Programmable Logic Controllers and Automation

Programmable Logic Controllers and Automation

Introduction to Programmable Logic Controllers (PLCs)

The programmable logic controller (PLC) is an industrial computer designed to operate in harsh environments and to execute logic, sequential, timing, counting, and analog control functions in real time. For the Red Seal exam, you must understand the PLC not as a simple electronic relay, but as a complete system for acquisition, processing, and control.

A PLC consists of four fundamental elements:

The scan time is the total time required to read the inputs, execute the program, and update the outputs. This time typically varies from 1 to 20 ms depending on program size and processor speed. A frequent exam error is confusing scan time with a sensor's response time — these are two distinct concepts.

PLC Operating Cycles

The operation of a PLC follows a repetitive cycle in three phases:

Critical point for the exam: during program execution, the physical inputs are NOT re-read. If an input changes state during the execution phase, the CPU will only detect it on the next scan cycle. This characteristic explains why outputs are consistent with an instantaneous state of the inputs, but can cause delays in high-speed applications.

I/O Numbering and Addressing

Each manufacturer uses a different addressing system, but the principle remains the same: physically and logically identifying each I/O point. For the exam, you must know the common conventions:

The image table is a memory area that contains a copy of the state of all inputs and outputs. This table is updated at the beginning and end of each scan cycle. Internal bits (or internal relays, markers) are binary memory locations used in the program to store intermediate states — they do not correspond to any physical input or output.

Programming Languages (IEC 61131-3)

The IEC 61131-3 standard defines five programming languages for PLCs. For the Red Seal exam, you must know them and be able to read them:

Ladder Diagram (LD)

The ladder diagram is the most widely used language in North America. It represents logic in the form of electrical circuits between two virtual power rails. The basic elements are:

Example of equivalent logic: a starter with a stop button (NC) and a holding contact.

| Stop Start Contactor |

In this example, if the stop button is pressed (NC contact opens), the circuit is opened and the contactor drops out. The auxiliary contact in parallel with the start button maintains the coil energized after the button is released.

Functions and Function Blocks

Function blocks (FBs) are graphical blocks that execute predefined functions. The most important ones for the exam:

Timer time calculation formula:

Actual time = PT × time base

If PT = 50 and time base = 100 ms, then the delay is 50 × 0.1 s = 5 seconds. A classic error is forgetting to multiply by the time base.

Sensors and Transducers

Sensors convert a physical quantity into an electrical signal. For the exam, you must know the following types and their characteristics:

Proximity Sensors

Temperature Sensors

Pressure and Level Transmitters

A transmitter converts a sensor signal into a standardized signal, typically 4–20 mA or 0–10 V. The 4–20 mA signal is preferred in industry because it allows detection of a broken wire (0 mA) and is less sensitive to electromagnetic interference.

Conversion calculation: if a pressure transmitter has a range of 0 to 100 kPa and produces 4–20 mA, the pressure for a current of 12 mA is:

P = (I − 4) × (P_max − P_min) / (20 − 4) = (12 − 4) × 100 / 16 = 50 kPa

Actuators and Power Interfaces

Contactors and Relays

The contactor is an electrically controlled switch used to switch power loads. Its essential characteristics:

Variable Frequency Drives (VFDs)

The variable frequency drive controls the speed of an induction motor by varying the frequency and voltage of the power supply. The fundamental relationship is:

N = 120 × f / p

where N is the speed in revolutions per minute, f is the frequency in hertz, and p is the number of motor poles.

For a 4-pole motor supplied at 60 Hz: N = 120 × 60 / 4 = 1800 RPM. At 30 Hz, the speed becomes 900 RPM.

V/f characteristic: to maintain constant torque, the voltage-to-frequency ratio must remain constant. If the rated voltage is 480 V at 60 Hz, then at 30 Hz the voltage must be 240 V.

Servomotors and Stepper Motors

Industrial Communication

Industrial networks allow communication between PLCs, drives, smart sensors, and supervisory systems (SCADA). For the exam, you must know the following protocols:

Modbus addressing: addresses are of type 0xxxx (discrete outputs), 1xxxx (discrete inputs), 3xxxx (input registers), 4xxxx (holding registers). Register 40001 corresponds to address 0 in the protocol.

Functional Safety and Emergency Stops

Functional safety aims to reduce risks to an acceptable level. For the exam, you must understand:

Golden rule: an emergency stop circuit must be hardwired and not dependent solely on the PLC program. Safety relays and light curtains must comply with applicable standards.

PLC Wiring and Installation

Canadian Electrical Code Requirements

The Canadian Electrical Code (CE Code) applies to the installation of PLCs and their circuits. Relevant rules include:

Circuit separation: power circuits (120 VAC and above) must be separated from low-voltage control circuits (24 VDC) to avoid electromagnetic interference. Use separate conduits or partitions in wireways.

Grounding and Shielding

Troubleshooting and Diagnostics

Troubleshooting a PLC system follows a systematic method:

Diagnostic tools:

Common Troubleshooting Errors

Calculations and Conversions for the Exam

Analog-to-Digital Conversion

An analog input module converts a 4–20 mA or 0–10 V signal into a digital value. The resolution is determined by the number of bits:

Digital value = (Signal − Signal_min) × (2^n − 1) / (Signal_max − Signal_min)

For a 12-bit module (0–4095) with a 4–20 mA signal and a reading of 12 mA:

Value = (12 − 4) × 4095 / (20 − 4) = 8 × 4095 / 16 = 2047

Scaling Conversion

To convert a digital value into a physical quantity:

Quantity = (Digital_value − Value_min) × (Quantity_max − Quantity_min) / (Value_max − Value_min) + Quantity_min

Output Circuit Load Calculation

To determine how many outputs a module can drive, calculate the total current:

I_total = Σ I_actuator

Verify that I_total ≤ I_max of the module and that each individual output respects its rated current.

Applicable Standards and Codes

For the Red Seal exam, you must know the following standards:

Important note: the Canadian Electrical Code, Part I applies to electrical installations in hazardous locations (classified areas). If a PLC is installed in a hazardous area, it must be certified for that area or installed in a pressurized enclosure.

Pitfalls to Avoid

Summary

To pass the exam, practice reading ladder diagrams, calculating timers and conversions, and identifying the applicable standards for each situation. Practice with concrete examples is the best preparation.