There must be at least one PID contol loop in the project with one analog measurement input and one analog control output! This is to have the deliverer group to learn how to implement a PID controller using PLC.

Even if PID control principle is familiar to you it is not so easy to implement a PID with a PLC for the first time. It is much more pleasant to learn it in a relatively stress-free environment than in the middle of real life project with your boss looking over your shoulder.

I/O space should lie within the following boundaries:

• 8-14 binary inputs
• 8-10 binary outputs
• 1-2 analog inputs
• 1 analog outputs

The Requirement Specification describes the ordered system extensively. The document includes but is not limited to following sections:

• Functional description
  • Begin with the big picture (why the process or device is needed; what it will do; how it will do it)
  • Description of all the different functions of the system (i.e. the subprocesses)
  • PI diagram (Piping and Instrumentation) - use Visio and search for Piping and instrumentation diagram when creating a new drawing
  • Whenever you refer to individual measurement, control output or control loop you have to mention the tag code referring to that specific item: "Mixing tank level LI-100 is controlled by LIC-100 that adjusts the position of valve V-100 with continuous control output LC-100."
  • Define typical process values along with allowed deviation - setpoint of circulating water temperature depends on the outside temperature according to the diagram below with alarm limits are set to +-5 degrees Celsius; furnace pressure -100 mbar with alarm limit at -300 mbar; etc.
  • Control diagrams and descriptions
  • Description of interlockings - verbal and, if needed, as logic diagrams: Feed screw start release = inlet air fan running AND flue gas temperature below 50 degrees Celsius.
  • Description of how the application should handle discrepancies in each possible case
  • How the process is run up and down
  • Emergency stop
• Hardware description
  • Description of the required characteristics
  • Description of the environmental conditions the system has to be functional in
• I/O table
  • Tag code of the loop (LIC-100) and individual measurements (LI-100) and controls (LC-100)
  • I/O type (binary/analog/...)
  • Type of signalling (4-20 mA/1-5 V/0-300 Hz/...)
  • Scaling of analog signal (0..20 kg/s) or operating point of binary signal (tank level above 60 % > contact opens)
  • Function (e.g. Mixer tank high level limit switch)