Peter Rohner

Design Method Considerations for Sequential Circuits

There is no universal circuit design method that suits all types of sequential circuits. Some methods are more suitable for complex circuits than others. Some methods are commonly used for compound circuits but would be too expensive for simple two-cylinder circuits. For this reason a circuit designer should be aware of at least two methods to make his circuits economical for particular applications (see figure 6.1). Figure 6.1 shows the four most commonly used design methods for sequential circuits, as well as the fragmentation of fluid power circuits.

Control Problem Analysis

The grouping of the total circuit into sub-circuits seems to be one of the main difficulties that confront circuit designers. However, these obstacles can be drastically reduced by analysing the total problem and splitting the design into the three main categories: the sequential circuit, the fringe condition modules, and the combinational circuit.

Sequential Logic Circuit Design with Karnaugh — Veitch Mapping Method

The graphical design method using Karnaugh-Veitch maps is the basis for sequential logic circuit design and a systematic algorithm for the use of such maps is presented in as simplified terms as possible. To demonstrate the method, ten practical examples are used and the fourth example is a specially selected control problem which provides the reader with all the possible variations occurring in sequential circuit design.

Pneumatic Control for Hydraulic Sequential Power Systems

More than 80 per cent of all industrial hydraulic systems and about 40 per cent of industrial pneumatic power systems are electrically controlled. Why do we find this high percentage of ‘foreign’ control in fluid power, when fluid power itself is quite capable of controlling its own power circuits? The main reason for this phenomenon is the lack of education in the fluid power field itself. For many years electrical control was considered to be ‘the only solution’ for fluid power sequential and combinational control. Electricians and electronic technicians were taught the rules and methods of logic industrial control. This promptly resulted in the use of electrical and electronic switching hardware for fluid power control. Electrical logic is still a compulsory subject in most technical education programs for machine design engineers, draughtsmen and technicians, whereas fluid power logic and methodical fluid power control is only an elective subject or, indeed, not taught at all. It is the aim of this chapter to remove this stigma and to promote the reputation of fluid power as an ideal means of controlling sequential and combinational hydraulic power systems.

Mixed Control Circuit Design

Industrial control systems often consist of a mixed circuit structure and each problem has to be solved according to its characteristics. This is not always easy, because the circuit designer has to know the limits and merits of various design methods, as well as the economics of the vast variety of hardware available today.

Pneumatic Symbols and Control Element Description

In order to understand a foreign people properly, one needs to learn their language. The same holds true for the field of circuit design. Unfortunately though, the symbolic language used by fluid power circuit designers has not yet been standardised to the point where one could use a symbol without creating ambiguity. This confusion originates from the fact that the technology in this field has developed so rapidly that standardisation has not kept up with progress. For this reason, many circuit designers use symbols which perfectly describe the hardware they use and this practice should not be condemned until uniformity in the field of standardisation is achieved.

Position-sensing and Pressure-sensing

In order to make pneumatics a viable means of machine control, design engineers as well as application engineers should have a thorough knowledge of the vast range of sensing equipment available today. Position sensors are built for two different forms of sensing, namely contact-type sensing and non-contact-type sensing.

Compound Circuit Design

Compound circuits are sequential control circuits consisting of several individual. sequential programs merged into one common circuit, with selection provision for each individual program. This type of circuit design is often used on multiple-purpose machines where a variety of products is produced with the same machine and where individual circuits have proved to be uneconomical. The Karnaugh-Veitch mapping method is ideal for this type of circuit and is (in the author’s opinion) the only feasible method of compound circuit design.

Circuit Layout, Presentation and Labelling

At present, no international standards exist for uniform circuit array and circuit labelling. However, a well-presented circuit can facilitate circuit construction for the engineer and fault-finding for maintenance personnel, as well as making the total control package a more marketable item.

Combinational Circuit Design

As discussed in chapter 5, combinational circuit output signals depend solely on the momentary state of their input signals or input variables. Time-delay and signal memorisation are not taken into consideration in combinational circuits. This fact makes combinational circuits relatively simple to design and the Karnaugh-Veitch map is once again used to minimise equations. A further helpful design tool for combinational circuits is the truth table, which is discussed in the following design problems.