Electronic and digital weight measurement on agricultural machinery
A wide range of sensors is required for data acquisition leading to operational decisions aimed at the application of Agriculture 4.0. Those measuring weight undoubtedly play a significant role in monitoring products harvested or distributed in the field by agricultural machinery. Strain gauge load cells represent the main category of this type of sensor
The so-called "Agriculture 4.0" is the evolution of precision agriculture, the "upgrading" of farm management strategies that use information technology to acquire data leading to finalized decisions. Basically, it relies on the automatic collection, integration, and analysis of data from the field, from sensors, and from any other third-party source to supplement the farmer's decision support with information from other actors in the supply chain beyond the boundaries of the individual enterprise. The aim is to increase farm profitability and economic, environmental, and social sustainability.
Among the most critical data coming from the field (in the broadest sense of the term) are definitely included the weight values of products harvested and/or materials distributed in various ways (organic and inorganic fertilizers, plant protection products in mixtures, ingredients of unifeed rations, etc.), which are often continuously monitored in real-time, depending on the area harvested or treated. In order to collect this category of data, we use sensors that, in most cases, are represented by strain gauge load cells, suitably connected to signal amplification and management circuits. These are then sent to computerized control units for their subsequent display, storage and management, often remotely and in the cloud. After a brief discussion about the construction and operation of these weight sensors, a brief overview of some of the most common implementations on agricultural machinery will be given.
The Wheatstone bridge
This is by far the most frequently adopted electrical scheme in strain gauge load cells for measuring forces, thus also weight force. It is based on a rhombus connection scheme of 4 resistors. A constant, stabilized electrical voltage is applied to one pair of opposite vertices of the circuit, and a voltmeter is connected to the remaining pair. If the resistors are identical (i.e., the bridge is "balanced"), their effect on the voltage is perfectly balanced so that the voltage will be zero on the meter. If, on the other hand, as an example, the ohmic value of one of the resistors is varied, the bridge will become unbalanced, and the voltmeter will indicate a proportional voltage accordingly. If one or more of the branches of the Wheatstone bridge are represented by strain gauges, the electrical voltage indicated on the voltmeter will be proportional to the mechanical stress on the structure to which they are bonded. Through the Wheatstone bridge, infinitesimal changes in resistance, even as small as 0.0001 Ω, and consequently minute magnitudes of force, can be measured with extreme accuracy.
The strain gauge load cell
It is an electronic transducer that measures the force applied on an object to which it is attached (or otherwise in contact) thanks to a series of strain gauges inserted inside it, almost always combined in Wheatstone bridge circuits. They are based on a metal body made of steel or aluminum, in which the strain gauges detect the mechanical deformations of the material, compression, tension, bending, shear, etc., translating them into voltage changes in the accompanying electrical circuit, which is finally correlated to the force parameter. Strain gauge load cells can be of several types.
Plate-bearing bending: suitable for small and medium capacity systems when the weighing surface must remain insensitive to the load's implementation point. They are generally made of aluminum and are not recommended for use in high-humidity environments or in the presence of chemically aggressive substances because of the inability to hermetically seal the internal strain gauges due to the special construction design.
Double bending: they are suitable for small capacities (2÷300 kg) and are characterized by good response even in highly dynamic systems. Since they are insensitive to the point of load implementation, they are also profitably used for weighing containers with a center of gravity subject to small displacements.
Shear: They are used for medium capacity weighing (0.5÷10 t) and require solid, firmly anchored mounting bases so as to withstand even extremely high bending moments. They are not suitable for measuring significant transverse loads.
Universal tension/compression: They cover a reasonably wide measurement range (100 kg÷10 t) and typically allow thrust and/or tension measurements in equipment installed on lifting equipment.
Compression Column: They are used for medium/large capacity weighing (10÷500 t). They are self-centering and allow free thermal expansion of the parts.
Pin-type:: They are used on lifting equipment and operating machinery. Their peculiar advantage is the possibility of replacing the original pivots without needing modifications. These are load cells that are usually specifically sized for each implementation. Other types of strain gauge load cells are also available for particular uses, such as toroidal, double shear, flange and washer, etc.
The weighing function and, more generally, force sensing is widely used on agricultural equipment. Some typical examples are briefly illustrated below.
Fertilizer spreaders
On modern single- or double-disc centrifugal spreading models, the quantity spread is monitored based on continuous weighing by the difference of the fertilizer content in the hopper, which is attached to the machine's main frame by a subframe, with the interposition of one or two load cells. Specifically, the dual-sensor solution allows the material to be weighed accurately, even when working on slopes. The load cells, often with a medium to full high scale, i.e., up to 10 t, transmit at a high frequency (100 times per second) to the control unit the data on the weight of fertilizer still in the hopper and, consequently, by the difference the one spread in the field.
In variable-rate fertilization, based on prescription maps of the amounts to be applied to the various zones of the plot, this data is essential (along with geolocation) to act appropriately on the fertilizer output dosing lights to be able to comply with the programmed directions.
Mixer wagons
In this case, the need is to be able to precisely weigh the various ingredients of the feed ration according to the different recipes previously defined for the various groups of animals. Therefore, the appropriate formulas are stored in the control unit according to the sequence of introduction of the mixture's components into the mixing wagon. All recent models of the mixer (chopping) wagon can store several combinations, each consisting of a good number of different ingredients. Therefore, by recalling the recipe to be prepared, the system triggers a certain amount of advance via an audible (and sometimes visual) alarm when the desired amount of loaded product is reached. The advance is necessary to ensure maximum accuracy in dosing to take into account the hysteresis of the operation, i.e., that amount of ingredient that nevertheless falls into the hopper even after the interruption of the delivery, since it has already been conveyed into the distribution pipes, starting from the storage site.
Again, the quantity of each product is measured by continuous measurement of the incremental difference of the total gross weight of the loading hopper from the main frame, thanks to several strain gauge cells interposed between the two parts of the medium. For this purpose, there may be 3 or 4 compression, bending, or shear load cells.
Balers
On some models of its big balers, New Holland has implemented the "ActiveWeigh" function, which is the ability to weigh the freshly packed and moving bale during unloading just before it is deposited on the ground, thanks to the installation of load cells integrated into the ejection chute. The system can operate regardless of bale length, field conditions, and baler movement. The data provided, i.e., the weight of the single bale, the total weight of the packaged product since the start of work, the average value of the bales produced, and the productivity in t/h are displayed on the IntelliView monitor and are helpful for complete monitoring of processing, with a view to "Agriculture 4.0."
Farm trailers
In addition to several other valuable features, more advanced models of farm trailers are equipped with a weighing system of the transported material, quite similar to that of mixer wagons. In this case, the value of the described function is even higher, taking into account that to circulate on public roads under the provisions of the Highway Code, it is essential not to exceed the maximum capacity of each model. Besides integrated solutions, already provided for by the manufacturer at the trailer's manufacturing stage, retrofit mounting kits can also be found on the market based on one (or more) strain gauges installed ad hoc in strategic parts of the machine's load-bearing structure, which can indicate on the relevant control unit the continuous weighing of the loaded product.
The strain gauge
The strain gauge operation is based on the principle of linear variation (within defined limits) of the electrical resistance of conducting materials in relation to their strain. Basically, it is a strain gauge grid, i.e., a long sheet arranged in loops of resistive material (usually Constantan, an alloy of copper and nickel) deposited by photoengraving on a flexible plastic substrate, which is carefully applied with a suitable glue to the structure whose (micro)deformation is to be measured. The strain gauge is then connected to a circuit to measure the change in resistance, which is finally related to the force that caused the corresponding deformation.
Basically, in the strain gauge grid, the cross-sectional area of the conductor varies as a function of stress: if it is stretched, the resistance increases, while if it is compressed, it decreases.
The strain gauge substrate must be flexible, elastic, strong and hopefully electrically insulate the grid from the underlying structure. The most commonly adopted material for this purpose is epoxy resin reinforced with glass fibers.
The adhesive for bonding the strain gauge must ensure an intimate and durable connection with the structure's surface to be measured: the most suitable products for the purpose are cyanoacrylate or epoxy and epoxy-phenolic compounds.
Finally, the strain gauge must be protected or encapsulated to protect its implementation from disturbing external agents (moisture, contact with oils and solvents, improper mechanical actions such as blows, cuts, etc.). Silicone or nitrile resin is used for this purpose.
