APPLICABILITY OF HYDRAULIC DYNAMOMETER FOR MEASURING LOAD MASS ON FORWARDERS

In the last few years, with the start of wood biomass production from wood residues, the need for determining the quantity of extracted wood residuals on a landing site has appeared. The beginning of intensive usage of wood residues for wood biomass starts in lowland forest where all wood residues are extracted with forwarders. There are several ways to determine load mass on a forwarder, first and probably most accurate is the use of load cells which are installed between forwarder undercarriage and loading space. In Croatia, as far as it is known, there is no forwarder with such equipment, although manufacturers offer the installation of such equipment when buying a new forwarder. The second option is using a portable measuring platform (axle scale) which was already used for research of axle loads of trucks and forwarders. The data obtained with the measuring platform are very accurate, while its deficiency is relatively great mass, large dimensions and high price. The third option is determining mass by using hydraulic dynamometer which is installed on crane between the rotator and the telescopic boom. The production and installation of such a system is very simple, and with the price it can easily compete with previously described measuring systems. The main deficiency of this system is its unsatisfying accuracy. The results of assortment mass measuring with hydraulic dynamometer installed on a hydraulic crane and discussion on factors influencing obtained results will be presented in this paper.


INTRODUCTION
Wood has always been a very important source of energy at all stages of society development.In the middle of the 19th century it was the source of almost 90% of the energy needed for humanity, while in the last decade of the 20th century in the developed countries its share was below 5%.With the beginning of energy crisis the share of wood in the energy balance of most developed European countries starts to rise, as well as the need for all other energy sources.
In the Republic of Croatia, which has 44% of its land territory covered with forest, there are numerous possibilities of using wood biomass, and there is a long tradition of using this renewable energy source, but mostly for household heating.The production of biomass is implemented into the Strategy of Energetic Development of Croatia, in which it is planed that by 2030 15% of all consumed energy comes from biomass and waste.The stated should be achieved through creating legislative conditions for an increased usage and production of biomass and waste energy, encouraging research activities in this area, and development of technologies for its gathering, conversion and usage.Furthermore, the idea is to enable information and knowledge exchange.
In 2007, the company "Šumska biomasa" Ltd. was established.The company is completely owned by the state forest enterprise "Hrvatske šume" Ltd.Zagreb, and it was established for the purpose of organizing, gathering and production of energy wood.Most part of its production, since its beginning, is the production of forest biomass from regeneration fellings from Croatian lowland forests, as a result of forest residues management.
Bosner et al. gives three basic reasons for determining the quantity of "green" forest biomass right after forwarding ends: 1. Volume of extracted forest biomass cannot be determined by measuring its length and diameter.

It was needed to establish the norms for
forest biomass extraction so that production and costs can be determined.3. The need for collecting the data on the quantities of forest biomass, for better production planning.For the purpose of measuring axle loads, respectively gathering informations about load which is transported, by now two models of portable measuring platforms were used in the administration unit "Vinkovci".The model described by Bosner et al. (2008) consisted of two portable scales WLS 101/R2K which was put into the metal housing, the maximum loading capacity of scales was 10 tons.The dimensions of the metal housing were such that only one axle was measured at a time.Because of its malfunctions, a larger, more robust portable platform was created.Its dimensions were 2.7×3.0 m, and it had the possibility to measure loads of up to 30 tons with the precision of 10kg.This measuring platform was created only for the administration unit "Vinkovci" and its needs for determining the mass of extracted energy wood and measuring axle loads of trucks for wood biomass transport.Because of its dimensions, with this measuring platform it was possible to measure both (rear) axles of trucks or entire bogi axle of a forwarder.The construction and detail technical characteristics were described by Zorić et al. (2012).Poršinsky (2005) speaks about forwarder carrying capacity, and he divides it into the theoretical capacity which is determined by forwarders technical characteristics and the practical capacity which is under the influence of terrain characteristics (soil condition, slope, and surface obstacles).These characteristics mostly decrease theoretical possibilities of the vehicle.Pandur (2013) researched the mass utilization of forwarders loading space on two different types of forwarder during extraction of technical roundwood, energy wood and forest residues.For his research he uses a portable measuring platform described by Zorić et al. (2012).
The aim of this paper is to determine the accuracy of mass measuring with a hydraulic dynamometer installed between the telescopic boom and rotator of a forwarder hydraulic crane.For this purpose a hydraulic dynamometer was tested using oak logs and the reason is easier manipulation with logs than with energy wood such as tree tops.

MATERIAL AND METHODS
The hydraulic dynamometer LM Multi was used for weight measurement, i.e. for measuring force on the forwarder or truck crane during loading or unloading of wood.It is composed of a measuring link (figure 1) installed right after rotator, hydraulic pipe which transfers oil pressure from measuring lint to the pressure sensor (figure 3) and measuring device (figure 4) with suitable software.It measures pressure in the hydraulic system of the machine during cargo lifting.Pressure sensor converts the created pressure into the weight or mass, which is displayed on the screen of the measuring device.
The hydraulic system of dynamometer uses oil from the crane.During installation of the instrument, it is necessary to install the connection of two hydraulic systems with the valve for opening and closing (figure 2).When the hydraulic pump is turned on and valve is open and system starts to fill with hydraulic oil.During crane manipulation, the valve should be closed.
The hydraulic pressure sensor should be set into a dry and protected place, to extend its working period.The best place for its installation is in the cabin of the machine.The sensor measures pressure using the strain gauge method and data are transferred to the measuring device by electric signals.The rest of the connections are used for connecting to the printer, keyboard (hand switch) or scaling pedal (leg switch).The keyboard or pedal are used for non-automatic measurement, when the load is steady on the crane, the scaling is conducted manually.In the automatic mode if the switch is on, scaling is not possible.
Load master has a 5.4˝ colored screen, with the menu in the Croatian language.The possibility of updating the device and data collection through USB connection is very useful.
The research is divided into three stages.The first and second stages of measurement are conducted in mechanical workshop where the dynamometer was installed, while the third stage was conducted in field working conditions.

Scaling of the whole load
The measurements of forwarder (Valmet 840.2) crane load were done in the mechanical workshop of the forest office "Otok".Fourteen oak logs were used for the measurement.The logs were felled a month before measurement and their dimensions are shown in table 1.
At the first stage of measurements the automatic mode was used during loading and unloading of the forwarder.During unloading the mass of 14 logs was 7,764 kg, while the loading mass was 7,508 kg.The mass of the loaded and unloaded forwarder was also measured by bridge scale.The loaded forwarder had a mass of 23,740 kg, while the unloaded forwarder had a mass of 15,340 kg.On the basis of that measurement the mass of logs was 8,400 kg and this was considered the most accurate data.

Testing of hydraulic dynamometer accuracy -Multiple scaling of only one log
During precision testing 1 , the log was grabbed by grapple in its middle (figure 7), mass center (figure 8), on the thicker end (figures 9-12) and thinner end (figures 13-16), and at the front of the log (figure 17).While the log was grabbed by it thicker and thinner end four measurements were conducted, for each measurement log was turned for 90˚ around rotator.The results of the measurement are shown in table 2.

1
Precision is a measure of reliability of measuring device, i.e. there is a possibility of measuring device to measure the same value with minimum changes, while accuracy is described as the deviation of measurement from the exact value, and unlike precision it can be defined by only one measurement.

Testing of the hydraulic dynamometer during wood extraction
For the purpose of the research during two working days forwarder extracted wood and accuracy of crane scale were tested.In total 12 loads were measured during loading and unloading.The first seven loads were measured with the automatic mode during loading and manual mode during unloading, while the next five loads were measured with the manual mode during loading and automatic mode during unloading.From the 5 th till the 12 th load, an axle scale was used for control measurements.The results of the measurements are shown in table 3. Before the measurement of loads 11 and 12, the measuring device was set to increase the measuring mass by 6%.The green bars represent load mass measured by axle scale.The yellow bars are load mass calculated from load volume and wood density of 1.150 kg/m 3 .Wood density was calculated from measured mass using axle scale and load volume (loads 5-12).

DISCUSSION WITH CONCLUSIONS
Values ranging from 510 to 818 kg were obtained by mass measuring of one log (11 measurements), and the volume of the log was 0.62 m 3 .In the measurement of that same log on axle scales, whose precision was 20 kg, its measured mass was 800 kg.The values measured while the log was grabbed in the mass center and at the front of the log were 810 kg and 807, respectively.It is obvious that the real mass of the log was 810 kg.The average of the rest of the nine measurements, when the log was grabbed outside of the mass center, was 723 kg, which is 10.7% less than the real mass of the log.The reason for that is dynamometer construction, which gives accurate values only when it is in position of plumb, at a 90˚ angle in relation to the ground surface.If the angle between the dynamometer and vertical axle is larger, the measured mass is lower.
On the basis of the results of measurements it is observed that, regarding the working mode, the measured values are larger during loading than during unloading.In all 12 loads, the loading had larger values by 13.2%.The reasons for that were twofold.The first one is that, when using the automatic mode, it is possible to measure one log several times.It appears during movement of the log to the forwarder and positioning of the crane for lifting.During movement of the log, a force appears, which is recognized by the dynamometer software as a measurement, and the device memorizes it as a mass measurement.The same thing appears when logs are manipulated in the loading space for better positioning.This type of error was observed in load 2. When using the automatic mode the mass of 20.672 kg was measured.It appears because there were several large logs during whose loading multiple measurements were obtained.
The error of multiple measurements while using the automatic mode can be avoided.The switch which is used during manual mode measurement for memorizing the measurement value in the exact moment has the same function while using the automatic mode, i.e. it blockades measurement when it is turned on.In this research it was not used because both hands of the driver were used for driving the forwarder.To solve this problem, it would be practical to install this switch as a foot switch.Nevertheless, it can be installed as both a hand and foot switch, so that it can be used in the manual mode by hand and by foot in the automatic mode.It is very important for the driver to be very careful, because it is not possible to delete the memorized value.
The second reason for an error appears during unloading in the automatic mode, when the obtained values are smaller than the real mass of logs.It happens because, when unloading, the log "flies" downwards together with the grapple and the measured values of the mass are lower.The software of the device, when loading and unloading, during several seconds takes multiple readings, and the memorized mass represents the average of those readings.
It can be concluded that the automatic mode should not be used when unloading, while it measures larger values when loading.The manual mode is more precise, but it requires a log and crane to be calm, which considerably slows down wood extraction.The error which appears when log is not grabbed in the mass center and which is always lower than the real mass, can be overridden manually by setting the device software to measure greater mass than real.Before that, it is necessary to determine the correction fac-tor by comparing the measured values with the real mass measured by a certificated scale.For a more accurate determination of the correction factor it is necessary to conduct a larger number of measurements, although on the basis of these measurements it is probably somewhere between 5-10% for the ways of measuring described in this research.It is likely to presume that the value of the correction factor is most affected by the tree species and shape of the log, as well as the driver of the forwarder.

Figure 1 .Figure 4 .
Figure 1.Measuring link installed on the rotator

Figure 19 .
Figure 19.Graph of the measuring results

Table 1 .
Assortments used for measurements

Table 2 .
Measuring of log no. 2, volume of the log was 0.62 m 3

Table 3 .
Measuring data of 12 loads