Technical Papers

Custom-made and Optimized Drive Technology

Frequency Converters for Multi-axis Applications Enable Large-scale Production of High Quality

When used in special machines or special applications conventional frequency converters will reach their limits – these demanding tasks require customized solutions that are suited exactly to the individual requirements. The German company SIEB & MEYER, a specialist for drive and control technology based in Lueneburg, implements these customized modules. One good example is a special frequency converter that ensures reliable operation of rotary transfer machines developed by the Swiss supplier PRECITRAME. The experts optimized the used drive technology regarding installation volume, communication, safety requirements and efficiency.

PRECITRAME MACHINES SA is a Swiss manufacturer of CNC transfer and polishing machines. In light of the continuous advancement of its products in 2011, PRECITRAME searched the market for new frequency converters to be used in its rotary transfer machines. For this purpose the company compiled a detailed list of requirements: Beside trouble-free installation and accurate fitting the frequency converters should be of very compact design size and must provide opportunities for customization. Other essential features were a flexible, digital communication interface, a sufficient drive modulation to operate motors at high speeds and the opportunity to select and control different drive motors (motor parameter selection). In addition, the option to operate synchronous motors at a later time was desired. "The frequency converter drives the main manufacturing spindles of the rotary transfer machine. These are 2 kW models with speeds up to 40,000 rpm," Daniel Kunz, responsible R&D at PRECITRAME MACHINES SA, explains. "That means, the frequency converter rotates the motors to which the milling and drilling tools are fastened and the tools process the work pieces. These tools must ensure optimal surface quality and high precision in addition to providing a long service life. The used frequency converter makes a significant contribution to all these requirements."

Flexible Drive Solutions with Manifold Characteristics

This challenge was perfect for SIEB & MEYER: The company specializes in customized solutions in the field of high-speed frequency converters and intelligent drive amplifiers. "This includes not only simple hardware adaptations of the existing series products but also completely new devices and functions," says Torsten Blankenburg, CTO at SIEB & MEYER. The individually developed solutions are based on well-proven standard devices – in this case on the digital drive system SD2 that is especially designed for multi-axis systems and high speeds.

"Our SD2 frequency converters meet all requirements of this customer," says Blankenburg. "They have a compact and lightweight design and provide sensorless control of both asynchronous and synchronous spindles." One power supply unit can supply up to 12 drives. Hence, the system requires only one line filter and one brake resistor. Up to 12 frequency converters can be connected to one Profibus gateway. In order to drive rotary transfer machines SIEB & MEYER added several features to the serial SD2: Speed zero detection, i.e. motor standstill detection for asynchronous and synchronous motors without rotary encoder system, and automatic detection of the used spindle motor were implemented; the Lueneburg based experts also allowed changing the motor data by the machine control via Profibus. Last but not least, SIEB & MEYER was able to optimize the efficiency of the system: The drive system requires less power because the braking energy is utilized. "With the traditional drive topology excess energy during braking is wasted, since it is converted into heat," Blankenburg explains. "This is not the case here – when the motor brakes, the generated energy is used for the drives."

Since 2016: More Safety with new Functions

Over many years the customized solution proved itself in the rotary transfer machines by PRECITRAME - however, the collaboration reached a new phase in the summer of 2016. After extensive tests, SIEB & MEYER delivered the SD2 with new hardware and software. Background: Now, PRECITRAME can provide the end customer with the new models which are equipped with the following TÜV-certified safety functions: SFM (Safe Frequency Monitor) and SLOF (Safely Limited Output Frequency). Both safety functions were especially designed for motors without speed sensors."The existing drive amplifiers were already equipped with the safety functions STO (Safe Torque Off) and a sensorless SLS (Safely Limited Speed)," Torsten Blankenburg explains. "In order to meet the consistently increasing demands on productivity and personnel safety, we additionally implemented the functions SLOF and SFM in all our SD2 models."

"This update came just in time for us, since according to safety standards the safety function SMF is obligatory by now," Kunz adds. "By means of the safety function SFM the operator can safely detect whether or not the speed of a sensorless spindle has dropped below a safe speed value after switch-off." The safety function SLOF on the other hand is used to make sure that critical speeds are not exceeded. This can protect equipment, for example, when a tool might break due to overspeed. If the set maximum rotating field frequency is exceeded because of a faulty input or malfunction of the drive amplifier, the function STO disables the output stage.

Integrated Safety, Space-saving Wiring

In order to implement the new, sensorless functions SIEB & MEYER integrated the required hardware and software in the frequency converters of series SD2. Hence, expensive external safety modules are no longer necessary. The outcome is a cost-effective solution for the machine manufacturer and suitable for sensorless systems. This makes using spindles and motors without speed sensors possible – a crucial point for application areas in which spindles/motors cannot be equipped with speed sensors for technical or financial reasons. The data of the safety functions is bundled and forwarded via Profibus gateways. A gateway can bundle up to 12 OSSD safety inputs and outputs each. "This reduced the cost substantially on account of much less wiring work and hence, saves space in the switch cabinet," Kunz explains. "Thanks to the combination of this construction and a certified Profibus gateway, the integration of the new SD2 frequency converters in the existing machine design is very easy.“ This is a win-win situation for both end customer and manufacturer of the rotary transfer machines.

 

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