PLC and Controls Exam Notes

Control System Dynamics

  • Control system dynamics encompasses process and motion control.

  • HMI and SCADA systems are integrated with ControlLogix PLCs for:

    • Data communication (DeviceNet).

    • Process Control.

    • Network Systems.

    • Motion Control.

    • SCADA.

  • Designing intuitive HMI screens and data visualization is crucial.

  • Troubleshooting and diagnostics are essential for maintaining control systems.

Understanding Controls in Industry

  • Continuous Process:

    • Raw materials enter at one end, and finished products exit at the other.

    • The process operates continuously.

  • Batch Processing:

    • No continuous flow of product material between process sections.

    • A set amount of input is received in a batch, and operations are performed on the batch to produce a product.

  • Discrete Manufacturing:

    • Characterized by individual or separate unit production.

    • A series of operations yields a useful output product.

    • Typically involves digital inputs to PLCs, activating motors and robotic devices.

  • Centralized Control:

    • Multiple machines or processes are controlled by a single central controller.

    • A single, large control system manages diverse manufacturing processes and operations.

Process Control Systems

  • Distributive Control System (DCS):

    • Operates on a network framework.

    • Requires multiple PLCs that interact to execute the full range of control activities.

    • Each PLC manages various processes locally while sharing data and process status updates via communication links.

Components of a Process Control System

  • HMI (Human Machine Interface):

    • Allows human input through switches, controls, and keypads.

    • Used to set up starting conditions or alter process control.

  • Sensors and Actuators:

    • Sensors provide inputs from the process and the external environment.

    • Actuators convert input and output signals into a usable form.

  • Controllers:

    • Make decisions based on input signals.

  • Signal Conditioning:

    • Involves converting input and output signals to a usable form.

Human Machine Interface (HMI)

  • HMI Definition:

    • Equipment providing a control and visualization interface between a human and a process.

    • Allows operators to control, monitor, diagnose, and manage applications.

  • Operator responsibilities may include:

    • Starting and stopping processes.

    • Operating controls and making adjustments, monitoring progress.

    • Detecting abnormal situations and undertaking corrective actions.

  • Graphical Display HMI:

    • Used extensively in the manufacturing industry for various operations.

Feedback Systems

  • Close-Loop Control System:

    • Utilizes feedback where the output of a process affects the input control signal.

    • Measures the actual output and compares it to the desired output.

    • The actual output is sensed and fed back to be subtracted from the set-point input.

    • Difference \rightarrow Controller Action \rightarrow Change Actual Output \rightarrow Difference = 0

  • Open-Loop Control System:

    • Operates based on predefined settings without feedback.

    • Directly influences the process without considering changes in output.

    • Less adaptable to disturbances or system variations.

  • Proportional Plus Derivative (PD) Control:

    • Used in process control systems with rapidly changing errors.

    • Combines proportional control with derivative control.

    • Controller output responds to the error rate of change and its magnitude.

  • PID Control:

    • A feedback control method combining proportional, integral, and derivative actions.

PID Controller

  • Proportional Action:

    • Provides smooth control without hunting.

  • Integral Action:

    • Automatically corrects offset.

  • Derivative Action:

    • Responds quickly to large external disturbances.

  • PID Controller Benefits:

    • Most widely used type of process controller.

    • Modes complement each other to reduce system error to zero faster than other controllers.

  • PID Feedback Loop:

    • Control can be changed during operation to tune the process.

    • The integral term improves accuracy, and the derivative reduces overshoot for transient upsets.

    • PID control allows the output power level to be varied.

Motion Control

  • Motion Control System:

    • Provides precise positioning, velocity, and torque control.

  • PLCs in Motion Control:

    • Ideally suited for both linear and rotary motion control applications.

  • Pick and Place Machines:

    • Used in the consumer products industry for product transfer applications.

  • Control Components Used:

    • Robots operation.

    • Programmable Logic Control.

    • Servo Motor.

    • Servo Drive.

    • Motion Module.

PLC Motion Control System Components

  • Basic Components:

    • Controller.

    • Motion Module.

    • Servo Drive.

    • Motors with Encoders.

    • Machinery being controlled.

  • Axis of Motion:

    • Each motor controlled in the system.

  • Bottle-Filling Motion Control Process Example:

    • Requires two axes of motion: the bottle filler mechanism and the conveyor speed motor.

Data Communications in PLCs

  • Data Communications:

    • Refers to the ways PLC microprocessor-based systems communicate with each other and other devices.

  • Types of Communication Links:

    • Point-to-point links.

    • Network links.

  • Serial Communications:

    • Used with devices such as printers, operator workstations, motor drives, bar code readers, computers, or another PLC.

    • Serial interfaces can be built into the processor module or come as separate modules.

    • A serial module in each controller is typically sufficient for two PLCs to establish a point-to-point link.

DeviceNet

  • DeviceNet:

    • An open device-level network.

    • Relatively low speed but efficient at handling short messages to and from I/O modules.

  • Increasing PLC Power:

    • PLCs control an increasing number of I/O field devices.

  • Wiring Considerations:

    • It may not be practical to separately wire each sensor and actuator directly into I/O modules.

  • DeviceNet Connection Benefits:

    • Reduces costs and time-consuming wiring.

    • Integrates all I/O devices on a 4-wire trunk network with data and power conductors in the same cable.

Troubleshooting and Diagnosing PLCs

  • PLC Troubleshooting Approach:

    • Employ a careful and systematic approach to resolve the problem.

  • PLC Advantages:

    • Relatively easy to troubleshoot as the control program can be displayed and watched in real-time.

  • Program Confidence:

    • If the system has been operating, the program logic is likely accurate.

  • New Systems:

    • Programming errors should be considered in systems that have never worked or are being commissioned.

Common PLC Issues

  • Hardware Failures:

    • I/O modules, power supply, or processor faults.

  • Software Errors:

    • Logic errors, incorrect data handling, or communication faults.

  • Environmental Factors:

    • Electrical noise, temperature extremes, or improper grounding.

Troubleshooting Steps

  • Preliminary Assessment:

    • Reviewing system logs and error codes.

  • Visual Inspection:

    • Checking for visible signs of damage or component failure.

  • Signal Tracing:

    • Testing input and output signals to identify faults.

  • Logic Verification:

    • Reviewing and testing program logic.

Processor Module Faults

  • Processor Role:

    • Responsible for self-detection of potential problems.

    • Performs error checks during operation.

    • Sends status information to indicators on the processor module.

  • Diagnostics Access:

    • Can diagnose processor faults or obtain detailed information through programming software.

Sample Diagnostic LEDs

  • Green Light (Run Mode):

    • On steady: Process is in RUN mode.

    • Flashing: Operation is transmitting data.

  • FLT Light (Run Mode):

    • Flashing: Major error in processor, chassis, or memory; OFF indicates no fault.

  • Battery Light:

    • OFF: Battery is functional.

    • On steady: Battery voltage is below a threshold or battery is missing.

Input Errors

  • Potential Issues When Outputs Fail:

    • Wiring for input and output between modules and field devices.

    • Power supplies for modules or field devices.

    • Input sensing devices.

    • Output actuators.

    • PLC I/O modules.

    • PLC processor.

  • Input Error Diagnosis:

    • If the status indicator on the input module does not illuminate when the input device is on, measure voltage across the input terminal.

    • If the voltage level is correct, the input module should be replaced.

    • If the voltage level is incorrect, the power supply, wiring, or input device may be faulty.

Program Control Instructions

  • Purpose:

    • Activate or deactivate specific sections of a logical program.

    • Facilitate the transfer of program execution from one point to another.

  • Functionality:

    • Manage the flow of a program's execution.

    • Direct program behavior based on specific conditions or requirements.

Control Relays

  • Role in Relay Control Circuitry:

    • Hardwired master control relays (MCRs) enable shutdown of an entire circuit by controlling input and output power.

  • Function:

    • Manage the flow of electricity within the circuit.

    • Allow for swift and effective shutdown when necessary.

  • Importance:

    • Ensure safety and proper functioning of circuitry.

    • Provide a centralized mechanism for power control.

MCRs (Master Control Relays)

  • Functions Beyond Power Management:

    • Isolating specific sections of a program as needed.

  • Examples of Use:

    • Inhibiting program zones during recipe loading processes.

    • Monitoring emergency stop conditions.

    • Establishing preconditions to synchronize machinery during start-up procedures.

  • Versatility and Importance:

    • Control and coordinate various aspects of industrial processes and operations.

Jump Instruction

  • JMP Instruction:

    • Facilitates skipping specific program instructions based on existing conditions.

  • Label Instruction:

    • Identifies the target destination ladder rung without affecting the logic continuity.

  • Roles:

    • Control the flow of a program's execution within ladder logic programming.

  • Functionality:

    • JMP allows for conditional jumps, enabling efficient program branching based on predetermined criteria.

    • Labels provide a clear reference point for directing program execution, aiding in code organization and readability.

Jump Operation

  • Common Use:

    • Hazardous areas where immediate action is needed regardless of the current process status.

  • Implementation:

    • Use the JMP instruction in conjunction with the LBL (Label) instruction.

  • Functionality:

    • When JMP is activated, all outputs between JMP and LBL are disabled until JMP is active.

    • Exception: Latch –(L)– outputs are not disabled.

Subroutine Instructions

  • Definition:

    • A concise set of instructions designed to execute a particular task or function.

  • Usage:

    • Utilized by the main program to delegate specific functionalities, enhancing code modularity and readability.

  • Benefits:

    • Segregate complex operations into manageable, reusable units, promoting code efficiency and maintainability.

  • Invocation:

    • Invoked by the main program as needed, enabling modular design principles and facilitating code reusability across various sections of the program.

  • Functionality Details:

    • Subroutines encapsulate specific functionalities, such as mathematical calculations, input/output operations, or complex algorithms.

    • Streamlines main program's structure and improves overall code organization.

  • Execution Flow:

    • When a subroutine is invoked, the program transitions away from the main sequence to execute specific functions within the subroutine before returning to the main program.

Specific Subroutine Instructions

  • JSR (Jump to Subroutine):

    • Jumps to a designated subroutine instruction.

  • RET (Return from Subroutine):

    • Exits current subroutine and returns to previous condition.

  • SBR (Subroutine):

    • Identifies the subroutine program.

Fault Routine

  • Non-Recoverable Faults:

    • In the case of a non-recoverable fault, the fault routine undergoes a single scan before initiating shutdown procedures.

  • Recoverable Faults:

    • If a fault is recoverable, the fault routine is activated to address the issue.

  • Processor Response:

    • Upon detection of a significant fault, the processor searches for an associated fault routine.

    • Execution proceeds if a routine is found; otherwise, the processor initiates a shutdown sequence.

  • Influence:

    • The utilization of a fault subroutine file dictates the processor's response to programming errors, influencing its behavior in handling faults and errors within the system.