Microcontrollers are special types of processor chips that are very small and somewhat versatile, due to their programmable nature. This type of processor is fully integrated, a "computer on a chip," unlike general purpose processors that people use in their home PCs and laptops. Programmable logic controllers (PLCs) are a subset of microcontrollers that are specifically designed to carry a set of instructions for manufacturing robots and industrial equipment designed for specific automated tasks.
Since microcontrollers are fully integrated onto one chip, they are cheap to manufacture. Microcontrollers typically have much lower specs than even a low-power consumer grade general CPU and a generally standardized architecture, making them even more easy to mass produce.
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Once a microcontroller or PLC has been programmed, typically they cannot be reprogrammed, since microcontrollers are controlled by Read-Only Memory (ROM) rather than Random Access Memory (RAM), which is intended for dynamically updating memory and is inappropriate for the repetitive tasks that microcontrollers are made to perform.
Many tasks performed by microcontrollers and PLCs are far too minute and repetitive for human attention, such as the assembly of small electronics. The programmable nature of these chips also allows manufacturing robots to reproduce these motions very quickly and consistently, increasing productivity.
Many microcontrollers, especially PLCs, are custom programmed for custom-built machines. Complicated manufacturing robotics designed for incredibly specific tasks, which can be extremely expensive. In many of these implementations, the expense of a custom built manufacturing robot also includes regular maintenance.