Chapter 2: Introduction to MicroprocessorBased Control

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Chapter 2Introduction to Microprocessor-Based
Control

• Adapted fromKilian, C. T. (2001), Modern
  Control Technology Components and SystemsDelmar

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Objectives

• Understand what a microprocessor is, what it
  does, and how it works.
• Understand the concepts of RAM and ROM computer
  memory and how memory is accessed via the address
  and data buses.
• Understand how parallel and serial data
  interfaces work.
• Perform relevant calculations pertaining to
  analog-to-digital converters and
  digital-to-analog converters.
• Understand the principles of digital controller
  software.
• Recognize and describe the characteristics of the
  various types of available digital controllers,
  that is, microcontrollers, single-board
  computers, programmable logic controllers, and
  personal computers.

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Introduction

• Microprocessors ushered in a whole new era for
  control systems electronics.
• Microprocessors require additional components to
  be useful RAM, ROM, etc.

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• Microcontrollers are essentially microprocessors
  with built-in features to be used independently.

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Reasons for Microprocessor Control

• Low-level signals converted to digital can be
  transmitted long distances error free.
• Micro can handle complex calculations.
• Memory is available for tracking and storage.
• Loading new programs for control change is easy.
• Easily connected to networks.

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• A computer is made up of four basic blocks
• Central Processing Unit (CPU)
• Does the actual computing.
• Arithmetic Unit performs math and logic
• Control Manages flow of data
• Memory Data is contained in memory locations at
  specified addresses.
• RAM volatile, read/write memory
• ROM nonvolatile, read only
• EPROM/EEPROM/Flash Erasable ROM

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• Input/Output ports Used for connections to
  devices.
• Busing
• Devices are multiplexed using 3 major buses
• Address Bus To specify the device or memory
  location to communicate with.
• Data Bus To transfer data between the CPU and
  device.
• Control Bus Timing and event control, such as
  read and write operations.

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Microprocessor Instructions Op-Codes

• Each processor has its own instruction set of
  commands to control its operation.
• Move data
• Perform math operations
• Perform logical operations
• Each instruction has a unique Op-code, a binary
  value associated to it.01001101 or 4Dh.
• An Accumulator is staging area for data data is
  moved into it, and operations are performed on
  that data.

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Machine Code/Mnemonics/PC

• Machine Code
• The program the CPU follows represented in binary
  or hex.
• Mnemonics
• Abbreviations representing an op-code. Programs
  written in assembly language use mnemonics.
• Program counter
• Used to point to the memory address of the
  instruction to be performed.
• Fetch-execute cycles
• Performed to bring an instruction into memory and
  execute it.

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• From another text

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DAC - Parallel

• Parallel Interface Transfers 8-bits (or more) at
  once.
• Digital-to-Analog Converter (DAC) converts
  8-digital data to analog.

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DAC Formula Resolution

• Vout Input x Vref 256 (for
  8-bit)
• Vout DAC output analog voltage
• Input Decimal value of binary input
• Vref Reference DC voltage
• Resolution
• The worst case error introduced when converting.
  In an 8-bit DAC, there are 255 possible steps.
  The resolution is the smallest step size, or
  1/255, 0.39.

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• A DAC has a 5V reference with a binary input of
  10010100, calculate the voltage output.
• If the binary input were 11111111?

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ADC

• Analog-to-Digital converter (ADC)A circuit that
  converts an analog voltage to digital.

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ADC

• Conversion Time The time required to convert an
  analog voltage to digital.
• For an 8-bit ADC
• Output Vin x 255 Vref
• Resolution in is 1/255 x 100 (for an 8-bit
  ADC).

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• The smallest step change in voltage for a DAC or
  ADC is the voltage resolution how closely a
  voltage can be resolved due to the digital
  quantization
• With a 5V reference, 1 LSB 5V/255 19.5mV
  5V x .39 .00195V)
• ADCs and DACs have a resolution error of ½ LSB.
• ½ LSB 9.7 mV
• In a ADC, the input voltage could be /- 9.7mV.

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• Given a 10-Bit ADC with a 5V reference,
  calculate
• The Resolution
• The LSB Value (volts)
• The ½ LSB Value (volts)
• The digital output for an input of voltage of
  3.2V

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• A 0 to 2000 PSI pressure sensor has an output of
  0 to 5V. This voltage feeds an 8-bit ADC with a
  Vref of 5V.
• Draw a diagram and a graph.
• What would be the resolution of the ADC in PSI?
• What would be the transfer function from the
  input pressure to the digital output?
• Given an input of 1250PSI, what would be the
  output of the ADC?
• What equation would convert the ADC back to PSI
  in the controller?

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Serial Interface

• Data is sent 1 bit at a time.
• Reduces number of cables or lines
• More easily shielded from noise.
• Existing data lines may be used (phone).
• Parallel data must be converted to serial to
  transmit, and vice-versa on receive.
• A UART (Universal Asynchronous Transmitter
  Receiver) is a device which performs this
  conversion.

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Asynchronous Transmission

• Data is sent with defined timing, termed a BAUD
  rate. 2400,9600, 19200, etc. Start Bit Stop
  Bit are used to frame the signal.
• A parity bit is used optionally for error
  detection.
• Common settings 9600 Baud, 8 bits, no parity, 1
  stop-bit -- 9600 8-N-1

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RS-232

• RS-232 is a specification which defines standard
  for serial interfaces between DTEs (Data Terminal
  Equipment Computers), and DCEs (Data
  Communication Equipment Modems, etc).
• DTE to DTE communications can be performed
  serially using a cross-over or Null-Modem cable.

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Synchronous Communications

• Unlike asynchronous, which relies on the timing
  of the data, with synchronous communications the
  clocks of the 2 devices are locked together.
• One means is to use a separate clock line to
  indicate individual bit positions.

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Networking

• Multiple devices are connected together.
• Serial data is passed between devices.
• Devices are provided individual address numbers
  to send data to a particular device.

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Controller Programmer

• Real Time control Program runs in a loop,
  sensing the current condition and calculating new
  output to the actuator.
• Each pass through the program is an iteration or
  scan.

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• The frequency at which new data is collected is
  the sampling rate (scan time).
• Time-delay loops may be inserted to slow the
  execution or scan time.
• Programs can be written at the lowest level
  (machine code, assembler) or high level languages
  (C), BASIC, etc.

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Microcontrollers

• A single-chip computer specifically designed for
  I/O control.
• On board RAM, ROM, possibly timers and ADCs.
• High speed is not required due to low complexity
  of tasks.
• Very large cost savings over microcomputers.
• Motorola 68HC11, Intel 8051, PIC 16C72, Atmel
  AVR, BASIC Stamp

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• BASIC Stamp

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Single-Board Computers

• A computer on a single board. Programmable for
  I/O control and the ability to use high level
  peripherals.

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Programmable Logic Controllers

• Self-contained microprocessor based controller.
• Designed for fast connection and control of
  processes.
• Used extensively in industrial control
  environments.
• Programs in relay-logic to be compatible to the
  more traditional electrical workforce.

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Personal Computers

• PCs with dedicated I/O and data acquisition cards
  and specialized software may be used as
  controllers.

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Objectives Review

• Understand what a microprocessor is, what it
  does, and how it works.
• Understand the concepts of RAM and ROM computer
  memory and how memory is accessed via the address
  and data buses.
• Understand how parallel and serial data
  interfaces work.
• Perform relevant calculations pertaining to
  analog-to-digital converters and
  digital-to-analog converters.
• Understand the principles of digital controller
  software.
• Recognize and describe the characteristics of the
  various types of available digital controllers,
  that is, microcontrollers, single-board
  computers, programmable logic controllers, and
  personal computers.