The basic components of each weighing system can be summarized in three fundamental elements: load cells, electronic unit and automation equipment. Let's see them in detail.
LOAD CELLS have a very precise operation: being transducers, their role is to detect a force applied to an object, by measuring an electrical signal that changes according to the deformation that this force produces on the object itself.
So we can simplify by stating that the load cells transform the measured weight into an electrical signal, which is usually low level (- mV dc). The load cells can be of different types, however the most used are the load cells with electrical strain gauges. Of different shapes and sizes according to the needs, the load cells are usually used in numbers from 1 to 4 even if, in specific cases, their number can even reach 8 or more.
The ELECTRONIC UNIT has the purpose of powering the load cells and transforming the voltage generated by the cells into a standard signal or digital information of the computer type. Depending on the needs, the electronic unit can be constituted by a normal blind transmitter for field mounting or by a weight indicator equipped with some automation functions, such as dosage or alarms of various types, dedicated exclusively to the single weighing loop. . In the event that, for particular needs, the electronic unit should be placed at a greater distance than the length of the cell cable, the use of a junction box is envisaged in order to connect the load cells in parallel and for the connection to the extension cable.
Finally, the AUTOMATION EQUIPMENT of industrial weighing systems consists of a PC or a PLC and performs the function, as the name already clearly defines, of integrating elements of automation of the weighing process that may be required by the process productive such as: dosages, adjustments, starting and stopping machines, notifications and alarms and much more.
LOAD CELL CONNECTIONS
Going deeper into the topic, let's now see what it means to connect load cells in parallel to electrical strain gauges. In concrete terms, the result of connecting multiple load cells translates into the constitution of a single load cell whose characteristics are defined by the qualities of the constituent cells. In other words, the nominal capacity of the totality of the connected load cells is equal to the sum of that of each cell while the sensitivity corresponds to the sensitivity of each constituent cell. It is important to remember that, in order for the system to be functional, all the cells must have the same sensitivity and the same range and the output signal must be strictly positioned in the center of the power supply.
There are two main solutions for connecting the cells to the electronic unit: four-wire connection and six-wire connection. Both configurations have strengths and weaknesses that we will deepen below.
1. Four-wire connection
The four-wire connection of the load cells is the most economical from the point of view of the cost of the extension cable and guarantees a lower incision of disturbances on the line. Measurements, however, can run into errors caused by the change in ambient temperature.
2. Six-wire connection
The main advantage of the six-wire connection for the load cells is the absence of errors due to the variation of the ambient temperature. However, due to the introduction of sensing conductors, the risk of disturbances increases and the cost is increased due to the longer length of the extension cable.
In the 4-wire connection errors appear due to the variation of the ambient temperature, which in turn generates changes in the resistance of the power supply cable to the cells and therefore in the supply voltage. The error can be obtained through the following equation:
ERR = 0,01491 Tv * Lc / (Rp * Sc)
Where is it:
• Err: measurement error (% gross weight measured)
• Tv: variation of the ambient temperature with respect to the calibration temperature (° C)
• Lc: length of the extension cable (m)
• Rp: Total resistance of cells in parallel (1 / (1 / Rp1 + 1 / Rp2 + .... 1 / Rpn) (ohm)
• Sc: section of the power supply conductors (mm2)
In case of need for very high precision measurements, it may be necessary to cancel the measurement differences at the corners of the platform caused, generally, by the differences in the nominal sensitivity of the load cells and the mechanical assembly differences.
The actions to be taken, in this case, concern the installation of trimmers in series with the power supply or in parallel with the signal. In detail:
• Trimmer in series with the power supply
It produces interactions between zero and field and shifts the signal level causing circulation of currents between the cells with following slight non linearity;
• Trimmer in parallel to the signal
reduces the maximum available signal and causes slight non-linearity.
It is important to specify that the trimmers to be used must have the lowest possible temperature coefficient. Alternatively, it is possible to use wire resistors (to be cut or scraped) with a very low temperature coefficient but not very convenient given the cost that could make the calibration of the angles exaggeratedly onerous.
ELECTRONIC UNIT CONNECTIONS
In conclusion, we will examine the basic electronic unit connection diagrams that have developed over the years thanks to technological innovation and are currently used in various industrial weighing systems. We can make a general subdivision as follows:
• 1970s scheme
Born as a simple low cost system for standard indications or alarms, it generates a mA signal requiring a simple shielded pair cable easily available on the market. There is a version with a frequency signal, usually used for complex weighing systems of the conventional digital type but has never had a wide diffusion on the market.
• 1980s - 1990s scheme
With this evolution, the weighing system enters by law in the field of industrial instrumentation and in the development of integrated systems with the general automation and plant supervision systems. Some types of these industrial weighing systems allow angle calibration without resorting to trimmers or wire resistors thanks to the measurement of the value of each individual cell.
• Classic scheme
This is the most used system in the case of single weighing loops, also known as “one cell, one instrument”. It risks becoming very expensive in case of long distances since the extension cable must travel a path well separated from the power or other cables to avoid possible cases of electromagnetic disturbance. The different shields applied on the cable also further affect costs to avoid any errors due to external disturbances, whether it is a four or six-wire connection.
• Future Scheme
Despite the name, this type of scheme is already used today in some industrial weighing systems but is not yet widespread on a large scale. With the development and consequent reduction in the size of integrated circuits, it became possible to equip the load cells with signal conversion and serial transmission circuits. Thanks to these integrations, the load cells become part of a computer network that can be connected both to classic electronic units, to large automation and supervision systems, and to the Internet.