机械工程英语——Lesson 21 Programmable Controller

Modern programmable controllers are no more than microcomputers used for control propose.

The use of the name “programmable controller” (PC), or more archaically, “programmable logic devices” (PLC), arises because programmable controllers, albeit very unsophisticated logic devices, were available before the microcomputer came on the scene. The retention of the name is more acceptable psychologically to engineers already versed in logic control.

Previous devices used binary switching, just as microcomputers do. The incorporated such components as relays, solid state (transistor) switches or fluidics (air flow logic). They were programmed to use the minimum number of components.

The performance was limited by speed of operation, sheer bulk and cost of components.

A microprocessor may contain 100,000 switches. Memory for programs is inexpensive and plentiful. The limitations of the earlier programmable controllers, therefore, disappear. Programs are now best written for ease of understanding, rather than to minimize components, and one may be as profligate with memory as one choose.

2. Facilities

Programmable controllers will usually have the following facilities:

a.Control interface which are readily available either integral with the basic unit or as plugs in extras.

b.Some means of permanent storage of developed programs which will be either RAM with battery or PROM.

c.Interaction with human is often by simple numeric keypad. Readout may be on a simple display of light-emitting diodes. A full keyboard and monitor can be used if appropriate.

d.They are robust in construction and well protected against electrical noise and interference.