OPERATOR PERFORMANCE IMPROVEMENT THROUGH TRAINING IN A CONTROLLED CABLE YARDING OPERATION Giovanna Ottaviani Aalmo1†, Bruce E. Talbot2 1,2 †
Norwegian Forest and landscape Institute, Norway.
[email protected],
[email protected] presenter
Introduction In Norway, some 150 million m3 (solid under bark) of timber is mature or maturing on slopes with an inclination steeper than 33% (steep), of which 62 million m3 are on slopes exceeding 50% (very steep). The lack of skilled yarder operators is a recognized bottleneck to achieving a higher efficiency level and running professional, year round operations. Many agricultural and forest workers also employed in Norwegian steep terrain harvesting sites, are migrant workers, seldom with an experience in this field. Higher productivity levels can be achieved through providing new/better equipment or tools, developing and implementing work standards or improving the performances of the harvesting crews. The skill levels of the workers can be improved through specific training. Training of new yarder operators traditionally takes place under normal working conditions but involves a certain level of risk to the personnel and equipment. This practice also presents several disadvantages mainly concerning decreased productivity, while sudden tension spikes can break the rigging, uproot tail spars, or snap one of the lines. Instead the use of a test rig could decrease down-time, repair and re-rigging costs while improving the safety conditions of the trainee operating the cable yarder. The effect of training can be evaluated using a learning curve, which illustrates the rate of learning for a given task . The main objective of this study was to investigate to what extent consecutive replications of the same yarding exercise improved operators’ skill levels. Materials and Methods The test was performed over two days; each participant was assessed 6 times. The time required for the yarding exercise and the subjects’ improvements in relation to their cumulative experience (six consecutive replications of the test) were investigated. Five subjects with varying levels of experience in operating similar joystick controls participated in the test. The machine used was a 1:3 true scale of an Owren 400 which is a popular tower yarder in the boreal zone, and was used in a running skyline configuration (Figure 1).
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Figure 1:: The Owren mini 400 used d as a test rig in the trials, detail of the tower and thee drums (photto: Morten Nitteberg))
The yarrder was riggged in a 42 2 m long coorridor with h a 17 % slope (Figuree 2). The en nd-block was fixeed 6.4 m abbove the gro ound. Maxiimum defleection at mid d-slope wass 5.2 m. Five small logs weere laid out on o permaneently markedd course.
Figure 2: The schematic representatio on of the corriidor layout, on n a 17 % slopee 42 meters lonng, five logs were w placed on permaanently marked d course, two between the toower yarder and a the obstacle and three bbetween the ob bstacle and the end trree.
This stuudy employeed a repeateed measure experimenttal design with w two inddependent variables. v The deppendent varriables in th his study weere the totall time to peerform the ttask and thee tension measureed during thhe exercise. Each of the subjects had to yaard all five logs to thee landing area inddividually, giving g one replication r oof five cyclees. In total there t were 6 replication ns of the 5 logs pper subject where w both time and ccorrespondin ng tension were w measuured. The fiive cycle times (11 for each log) were su ummed to ggive the rep plication tim me. In measuuring the teension, 6 replicatiions of the five logs were w recordded. The ten nsion (N) was w quantifiied using a wireless 3.5 kN dynamometter with con ntinuous loggging. This was attach hed to the ennd block to monitor the tenssion in the haul-back h line (equivallent to that in i mainline), and proviide some in ndication of the ‘‘smoothnesss’ of the op peration. T Tension spik kes should be avoidedd as they caan cause 2
system failure, breaking the guy-lines or snapping the mainline or haul-back line. The ability to operate with low tension is one of the main advantages of the running skyline. We analyzed the data by using linear mixed effects models. We used Subjects as random effects. As fixed effects, we included Trial number, Previous Experience and the interaction of the two into the model. We checked for normality and homogeneity by visual inspections of plots of residuals against fitted values. To assess the validity of the mixed effects analyses, we performed likelihood ratio tests comparing the models with fixed effects to the null models with only the random effects. Results The expectation that the mixed model would show a decrease in total time according to the different categories of ‘previous experience’ was confirmed. The plot shows a learning pattern with subjects improving their performance (less time used to perform the same task) for each replication over the previous one. The improvement looks fairly linear. The intercept is higher for subjects with less experience and closer to zero for the control (Instructor).
STANDARD DEVIATION (N)
The standard deviation of the Tension for the 6 trials decreases with increasing trial number, getting into the range of the control by the 6th repetition (Figure 3).
5000 4000 3000
Subject 2
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Subject 4
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Control
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REPLICATION
Figure 3: Standard deviations around the mean tension, by subject and replication
Conclusions This paper quantifies the learning effect of repetitive training of 5 subjects on a mechanical tower yarder simulator. The improvement in task completion time as a function of replication number and previous experience, together with the reduction in tension variability, provide support for the utility of learning curve theory in the prediction of future productivity with a training intervention.
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