Saturday, August 28, 2010

Just Beginning

I have a web site,, where I try to present some of my observations on the PLC systems I have used, along with various things that interest me from time to time. But I'm going to use this blog as a 'how did I get here'. But first an introduction. (Those familiar with the basics can skip.)

I can't imagine someone coming here without a familiarity with PLCs but for those who don't, let me give a brief explanation. Current machines of any moderate complexity and up have a computerized controller at their heart.

Large machine used to, up to the middle 1960's, have banks of relays, timer units and counter units occupying huge cabinets to control part of their operations. As solid-state electronics were developed the logical sequence of processes were transferred to banks of 'chips' to perform the decision processes. The MODICON controller (mid 1960's) was one of the first of these solid-state controllers.

Later, as microprocessors, essentially 'computers-on-a-chip' were developed ( mid 1970's), the control was transferred to computer programs running using these new devices. The development of small computer systems has encouraged even smaller machine developers to use the ever cheaper computerized control systems to use these units. Current technology produces controllers for little more than the cost of a couple of relays. Entry systems may feature free software to program them.

The whole family of computers which are designed for machine control are labeled PLC or Programmable Logical Controller. (The label was originally 'PC' for Programmable Controller until some upstart company called IBM appropriated that acronym for their product, the Personal Computer.) As an ironic development the IBM PC format has become the development platform usually used to generate the 'programs' mentioned below.

A PLC controls a machine or process by first using 'inputs'. This is information from the machine or operator. These 'inputs' are usually in the form of individual ON/OFF states (indicating the presence or absence of a state) or a number (indicating the level of a state). These can come from sensors on a machine or as data from an operator via pushbuttons or an operator display (sometimes called a Human-Machine-Interface or HMI).

The PLC then, using rules established in a 'program' (which can be written in a number of competing control languages - each claiming to be better than the others) processes the 'inputs' and remembered information.

Using the results of the program 'rules' the PLC controls 'outputs'. These, again, may be discrete ON/OFF outputs or levels to devices which can actually affect operations on the machine.