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 The soil compaction plays an impor-tant part for the life of a road or a building, which is constructed on the soil. The best quality for the soil is reached by homogenous compaction. Tensions of the pavement are minimised also at intensive loads. When the road or the building shows defects even after a short time the reason is often a bad and inhomogeneous compaction of the soil. In this case the repair costs are extremely high. Neither building sponsors nor contractors want to take any risk and therefore demand certificates of the soil compaction. Conventional methods for certification take a lot of time, as e.g. the dynamic plate loading test. Measuring is done at random or systematically chosen points depending on the demands. These tests cannot be made for the complete area. Moreover the construction work has to be interrupted for the measurements. A continuous compaction control system (CCC) certification of the complete area is possible. Such measurement systems register the movement of the roller drum and determine the soil compaction from it. The ASC-sensor by Völkel is used for continuous measurement of the soil compaction for vibratory rollers. The roller operator is able to use the data of the sensor to recognize where the compaction work is finished and to avoid over-compaction. Furthermore during work they can find weak spots, which have to be repaired. The compaction sensor is mounted near the bearings of the drum. An acceleration sensor, which is integrated into the ASC-sensor housing measures the vertical acceleration of the drum and transforms it into a voltage signal, which is transmitted to the signal processor of the compaction sensor. The processor uses the fact that the drum of a vibratory roller reacts differently to the various kinds of soil consistency. Therefore data pertaining to compaction value, jump value, vibrator frequency and oscillation amplitude can be calculated. This data is transmitted via CAN to the display in the driver cabin, so that the roller operator can see the compaction result immediately.
The ASC-sensor uses the CANopen protocol to communicate with a display or a vehicle control unit. In addition a part of the EEPROM memory of the ASC-sensor can be used to store machine data in it. The memory can be accessed via object directory of CANopen. The value for soil compaction increases with the stiffness of the ground material. A non-increasing value from the last pass to the actual pass of the vibratory roller shows the maximum compaction. More passes are not necessary.
At larger building sites or road works a reference area is used to determine the set value for the complete area. The soil material of the reference area has to correspond to the ground material of the complete area. If the compaction value reaches the set value compaction work is finished, too. If the compaction value does not increase during the first passes and the value is much lower than the set value a fault area is detected.
The reasons can be that the soil is too soggy or of the wrong material. In the worst case the soil has to be replaced. The drum of a vibratory roller can start to jump if the soil is too hard, because the vibration energy cannot be transferred into compaction energy. In case of jumping the drum lifts off from the ground during a complete turn of the vibrator. This condition is bad for the driver and the mechanics, especially the bearings of the roller, therefore it should be avoided. If the jump value is beyond zero, the driver has to change the amplitude or vibration frequency or switch vibration off. Newer roller types are able to adjust the vibration amplitude or the effective direction of the vibration continuously. The jump and the compaction value can be used for an automatic control to avoid the jump condition and to make the soil compaction more homogenous and better.
The roller operator can use the continuously measured data from the ASC-sensor to save time, avoiding unnecessary passes, to remediate weak spots immediately and to extend the life of the roller by minimizing the jump situation. The basic system consists of the sensor, which is mounted on the drum and a display in the driver cabin. On most compaction measurement systems the evaluation electronic is placed into the driver cabin. The loss-free transmission of the analog acceleration signal from the drum to the driver cabin is not a trivial problem. The rough machine environment can interfere the signal. The long cable absorbs the analog signal. Interfering impulses of other cables can be injected into the signal in an inductive or capacitive way. Therefore the acceleration sensor and the signal processor are placed on the same board and combined in a minimized case. In this way the analog signal of the acceleration sensor can be transferred directly to the signal processor without losses or interferences. The result is an optimal base for the calculation of the compaction values.
The transmission of the calculated compaction values to a display or a vehicle control unit is done via CAN bus. This digital transmission is loss-free as well. The maximum distance for transmission is specified by the CAN bus, so up to 1000m are possible. The combination of the acceleration sensor and signal processor has many more advantages. The usage of more compaction sensors in one system is possible. Both drums of a vibratory tandem roller can be measured and data can be logged at the same time. Furthermore measurement is possible at the drum of a roller and in addition at coupled vibrating plates. The compaction sensor is easy to integrate in a machine-specific bus system. The compaction data can be used for control processes such as the control of the vibration frequency or the vibrator amplitude. Controlling the vibrator amplitude can avoid jump situations and ensures homogenous compaction results with high quality.
This article has been published in CAN CiA’s News Letter.
[CAN in Automation (CiA – www.can-cia.org) was formed in 1992 with a view to help promote, standardise and foster development in new applications of this innovative technology called CAN (Controller Area Network). CiA has appointed Global Technology in Mumbai to help CAN Community in India. Contact: Global Technology (CiA India Office). Email: info@ globaltechnology.org.in]
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