Optimum power density, efficiency and precision
In order to accommodate servo drives of increasingly shorter design, the length and diameter of the inductive multiturn rotary encoders used must also be reduced.
Figure 1- EBI 1135
Manufacturers of robots and CNC machine tools require precise and compact drives with high power efficiency. To meet these and future demands, STÖBER Antriebstechnik GmbH & Co. KG decided to develop a completely new family of servo motors with three different motor sizes, each available in four different lengths for torque values of 2.8 to 31 Nm in various versions. Thanks to their innovative motor design, the synchronous servo motors of the EZ series provide a very high power density in an extremely compact form. The positioning and speed of the servo motors are controlled with the battery-backed Multiturn Rotary Encoder EBI 1135 likewise newly developed by DR. JOHANNES HEIDENHAIN GmbH.
New manufacturing method
Smaller, lighter, more dynamic and more precise are the common demands made by customers to Heinz Bäzner, Head of Cost Center, Motors, at STÖBER. These market demands led to the completely new development of the EZ series of servo motors which have now been on the market since the beginning of 2011 in flange sizes of 98 mm to 145 mm. The synchronous servo motors for speeds of up to 6,000 rpm are extremely short with a high power density. This has been made possible by the innovative method of manufacturing the stator winding.
"Previously, with the STÖBER servo motors, we used a complete laminated stator core housing into which the windings were introduced individually," said Heinz Bäzner. "In the case of the EZ motors, however, tooth winding was used, in which the winding was produced as a multi-layer winding (orthocyclic winding). This ensures a very high copper fill factor."
Furthermore, the stator plates are no longer welded or riveted, but are bonded together directly when punched. This means that the plates are placed so closely together that a significantly higher layer factor than with punching or riveting is achieved. This manufacturing method ensures higher stability and thus less vibration, less detent torque and a smoother surface without rivets or welding seams, which in turn prevents eddy currents. The higher layer factor also means less loss in the laminated stator core and a higher power density.
"Another highlight" for Heinz Bäzner is "the encapsulating of the winding in plastic." This ensures even better heat removal. The higher copper fill factor, the new patented procedure for stator plate production and subsequent encapsulating of the winding as well as other measures such as the use of high-energy neodymium iron boron magnets permit the volume to be reduced by about 50 % with the same torque compared with previously manufactured servo motors. "These motors' high efficiency rate of 91 to 95 percent is therefore better than required by the IE4," emphasized Heinz Bäzner. The weight of the motors is also reduced by half thanks to the lower volume. The newly calculated rotor design and optimized rotor gaps also played their part in reducing the moment of inertia and thus improving the dynamics of the motors accordingly.
Compact motors need compact components
"The extremely compact design of the motors forced us to solve a number of design problems," said Heinz Bäzner. "For instance, the rotor bearings are deep inside the end winding." For this reason a very small encoder was needed for mounting on the cam. The multiturn rotary encoders with mechanical transmission used previously in the STÖBER servo motors were no longer suitable due to their size.
Heinz Bäzner continued, "We therefore asked HEIDENHAIN, with whom we have enjoyed successful cooperation for many years, whether they had a rotary encoder that can fit into this space and can fulfill our high requirements. Finally, after intensive tests, it became clear that the newly developed inductive rotary encoder EBI 1135 was the optimum solution for our EZ series of servo motors."
Figure 2- Motor cross-section
The EBI1135 encoders are the first generation of inductive multiturn rotary encoders from HEIDENHAIN with battery buffering," explained Dr.-Ing. André Schramm from Marketing and Product Management at HEIDENHAIN. "Thus, compared with our other multiturn rotary encoders, all with mechanical transmission, we can achieve comparably high degrees of control and precision with the battery-buffered devices despite their small size, which means that a high level of control performance can also be achieved."
"It is just a pity," added Heinz Bäzner with a laugh, "that we did not come across the encoder before we had finished the design phase. Otherwise we could have made the motor another 10 mm shorter." The number of revolutions is generated inductively with this new generation of encoders and buffered with the battery. This means that even if there is a power failure, the current position of the servo motor can always be read out. Here, the battery is mounted externally, as with all encoders with battery buffering, because the temperatures are too high where the encoder is incorporated in the motor. At STÖBER, the battery is mounted in the likewise newly developed Absolute Encoder Support (AES). The AES is installed between the servo controller and the rotary encoder cable. The advantages of this solution are that the battery is easy to access and the absolute position of the servo motor is safely buffered even when the servo controller is replaced.
Figure 3- Absolute Encoder Support
Small rotary encoder with extraordinary performance data
The EBI 1135 absolute multiturn encoder attains a total resolution of 34 bits (singleturn: 18 bits, multiturn: 16 bits). Thanks to its modular design without ball bearings and transmission, it is one of the world's smallest absolute rotary encoders with a length of 12.3 mm and outside housing diameter of only 36.83 mm. It is therefore predestined for use in highly dynamic servo motors of small size for automation technology and industrial robots.
The new rotary encoder with battery-buffered revolution counter distinguishes itself in particular with optimized inductive scanning evenly around the circumference together with a sturdy design. This produces a high degree of precision of ± 120", good controller quality and a large permissible axial mounting tolerance of ± 0.3 mm. The large permissible axial mounting tolerance of ± 0.3 mm, the wide voltage range of 3.6 to 14 V and, compared with multiturn scanning with a mechanical transmission, the greatly expanded multiturn value range of 65,536 distinguishable revolutions increases the reserves in the application available to the customer. The electronic multiturn scanning without additional mechanical components also improves the shock and vibration compatibility and ensures low-noise operation.
According to Dr. André Schramm, "The EBI 1135 is the first inductive encoder that is equipped with the EnDat 2.2 interface." This purely serial interface permits fast and secure data transmission at clock-pulse rates of up to 8 MHz for high drive dynamics even in environments with potentially high electromagnetic interference (e.g. welding robots). In addition, the greater voltage range of 3.6 V to 14 V means that there are no longer any problems with voltage drop in the cable.
The multiturn function of the EBI 1135 is realized through a revolution counter. To prevent loss of the absolute position information during power failure, the multiturn must be driven with an external buffer battery. Compared with the absolute value encoders with battery-buffered revolution counter currently in widespread use in Asia, two features of the EBI 1135 are to be highlighted in particular. One the one hand, the power consumption in battery-buffering mode was lowered to 12 µA (the usual value here is about 25 µA) and a longer battery life of approximately 10 years could be achieved using a buffer battery with 1500 mAh and 3.6 V. On the other hand, the full speed of 12,000 rpm is permissible also in battery-buffering mode (in many cases, devices from the competition greatly reduce the permissible speed in battery-buffering mode).
Summary and outlook
The EBI 1135 multiturn rotary encoders are some of the world's smallest. Only thanks to them are such drastic developments possible, as in the case of the EZ series of synchronous servo motors with regard to length and power density. The servo motors are now available in a wide variety of different versions. For example, there are versions with holding brake, with liquid-cooled flange or external fan, or with hollow shaft. In the near future there will then also be servo motors with spindle drives as rotating spindles or spindle nuts—and once again the EBI 1135 will be implemented.