The technology behind the servo press
Why a servo press
Nowadays manufacturers to be competitive shall enhance products quality and reduce the cost. Standard solutions as hydraulic press or pneumatic press show the limits in high performances, flexibility and accuracy in executions. Servo press provide at first instance all requirement described above but furthermore provide a pressing process monitoring embedded in the servo drive to seek defected parts and process abnormalities in run.
How is made a servo press
Frequently servo press is confused with servo toggle press, both are powered by a servo motor but the device used to transform the motor rotation to a linear movement is different. The servo press to transform the movement use the ball screw, instead the toggle servo press use the mechanism rod with crankshaft. The servo press was be developed to provide a flexible module easy to install in whichever machine and need few time to set up. The servo press is powered by a brushless motor reduced by a satellite gearbox and then linked to the ball screw through a belt or by a joint coupling, Auloma provide both solutions with component without backlash. The ball screw is supported by rugged bearing and guides system that supply also the anti twist ram function. To complete the opera inside the module are installed the sensors to detect the two magnitudes force and displacement to execute the analysis of the pressing process.
One of the main feature of a servo press is the ram accuracy, Auloma can provide solutions with a ram repeatability and accuracy of ±3 microns, that allow precise pressing operations. The use of servo presses in applications of metal forming as springs or other stamping process enhance the quality of the products, because the ram accuracy interact directly on the precision of the executed part. The capability of the ram thrust depends by the motor power, for the rule of energy conservation it's easy calculate the power necessary to execute your application, is enough multiply force * ram stroke * cycle time to understand approximately which power the motor shall provide. The brushless synchronous motor available today in the market can perform pressing cycle with a force no more 500 kN (50 ton). This physical limit cut off the servo press in heavy duty applications.
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The device has embedded some sensors to measure two magnitudes, Force and displacement, that occur during the pressing operations. The servo press is controlled by a software installed in the servo drive CPU. This software is capable of control the drive profile of the press, according the parameters defined by the process control. During the press cycle the sensors measure with an high frequency, thanks to the huge sampling of the CPU, the reaction force that occur against the ram during its stroke. The values of force vs displacement gathered are analysed by the software and through its advanced algorithms can detect any variation of the force vs displacement waveform. If the variation detected by software are out of the tolerated limits, the system provide immediately through a digital output or by communication FieldBUS a warning signal to the PLC master that control the machine. As you can see, more are precise the sensors, more accurate will be the analysis. Auloma mount force sensor made appositely for its applications, thus very precise with a total error of 0,2% on the full scale. This advantage put Auloma on the top of servo press manufacturers, because the force measured don't depends by standard force sensor purchased on the market.
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Statistical process control
Since start the industrial revolution, the customers are demanding zero defects. Product quality has to be at the highest levels ever. Being “good enough” is no longer good enough. Sixty years ago, Dr. Edward Deming’s introduction of Statistical Process Control methods simplified the management of quality. We now have a wide assortment of quality tools including Six Sigma and Lean Six Sigma. Certainly product defect rates should be well under 3.4 defects per million, with a corresponding reduction in costs and prices. The data gathered by the process analysis through the servo press can be provided in CSV format to server machine where is installed a SPC software, in this way will be possible a statistical analysis and and prevent that defects became a quality issue.