Ergonomics in machine design

7. 1. INTRODUCTION.

Mechanization not only changes the character and structure of labour in agriculture and industry, but also influences the nature of the work load. Instead of delivering energy, the main issue now is the perception and processing of information, as well as controlling and regulating the work being done by machines. More and more the accent is shifting to tasks which appeal to the capacity of man to process information.

Supplying the requirements for increased capacity is possible by increasing the ground speed and/or the working width, which, however, involves higher information speed, more actions for operation per time unit and a higher operator's work load. Human beings should not adapt themselves to the machine and - as a consequence of too high a load - will hazard occupational diseases and reduced performance.

By means of principles, parameters, procedures and criteria from various disciplines - anthropometry, perception, selection and action - ergonomics can contribute to an optimum man - task system with a high capacity and a favourable operator's load.

This study deals with the ergonomic system-analysis of the operation of a self- propelled combine harvester. The operator of this machine regulates the movement - direction and speed -, the mowing and threshing, the separation of kernel and straw, as well as the delivery of the product. Based upon the information obtained from crop, terrain, plot and machine the operator has to decide on such an adjustment of the machine, which results in a process with a high capacity and a high quality of work.

7.2. ANTHROPOMETRY.

The characteristics of the human body determine the location and displacement of the operating controls, as well as the forces to be exerted, the nature and frequency of movements. Based upon the relevant measurements and motions of the human body, the anthropometric data are given for designing an optimum work- space lay-out of a combine harvester.

1. Lay-out studiesTo determine the location and displacement of the controls, as well as the forces to be exerted, and the location of the foot-board and cutterbar, special measuring equipment and methods have been developed.The work-space lay-out - specific for each machine - appeared to differ considerably. It is recommendable, that frequently used controls are located close to each other and within the normal area.

The forces needed for steering are very low, because of the application of power steering. The forces needed for operating the clutch and the brake-Pedal are too high for the older machines; for the newer combines they meet the requirements.

2. FrequencyThe frequency with which different operations and movements have to be carried out, determines mainly the operator's work load. The data, collected in measuring the operation frequency, indicate the priority in the lay-out of controls.

The studies show, that the header height, reel position and ground speed control are most important. The location and displacement of these controls deserve priority in the design of the work-space lay-out.The movements of the steering wheel only have a correcting character.

3. M.T.M.-analysisBased upon the results of the lay-out studies and the frequency-analysis, the operating element time has been calculated by means of M.T.M.; these timevalues can be used to obtain reliable data of the ergonomic quality of the lay-out of a certain work-space.

The study shows, that there are great differences between the various machines; a small modification in the work-space lay-out leads to a favourable change in the operating element time. A development in the direction of fingertip control - i.e. header height, reel position and ground speed control grouped together - is desirable.

7.3. PERCEPTION.

The perception of information, that a worker receives from work-space and environment, takes place via the senses, which are sensitive to specific impulses. The reaction to the information depends upon the sense organ that is stimulated, the strength of the stimulus and the place where the stimulus arrives.

1. SightThe perception of visual information depends upon the object, the environment and the distance between the eye and the object.

For combining it is necessary to obtain information from the feed table auger just in front of the conveyor chain and of the separation from the previously cut swath. On the machines examined the visual angle, as well as the horizontal distance between the eye and the cutterbar, are favourable in the horizontal plane. In the vertical plane the view angle is extremely unfavourable; besides, the distance between the eye and the cutterbar is too long, which is caused by the large vertical distance.

Improvement of the visibility is possible by reduction of the view angle and the vertical distance. The operator's platform must be brought closer to the field. An eccentric location of the operator's platform offers favourable possibilities, since the platform can be located next to the conveyor chain.

2. HearingHearing ensures mutual communication between man, machine and environment. Depending on frequency, sound pressure level and duration of the exposure, sound has an annoying or a damaging effect.

By the functioning of the engine and other parts of the machine, in particular: the threshing mechanism, the operators of combines are submitted to sound. Measured at the operator's ear, the sound pressure levels of the machines examined are too high, whilst the presence of a cabin does not lead to an important decline of the sound pressure level. Moreover, the ground speed during combining does not affect the sound pressure level.

The most common method of solving the sound problem is the enclosure of the source to reduce the sound pressure level to bearable levels.

3. Scent and tasteFrom an ergonomic point of view scent and taste are less important, because only a small number of actions appeals specifically to these senses. Attention must be paid to the dust in the air around an operator of a combine.

The dust concentration in the air around operators of the machines examined is too high; with respect to the duration of the harvesting period the possibility of developing silicosis must be considered to be low. On a machine without cabin the dust concentration is considerably higher than on a machine with cabin; during harvesting barley less dust is circulating around the operator than during harvesting wheat.

Protection of the operator of a combine against dust is possible by ventilation (blowing away or sucking off the particles) and personal protective equipment. The ideal concept is the enclosure of the operator in a cabin with a small overpressure, so that particles cannot penetrate into the work-space.

4. FeelingBy uneveness of terrain and road surface, the profile of the tyres and the functioning of the engine and other parts of the machine, the operators of machinery are submitted to mechanical vibrations, which adversely affect their health and performance. The effect and impact of vibrations varies with the frequency, as the human body is most sensitive to mechanical vibrations within the frequency range from 2 Hz to 6 Hz. It is necessary, that the curve K = 4 of the VD1- Recommendation is not exceeded in designing the work-space of farm machinery.

The intensity of mechanical vibrations on combine harvesters is low, whilst the movements in the vertical direction are more numerous than in the other directions. Combining at different ground speeds appears to affect hardly the intensity; with greater working width the intensity is slightly smaller. When driving on the road with increasing ground speeds there appears an increasing intensity, which is lower than during combining. The spectral distribution indicates the existence of specific frequency ranges, which are explicable from the machine.

At present, the most obvious way to reduce ride vibration is to fit a correctly designed suspension seat. The benefit of a good suspension seat is limited by the increasing movements between the operator and the controls; to eliminate this the vibrations of the whole vehicle or the whole of the operator's work-space should be reduced. A suspended enclosed cabin - in which controls, seat and operator move simultaneously - is an approach, which combines the need for improving ride with greater protection from sound and dust.

Since machines, performing functions of perception, do not exist or are very rare, the designer of a man-task system must take into account the attainments of the investigations regarding perception. For the combine this means primarily an adequate presentation of the relevant information; by improving the visibility of the cutterbar (reduction of the view angle and the vertical distance) this can be realized. Besides, the redundant and non-relevant information - sound, dust and mechanical vibrations - has to be eliminated; the suspended cabin - in which the controls, the seat and the operator move simultaneously - is an approach, which combines the need for improving ride with greater protection from sound and dust.

7.4. SELECTION.

The transport of data from the stimulated sense organs ('input') to the effectors ('output') is not simply and solely transport. The data are processed, from the various possibilities the right answer is chosen ('choice') and transformed into action.

1. TheoryIn task performance the number of directed switches from input to output determines information processing and mental load, because all signals make use of 'a single channel decision mechanism' with a limited capacity. Generally, a value of two to three bits per second is assumed to be the limit for adequate information processing.

For the assessment of mental load a well-functioning parameter is not yet available; most can be expected of the method with dual tasks and the heart rate.

2. Indoor experimentsFor studying a man-task system under controlled conditions and to obtain information regarding the ergonomic qualities of the work-space lay-out of a certain machine, a simulator is built.

The studies led to the following conclusions:a. In simulating the operation of a combine harvester the performance and the load depend upon the experimental distraction and the anthropometric qualities of the work-space lay-out. The steering accuracy detiorates and the load increases as a function of the number of signals per minute in the secondary task. There is a significant difference between the machines, which increases as the experimental distraction increases.

b. By executing an identical series of information a learning curve appears; the performance increases and the load decreases as a function of the number of cycles. The appearance of the learning curve is disturbed by different series of information in an experiment. There is a significant difference between the machines.

In simulating the operation of a combine harvester, the performance and the load depend upon the experimental distraction (the number of signals per minute in the secondary task and the series of information in an experiment) and the anthropometric qualities of the work-space lay-out of the machine. The results of the indoor experiments confirm the statements - endorsed completely by the results of the M.T.M.-analysis -, that subjects are more quickly familiar with one machine than with another, which causes a more efficient performance.

The conclusion can be made, that the performance and the load are primarily determined by the work-space lay-out.

7.5. ACTION.

The choice of the decision mechanism is transported to the effectors and transformed into voluntary muscle movements, which are necessary for the output or performance of a man - task system.

1. TheoryThe muscle movements demand energy and this leads to the physical load of man, whose working capacity is limited. Besides, the mechanical effect of human labour is very slight. An energy consumption of 20 kJ (= 4,8 kcal) per minute, including a basal metabolic rate of about 4,2 kJ (= 1 kcal) per minute, has been accepted as the maximum consistent level, that an adult man should be expected to expend.

For the assessment of physical load well-functioning parameters are available. In all kinds of tasks, even over a long period of time, the heart rate can be faultlessly registered.

2. Field experimentsFor collecting data about the character and extent of loading components during combining, as well as the influence they have on the output of the system, field experiments have been carried out.

The studies led to the following conclusions:a. During the operation of a combine harvester the performance and the load depend upon the machine - in particular: the operator's platform-, the stubble height, the ground speed, the working width and the crop. There is a significant difference between the operator's platforms, which increases as the ground speed increases. The interaction between ground speed and working width, representing capacity, is significant; realizing a certain level of capacity, it is preferable to work at a low ground speed combined with a greater working width.

b. Using an automatic header height control system the operation frequency and the load decrease. The advantage of an automatic system becomes greater when working at higher ground speeds.

The results of the field experiments endorse completely the results of the anthropometric studies and the indoor experiments. There are differences between the machines; at an increasing information speed - indoor: increasing experimental distraction; field: increasing ground speed - the differences between the machines become greater.

The conclusion can be repeated, that the performance and the load are primarily determined by the work-space lay-out.

7.6. DISCUSSION.

Human beings should not adapt themselves to the machine and - as a consequence of too high a load - will hazard occupational diseases and reduced performance. By means of principles, parameters, procedures and criteria from various disciplines - Anthropometry (Chapter 2), Perception (Chapter 3), Selection (Chapter 4) and Action (Chapter 5) - the ergonomic factors, influencing the output or performance in operating a self-propelled combine harvester, have been studied by means of lay-out studies, indoor and field experiments. Based upon the results of these studies the conclusion can be made, that the performance and the load in a man-task system are primarily determined by the workspace lay-out. Recommendations for the ideal concept of the operator's platform of the self-propelled combine harvester are given (Chapter 6).

Supplying the requirements for an increase of capacity it is necessary, that the designer of a self-propelled combine harvester pays attention to an adequate processing of the increasing information speed. This is possible by using machines with an optimum work-space lay-out and eliminating of redundant and non-relevant information. Moreover, the introduction of automation, in which many processes - combine: moving, mowing, threshing, cleaning and maintenance - are executed and corrected without human interference, offers favourable advantages.

In the near future the 'supervisor of harvesting' has to settle - based upon the conditions of crop, terrain and climate - the norm and the tolerance of the whole process. Next to it, the supervisor has to control - sitting at home, in front of a correctly designed console - the processes and, if necessary, has to re-adjust by means of remote-control.