Introduction
In the past, control equipment for dosing systems employing belt weighers was usually carried out with microprocessor circuits developed by each manufacturer for the exclusive control and adjustment of their own dosing systems. 
    Although application processes always demand a high level of reliability and precision, this system did not change because the market did not offer open systems or components for general use, which could be programmed to be able to control weighing systems with the correct level of reliability.
    This meant that customers found themselves with equipment that was difficult to access and handle and which required the permanent presence of highly qualified technical personnel to operate and maintain it. If not, it was necessary to depend on the services of the supplier, with all the attendant costs and waiting times. Through direct contact with its customers,
     Vidmar learnt what its customers wanted, which was not black boxes to control equipment that they could not access directly to resolve any problem and the need to keep large quantities of spares for each item of equipment.

Greater possibilities
However, once PLCs appeared on the market, the possibilities available for controlling any process offered the company a wide range of applications. Vidmar understood this and focused its efforts on developing all its weighing and dosing control systems, both continuous and static, with standard PLCs.
    However, it did not concentrate on a specific make. Instead it concentrated on developing powerful soft-ware that could offer all the basic services of any commercial control equipment and was also sufficiently flexible to adapt to the particular needs of each customer. It also needed to be easily adapted to the programming languages of each PLC manufacturer.
      Today, the company claims to offer even more functions than any commercial equipment available on the market. This is because the PLC is an open system therefore any expansion and/or customisation with specific functions for each installation can be made while at the same time remaining an open system that is easy to handle and maintain by the technical personnel.
     With specific reference to the cement plant and depending on the make of PLC normally used by the customer, the company chooses the CPU model best suited to the plant’s needs and capacity. Normally,  a   PLC

Figure 1. A control circuit for various dosing weighers.

    
for  each   section   is   used, i.e. cement mill, crude mill and supply of crude to the kiln. The capacity of the PLC is chosen according to the number of machines and/or dosing weighers to be controlled.
     The company consider it very important to choose a PLC that has a communications port in a standardised network for the entire plant, so that it becomes a remote terminal of the general control PLC for the section or for the central monitoring and control system of the plant. This means that the entire cable interconnection between the two systems is no longer needed and has the additional benefit that large volumes of information can be easily interchanged. This is very  useful for analysing problems and processing statistics as well as achieving easy and extensive monitoring of the equipment from the control room.
     As with any size of PLC, the basic needs to be taken  into account are as follows.

Weight reading
High resolution analogue inputs. At the moment, the majority of PLCs have analogue inputs of between 14 and 16 bits (internal resolution of 16 368 to 65 472).  This ensures a highly precise weight reading, although  the precision normally guaranteed for continuous weighing systems can be achieved with 12 bit inputs (resolution 4092). The 4 to 20 mA signal is obtained by means of a standard amplifier that supplies the weighing cell with +10 Vdc, reads the signal of this cell in mV and converts it into a universal 4 to 20 mA signal. Although Vidmar has is own weight amplifier (MPA-50),  any amplifier available on the market can also be used for this function.

Speed reading
The speed information is transmitted by an encoder (digital) which, in the case of the company’s weighers, is mounted directly on the rear section of the motor. For speed reading, a rapid reading module is used and is specifically designed for this purpose. Each PLC manufacturer has this equipment as standard. Although the majority of the rapid reading modules available on the market can read signals at frequencies up to 100 Khz, in the company’s case it manages inputs for frequencies of between 5 and 10 Khz.

Control signal
Analogue outputs: normally, the resolution of the majority of analogue output modules on the market is 12 bits, which is quite sufficient for the precise control of a dosing weigher. Although speed controllers can be used in some cases, which can communicate with the control PLC via field buses or communication net-works a 4 to 20 mA analogue signal is normally employed. This means that any speed controller on the market can also be used. The analogue output is con-trolled logically via a PID inside the PLC.

In the last instance, the motor speed is determined by a controller, usually of a frequency reached by the 4 to 20 mA analogue signal controlled from the PLC. This controller is commonly available on the market and is chosen by the customer.

Confirmation signals
     Digital inputs: these are normally 24 Vdc, but can  be changed according to the usual service voltages at each plant and the number of inputs per module required. The most common input signals of a simple dosing weigher are movement of the belt,  speed control, confirmation of operation, control faults and external interlocks.
    Command signals: like confirmation signals, these are normally 24 Vdc, but can be changed according to the usual service voltages at each plant and the number of inputs per module required. Normally, the digital outputs are used for the operating commands for each item of electrical equipment included in the system. Intermediate relays are responsible for the connection of the digital output to the power components.
      Local control system: although the majority of the data calculated on the PLC can be displayed on Scada monitors via the communications network installed at the plant, it is very important to have a local display, setting and control system to make adjustments, calibrations and corrections to programme times and to operate the weigher(s) in the local mode. To achieve this, touch panels are used that provide a man/machine exchange of information to make a simple adjustment to one or more dosing systems that make up the section. This screen normally communicates with the PLC through the same programming port or via an additional module, an RS-232/RS-422/RS-485 or other type of interface specific to each manufacturer.
Figure 1 shows a control circuit for various dosing weighers.

Conclusion
Due to this philosophy of offering open systems based on equipment and spares that are readily available on the market, and with a general standardisation of parts (because the same input/output, digital/analogue modules used for adjusting and controlling the dosing weighers can also be used for other equipment at the plant), Vidmar has succeeded in establishing itself not only in Spain, but also in other countries such as Mexico, Venezuela, Colombia, Morocco, Philippines, Vietnam, Jordan, Indonesia and Honduras. Its main national and international customers are the world's leading cement groups, such as Lafarge, Cemex, Italcementi, Cimpor and Holcim.
   Vidmar is a Spanish company and was founded in Barcelona in 1985. Its core competence is the design, manufacturer, supply and servicing of continuous and static industrial dosing and weight systems using its own technology.

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