Hard-wired systems

Electronic modules are the industry standard for controllers employing a wide range of control strategies. Although, more recently, there has been rapid development of microprocessor-based controllers (see Sections 7.20 and 7.21) where control actions are simulated using software, hard wired systems based upon the integrated circuit (IQ and operational amplifier (op-amp) are still much in evidence. 

A hard wired system is one made up from a fixed set of electronic components whose characteristics cannot be changed by reprogramming software—if, indeed, software is available within the system. 

Cr Cr- l Capacitance of amplifier feedback circuit in hard-wired system generating PI action Capacitance of voltage input circuit in hard-wired system generating PD action i, etc. Elements in Routh-Hurwitz array F (farad) F (farad) M L-2T[Pg.731]

Sequence control of process operations, and recipe/batch management and tracking. Alarm and device interlocking (often in addition to separate hard-wired systems). Event and alarm recording, and historical trend recording of process variables. 

The CEO and board members interact frequently with each unit, using hard-wired systems to track performance, and spending a significant amount of their time on this activity. 

UPS batteries and power converters to replace interrupted power for the computer systems and counting room instruments. The batteries are continually charged by the regular service system. During power loss, they support orderly shutdown of computers and instruments with preservation of accumulated data and instructions for calculations. An individual system is recommended for each piece of equipment instead of a large hard-wired system to avoid catastrophic data loss if the hard-wired system fails. 

Hard-wired systems persisted and still do to this day. At the same time, various technical experts and regulators reviewed codes of practice and soimd design principles, eventually leading to the 1999 document from the International Electrotechnical Commission in Geneva, lEC 61508 (Ref 1). Since then a number of sector-specific standards have been developed, each referencing the generic standard lEC 61508. 

These advantages are counterbalanced by a number of disadvantages. Software implementation tends to be more complex and therefore more prone to design errors than implementation of purely hard-wired systems. Moreover, software implementations are discrete logic models of the real world. This has two types of consequences. Software is more sensitive (i.e. less tolerant) to small errors. It is also more difficult to test, because interpolation and extrapolation are much more difficult to apply to computer based systems than to traditional analog systems, and ultimately are not entirely valid. 

Conventional reliability assessments of random failure rates for hard-wired systems are based on measured failure rates for all the system s components and an assumption of perfect routine testing (i.e., the routine testing detects all latent faults). This assessment approach can often yield umealistically low predictions for actual systems failure rates. The system failure rates will in practice be dominated by common-mode failures and not by random failures. 

Where the necessary integrity of a computer based system that is intended for use in a reactor protection system cannot be demonstrated with a high level of confidence, diverse means of ensuring fulfilment of the protection functions (e.g. hard wired systems) shall be provided. 

Another common interlock configuration is to locate a solenoid switch between a controller and a control valve. When an alarm is actuated, the solenoid trips and causes the air pressure in the pneumatic control valve to be vented consequently, the control valve reverts to either its fail-open or fail-close position. Interlocks have traditionally been implemented as hard-wired systems that are independent of the control hardware. But, for most applications, software implementation of the interlock logic via a digital computer or a programmable logic controller is a viable alternative. Programmable logic controllers (PLCs) used for batch processes are considered in Chapter 22 and Appendix A. 

Close attention to detail is essential in the design of all safety-related control systems, whether they are simple hard-wired systems, or complex systems implemented by software. It is important that safety analysis techniques are used to ensure that the requirements in the specification are met, and that the foreseeable failure modes of the control system do not compromise that specification. Issues of concern, which have been identified, include an over-optimistic dependence on the safety integrity of single channel systems, failure to adequately verify software, and poor consideration of human factors. Good design can also eliminate, or at least reduce, the chance of error on the part of the operator or maintenance technician. ... 

Electronic Connectors. The complexity and size of many electronic systems necessitate constmction from relatively small building blocks which ate then assembled with connectors. An electronic connector is a separable electrical connector used in telecommunications apparatus, computers, and in signal transmission and current transmission <5 A. Separable connectors ate favored over permanent or hard-wired connections because the former facilitate the manufacture of electronic systems also, connectors permit assemblies to be easily demounted and reconnected when inspection, replacement, or addition of new parts is called for. 

Being excellent at discrete logic, PLCs are a potential candidate for implementing interlocks. Process interlocks are clearlv acceptable for implementation within a PLC. Implementation of safety interlocks in programmable electronic systems (such as a PLC) is not universally accepted. Many organizations continue to require that all safety interlocks be hard-wired, but implementing safety interlocks in a PLC that is dedicated to safety functions is accepted by some as being equivalent to the hard-wired approach. 

The difference in the nature of process controls and safety interlock systems leads to the conclusion that these two should be physically separated (see Fig. 8-89). That is, safety interlocks should not be piggy-backed onto a process-control system. Instead, the safety interlocks should be provided by equipment, either hard-wired or programmable, that is dedicated to the safety functions. As the process controls become more complex, faults are more likely. Separation means that faults within the process controls have no consequences in the safety interlock system. 

Although the traditional point of reference for safety interlock systems is a hard-wired implementation, a programmed implementation is an alternative. The potential for latent defects in software implementation is a definite concern. Another concern is that solid-state components are not guaranteed to fail to the safe state. The former is addressed by extensive testing the latter is addressed by manufacturer-supplied and/or user-supplied diagnostics that are routinely executed by the processor within the safety interlock system. Although issues must be addressed in programmable implementations, the hard-wired implementations are not perfect either. 

In addition to the basic control loops, all processes have instrumentation that (1) sounds alarms to alert the operator to any abnormal or unsafe condition, and (2) shuts down the process if unsafe conditions are detected or equipment fails. For example, if a compressor motor overloads and the electrical control system on the motor shuts down the motor, the rest of the process will usually have to be shut down immediately. This type of instrumentation is called an interlock. It either shuts a control valve completely or drives the control valve wide open. Other examples of conditions that can interlock a process down include failure of a feed or reflux pump, detection of high pressure or temperature in a vessel, and indication of high or low liquid level in a tank or column base. Interlocks are usually achieved by pressure, mechanical, or electrical switches. They can be included in the computer software in a computer control system, but they are usually hard-wired for reliability and redundancy. 

First, computer circuits are hard-wired at the factory. A computer s circuitry never changes. In contrast, as we noted previously, brain circuitry is constantly adapting through a process of pruning and reestablishing connections. This pruning affords the nervous system a plasticity that enables it to adapt continually to the demands of an ever-changing environment. 

One important application of pneumatic transmission is in the operation of diaphragm actuators. These are the elements generally employed to drive the spindles of control valves (Section 7.22.3) and, if hard-wired transmission systems are employed, require devices which convert electric current into air pressure or air flowrate, i.e. electropneumatic (E/P) converters. The basic construction of a typical E/P converter is illustrated in Fig. 6.77. A coil is suspended in a magnetic field in such a way that when a current is passed through the coil it rotates. This rotation is sensed by a flapper/nozzle system (Section 7.22.1). The nozzle is supplied with air via a restrictor and its back pressure actuates a pneumatic relay. The output from the latter is applied to the feedback bellows and also acts as output from the E/P converter. Electropneumatic valve positioners employ the same principle of operation. 

Classically, the central nervous system has been envisioned as a series of hard-wired synaptic connections between neurons, not unlike millions of telephone wires within thousands upon thousands of cables (Fig. 1—4). This idea has been referred to as the anatomically addressed nervous system. The anatomically addressed brain is thus a complex wiring diagram, ferrying electrical impulses to wherever the wire is plugged in (i.e., at a synapse). There are an estimated 100 billion neurons, which make over 100 trillion synapses, in a single human brain. 

The reader should now appreciate that chemical neurotransmission is the foundation of psychopharmacology. It has three dimensions, namely, space, time, and function. The spatial dimension is both that of hard wiring as the anatomically addressed nervous system and that of a chemical soup as the chemically addressed nervous system. The time dimension reveals that neurotransmission can be fast (milliseconds) or slow (up to several seconds) in onset, depending on the neurotransmitter or neuromodulator, of which there are dozens. Neurotransmission can also cause actions... 

Remote login is the ability of a computer user in one location to establish an on-line connection with another computer elsewhere. Once the connection is established, the remote computer is used as if it were a hard-wired terminal of that system. Within the Transmission Control Protocol/Internet Protocol (TCP/IP) suite, this facility is called Telnet. Using Telnet, an Internet user can establish connections with a multitude of library catalogues, other bibliographic databases, university information systems, full text databases, data files (eg, statistics, oceanographic data, meteorological data, and geographic data), and other on-line services. Many of these connections are available to any Internet user and can be accessed without an account. 

When 8-bit microprocessor-based computers became available, we decided that such a computer, even though slower than minicomputers, would be adequate to operate the OMA and our interface controller with the ultracentrifuge. We now have in the system an Altair 8800 computer with 28K of memory (MITS, Albuquerque, NM), a hard-wired arithmetic board and two minifloppy disk drives (North Star, Berkeley, CA), a 700 ASR Terminal (Texas Instruments, Dallas, TX), and a 7202A Graphic Plotter (Hewlett-Packard, Palo Alto, CA). Appropriate software, written in Basic, has been developed to collect intensity data from the OMA automatically and also to treat and plot the data at the end of the experiment. Details of the system and software will be published elsewhere. We also have an improved illumination system with a 200 W Hg-Xe arc lamp and a Model H-20 monochromator with a holo-... 

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