There are two types of controller setups that can be used to control an industrial robot – PLC's and PC based systems.
PLCs (Programmable Logic Controllers)
A PLC (Programmable Logic Controller) is a small processing unit used to control industrialequipment. The units are often compact, designed to withstand inhospitable environments and are immune to a high level of noise. They operate in real time running a program written to battery-backed memory and trigger outputs using incorporated I/O relays.
The programmes are written in a number of ways but are traditionally written in Ladder Logic which is a language resembling a schematic diagram of relay logic. They can also be written in C, Basic and State Logic, a very high level language designed to program PLC's based on State Transition Diagrams.
The IEC 61131-3 standard describes four languages used to program PLC's which are as follows:-
- Function Block Diagram (FBD)
Uses a broad function library to build complex procedures in a graphical format. Standard math and logic functions may be coordinated with customisable communication and interface functions.
- Structured Text (ST)
A text language used for complex mathematical procedures and calculations less suited to graphical languages.
- Instruction List (IL)
A low-level language similar to assembly code. It is used in relatively simple logic instructions.
- Ladder Diagram / Logic (LD/L)
The primary programming language for PLC's.
Generally the program is written using a PC and the supplied development software and downloaded to the PLC. However ladder logic can be created and altered on the PLC itself if it has the required features.
PLC's offer a low cost control solution and remain adaptable to future system changes which could take place, meaning the system functionality could be modified without the need for a new PLC unit. The downside of PLC's is that they are not as powerful at processing as PC's, especially in applications involving large amounts of data.
With a PC controlling a robot, greater flexibility of software and hardware choices are introduced. A system can store much more information and process larger amounts of data quickly. The PC environment is one with which many people are familiar. This can be comforting when trying to come to terms with new systems.
The reliability of PC controllers and there inability to perform in a deterministic manner is a commonly cited reason for not adopting this kind of technology for industrial control. While this may have been true in the past modern PCs and operating systems such as Wind River's VxWorks can offer real time operation comparable with PLCs. PC's can be embedded in a tablet which is attached to the robot cabinet allowing versatile, on-site control as well as the Desktop PC alternatives which can control machinery from a distance or over a network.