5 Control Task Basics
In this chapter, we begin programming a process. The first task is to conceptualize the problem, then move to describe a solution with combinational and then sequential logic. Combinational logic is simply And/Or logic. This chapter will discuss combinational logic and use the DeMorgan Theorem to practice the use of Ladder contacts and coils. We will turn to Chapter 6 to discuss sequential or memory logic. Addressing of variables is also formally introduced.
Modelling the Control Task
Most verbal descriptions of a technical task are not effective in their scope and are unreliable and not clear-cut. A technical sketch, on the other hand, is reliable but lacks description of the human and therefore may miss important details. Several approaches are the usual best approach to describing a process to be programmed. All these types of charts, descriptions, sketches, etc are best in describing the engineering model. Even a mathematical equation is acceptable to help the process.
The engineering model must be complete and exact. What is described must work in all circumstances and under all conditions and produce a safe result (that also, in this world, must make a profit).
| Input (from customer | Phases | Activities | Output (to customers) |
| Inquiry | Analysis | Problem Analysis Requirement, Analysis Cost Calculation | Quotation |
| Order | Design | Requirements, Definition/Design | Construction Documents |
| Approval of Design Documents | Implementation | Realization, Production Including Testing | Product |
| Delivery/Commissioning | Installation | Erection in Operational Environment | Usable Facility |
| Acceptance, Commitment | Operation | Service | Customer Benefits |
Table 5-1 The Engineering Process
A description of the engineering process may be described as follows:
Several methods exist to describe a technical task. Some are more closely linked to the technical implementation such as Ladder Logic, Function Block Diagram, and a procedural language such as C or C++. It is always advisable to start with a drawing of the process with the inputs and outputs shown. A formal drawing may be prepared – referred to as a P&ID – to describe a process or an informal drawing such as the one below may be used.
The Juice Condenser
A description of the above process is as follows:
For saving transportation cost for apple juice, the juice is condensed in a process of evaporation. The water is evaporated in the tank using heat. The process of the process includes the following steps:
- Operator pushes the start pushbutton.
- Valve V-2 opens and fills to the high level switch and then closes.
- Heating occurs with the heat element on and stays on until the level reaches the half level or the temperature rises above 80o C. The temperature switch turns on when the temp reaches 80o C and turns off when the switch falls below 80o C.
- Heating is enabled by the high level switch on and the agitator is always on as long as the half level switc