Flashcards for:
Understanding PLCs and Building Automation Systems

A PLC (Programmable Logic Controller) is a small computer that carries out pre-programmed outputs based on inputs and specific rules, requiring minimal to zero manual intervention.

Scan time is significant because it affects the performance of the PLC, with different applications (such as controlling a water tank versus managing room temperature) requiring different scan times to optimize their control processes.

Advantages include local storage of software, reduced physical wiring, smaller installation footprint, easier reprogramming and fault finding, and easy expansion with additional input/output cards.

The basic steps of PLC operation include: Input scan, Program scan, Execute logic, Update outputs, and Housekeeping. Scan time can affect performance and varies depending on the application.

PLCs enhance fault detection and system reprogramming by using software connections instead of physical wiring, allowing for easier identification of issues and easier updates to the control logic.

Input modules can be digital (on/off) or analog (ranges like 0-100%). Tasks include sensing signal reception, converting signal voltage to CPU format, isolating PLC from voltage/current fluctuations, and sending the corrected signal to the CPU.

Local storage allows a PLC to operate independently of building management systems, which enhances reliability by not relying on external network connections and enables more straightforward fault finding and maintenance.

In a PID loop control scenario, a thermistor provides temperature data to the PLC, which then uses logic to proportionally open an actuator valve gradually, accurately controlling room temperature.

Before PLCs, control systems operated via banks of relays where each relay controlled dedicated inputs/outputs based on physical wiring, making them complex and extensive, which complicated operation changes or fault detection.

PLC input modules may include components to sense incoming signals, convert signal voltage to the correct CPU format, and isolate the PLC from voltage/current fluctuations before sending the corrected signal to the CPU.

Solid-state electronics and microchips replaced relay banks with software logic in the post-PLC era, allowing more streamlined operations through inputs, rules application, and output commands managed by software.

A PLC optimizes building heating by using software to calculate optimal start times based on factors like room temperature, outside temperature, and building occupancy, thus ensuring efficient and effective heating.

Output modules send signals to controlled devices such as lights, solenoid valves, and motor starters. They may also include additional components like a battery for power failures, a user interface screen, a time clock, and a power supply.

In a simple boiler control scenario, a bimetallic strip temperature sensor signals the PLC, which controls the boiler operation based on factors like room occupancy and time of day.

The CPU (Central Processing Unit) of a PLC includes a microprocessor for executing work based on inputs and logic, a memory chip for storing programs, output history, faults, and alarms, and other integrated circuits for Modbus and LAN connections.