PLC structure

A PLC is a computing platform with external interfaces for inputs and outputs. Unlike a consumer PC (that may only have a few years of total run time and regular reboots) PLCs are purpose designed and built for industrial environments and applications where a 10 year of up time running 24 hours 7 days a week is considered the norm.

Structurally PLCs can be divided into a few broad sections:

• Input - sensor data and communication signals. Inputs can either be analog signals or digital signals.
• Output - process commands and communication signals (to other networked devices or electromechanical devices [such as relays, valves motor drives, or actuators]).
• Processor & Memory - evaluates incoming data and execute the program instructions.
• Power supply – provides the correct voltage, current, and power requirements, for the PLC.
• Application / Programming / Human Machine Interface (HMI) - HMI as the name suggests, allows humans to interact with the PLC easily to check conditions and operating status (of the PLC). As well, the HMI, can be used to update programing of the PLC. HMIs may range from a simple keyboard to a full touchscreen display.

Programmable Logic Controllers or PLCs are the basic controller for industrial systems

Environmental conditions

Due to the industrial environments where PLC most often operate, they are constructed with higher standards then most consumer rated electronic devices. PLCs are required to be consistent and reliable. They are business critical devices. Any downtime with a PLC often results in a material impact to the business operating it.

Key features to endure these environmental conditions typically include:

• Circuit components with high operating temperature ratings (suited for -40°C to +85°C [-40°F to 185°F]).
• Conformal coatings on circuit boards to withstand high humidity and moisture content.
• Dust filters on all fan intakes or passive cooling heat sinks.
• Electrical isolation on I/O ports and surge protection in PLC circuity (to handle unexpected voltage spikes such as when industrial equipment powers up or down).

Connecting to the Network

PLCs often operate as a hub for communications within a factory, and more recently have become a conduit for information back to enterprise systems (residing in the cloud or back at headquarters).

Sourced from: Build Connected Intelligent Field Instruments Quickly with Comprehensive Solution Sets

Ladder Diagram (LD) / Relay Logic / Ladder Logic

Picture from PLC Programming Language Introduction

Ladder Diagram, also known as Ladder Logic or Relay logic is a widely used graphical programming language that mimics the original relay logic and wiring diagrams that were used in process control before PLC devices were developed. Great for simple on/off interfaces, logic control, and safety interlocks, LD is still favored in modern PLC programming as it easy to get started with and logically consistent with each rung of the ladder detailing the input and control devices as read from top to bottom and left to right.

Function Block Diagram (FBD)

Picture from User-Defined Function Block (UDFB) in Function Block Diagram (FBD) Using Siemens TIA Portal

Function Block Diagram (FBD) is graphical programming language for PLCs that describes the functional relationship between input and output variables of the control process.

Each block used in the graphical diagram are used to represent different logical operations or coding functions. The left to right connections and flow lines between these blocks show the path data takes flowing between these functions. FBD is great for process control and PID loops as the function blocks and looping paths closely match.

Sequential Function Chart (SFC)

Picture from Introduction to Crouzet PLC Sequential Function Chart (SFC) Programming

Sequential Function Chart (SFC) is a graphical programming language used in PLCs useful for operations that have multiple states of operations or require state machine diagrams. SFC can look like a flowchart, with boxes representing steps (with actions required), transition functions (controlling the process flow between steps), and branch (if the process needs to operate in parallel sequence). SFC is great for modeling state machines, and machine sequences as the process can be modeled with logical control inputs and operations.

Structured Text (ST)

Picture from Building a Structured Text User-Defined Function Block (UDFB) using Siemens TIA Portal

Structured Text (ST) is a text-based programming language for programming PLCs that more closely resembles common computer programing languages such as C++ or Pascal. Due to ST's syntax structure and list of high-level programming statements, the implementation of coding loops, logical value calculations, and data handling routines is easily accomplished which makes it a good starting point for PLC programmers that have background in more traditional computer programming.

Instruction List (IL)

Instruction List (IL) is a text-based programming language for PLCs. A low-level coding language (IL) can be though to resemble Assembly programming language, with direct operating statements and data control actions.

As of the third edition of the IEC 61131-3 standard released in 2012, IL was deprecated and not recommended for new projects. Still available and supported by some PLC hardware due to the long support time of industrial PLC hardware. It is becoming phased out and being replaced by other more flexible programming languages.

CODESYS

Software development environment that is IEC 61131-3 compliant. It's accepted by multiple different PLC manufacturers and therefore allows a developer to have a development environment that is not solely based on a single manufacturer.

Other Languages

As PLC processing power increases, more programming languages are becoming commonly used for PLC programming (including C++ and Python). These flexible, modern languages are acting as the intermediate layer between the PLC and other applications such as networking with PCs and other workstations. That said, there are drawbacks to consider for real time control applications (for example, redundancy and life cycle upkeep costs) when using a non-IEC 61131-3 language.

Motor Control / Electrical motors

Electrical motors are one of the simplest applications providing rotational power for Fans, pumps or conveyor belts.

Most motors require additional circuity and components such as motor drives for DC motors or Variable Frequency Drives (VFD) for controlling large AC motors

Solenoids / Relays / Contactors

Electromechanical devices that pass electrical current through a coil to provide movement.

A solenoid is made up of a coil of wire and the moveable plunger placed inside the coil also called an armature. When electrical current is applied to the end of the coil a magnetic field is generated that moves the armature, turning electrical energy into mechanical force. With a quick response time, compact size and simple operation, solenoids are a popular choice in industrial applications such as valve control for fluids and gases.

A relay is a device that uses low power electrical signals to control and switch higher electrical power circuits. A Relay is made up of a coil of wire generating a magnetic field and a separate switch circuit that closes an electrical circuit when the magnetic field is generated. Typically, relays can switch up to a few amps of current and a hundred of volts on the high-power side, but exact power ratings are dependent on the device and application. Relays simplicity and reliable design make them a popular choice for power switching for industrial and automotive applications.

A contactor is a device that uses low power electrical signals to control and switch much higher electrical power circuits. Unlike relays that switch a few amps of electrical current on the switching side of a Contactor. A contactor can switch dozens or hundreds of amps of electrical current and thousands of watts of electrical power. The robust design and internal components of most contactors make them preferred when switching high power electrical loads such as fans and motors.

Electrical Actuation

Mechanical positioning devices, usually powered by a rotational motion source like an electrical motor which is mechanically converted to create linear motion. This includes

• Ball screw driven using ball bearings between the central shaft and nut used in applications needing a high level of accuracy and stability
• Lead screw driven for applications requiring a low-cost and simplicity 
• Belt driven using a toothed belt to provide motion

Fluid power (Pneumatics / Hydraulics)

Some automation systems are powered by the movement of pressurized fluid (this could be gases like air or oil). These systems Pneumatic (air-driven) or Hydraulic (fluid-driven) are connected components that can be quickly installed and provide either linear or rotational power to devices.

Pneumatic (air-driven) systems typically operate under low pressure of 60 to 120 PSI powered by an air compressor and properly sized storage tank. Hydraulic (fluid-driven) systems operate at much higher pressures anywhere from 800 to 5000 PSI and have oil as a working fluid. Products include:

• Grippers
• Compressors
• Manifolds
• Tubing

Indicators / Alarms

There are several devices that can be connected to automation systems to inform operators of system status and operating conditions. The simplest and most direct are indicator lights and audio alarms.

The classic tower lights can offer a color-coded status light informing staff on the floor of the machine operating status or if there are any error that need to be cleared. While some are simple indicator lights, other tower light devices are networked end terminals and can be remotely controlled or send messages across a network.

Audio devices such as buzzers, speakers or other sound generating can also be used for indicating machine status generating attention on a noisy shop floor.

Pressure

Pressure sensors can measure the amount of pressure (of gases or liquids). This is usually done to maintain safe and efficient operation within a system. There are a few different types of pressure sensors depending on measuring method being used. Some of the most common include:

• Gauge - relative to current local atmospheric pressure
• Absolute – Measures pressure relative to a perfect vacuum
• Differential – measured the pressure difference between two pressure points, typically between two vessels

Photoelectric

Photoelectric sensors can determine the presence of an object (or measure a distance) by measuring the changes in the transmitted light beam. When the beam of light has been interrupted or blocked from the receiving sensor an object can be inferred, and action can be taken.

Time of flight and distance sensors are similar although they project and receive the light beam back. By measuring the time it takes for a light beam to be reflected off an object a distance measurement can be determined.

Proximity

Proximity sensors can detect presence and distances to objects without physically touching them. Useful for object detection safety interlocks and verifying objects are present (for quality control).

Here are some common methods that proximity sensors operate based on:

• Inductive - by detecting changes in rapidly changing electromagnetic field project outward from the sensor, an inductive sensor can detect the present of a ferrous (iron) and non-ferrous (non-iron based) metal objects.
• Capacitive - as a metallic and non-metallic object approaches the capacitive sensor it disrupts the electrostatic field by changing .
• Magnetic - the presence of a magnetic field approaching the sensor face can be detected by the changes in output by Giant magneto resistive (GMR) sensors or resonant circuits, useful for detecting when a known object is approaching like a safety door or tool head.

Temperature

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Temperature sensors measure thermal conditions and local temperature in process for control and monitoring and to avoid overheating and thermal runaway conditions. Temperature sensors can either be contact and changing electrical properties or by measuring the infrared energy given off.

Distance

Sample Image Text

Determining the distance from a sensor to an object (either stationary or moving) is of great importance to the modern factory. AMRs and other robotic platforms can navigate and make sure the work area is clear of any obstructions.

• Ultrasonics: A transmitter emits a short burst of high-frequency sound waves. When this pulse of sound hits an object, some of the sound waves are reflected to the receiver. A common application for ultrasonic sensors is contactless liquid and fluid level measurement within a tank.
• Radar: Like ultrasonics, radar sensors send out short bursts of high-frequency energy and measure distance based on the time it takes for the reflection to bounce off the surface. Instead of sound waves, radar uses microwave radio waves.
• Lidar: By rapidly spinning a light source like a laser and calculating the time it takes for the light to return to the receiver Distance from the LiDar sensor can be found. Much like Radar it allows for high accuracy.

Rotational

The speed of a rotating shaft or position of the slider can be measured by an encoder, which is an electro-mechanical device that converts the position or motion of a shaft or axle to analog or digital output signals.

Force and Load Cells

Load cells convert forces (like weight, compression, tension, or torque) into measurable signals. As the force applied changes, the output signal changes proportionally. Load cells are commonly used in industrial applications to control weight capacity, measure products, or track weight changes over time.

Regulatory Agencies

• Occupational Safety and Health Administration (OSHA) - United States government agency within the US Department of Labor sets workplace safety regulations designed to protect worker and prevent workplace accidents
• Underwriter Labs (UL) – global organization that tests and certifies electronic products for quality and safety.
• National Electrical Manufacturers Association (NEMA) – association electrical equipment that publishes safety standards.
• The International Electrotechnical Commission (IEC) - is an international standards organization that prepares and publishes international standards for all electrical, electronic, and related technologies.

Safety Integrity Level (SIL)

The Safety Integrity Level (SIL) is a functional safety term for the level of risk reduction provided by safety equipment. Based on the IEC 61508 standard there are four levels of SIL with level 1 being the least dependable and level 4 being the most dependable level of safety.

For PLCs this can mean detecting faults, executing safe state, and maintaining the certified operation.

Performance Level (PL)

Performance level (PL) is used to define the likelihood of products to perform safely (thus avoiding dangerous failures). PLe is the lowest likelihood of dangerous failure (0.000001% to 0.00001% probability).

Products

Safety light curtains are optoelectronic devices used to protect personnel from hazardous machinery. They create an invisible barrier of infrared beams between a transmitter and a receiver pair of devices. When any beam is interrupted, the system sends a signal to stop the machine immediately.

Operator presence mats are pressure-sensitive safety devices placed on the floor near hazardous machinery. When stepped on, they detect the presence of a person and trigger a control signal, typically to stop or prevent machine operation.

Safety interlocks are mechanical or electrical, based devices that prevent machinery from operating under unsafe conditions. They are typically installed on movable guards, doors, or access panels to prevent operations when open or misaligned and are integrated into a machine's control system.

Emergency stop switches (often called E-Stops) are manually operated safety devices and mechanical switches that are designed to immediately halt machinery or equipment in the event of a hazard.

Lockout devices are physical safety tools used to isolate switches and prevent the accidental startup of machinery or equipment during maintenance, cleaning, or repair. They are a core part of Lockout/Tagout (LOTO) procedures, which are mandated by OSHA under standard 29 CFR 1910.147.

Laser scanners are non-contact protective devices that use laser beams to monitor defined zones. If a person or object enters a protected area, the scanner sends a signal to stop nearby machinery, preventing injury or damage.