Mechatronics, Sensors and PLC

• The file contains information about Mechatronics, Sensors, Transducers, PLC and Electrical Actuation Systems

Introduction to Mechatronics

• Mechatronics is a combination of mechanics, electronics, computing and control engineering
• A system has an input and an output, and its internal workings are not the main concern

Basic Elements of a Mechatronic System

• Digital and analog sensors and actuators connect to a mechanical system, controlled by a microprocessor
• Control systems use components to maintain system performance close to desired specifications

Modelling Systems

• Modeling System response to an input signal is not instantaneous
• Measurement systems consist of a sensor, a signal conditioner, and a display system
• A sensor responds to the measured quantity and outputs a related signal
• A signal conditioner adapts the sensor signal for display or control
• A display system presents the conditioned signal for display or control

Feedback Control

• Feedback control involves feeding output signals back to adjust the system, restoring it to a normal value

Open & Closed Loop Systems

• Open Loop Systems lack feedback to adjust and maintain constant output
• Closed Loop Systems use feedback to adjust the input based on the required output value

Basic Elements of a Closed-Loop System

• Comparison Element compares the required value with the achieved value
• Control Element decides action based on the error signal
• Correction Element changes the process to correct the controlled condition
• Process Element is the system being controlled
• Measurement Element produces a signal related to the condition being controlled

Analog & Digital Systems

• Analog Systems use continuous-time signals whose size measures the variable
• Digital Systems use on/off signals, and the variable's value is represented by on/off pulse sequences

Programmable Logic Controller

• A Programmable Logic Controller (PLC) is a user-programmable system for control logic, sequencing, timing, and data manipulation

Sensors & Transducers

• Sensing is the process of collecting information about the world

Performance Terminology

• Range and Span is the max input value minus the min value
• Error: difference between measured and true value
• Accuracy indicates how correct a measurement system is

Input Devices

• Toggle Switch devices uses a lever to make/break electrical connections
• Limit Switch detects when system elements reach a certain position
• Reed Switches sense position using an actuating magnet

Proximity Sensors

• Proximity Sensors detect object presence using electromagnetic fields, light or sound

Types of Proximity Sensors

• Optical (Photoelectronic) Proximity Sensors are versatile
• Though-beam sensors have long sensing distances except with very transparent material

Optical Sensors

• Retro-reflective sensors have medium sensing distance, unsuitable for reflective or transparent targets
• Diffuse sensors have short sensing distance, dependent on color and surface reflectivity

Proximity Sensors

• Inductive Proximity Sensors use an alternating current coil to produce an oscillating magnetic field
• Capacitive Proximity Sensors detect any object with no contact
• Ultrasonic (Sonar) Sensors use reflected 20kHz sound waves and work for transparent targets

Sonar vs IR Sensors

• Sonar has a wider detection cone and longer range than IR sensors
• Sonar devices can interfere with each other

Ladder Logic (Introduction to PLC)

• A PLC is a digital apparatus with programmable memory for implementing control functions
• General Motors developed PLC in 1968
• PLCs need to be easy to program, avoid rewiring, be smaller, cheaper and highly reliable
• PLCs should have simple construction, low maintenance and low cost

PLC Sizes

• Small PLCs have up to 128 I/Os and 2Kb of memory, for simple to advanced machine controls
• Medium PLCs have 2048 I/Os and 32Kb of memory
• Large PLCs are sophisticated, with up to 8192 I/O and 750Kb memory for complex control

PLC Hardware Design

• Processor Architecture is part of PLC Hardware Design

Design of Basic Microcomputer

• A Basic Microcomputer includes CPU, ROM, RAM, Input, and Output connected via buses
• Central Processing Units (CPU) controls/supervises operations within a PLC
• Memory Unit stores programs

Types of Memory

• RAM (Random Access Memory) data can be read from and written to, is used for user programs, but the contents are lost without power
• ROM (Read Only Memory), data can only be read, and is for system programs; its content is saved even with power failures

PLC Parts & Functions

• Input Unit connects external input devices
• Output Unit connects external output devices to the CPU within PLC
• Console's LCD Display is used with Mnemonic Code Programming
• Monitor Screen is uses software programming
• Housing Unit protects PLC circuits
• Programming Unit is the programming console and or computer
• Secondary Storage is related to CPU where information is kept
• Power Supply provides DC power
• Printing Unit prints control system programming

Ladder Diagrams

• Ladder Diagrams document industrial control logic systems
• Ladder Diagrams consist of vertical power rails and horizontal rungs that are control circuits
• Ladder Diagram programming language originated from electrical control system design using relays
• Relays were replaced with logic circuits and CPUs

Analogue to Digital Signal

• Devices are wired to buffer transistors; control performed via programming
• Relay Logic Representation makes program creation easier

Operations on Ladder Logic

• Series Operation requires all inputs to be true for outputs to execute (AND)
• Parallel Operations: any true input executes the outputs (OR)
• Ladder Logic Execution reads rungs left to right, top to bottom
• Branches solve top left to bottom right,

Elements of Ladder Diagram Rungs

• Normally Open Contact (||) enables the rung if the left is enabled and underlining bit is set (1)
• Normally Closed Contact (|/|) enables the rung if the left is enabled and underlining bit is reset (0)
• Coil (()) sets a bit true when the rung is true (1) , and resets false (0).

Digital Signals

• Digital Signals, analog sensor outputs are converted to digital form
• Binary numbers are 0 and 1, represented by 0V and 5V signals

Binary Units

• LSB (Least Significant Bit) has the lowest value, usually the right-most bit
• MSB (Most Significant Bit) has the highest value, usually the left-most bit

Converting Analogue to Digital

• ADC samples analog signals with regular time pulses
• Sampling speed is measured in Hertz (Hz)

Quantization

• Quantization converts analog signal points to near stabilized values
• Digital to Analog Conversion (DAC) outputs an analog signal representing the weighted sum of non-zero bits

Digital Devices

• Multiplexers inputs data from many sources and outputs one selected input channel
• Serial Communication: data transferred as a group of bits is sent one bit at a time
• Synchronous Serial Communication: data is sent continuously at a constant rate
• Asynchronous Serial Communication transmits data without regular clock signals

Microprocessor Systems

• Microprocessor (µP) is a digital electronic component with miniaturized transistors on a semiconductor IC; serves as a CPU
• It is a programmable, clock-driven chip that accepts binary data and outputs data as per instructions

Basic Microcomputer Parts & BLock Diagram

• A Basic Microcomputer includes ALU, Control Unit, and Register Array

Computer Functions

• Arithmetic Logic Units (ALU) performs math & logic on memory or input
• Register Array consist of registers for temporary data storage
• Control Unit controls data flow and instructions

The Fetch-Execute Cycle

• Fetch: CPU retrieves instructions from memory
• Decode: CPU determines components needed
• Execute: CPU assigns actions
• Store: The processed data is written to memory

Terms Used in Microprocessor

• Instruction Set: Instructions the microprocessor knows
• Bandwidth: bits processed in an instruction
• Clock Speed: operations per second, in MHz or GHz
• Word Length: width of internal data bus
• Data Types: formats like binary, BCD, ASCII

Microprocessor Features

• Microprocessors are cost-effective, small sized and portable.
• Microprocessors use low power and are reliable

Programmable Logic Controller

• Microcontrollers control electronic devices; emphasizing self-sufficiency and cost-effectiveness

PLC Structure & Instruction

• A PLC uses programmable memory for control functions
• Logic control is based on switching operations

PLC Structure

• PLC includes a CPU, memory, I/O modules, and a power supply

Input/Output Processing

• Input / Output Processing involves continuously running through its program and updating it as a result of the input signals
• Continuous Updating: CPU scans input channels per program instructions with a built-in delay
• Mass Input/Output Copying: time taken to examine 100’s of I/O points get long due to updating delay

Input/Output Processing Sequence

• Scan all inputs and copy into RAM
• Fetch, decode, and execute program instructions, copy output instructions to RAM
• Update all outputs

PLC Addressing & Functions

• Input / Output Addresses identify each I/O via addresses
• Ladder program instructions translate to Logic Functions (AND, OR, NOR, NAND, XOR)
• Latching and Internal Relays are self-maintaining circuits
• Internal Relays act as relays within PLCs

PLC Timers & Counters

• Timers are outputs that control contacts elsewhere
• Counters count a specified number of contact operations with Up-Counter, Down-Counter functionality
• Shift Registers store a sequence of individual bits
• Master Relay controls an entire block of outputs
• Jumps: a function provides conditional jumps

Electrical Actuation Systems

• Electrical Actuation Systems are used to give outputs to systems

Data Movement

• Data Movement contains the move data instruction along with the source and destination addresses

Data Comparisons & Operations

• PLCs can do comparisons (less than, equal to, etc.)
• PLCs perform arithmetic operations

Mechanical Switches & Relays

• Mechanical Switches are often used as sensors input to systems
• Relays are used as electrically operated switches that actuate when there is a change in current

Solid State Switches

• Diodes act as one-way switches for current
• Thyristors are diodes with a gate for controlling its on/off conditions
• Bipolar Transistors can be npn and pnp types

Transistors

• NPN Bipolar Transistors, main current flows from collector to emitter, controlled by base signal
• PNP Bipolar Transistors, main current flows from emitter to collector, also controlled by base signal
• Power MOSFETs have n and p channel types, use voltage instead of current to exercise control
• Solenoid is an electromagnet that is used as an actuator
• Solenoid Valve is an electromechanical valve that controls gas/liquids with electrical signal
• Direct Current Motors are used as control elements for position or speed
• Alternating Motors are commonly used and driven by an alternating current
• Stepper Motors rotate through steps for each digital pulse
• Servo Motors rotate levers or parts of machines and usually only rotate between 90 and 180 degrees