A PLC chassis is the essential physical framework or rack that serves as the backbone for a Programmable Logic Controller (PLC) system, housing the various components.
In the context of a PLC, the chassis is the structure that physically contains the different electronic modules required for the system to operate and interact with external equipment.
Key Role and Function
Based on the provided reference, the primary function of the chassis is to act as the holder for the system's modules:
"The Chassis is what will physically hold the modules which allow the system to interface field devices: EtherNet, ControlNet, DeviceNet, Analog Inputs, Digital Outputs, etc."
This means the chassis provides the slots and connections necessary for modules such as:
- CPU Modules: The brain of the PLC.
- I/O Modules: Digital and analog input and output modules that connect to sensors and actuators in the field.
- Communication Modules: Modules like EtherNet, ControlNet, and DeviceNet that facilitate communication with other PLCs, HMI systems, or devices on a network.
- Specialty Modules: Modules for specific tasks like motion control, high-speed counting, etc.
Beyond simply holding the modules, the chassis often contains a backplane which is a printed circuit board with connectors and communication pathways. The backplane serves several crucial functions:
- Power Distribution: It distributes power from the power supply module to all installed modules.
- Data Communication: It provides the high-speed communication bus that allows the CPU module to exchange data with the I/O and communication modules.
Selecting a PLC Chassis
Choosing the right chassis depends on the specific requirements of the automation system. As the reference states, "The chassis can be selected based on the constraints of the system as well as the mechanical layout of the panel."
Factors influencing chassis selection include:
- Number of Modules Needed: The total number of input/output points, communication interfaces, and special functions determines how many slots are required, thus dictating the chassis size (e.g., 4-slot, 8-slot, 17-slot).
- Types of Modules: Some modules might require specific types of slots or have size constraints.
- Physical Space: The available space within the electrical panel where the PLC will be mounted is a critical constraint. The dimensions and mounting orientation (vertical or horizontal) of the chassis must fit the panel layout.
- Environment: While the chassis itself is often just a metal structure, the overall PLC system's environmental rating (e.g., temperature, vibration) might influence the choice of chassis or the need for protective enclosures.
Typical Chassis Structure
| Component | Description |
|---|---|
| Slots | Receptacles where modules plug in. Number varies by chassis size. |
| Backplane | Internal communication bus and power distribution board. |
| Mounting Hardware | Brackets or holes for mounting in an electrical panel. |
| Grounding Point | Connection point for system ground. |
In essence, the PLC chassis provides the physical and electrical foundation upon which a modular PLC system is built, ensuring reliable communication and power distribution between the CPU and its connected modules.
