Programmable Logic Controllers (PLCs)
The “Brains” of a machine, a Programmable Logic Controller or PLC is a digital computer used for the automation of industrial processes, such as control of machinery on an assembly line. They are programmed to operate in a very reliable specific fashion (turning on and off outputs) based on inputs, timers, and other internal logical controls. Early Programmable Logic Controllers were designed to replace electro-mechanical relay logic, and the basic programming structure of ladder logic’s contacts and coils is a holdover from those electrical diagrams. Today there are a wide range of Programmable Logic Controller (PLC) hardware options, from very simple smart relays to very complex, factory-wide control platforms. Commonly a user will choose a hardware platform based on the number and type of inputs and outputs they need for a machine, along with functions for data-storage, security, and communications.
A type of programmable control system designed for the control of multiple axes of motion, a Motion Controller typically offers specific programming function blocks for things like coordinated motion and circular interpolation, in addition to the normal input/output functions of a PLC. In fact, today the line between PLC and Motion Controller is frequently blurred with many vendors offering motion-control functions in their higher-end PLC’s, and vice versa.
An operator’s interaction with a machine can be done through buttons, switches, and panel meters, or through a programmable (often touchscreen) graphical interface, capable of displaying animations and offering in-depth diagnostics, data-collection, and communications to other devices. The Operator Interface (or “HMI”, Human-Machine Interface) communicates with the PLC and displays relevant information to the user about machine operation, such as units produced, motor speeds, or problems that may arise with operation.
Remote (or Network) I/O
Traditionally, every input and output was wired directly to a centralized PLC rack. On larger machines with hundreds or thousands of I/O points, this poses a logistical problem. One solution to this is Remote I/O, which allows for distributed blocks of I/O to be placed around a machine or factory, which then communicates back to the PLC via a network connection – commonly an industrial protocol like DeviceNet, EtherCAT, or Modbus/TCP. These come in many flavors (including for Safety devices), and can be very cost effective.
HMI + PLC Combos
Often on smaller machines, it can be cost effective and simplify both programming and wiring to use a HMI + PLC Combination unit instead of two discrete pieces of hardware (frequently from different manufacturers). These offer a single integrated environment to program both the logical controls (PLC) and user interface (HMI) aspects of machine operation, and a single point to wire I/O and network connections.