver and the heater turns on or turns off. Figure 5.2

shows the photograph of the two stage plastic extruder with gear setup.

The photograph of the plastic extruder is with two stage heaters, gear

setup and an induction motor. Thermocouples are attached with the

heater and sensing the temperature. Microcontroller produces control

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signal to the heater depends on the error signal. The circuit diagram for

the PIC 16F877A controller based temperature controller model for the

plastic extruder is shown in Figure 5.3.

Figure 5.1 Block Diagram of the PIC 16F877A Temperature

Controller for the Plastic Extruder

Figure 5.2 Photograph of the Two Stage Plastic Extruder with Gear

Setup

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Figure 5.3 Circuit Diagram for the PIC 16F877A Temperature

Controller for the Plastic Extruder

5.4.1 Power Supply

The power supply circuits are built using rectifiers, filters and

voltage regulators. Starting with an ac voltage, a steady dc voltage is

obtained by rectifying the ac voltage after filtered to a dc level and

finally regulated to obtain a desired fixed dc voltage. The regulation is

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usually obtained from an IC voltage regulator unit, which takes dc

voltage and provides sufficient lower dc voltage, which remains the

same even if the input dc voltage varies, or the output load connected to

the dc voltage changes.

The ac voltage, typically 120 V rms, is connected to a

transformer, which steps the ac voltage down to the level for the desired

dc output. A diode rectifier then provides a full-wave rectified voltage

that is initially filtered by a simple capacitor filter and smoothing of dc

voltage. This resulting dc voltage usually has some harmonics.

A regulator circuit is used for this dc input to provide a dc

voltage that has not only much less ripple voltage but also remains the

same dc value even if the input dc voltage varies to some extent, or the

load connected to the output dc voltage changes. This voltage regulation

is usually obtained using one of the popular voltage regulator IC units.

Voltage regulators comprise a class of widely used integrated

circuit. Regulator IC units contain the circuitry for reference source,

comparator amplifier, control device, and overload protection all in a

single IC. The internal construction of the IC is different from that

described for discrete voltage regulator circuits; the external operation is

mostly the same. IC units provide regulation of either a fixed positive

voltage, a fixed negative voltage, or an adjustable set voltage. The

regulators can be selected for operation with load currents from

hundreds of milli Amperes to tens of Amperes, corresponding to power

ratings from milli watts to tens of watts.

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5.4.2 Thermocouple

The temperature sensor used for the measurement is J type

thermocouple. A practical thermocouple consists of two wires of

dissimilar metals that are electrically joined. Thermoelectric voltage is

produced by the temperature gradient along the thermocouple wires. A

thermocouple is a junction between two different metals that produces a

voltage related to a temperature difference. Thermocouples are a widely

used type of temperature sensor for measurement and control and can

also be used to convert heat gradient into electricity.

They are inexpensive and interchangeable and are supplied

along with standard connectors that can measure a wide range of

temperatures. Thermocouple output signals are typically in the millivolt

range and generally have a very low temperature to voltage sensitivity.

Two temperature sensors used here is J type thermocouple.

Thermocouples are still the cheapest method for measuring high

temperatures, and accuracy of even ±5 Degree Celsius is often not a

problem when measuring temperatures greater than several hundred

Degree Celsius.

Junction of dissimilar metals will produce an electric potential

related to temperature. Thermocouples for practical measurement of

temperature are junctions of specific alloys which have a predictable and

repeatable relationship between temperature and voltage.

Thermocouples are widely used in industry; applications include

temperature measurement for kilns, gas turbine exhaust, diesel engines,

and other industrial processes.

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The conductor is subjected to a thermal gradient. It will

generate a voltage known as the thermoelectric effect or Seebeck effect.

Using a dissimilar metal to complete the circuit creates a circuit in

which the two legs generate different voltages, leaving a small

difference in voltage available for measurement. That difference

increases with temperature, and is between 1 and 70 microvolt’s per

Degree Celsius (µV/°C) for standard metal combinations.

For typical metals used in thermocouples, the output voltage

increases almost linearly with the temperature difference (∆T) over a

bounded range of temperatures. For precise measurements or

measurements outside of the linear temperature range, non-linearity

must be corrected. The nonlinear relationship between the temperature

difference (∆T) and the output voltage (mV) of a thermocouple can be

calculated by a polynomial shown in Equation (5.1)

N

n n

n 0

T a U=

∆ =∑ (5.1)

The coefficients are given for n from 0 to between 5 and 13

depending upon the metals. In some cases, better accuracy is obtained

with additional non polynomial terms. In modern equipment the

equation is usually implemented in a digital controller or stored in a look

up table and older devices use analog circuits.

Type K (chromel{90 percent nickel and 10 percent

chromium}–calomel)(Calomel consisting of 95% nickel, 2%

manganese, 2% Aluminum and 1% silicon) is the most common general

purpose thermocouple with a sensitivity of approximately 41 µV/°C,

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chromel positive relative to calomel. It is inexpensive, and a wide

variety of probes, available in its −200 °C to +1350 °C / -328 °F to

+2462 °F range. Type K was specified at a time when metallurgy was

less advanced than it is today, and consequently characteristics may vary

considerably between samples. One of the constituent metals is nickel.

The characteristics of thermocouples made with magnetic

material are that they undergo a step change in output, when the

magnetic material reaches its Curie point. Type E (chromel–constantan)

has a high output (68 µV/°C) which makes it well suited to cryogenic

use. Additionally, it is non-magnetic. Type J (iron–constantan) has a

more restricted range than type K (−40 to +750 °C), but higher

sensitivity of about 55 µV/°C. The Curie point of the iron (770 °C)

causes an abrupt change in the characteristics, which determines the

upper temperature limit.

The temperature range of J type Thermocouple is 0°C To 724°C.

The temperature range of K type Thermocouple 0°C To 1260°C. Both

types are producing same accuracy output. The experimental set up is

using J type thermocouple. Cost of J and K type thermocouples are

same. The temperature used in the research work is 70°C to 200°C. So, J

type Thermocouple is chosen for the research work.

5.4.3 Ultra Offset Operational Amplifier

The sensor output is very low voltage and amplified for data

acquisition. The amplification is achieved through the ultra offset

operational inverting amplifier. This is achieved using Op07 amplifier.

Figure 5.4 shows the circuit diagram of the operational amplifier. The

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OP07 has very low input offset voltage (75 µV max for OP07E) that is

obtained by trimming at the wafer stage. The OP07 also features low

input bias current (±4nA for the OP07E) and high open-loop gain (200

V/mV for the OP07E). Figure 5.5 shows the photograph of the

temperature sensing and operational amplifier.

Figure 5.4 Circuit Diagram of the Operational Amplifier

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Figure 5.5 Photograph of the Temperature Sensing and

Operational Amplifier

5.5 CIRCUIT OPERATION

The temperature measurement by the thermocouple is

amplified and converted to digital signal using analog to digital

converter. The amplified and converted output is given as input to the

port A pin of PIC16F877A microcontroller. The microcontroller

PIC16F877A set point temperature is compared with the measured

value. The microcontroller has inbuilt ADC. This converts analog signal

to 8/10 bit digital signal.

According to the error, the control signal is generated and

controls the respective driver. The driver actuates the respective heater.

The relay is connected to the microcontroller, so the heater switching

can be controlled. The relay is not connected directly to the

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microcontroller because the outputs are not sufficient to drive, so, relay

driver is used.

The driver is used as the control actuating device for the

heater. The relay with 12V is used, that cannot be controlled by the

microcontroller directly. Upper limit of normal 2003 (14pin dual inline

package) relay driver IC is used to drive relay. The keypad is used to set

the temperature values. The rating of the heater is with 1.0KW. The real

time implementation takes two reference set point temperatures 70oC,

100oC for the plastic extrusion model.

Push button switch is used as the keypad and with the pull up

resistors (10 Kilo Ohms) the switch is connected to the microcontroller.

When the button is pressed the port is pulled down and pin to 0v or

ground. The liquid crystal display is interfaced with the microcontroller

and act as a display element.

The display is used to know the set point and current

temperature values. LCD control pins such as E, RS and RW are

connected to microcontroller, so the controlling data displays the status

of the LCD. PIC controller is interfaced with LCD through the 8 bit

mode. Figure 5.6 shows the photograph of the temperature controller set

up for plastic extruder. Figure 5.7 shows the photograph of embedded

controller setup.

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Figure 5.6 Photograph of the Temperature Controller Set Up For

Plastic Extruder

Figure 5.7 Photograph of Embedded Controller

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5.6 RELAY DRIVER CIRCUIT

The ULN2803A is a monolithic high voltage, high current

Darlington transistor array. The device consists of eight NPN Darlington

pairs that feature high voltage outputs with common cathode clamp

diodes for switching inductive loads. The collector current rating of each

Darlington pair is 500 mA.

ULN2803 is an integrated circuit chip with eight Darlington

pair. So 8 relay can be derived. Relay driver IC (ULN2803) is used to

drive relay, since the output voltage from microcontroller will be 5V and

source & sink current will be only 25mA and that is not sufficient to

drive relay. The output from relay driver was about 12V and the

collector current rating is about 500mA that is sufficient to drive the

relay.

The Darlington pairs are parallelly connected for higher

current capability. They are used for relay drivers, hammer drivers, lamp

drivers, display drivers, line drivers, and logic buffers. The ULN2803A

has a 2.7 Kilo Ohms series base resistor for each Darlington pair for

operation directly with TTL or 5-V CMOS devices. The ULN2803A is

packed in a standard 18-pin dual in-line (N) package. The driver is used

as the control actuating device for the heater.

5.7 INTERFACE HARDWARE TO PIC16F877A

The HD44780-LCD display (16x2) used for the displaying the

characters. The data transmitted in either two 4 bit operations or one 8

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bit operation, thus allowing interfacing with 4 or 8 bit MCUs. The

display interfaced in 4 bit mode using (D4-D7) data pins and RS, R/W,

EN were the control pins used to control the display to initialize and

write the data over the display. EN pin used to enable the LCD to

initialize and write the data over RAM or to read internal register such

as busy flag. The register selects RS used to select instruction register or

data register.

5.8 MPLAB SOFTWARE AND APPRAISAL

The neuro fuzzy temperature controller for the plastic extruder

using embedded microcontroller is implemented through the PIC

16F877A microcontroller. The control algorithm has been written in

Hi Tech C code in MPLAB software. After developing the application

program, it has been downloaded to selected target machine.

5.8.1 Implementation Steps

• Initially ports of 16F877A are configured using TRIS registers.

This is to choose I/O pins and analog input pins.

• The ADC is configured for using internal clock as source for

conversion. Two analog inputs are given to A/D converter of

microcontroller.

• Sampled digital data read from ADRESH, ADRES registers.

• T1 and T2 temperature set through the keyboard.

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• The measured temperature compared with the set value. Depend

upon the difference the relay is on or off the heater. Precision of

the temperature control is high because of 8bit digital output with

low acquisition time.

The set points temperatures 70oC, 100

oC are applied for the

experimental setup. Relay method is used for controlling the operation

of the heater. The performance of the developed embedded conventional

and neuro fuzzy control system has been tested experimentally to

regulate the temperature of a plastic extrusion system.

The temperature control system is built and tested to evaluate

the performance of a novel neuro fuzzy logic control algorithm. From

the experimental results, it has been seen that the controller performance

is superior to the already existing controllers. The neuro fuzzy controller

violates the values beyond boundary and limits the set point. The

controller is programmed for define set of values and attains the set

point temperature quickly; damping ratio is very less and closing to the

MATLAB/Simulink results. The experimental results show the

controller algorithm regulates the temperature without any overshoot,

unlike the conventional controllers. The experimental results for the

neuro fuzzy controller are shown in Figure 5.8.

The MATLAB/Simulink and experimental results are similar

to each other. This means the model of the plastic extruder behaves

similar to the real plastic extruder. Naturally, the experimental graphical

results are not as smooth as the MATLAB/Simulink graphical results.

Two set point temperatures are applied as 70oC, 100

oC settling

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temperature for experimental results. Relay method neuro fuzzy

controller enables the best performance with time domain specifications.

In hardware relay method, neuro fuzzy controller gives the best response

among all the controllers for both MATLAB/Simulink and experiments.

Figure 5.8 Experimental Results for the Neuro Fuzzy

Controller

5.9 ENCAPSULATION

In this chapter, a temperature controller for a plastic extruder

using a digital controller has been developed and implemented. The

controller employs neuro fuzzy logic digital temperature controller. The

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experimental hardware results are composed with the

MATLAB/Simulink results.

Experimental results in this chapter show that the temperature

control for the various temperature set points can be achieved

effectively. From the results, it is shown that almost perfect temperature

control can be achieved at the different temperature set points. The

transient is not present in both the hardware and MATLAB/Simulink,

and should also influence the temperature control on the set point

temperatures.

It is concluded that digital neuro fuzzy controllers are effective

in dealing with the highly nonlinear characteristics of the temperature

control for the plastic extrusion system. The hardware significantly

decreases at runtimes. The controller provides a quick, accurate set point

tracking with reference temperature. For the different temperature set

points the hardware results data are close to the MATLAB/Simulink

results. The hardware neuro fuzzy controller relay method gives the best

response among all the controllers in experiments.