Roller mill; tandem mills; frequency inverter; programmable logical controller, automation; control; supervising system.
Tandem Mills with combined activation or individual roll activation are cases that require different approaches for automation and control. On rollers with combined action through the same turbine or electric motor the speed on each roller is similar, with the same linear speed for all of them (tangential speed) and a different angular speed with a proportional relation between them (according to the mechanical structure of the reductors). In some facilities these cases are being replaced for the individual roller activation by electric motor, at same time keeping the linear (tangential) speed equal on all of them, while the different angular speed between them is kept proportionally (by a relation).
This article presents a case study on Usina Trapiche, in Brazil, where was applied the control by linear (tangential) speed in rollers individually activated. The use of linear roller speed using other process variables is an interesting solution in order to achieve optimum milling yields and juice extraction.
The combined tandem roller activation (Figure 1) is easily encountered in sugar and ethanol mills in Brazil and elsewhere. Usually there are activations where a same turbine or electric motor activates two rollers at the same time, or just one. In lesser cases there is the same activation for all of the tandem mills.
Figure 1: Examples of combined tandem mills activation
Due to the need for improving sugarcane milling, some mills have adopted a far more complete way of speed control for each tandem mill by using the individual activation of each tandem roller. In order to keep the constant flow of sugar cane (in ton per hour –tc/h) and the bagasse humidity during the crushing process, Usina Trapiche, located at the municipality of Sirinhaém, Pernambuco state, invested for the electrical activation of its milling. Until the 20092010 crop there was one turbine for each tandem mill, i.e., each turbine activated simultaneously every tandem roller (pressroller, bottom roller, top roller and discharge roller).
On the 20102011 crop motors were implemented on the tandem mills 1, 2 and 5, so that each tandem mill had one motor for the pressroller, another one for the discharge roller and kept the turbine for the bottom and top rollers (Figure 2). Hence, automation arrived to control these individual activations and keep the rollers synchronized according to predefined mathematical calculations, with the adoption of the linear (tangential) speed.
Figure 2: Sketch (top view) of a tandem mill activation
The electrification of the individual roller activation eliminated the rotors(?) that were used previously. The rotors are a transmission gear with low efficiency and yield used for activation, and their absence reduces from 6 to 8% the tandem mills consumption power.
Usina Trapiche defined that the control of the tandem mills should be done in function of the linear (tangential) speed of the rollers that sets the cane progression speed (the quantity of the shredded cane input in each tandem mill equals the cane bagasse at the output plus the drained juice). The choice for the linear speed is due to the possibility of taking advantage of the crushing jackets(?) by matching their speed with the other rollers individual activation, hence obtaining a bagasse constant humidity value after being crushed between the top roller and the discharge roller.
In a joint work with SMAR, Usina Trapiche defined the mathematical calculations to be adopted on the project. The speed of the top roller activated by the turbine was the reference for the control of the speed of the roller activated by motors (pressroller and discharge roller). Based on this, the following formulas were used:
where:
linear speed (in m/s) 

angular speed (in rpm) 

radius (in meters) 

diameter (in meters) 
The following equation was adopted for the roller rotation synchronism:
where:
roller linear speed (in ms) to consider (pressroller or discharge roller)  
top roller linear speed (in m/s) used for reference


factor for the roller linear speed relation to consider, in function of the top roller 
With the application of formulas 1, 2 and 3 the following equation is reached:
Where:
roller angular speed (in rpm) to consider (pressroller or discharge roller) 

top roller linear speed (in rpm) used for reference 

top roller diameter (in meters) used for reference


roller diameter (in meters) to consider (pressroller or discharge roller)


factor for the roller linear speed relation to consider, in function of the top roller 
Due to the reductor present between the turbine and the top roller, we have the following reduction relation:
where:
top roller angular speed (in rpm) used for reference  
angular speed (in rpm) of the top and bottom rollers turbine 

 reduction relation of the reductor between the turbine and the top 
Therefore, we have the following final equation for each roller (pressroller and discharge roller):
The turbine angular speed() is acquired by a field rotation sensor connected to the module of the CLP analog input (a frequency converter for 4~20mA is used); the reduction relation of the reductor between the turbine and the top roller, (), the top roller diameter () and the diameter of the roller to consider () are fixed values according to the construction characteristics of the equipment (values inserted by the milling operator via supervisory); the relation factor () is defined by the mill, so that it enables the desired speed for each roller (value inserted by the milling operator via supervisory). To arrive at the ideal values of ()and the ideal turbine rotation, several mathematical calculations and simulations were performed and analyzed with basis on the reduction relation of the reductors, number of pinion teeth (?) and volandeira, among others (figure 3). (Não achei no Google os termos em inglês para dentes de pinhão e volandeira)
Figure 3: Simulation of the tandem rollers rotation with electric activation on the pressroller and the discharge roller
A Programmable Logic Controller is indicated to deal with systems characterized by discrete events, i.e., with a process whose variables assume values zero or one ( or socalled digital variables). It may also deal with analog variables defined by current values or electric voltage intervals. Digital inputs or outputs are the discrete elements, analog inputs or outputs are the variable elements between the known tension or current values.
The automation of the Usina Trapiche milling area was implemented by SMAR in 2007, with the programmable logic controller LC700 (Figure 4), using analog signals (4~20mA inputs and outputs) and digital signals. The milling process electrification kept the LC700 with the improvement of its hardware with E/S, racks and accessories.
The LC700 is applicable in several simple and complex installations and a wide range of industries. As it is a PLC, the LC700 was designed to incorporate the traditional discrete functions of manufacture, automation tasks, continuous regulatory process control and batch control. The LC700 hardware platform has a wide range of E/S modules combined with a large set of programmable function blocks and logic elements that turn it highly versatile.
Figure 4: SMAR LC700
For an application as complex as the electrification of Usina Trapiche milling, the LC700 responded satisfactorily and executed the control logics (Figure 5) according to the mathematical simulations, in spreadsheets (Figure 6).
Figure 5: Partial SMAR LC700 logic ladder used in the automation of USINA TRAPICHE (CONF700 software).
Figure 6: Simulation of the mathematical calculations of the speed relation between the tandem rollers.
Usina Trapiche implemented the Profibus DP protocol on the activation of tandem rollers 1, 2 and 5, besides acquiring multiple statuses at the Power House with a remote Profibus DP. The plant is devising that in the near future all of the tandem rollers be electrified and controlled by Profibus DP network.
In parallel with the LC700, SMAR installed the Profibus DF73 master controller that executed motor information (Figure 9) and cubicle information (Figure 9) over the network.
Figure 7: SMAR DF73 Profibus Master
Figure 8: Popup of the CFW11 supervisory of the 1^{st} tandem discharge roller, with indications from the frequency inverter acquired by the Profibus DP network.
Figure 9: Unifiliars of tandem roller cubicles for motor activation
This system was configured by SYSTEM302 and combines in a single environment software for devising process control strategies (Syscon), control strategies using logic ladder (Logic View), in addition to software for asset management (AssetView), plant information management (Equipment Database), among others. The SYSTEM302 aims at the convergence of the automation and information technologies, resulting in a robust, safe and integrated architecture (Figure 10).
Figure 10: Architecture of Usina Trapiche Profibus DP network
The Indusoft Web Studio is a powerful collection of automation tools that make possible developing IHM, SCADA applications for embedded systems and instrumentation systems.
Since 2007, when SMAR implemented the USINA TRAPICHE automation, the supervisory software has been the Indusoft Web Studio, which communicates with the LC700 through the OPC open protocol (OLE
for Process Control. The operational station enables the milling operators to supervise and command the plant through synoptic screens, tuning control circuits, alarms, history, on/off motors, etc. (Figures 11, 12, 13 and 14). The milling electrification was implemented on the supervisory with synoptic screens and friendly commands (Figures 15 and 16).
Figure 11: Screen with the cane preparation and the tandem mills at Usina Trapiche
Figure 12: Control circuit tuning screen.
Figure 13: Alarm screen.
Figure 14: Motor activation screen.
Figure 15: First tandem rollers activation screen.
Figure 16: Screen with speed variables and commands of the discharge roller and the pressroller.
The Usina Trapiche CCM that provides the milling electrification is divided today in four cubicles, one for each tandem (1, 2 and 5) and one backup cubicle. Each cubicle has a CFW09Weg frequency inverter for the pressroller and a CF311Weg for the discharge roller. The backup cubicle is available in the system to replace another with failures or problems. In order to prevent that cables connected to the LC700 are relocated from one cubicle to another, all of them are interconnected and the inverters frequency on the system is done via CLLP/supervisory (Figure 17). The milling operator is free to choose which cubicle to use. This condition minimizes the time spent on this transition phase, speeds up the exchange and increases the system availability.
Figure 17: Screen with the activation of tandems 1, 2 and 5, with option to select the backup cubicle.
SMAR Installation and Technical Assistance Department (DIAT) commissioned the plant with agility and simplicity. It performed all of the control logic tests and simulated the system according to the mathematical calculations previously defined. After the successful tests and simulations, the startup was carried out to prove the efficiency of the implemented system.
According to Usina Trapiche´s industrial manager, Mr. Eduardo Mota Valença, these implementations improved the milling control, as it enabled the activation of the rollers speed independently, hence providing better extraction and less bagasse humidity. “The milling capacity increased as the activation phase was eliminated, with less power consumption and friction, and the activation was directed to the extraction itself”, he concluded.
The results obtained after the Usina Trapiche startup system showed that this system enables a reliable, flexible and technically qualified solution. The possibility of predefining the cane flow process desired and the mathematical calculations of each roller independent rotation provide unique quality and yield of cane juice for sugar and ethanol production, besides the best characteristics of bagasse for boiler combustion.
Derek Stesse, Eletrical Engineer from UNESP – São Paulo State University (Bauru/SP Campus) is SMAR Application Engineer on the Sugar & Ethanol area.
Email: derek@smar.com.br.
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