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Summary

In Chapter 14 we saw how a finite-state machine can be synthesized from a state diagram by writing down a table for the next-state function and synthesizing the logic that realizes this table. For many sequential functions, however, the next-state function can be more simply described by an expression rather than by a table. Such functions are more efficiently described and realized as datapaths, where the next state is computed as a logical function, often involving arithmetic circuits, multiplexers, and other building block circuits.

COUNTERS

A simpler counter

Suppose you want to build a finite-state machine with the state diagram shown in Figure 16.1. This circuit is forced to state 0 whenever input r is true. Whenever input r is false, the machine counts through the states from 0 to 31 and then cycles back to 0. Because of this counting behavior, we refer to this finite-state machine as a counter.

We could design the counter employing the methodology developed in Chapter 14. A VHDL description taking this approach for a three-bit counter (eight states) is shown in Figure 16.2. A three-bit wide bank of flip-flops holds the current state, count, and updates it from the next state, nxt, on each rising edge of the clock. The matching case statement captures the state table, specifying the next state for each input and current state combination.

While this method for generating counters works, it is verbose and inefficient. The lines of the state table are repetitive. The behavior of the machine can be captured entirely by the single line

nxt ≤ (others => ’0’) when rst else count+1;

We use array aggregate notation, “(others => ’0’)”, to specify a std_logic_vector value with all elements equal to ’0’. This is a datapath description of the finite-state machine in which we specify the next state as a function of the current state and inputs.

Summary

What happens when we violate the setup- and hold-time constraints of a flip-flop? Until now, we have considered only the normal behavior of a flip-flop when these constraints are satisfied. In this chapter we investigate the abnormal behavior that occurs when we violate these constraints. We will see that violating setup and hold times may result in the flip-flop entering a metastable state in which its state variable is neither a 1 nor a 0. It may stay in this metastable state for an indefinite amount of time before arriving at one of the two stable states (0 or 1). This synchronization failure can lead to serious problems in digital systems.

To stretch an analogy, flip-flops are a lot like people. If you treat them well, they will behave well. If you mistreat them, they behave poorly. In the case of flip-flops, you treat them well by observing their setup and hold constraints. As long as they are well treated, flip-flops will function properly, never missing a bit. If, however, you mistreat your flip-flop by violating the setup and hold constraints, it may react by misbehaving – staying indefinitely in a metastable state. This chapter explores what happens when these good flip-flops go bad.

SYNCHRONIZATION FAILURE

When we violate the setup- or hold-time constraints of a D flip-flop, we can put the internal state of the flip-flop into an illegal state. That is, the internal nodes of the flip-flop can be left at a voltage that is neither a 0 nor a 1. If the output of the flip-flop is sampled while it is in this state, the result is indeterminate and possibly inconsistent. Some gates may see the flip-flop output as a 0, while others may see it as a 1, and still others may propagate the indeterminate state.

Consider the following experiment with a D flip-flop. Initially both d and clk are low. During our experiment, they both rise. If signal d rises ts before clk, the output q will be 1 at the end of the experiment. If signal clk rises th before d, the output q will be 0 at the end of the experiment.

Summary

In this chapter we work through several examples of combinational circuits to reinforce the concepts in the preceding chapters. A multiple-of-3 circuit is another example of an iterative circuit. The tomorrow circuit from Section 1.4 is an example of a counter circuit with subcircuits for modularity. A priority arbiter is an example of a building-block circuit – built using design entities described in preceding chapters. Finally, a circuit designed to play tic-tac-toe gives a complex example combining many concepts.

MULTIPLE-OF-3 CIRCUIT

In this section we develop a circuit that determines whether an input number is a multiple of 3. We implement this function using an iterative circuit (like the magnitude comparator of Section 8.6). A block diagram of an iterative multiple-of-3 circuit is shown in Figure 9.1. Each stage performs, in binary, a step of long division of the input number by 3, passing along the remainder but discarding the quotient. The circuit checks the input number one bit at a time starting at the MSB. At each bit, we compute the remainder so far (0, 1, or 2). At the LSB we check whether the overall remainder is 0. Each bit cell takes the remainder so far to its left, and one bit of the input, and computes the remainder so far to its right.

The VHDL design entity for the bit cell of our iterative multiple-of-3 circuit is shown in Figure 9.2. The remainder in remin represents the remainder from the neighboring bit to the left, and hence has a weight of 2 relative to the current bit position. In our neighboring bit this signal represented a remainder of 0, 1, or 2. However, in the present bit, this value is shifted to the left by one bit and it represents a value of 0, 2, or 4. Hence we can concatenate remin with the current bit of the input, input, to form a three-bit binary number and then take the remainder (mod 3) of this number. A case statement is used to compute the new remainder.

Summary

The specification for a digital system typically includes not only its function, but also the delay and power (or energy) of the system. For example, a specification for an adder describes (i) the function, that the output is to be the sum of the two inputs; (ii) the delay, that the output must be valid within 1 ns after the inputs are stable; and (iii) its energy, that each add consumes no more than 2 pJ. In this chapter we shall derive simple methods to estimate the delay and power of CMOS logic circuits.

DELAY OF STATIC CMOS GATES

As illustrated in Figure 5.1, the delay of a logic gate, tp, is the time from when the input of the gate crosses the 50% point between V0 and V1 to when the output of the gate crosses the same point. Specifying delay in this manner allows us to compute the delay of a chain of logic gates by simply summing the delays of the individual gates. For example, in Figure 5.1 the delay from a to c is the sum of the delay of the two gates. The 50% point on the output of the first inverter is also the 50% point on the input of the second inverter.

Because the resistance of the PFET pull-up network may be different than that of the NFET pull-down network, a CMOS gate may have a rising delay that is different from its falling delay. When the two delays differ, we denote the rising delay, the delay from a falling input to a rising output, as tpr and the falling delay as tpf, as shown in Figure 5.1.

We can use the simple switch model derived in Section 4.2 to estimate tpr and tpf by calculating the RCtime constant of the circuit formed by the output resistance of the driving gate and the input capacitance of its load(s).1 Because this time constant depends in equal parts on the driving and receiving gates, we cannot specify the delay of a gate by itself, but only as a function of output load.

Summary

Digital systems are pervasive in modern society. Some uses of digital technology are obvious – such as a personal computer or a network switch. However, there are also many other applications of digital technology. When you speak on the phone, in almost all cases your voice is being digitized and transmitted via digital communications equipment. When you listen to an audio file, the music, recorded in digital form, is processed by digital logic to correct errors and improve the audio quality. When you watch TV, the image is transmitted in a digital format and processed by digital electronics. If you have a DVR (digital video recorder) you are recording video in digital form. DVDs are compressed digital video recordings. When you play a DVD or stream a movie, you are digitally decompressing and processing the video. Most communication radios, such as cell phones and wireless networks, use digital signal processing to implement their modems. The list goes on.

Most modern electronics uses analog circuitry only at the edge – to interface to a physical sensor or actuator. As quickly as possible, signals from a sensor (e.g., a microphone) are converted into digital form. All real processing, storage, and transmission of information is done digitally. The signals are converted back to analog form only at the output – to drive an actuator (e.g., a speaker) or control other analog systems.

Not so long ago, the world was not as digital. In the 1960s digital logic was found only in expensive computer systems and a few other niche applications. All TVs, radios, music recordings, and telephones were analog.

The shift to digital was enabled by the scaling of integrated circuits. As integrated circuits became more complex, more sophisticated signal processing became possible. Complex techniques such as modulation, error correction, and compression were not feasible in analog technology. Only digital logic, with its ability to perform computations without accumulating noise and its ability to represent signals with arbitrary precision, could implement these signal processing algorithms.

In this book we will look at how the digital systems that form such a large part of our lives function and how they are designed.

Summary

This chapter gives some additional examples of sequential circuits. We start with a simple FSM that reduces the number of 1s on its input by a factor of 3 to review how to draw a state diagram from a specification and how to implement a simple FSM in VHDL. We then implement an SOS detector to review factoring of state machines. Next, we revisit our tic-tactoe game from Section 9.4 and build a datapath sequential circuit that plays a game against itself using the combinational move generator we previously developed. We illustrate the use of table-driven sequential circuits and composing circuits from sequential building blocks like counters and shift registers by building a Huffman encoder and decoder. The encoder uses table lookup along with a counter and shift register, while the decoder traverses a tree data structure stored in a table.

DIVIDE-BY-3 COUNTER

In this section we will design a finite-state machine that outputs a high signal on the output for one cycle for each three cycles the input has been high. More specifically, our FSM has a single input called input and a single output called output. When input is detected high for the third cycle (and sixth, ninth, etc.), output will go high for exactly one cycle. This FSM divides the number of pulses on the input by 3. It does not divide the binary number represented by the input by 3.

A state diagram for this machine is shown in Figure 19.1. At first it may seem that we can implement this machine with three states; however, four are required. We need states A to D to distinguish having seen the input high for zero, one, two, or three cycles so far. The machine resets to state A. It sits in this state until the input is high on a rising clock edge, at which time it advances to state B. The second high input takes the machine to C, and the third high input takes the machine to D, where the output goes high for one cycle. We can't simply have this third high input take us back to A because we need to distinguish having seen three cycles of high input – in which case the output goes high – from having seen zero cycles of high input.

Summary

This book has been especially written and prepared for undergraduate and post-graduate students of industrial engineering, production engineering, industrial and production engineering, mechanical and industrial engineering or mechanical engineering, pursuing a one semester course on work systems design, work safety and ergonomics, work design and ergonomics, work study and ergonomics, industrial ergonomics, occupational health and ergonomics, or industrial engineering and ergonomics. Normally the course is offered as a single semester course. But in the present text it has been the author's endeavour to cover not only what can be taught in a single semester but also provide some additional material needed by an ergonomist or an occupational safety engineer. The present text is supplemented with the case studies and live examples from the industry. Hence, human factor engineers or occupational safety engineers or work study engineers will benefit from the knowledge on practical applications of Work Study and Ergonomics.

Chapter 1 explains the basic concepts, definitions, scope and importance of productivity, reasons for lower productivity, and methods to improve the same followed by integration of work study and productivity. Chapter 2 describes the role of human factors in work study, and the qualities of a good work study man. Chapter 3 deals with the concepts, definitions and applications of method study, procedural steps to conduct methods' study, different tools and techniques for methods' study; followed by principles of motion economy, therbligs and work simplification. Subsequently Chapter 4 focusses on basic techniques of work measurement, work sampling, and stopwatch time study procedure, equipment and forms for time study, allowances, rating scales, calculation of basic and standard time; followed by some numerical problems. As the standard time calculated through work sampling or stopwatch time study is useful for deciding wage rates and incentives for the workers and employees, hence Chapter 5 is dedicated to various wage and incentives schemes and programmes.

By the end of Chapter 5 the students would have acquired an understanding of work study, its importance and its role in industry. Consequently they will be able to conduct motion studies, time studies, set time standards and apply them in fixing wages and incentives for the employees/workers of an organization. Given this, Chapter 6 of the book will introduce the students to the basic concept of ergonomics, to the historical evolution, scope and objectives of ergonomics, …

Summary

Introduction

This chapter focuses upon the physiological aspects of muscular work, and it describes how various physiological systems work together to meet the energy expenditure requirements of work load and how these requirements can be measured quantitatively and considered in the analysis of physical work. Whereas in subsequent chapters the focus is on the mechanical aspects of physical work and awkward postures and heavy exertion forces that can lead to severe musculoskeletal problems, such as low-back pain and upper-extremity disorders.

This chapter begins with a depiction of the physiological structure of muscles and how energy is generated and made available for use by the muscles. Subsequently, it describes how the raw materials for energy production are supplied and its waste products removed by the circulatory and respiratory systems. The human body has a musculoskeletal system of bones, muscles and connective tissues which enable it to uphold the body posture, walk and run and lift and carry objects. Physical work is possible only when there is sufficient energy/vigour to support muscular contractions. Energy expenditure requirements of various types of activities are then described, together with a discussion about how the levels of energy expenditure can be measured quantitatively. Clearly, there are upper limits of energy production and muscular work for each individual. The implications of these work capacity limits for ergonomic job design are discussed in the last section of the chapter.

Muscle Structure

The primary role of muscle is to generate force and produce movement by exerting forces such as pull and push. The human body has mainly three types of muscle cells, also called as muscle fibres. However, this chapter is primarily focused upon the skeletal muscle, since it is directly responsible for physical work, these are described below.

Summary

Introduction to Work Measurement

It has already been described in the previous chapter that work study is mainly composed of method study and work measurement. Both these components are very closely linked with each other. Method study is mainly concerned about reducing the ineffective movements and time related to the job. Whereas work measurement is investigation of ineffective time related to a task, and subsequently establishment of time standards for the job under consideration. In broader terms ‘work measurement can be defined as the application of ways or methods designed to determine the required (standard) time for a competent operator to perform a task at a distinct rate of operating’.

Therefore, work measurement techniques are focused to find out the time required for completing an operation or an element by a qualified operator working at a standard pace and using the standard method. The time thus calculated is known as standard time. The method to do the job is normally standardized by using motion study procedure before carrying out work measurement or time study. The different commonly applied work measurement techniques are as follows:

Summary

Introduction

In chapter 17 work related physical conditions for the protection of workers has been investigated in small and medium enterprises, which explored that managing of various enterprises lacking within this concern. Before that, it is also observed in chapters 14, 15, and 16 that employees of steel industry were highly exposed to noise, dust, and heat stress especially during the extreme weather conditions in summer season. Thus, workers working in low resource settings were found highly exposed to onsite hazardous work conditions. The present chapter exhibits a case study which investigated the effect of onsite work conditions and physical activities on cardiovascular parameters of industry workers. This chapter comprises two parts: first part explains general background of cardiovascular health followed by methods to assess the cardiovascular functions. Second part is dedicated to a case study of comparison between exposed and control group, process wise variation in the parameters and effect of duration of work experience on the cardiovascular functions (parameters). Subsequently, it describes about the influence of alcohol, smoking and tobacco consumption on the cardiovascular parameters.

Statistical Tools used for Analysis

As mentioned above in the introduction that, for each type of parameter two groups of population were included: one group of male subjects (workers) from casting and forging firms, i.e., exposed group and another group of male subjects from the randomly chosen non-exposed population, i.e., control group. All the parameters were statistically investigated using the t-test and ANOVA for multiple comparisons among various sub-groups with a confidence level of 95 per cent. The results were also analyzed for association of physiological parameters with habits like alcohol, smoking and tobacco ingestion. The conclusions were drawn on the basis of results and analysis.

Cardiovascular Autonomic Control (HRV) and Work Conditions

The working environment of steel SMEs is very risky and unavoidable. The work-related physical condition and safety measures are ignored at the workplaces (Singh et al., 2010). It has been established by Togo and Takahashi (2009) that the occupational exposure to hazardous settings along with work related stressors, and work timing had been associations with low HF component of power spectral density. The workers are exposed to noise, heat stress, dust and fumes, etc. which are known factors leading to decreased parasympathetic activity (Shannon et al., 2001 and Fujino et al., 2007).

Summary

Background of Ergonomics

Ever since human first began to interact with his environment in any complex way, the tendency to produce conformity by aggressive or illogical means has been widespread and same is called procrustean approach. Particularly the industrial worker has been ‘customized to fit the demands of his physical work, consequently with most sufferers usually accepting a significant degree of discomfort and disability without too much argument. The procrustean approach explained through a situation like the arms of an operator have elongated to reach inaccessible controls and perceptual abilities stretched to be able to hear or to see virtually inaudible or invisible signals. At the other end of the procrustean range, legs have cut down to fit confined workplaces and cognitive capacities shrink to fit tedious tasks.

The problem has become increasingly important since the industrial revolution, particularly with the expansion in the complexity of both work and machines. Because of a poor ‘fit’ between the human operator and his environment, lives have been lost, productivity reduced and errors have been vital, the needs and abilities of the man and compatibility with in the environment have been as secondary importance.

Training the operator in tasks is no doubt difficult to carry out, but also a costly procedure. In most of the cases, training and production time can be reduced to a significant extent by designing the machine in harmony with the operator's capability. Even though it is possible to train an operator to a high level of competency and skill, training alone will not solve their problems and increase their output. Moreover with the increasing complexity of industrial setup, it is impractical to assure operator's effectiveness without creating the appropriate environment. Further, he needs help in the form of suitable designed controls, information displays and other aspects of his environment for maximizing the effects of training. Chaney and Teel (1967) performed a study and compared the detection efficiency of skilled inspectors of machine parts, either after a four-hour training programme or after giving a set of specially designed visual aids and display to help them for defect detection. The result of the study revealed that, the training programme resulted in a 32 per cent hike in defect detection, whereas the use of the suitable visual aids resulted in 42 per cent increase.

Summary

Introduction

In general, the upper limb or distal upper extremity (DUE) includes the muscle forming the rounded contour of the shoulder to the hand which is called deltoid region. Therefore, DUEs are the limbs like armpit, upper and lower arm, elbow, wrist and hand. These are perceived to be more associated with nerve injury. The risk to these DUE disorders can be assessed using some qualitative and quantities methods. Moore and Garg (1995) proposed a method ‘The Strain Index’ to analyze jobs for risk of distal upper extremity disorders and to distinguish between jobs which are associated with distal upper extremity disorders from those which are not. However, Drinkaus et al. (2005) proposed that the Strain Index (SI) may be modified to estimate the risk of distal upper extremity injury. The SI was developed and validated using single task jobs. It was proposed that the SI would be more useful; however, if it could be extended to estimate the risk of a job with more than one task. Two methods were mentioned: the first is maximum task and the second is CARD (Cumulative Assessment of Risk to the Distal Upper Extremity) approach. Stephens et Al. (2009) evaluated the SI for various worksites with job titles including assemblers, painters and office workers (which performed very different tasks between the different worksites even with the same job titles), and some unique job titles to specific worksites such as hook fabricator and deck hand. These authors used different SI computation methods and resulted in significantly different SI scores along with different risk level classifications calculated by the different computation methods. Vishal et al. (1999) developed a cumulative trauma disorders (CTD) risk assessment model for predicting injury incidence rates. The model was found best suited for job tasks with cycle times greater than 4 sec. Moore and Garg (1998) evaluated the effectiveness of a corporate ergonomics programme that used a participatory approach to solving problems related to musculoskeletal hazards. The corporation experienced a significant decrease in the percentage of recordable disorders related to musculoskeletal risk factors, a marked decrease in the lost-time incidence rate, and a marked decrease in total and per capita annual workers' compensation costs. Garg et al. (2007) presented three different studies showing that the SI is capable of identifying jobs with no distal upper extremity morbidity as ‘safe’ and jobs with distal upper morbidity as ‘hazardous’.

Summary

Introduction

The implementation of occupational hygiene and pollution control measures is still ignored in various developing countries like India. The agenda of occupational safety is not being taken as seriously in small scale industries, this may be due to illiteracy and cheap availability of workers from a large population. The preventive measures and equipment are taken as unwanted and sometimes not provided by the organization. Employees of small scale industries (SMEs) are mostly migrants and they agree with working environment and never consult about preventive measures. Hence, workers engaged in these units suffer from various occupational health hazards and risks. This chapter mainly focuses upon the occupational heat stress assessment and its hazardous effects.

Heat Stress and Human Performance

All human beings are largely affected by environmental temperature. The physical and mental performance deteriorates because of complex association of physiological and pathophysiological processes. The lack of fluid in human body occurs due to a long exposure to heat stress that lowers patience, possibly affecting mental and psychomotor functions. The assessment of heat stress and physiological response of worker is very vital to maintain optimal working environment for human health and productivity in developing country like India.

Gomes et al. (2002) compared the occupational exposure to heat and protective measures used by the workers of foundry unit and a bottling plant. The foundry workers were found at higher risk of exposure to heat, noise, IR and UV radiations as compared to the workers engaged in the bottle filling unit.

The level of heat stress and noise exceeds the permissible limit. Simultaneously there were observations of high rate of muscle spasm, hearing loss and visual disability (Gomes et al., 2002). The skin temperature is significantly affected by body activities and clothes; thus an index that recognizes work activity and clothing is more appropriate. There must be provision of sufficient rest breaks after a continuous work time so as to minimize the effect of metabolic heat (Onder et al., 2005). The standards of care and awareness will refuse due to any physical discomfort in any working environment which may increase the risk of health and safety in mining (Onder et al., 2005). The systolic blood pressure (SBP) changes with the induced heat stress (Cui et al., 2004).

Summary

Introduction

This chapter comprises of three parts; first part of the chapter briefly describe the environmental law w.r.t. legal control of hazardous substances and processes, followed by basics of safety management. The second part describes about scenario of occupational health and safety in India and abroad. The third part of the chapter represents the results of exploratory case study to bring out the occupational health and safety practices in SMEs and addresses some important issues such as status of health and safety practices in Casting & Forging SMEs, level of use of PPEs, exertions, fatigue and anxiety due to work exposure, work schedule, overtime, alcohol/tobacco/smoking etc intake, work posture and musculoskeletal disorders, health symptoms like respiratory symptoms, BP, hearing disability, etc. and at the end the conclusions of the case study.

Environmental Law – Legal Control of Hazardous Substances and Processes

There is a huge number of chemicals along with extremely toxic substances increasing at a fast pace and their control and effective management is becoming extremely difficult.

About 5 million chemicals have been synthesized in the past 4-5 decades at the rate of 120,000 every year; about 50–70 thousand chemicals are being used extensively in millions of commercial products. There has been a tremendous increase in the chemical and alkali industry since independence. A total of 515 units spread over 20 states in India have been identified as hazardous installations by a nationwide survey undertaken by ILO.

Today India is the leading producer of pesticides with more production in South Asia. There are some reports available which reveal that many factories near Udaipur are engaged in making acids, secret trade name, and highly toxic chemical for exports. It is a banned item in the western countries because of toxicity and poisonous gases and liquid effluents that emanate from it during its manufacture.

Under the confidentiality clause for protection of trade secrets, the chemical formula has been kept a secret from public and officials and consequently the regulatory agencies have failed to exercise control over them. Therefore in this way some laws are itself hindrances against the legal control of hazardous substances in the country.

The idea of management of hazardous substances is relatively new to the Indian industrial, administrative and legal backgrounds. However, the problem of toxic hazards has already touched dangerous proportions. Therefore, there is a great need of emphasis on these issues.

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Contents

Part VIII - Appendix: VHDL coding style and syntax guide

16 - Datapath sequential logic

Summary

28 - Metastability and synchronization failure

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9 - Combinational examples

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5 - Delay and power of CMOS circuits

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1 - The digital abstraction

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19 - Sequential examples

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Preface

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7 - Work Physiology

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Contents

4 - Work Measurement

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18 - Cardiovascular Health of Steel Workers

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Acknowledgments

6 - Introduction to Ergonomics

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9 - Risk Assessment for Distal Upper Extremities (DUE) Disorders

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15 - Occupational Heat Stress Exposure

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17 - Occupational Health and Safety

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List of Tables

3 - Method Study (Motion Study)

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