NBK America LLC XG Series White Paper
Summary Abstract The semiconductor, robotics, automation, medical, and precision machinery industries have equipment with the latest servomotor technology possible to meet demanding performance criteria. As gain settings are increased, precision accuracy and speed are critical aspects in which coupling technology must advance to ensure peak servo system performance. In a new white paper, NBK addresses some important considerations you should keep in mind when specifying a servomotor coupling that meets the demands for high servomotor specification requirements that continue to evolve.
Advancements in Servomotor Coupling Technology
Flexible couplings are machine elements that fasten drive shafts and transmit torque while allowing for multiple forms of misalignment such as lateral, angular, and axial. Various flexible coupling types have been developed based on application demands. Couplings used in servo systems with specific feedback mechanisms are often selected due to their static torsional stiffness and backlash-free features which are integral requirements for high precision and high-speed applications. Typically, users are accustomed to utilizing the disc coupling with static torsional stiffness capability (Pic.1*). Recent technological improvements in servomotors have led to dramatic improvement in speed response frequency. Vibration (aka “hunting”) occurrence tends to arise as when increased gain settings are applied to servo systems using high static torsional stiffness coupling such as the disc or bellows type (Pic.2**). A potential solution to resolve hunting while operating servo system at high gain settings involves equipping the coupling with vibration damping technology. Further discussion will analyze the extent of the high-gain rubber type coupling’s features as a solution for high response mandatory servo systems designed for the semiconductor manufacturing equipment as well as many other automation fields.
*Pic. 1 Disc type
**Pic. 2 Bellows type
***Pic. 3 High-gain rubber type
Stabilization Time’s effect on productivity Integrated dampening technology has the ability to greatly reduce stabilization time for speed purposes, but increase productivity as whole. The interplay between a feed screw machine with a ball screw and servomotor system illustrates potentially an operation issue. For feed screw related applications
Formatted: Indent: First line: 1 ch
which utilize both servomotors and ball screws, the ideal scenario dictates that operation proceeds in exact accordance with the servomotor's commands; however, real life scenarios may experience a situation where executed commands are delayed. Delayed response is a factor in errant positioning and this delayed response is known as stabilization time (See Fig. 2). Higher servomotor gain and high-response operation are required for reduced stabilization time (Fig. 3), but substantial gain increases are likely to lead to the counterproductive hunting phenomena. Hunting in effect will distort the application’s operating equilibrium and disable servomotor control. Increasing the gain while suppressing hunting requires systematic adjustments to the servomotor parameters such as reviewing the coupling’s mechanical characteristics. Tests have shown that when a servomotor contains a Disc or Bellows couplings that raising the gain tends to cause hunting occurrence much more readily. When hunting occurs, conventional wisdom proposes switching to a coupling with higher torsional stiffness to increase the rigidity of the entire rotating system. However, the torsional rigidity of the entire system is dependent on the torsional rigidity of the ball screw. Table 1 shows the calculated values of the torsional rigidity of the entire system utilizing different couplings. Torsional rigidity value of the disc coupling is 450 N.m/rad compared to the high-gain rubber type coupling value of 240 N.m/rad. Strictly examining the values, one would conclude that the disc coupling has 1.9 times higher torsional stiffness values. Further examination reveals that the torsional rigidity of the entire system with a disc coupling is 79 N.m/rad for the former and 68 N.m/rad for the latter, making the actual difference about 1.2 times. In other words, the torsional rigidity of the ball screw is the greater determining factor for the torsional rigidity of the entire system as opposed to the coupling. Simply changing the coupling with inherent higher torsional stiffness values may not sufficiently improve the torsional rigidity of the entire system nor provide protection against hunting occurrence. Technical advancements in servomotors especially in speed response frequency (Fig. 1) have made the need for increased vibration dampening capability even more pronounced to avoid hunting. Ultimately, vibration dampening along with adequate statistical torsional stiffness allows for accurate repeat position repeatability to ensure positioning accuracy.
Speed response frequency (Hz)
3500 3000 2500 2000 1500 1000 500 0 1990
1995
2000
Year
2005
2010
2015
Fig. 1 Industry changes in servomotor speed response frequency* (Note: Based on servomotor speed response frequency manufactures’ catalog values)
Fig. 2 Stabilization time
Fig. 3 Change in stabilization time due to gain
Table 1 Torsional rigidity of the entire system Disc t ype Torsional rigidity of coupling: Kc
450
High-gain rubber type 240
(N.m/rad) Ball screw groove diameter (mm)
7.8
7.8
Support bearing - Nut distance (mm)
300
300
Torsional rigidity of ball screw: Kbs
96
96
79
68
(N.m/rad) Torsional rigidity of entire system*: K (N.m/rad)
*Torsional rigidity of the entire system: 1/K = 1/Kc + 1/Kbs
1. Purpose Testing was conducted to see the relationship between a coupling's Static Torsional Stiffness and positioning repeatability on an actuator
Formatted Table
2. Testing equipment Equipment
Part No
Maker
Note Positioning repeatability：
Actuator
KR30H
THK
±0.005(mm) Position accuracy：0.1(mm)
Motor
HF-KP013
Coupling
NBK
XBW-25C2-6×8
NBK
6×8 Sensors
Electric
XGT-25C-6×8
MJT-20C-RDLaser Displacement
Mitsubishi
XL-80
NBK
Static Torsional Stiffness： 170(Nm/rad) Static Torsional Stiffness： 850(Nm/rad) Static Torsional Stiffness： 55(Nm/rad)
RENISHAW
3. Method Testing methodology adheres to JIS B 6192 protocols and the testing equipment used the XGT, XBW and MJT couplings where the accuracy of stop position accuracy was measured 7 times. The gap of Max. and Min. values was calculated and respective values were compared (See Test results).
The origin of position is set at both the center and edge for the max range liner stroke. Max value as testing parameter was written in ±with its half value as below
4. Testing Conditions Motor speed
3000 [min-1]
Acceleration/deceleration time
50 [ms]
Positioning location
20，170，350 [mm]
Loaded object
3 [kg]
5. Testing result Positioning location 1st 2nd 3rd 4th 5th 6th 7th Max. Min. Gap Positioning repeatability
[mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm]
20 19.9983 19.9983 19.9983 19.9983 19.9983 19.9984 19.9983 19.9984 19.9983 1E-04
XGT 170 170.0387 170.0394 170.0385 170.0391 170.0396 170.0387 170.0394 170.0396 170.0385 0.0011 0.0015
350 350.0446 350.0456 350.0456 350.0452 350.0439 350.0449 350.0469 350.0469 350.0439 0.003
20 19.9989 19.9988 19.9987 19.9988 19.9989 19.9989 19.9988 19.9989 19.9987 0.0002
XBW 170 170.0382 170.0387 170.0375 170.0371 170.0376 170.0371 170.0376 170.0387 170.0371 0.0016 0.0012
350 20 350.0413 20 350.042 20 350.0436 20 350.0433 20 350.0433 20 350.0432 20 350.0426 20 350.0436 20 350.0413 20 0.0023 7E-04
MJT 170 170.0358 170.0363 170.0366 170.0357 170.0356 170.0359 170.0368 170.0368 170.0356 0.0012 0.0012
6. Conclusion Static Torsional Stiffness does not gravely impact positioning repeatability on actuators. Sub-micron value variations referenced above are likely due to the precision performance of the actuator.
High-Gain Rubber Coupling’s Unique Construction The high-gain rubber type coupling has a completely integrated structure in which aluminum hubs on both sides are molded with a vibration-reduction rubber that prevents backlash yet remains flexible. As seen in picture and figure 4, the internal claw-like structure lined with rubber allows for optimal torsional rigidity and damping.
350 350.0456 350.0459 350.0459 350.0475 350.0466 350.0474 350.0481 350.0481 350.0456 0.0025
Pic.4 Structure of high-gain rubber type
Fig. 4 Coupling damping performance comparison The Bode plot (Fig. 5) brilliantly illustrates why the high-gain rubber type coupling has the advantage for increased servomotor gain well beyond the capacity of the disc coupling with higher torsional stiffness values. Gain width between 0 dB and the point where there is a phase delay in the Bode plot is -180° and this is known as the gain margin. General guidelines for servo systems stipulate for setting the gain margin between 10 and 20 dB. As the servomotor gain is increased, the gain margin decreases. When the gain margin falls below 10 dB, hunting tends to occur. Comparing the limit gain (servo gain at which hunting occurs) of the disc type to the gain margin of the high-gain rubber type, the high gain rubber type at 17.40 dB surpasses the disc type's 9.90 dB value. Since the gain margin is above 10 dB. the servomotor gain of the high-gain rubber type can be increased beyond that of the disc type thus reducing the stabilization time to allow for increased productivity.
Fig. 5 Bode plot Table 2 shows the difference in stabilization time according to the difference in coupling types and servomotor gain. If the servomotor gain is the same, there is no difference in the stabilization time due to the difference in couplings. However, when comparing the servomotor limit gain, the stabilization time is 12 ms for the disc type limit gain (25) and 3 ms for the high-gain rubber type limit gain (32). The high-gain rubber type which suppresses hunting and improves servomotor gain can more effectively reduce the stabilization time.
Formatted: Indent: First line: 1 ch
Table 2 Difference in stabilization time according to coupling type and servomotor gain Servomotor gain*
Disc type
High-gain rubber type
25
12 ms
12 ms
32
Occurrence of hunting
3 ms
* Values with all gains, such as position control gain and speed control gain, adjusted
Additional testing results in an actual system were completed and are shown below as an example. Ball screw shaft diameter: φ15 Servomotor: 100 W Motor revolution: 3,000 min-1 Acceleration/deceleration time: 50 ms Workpiece load: 3.0 kg Ratio of moment of inertia of load: 3.5
Rubber Durability Rubber is used as a component to impart damping properties and data on the durability has been accumulated since the advent of high-gain couplings in 2007. Fig. 6 shows that even after 108 drive tests, there is no drop in performance due to rubber deterioration. 5.0
Torque [N.m]
Torque [N.m]
5.0
0.0
-5.0 -0.05
0.00 Angle [rad]
0.05
0.0
-5.0 -0.05
0.00 Angle [rad]
0.05
After
Before Fig. 6 Results of 108 drive tests
Future Issues Servomotors are expected to display even higher frequency response in the future paired with demands for higher precision and higher speed. Couplings technology will continue to require a meld of high torsional rigidity and high-level damping properties to help maintain peek servo system performance.
References Atsushi Matsubara: Design and Control of Precision Positioning and Feed Drive
Systems, 2008 Nabeya Bi-tech Kaisha: NBK General Catalog, 2017
NBK America LLC, Hiroki Goto, Engineering Specialist Paulo Castelo, Technical Sales Solution Surpervisor
Formatted: Left, Indent: First line: 0 ch
Advancements in Servomotor Coupling Technology
Flexible couplings are machine elements that fasten drive shafts and transmit torque while allowing for multiple forms of misalignment such as lateral, angular, and axial. Various flexible coupling types have been developed based on application demands. Couplings used in servo systems with specific feedback mechanisms are often selected due to their static torsional stiffness and backlash-free features which are integral requirements for high precision and high-speed applications. Typically, users are accustomed to utilizing the disc coupling with static torsional stiffness capability (Pic.1*). Recent technological improvements in servomotors have led to dramatic improvement in speed response frequency. Vibration (aka “hunting”) occurrence tends to arise as when increased gain settings are applied to servo systems using high static torsional stiffness coupling such as the disc or bellows type (Pic.2**). A potential solution to resolve hunting while operating servo system at high gain settings involves equipping the coupling with vibration damping technology. Further discussion will analyze the extent of the high-gain rubber type coupling’s features as a solution for high response mandatory servo systems designed for the semiconductor manufacturing equipment as well as many other automation fields.
*Pic. 1 Disc type
**Pic. 2 Bellows type
***Pic. 3 High-gain rubber type
Stabilization Time’s effect on productivity Integrated dampening technology has the ability to greatly reduce stabilization time for speed purposes, but increase productivity as whole. The interplay between a feed screw machine with a ball screw and servomotor system illustrates potentially an operation issue. For feed screw related applications
Formatted: Indent: First line: 1 ch
which utilize both servomotors and ball screws, the ideal scenario dictates that operation proceeds in exact accordance with the servomotor's commands; however, real life scenarios may experience a situation where executed commands are delayed. Delayed response is a factor in errant positioning and this delayed response is known as stabilization time (See Fig. 2). Higher servomotor gain and high-response operation are required for reduced stabilization time (Fig. 3), but substantial gain increases are likely to lead to the counterproductive hunting phenomena. Hunting in effect will distort the application’s operating equilibrium and disable servomotor control. Increasing the gain while suppressing hunting requires systematic adjustments to the servomotor parameters such as reviewing the coupling’s mechanical characteristics. Tests have shown that when a servomotor contains a Disc or Bellows couplings that raising the gain tends to cause hunting occurrence much more readily. When hunting occurs, conventional wisdom proposes switching to a coupling with higher torsional stiffness to increase the rigidity of the entire rotating system. However, the torsional rigidity of the entire system is dependent on the torsional rigidity of the ball screw. Table 1 shows the calculated values of the torsional rigidity of the entire system utilizing different couplings. Torsional rigidity value of the disc coupling is 450 N.m/rad compared to the high-gain rubber type coupling value of 240 N.m/rad. Strictly examining the values, one would conclude that the disc coupling has 1.9 times higher torsional stiffness values. Further examination reveals that the torsional rigidity of the entire system with a disc coupling is 79 N.m/rad for the former and 68 N.m/rad for the latter, making the actual difference about 1.2 times. In other words, the torsional rigidity of the ball screw is the greater determining factor for the torsional rigidity of the entire system as opposed to the coupling. Simply changing the coupling with inherent higher torsional stiffness values may not sufficiently improve the torsional rigidity of the entire system nor provide protection against hunting occurrence. Technical advancements in servomotors especially in speed response frequency (Fig. 1) have made the need for increased vibration dampening capability even more pronounced to avoid hunting. Ultimately, vibration dampening along with adequate statistical torsional stiffness allows for accurate repeat position repeatability to ensure positioning accuracy.
Speed response frequency (Hz)
3500 3000 2500 2000 1500 1000 500 0 1990
1995
2000
Year
2005
2010
2015
Fig. 1 Industry changes in servomotor speed response frequency* (Note: Based on servomotor speed response frequency manufactures’ catalog values)
Fig. 2 Stabilization time
Fig. 3 Change in stabilization time due to gain
Table 1 Torsional rigidity of the entire system Disc t ype Torsional rigidity of coupling: Kc
450
High-gain rubber type 240
(N.m/rad) Ball screw groove diameter (mm)
7.8
7.8
Support bearing - Nut distance (mm)
300
300
Torsional rigidity of ball screw: Kbs
96
96
79
68
(N.m/rad) Torsional rigidity of entire system*: K (N.m/rad)
*Torsional rigidity of the entire system: 1/K = 1/Kc + 1/Kbs
1. Purpose Testing was conducted to see the relationship between a coupling's Static Torsional Stiffness and positioning repeatability on an actuator
Formatted Table
2. Testing equipment Equipment
Part No
Maker
Note Positioning repeatability：
Actuator
KR30H
THK
±0.005(mm) Position accuracy：0.1(mm)
Motor
HF-KP013
Coupling
NBK
XBW-25C2-6×8
NBK
6×8 Sensors
Electric
XGT-25C-6×8
MJT-20C-RDLaser Displacement
Mitsubishi
XL-80
NBK
Static Torsional Stiffness： 170(Nm/rad) Static Torsional Stiffness： 850(Nm/rad) Static Torsional Stiffness： 55(Nm/rad)
RENISHAW
3. Method Testing methodology adheres to JIS B 6192 protocols and the testing equipment used the XGT, XBW and MJT couplings where the accuracy of stop position accuracy was measured 7 times. The gap of Max. and Min. values was calculated and respective values were compared (See Test results).
The origin of position is set at both the center and edge for the max range liner stroke. Max value as testing parameter was written in ±with its half value as below
4. Testing Conditions Motor speed
3000 [min-1]
Acceleration/deceleration time
50 [ms]
Positioning location
20，170，350 [mm]
Loaded object
3 [kg]
5. Testing result Positioning location 1st 2nd 3rd 4th 5th 6th 7th Max. Min. Gap Positioning repeatability
[mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm]
20 19.9983 19.9983 19.9983 19.9983 19.9983 19.9984 19.9983 19.9984 19.9983 1E-04
XGT 170 170.0387 170.0394 170.0385 170.0391 170.0396 170.0387 170.0394 170.0396 170.0385 0.0011 0.0015
350 350.0446 350.0456 350.0456 350.0452 350.0439 350.0449 350.0469 350.0469 350.0439 0.003
20 19.9989 19.9988 19.9987 19.9988 19.9989 19.9989 19.9988 19.9989 19.9987 0.0002
XBW 170 170.0382 170.0387 170.0375 170.0371 170.0376 170.0371 170.0376 170.0387 170.0371 0.0016 0.0012
350 20 350.0413 20 350.042 20 350.0436 20 350.0433 20 350.0433 20 350.0432 20 350.0426 20 350.0436 20 350.0413 20 0.0023 7E-04
MJT 170 170.0358 170.0363 170.0366 170.0357 170.0356 170.0359 170.0368 170.0368 170.0356 0.0012 0.0012
6. Conclusion Static Torsional Stiffness does not gravely impact positioning repeatability on actuators. Sub-micron value variations referenced above are likely due to the precision performance of the actuator.
High-Gain Rubber Coupling’s Unique Construction The high-gain rubber type coupling has a completely integrated structure in which aluminum hubs on both sides are molded with a vibration-reduction rubber that prevents backlash yet remains flexible. As seen in picture and figure 4, the internal claw-like structure lined with rubber allows for optimal torsional rigidity and damping.
350 350.0456 350.0459 350.0459 350.0475 350.0466 350.0474 350.0481 350.0481 350.0456 0.0025
Pic.4 Structure of high-gain rubber type
Fig. 4 Coupling damping performance comparison The Bode plot (Fig. 5) brilliantly illustrates why the high-gain rubber type coupling has the advantage for increased servomotor gain well beyond the capacity of the disc coupling with higher torsional stiffness values. Gain width between 0 dB and the point where there is a phase delay in the Bode plot is -180° and this is known as the gain margin. General guidelines for servo systems stipulate for setting the gain margin between 10 and 20 dB. As the servomotor gain is increased, the gain margin decreases. When the gain margin falls below 10 dB, hunting tends to occur. Comparing the limit gain (servo gain at which hunting occurs) of the disc type to the gain margin of the high-gain rubber type, the high gain rubber type at 17.40 dB surpasses the disc type's 9.90 dB value. Since the gain margin is above 10 dB. the servomotor gain of the high-gain rubber type can be increased beyond that of the disc type thus reducing the stabilization time to allow for increased productivity.
Fig. 5 Bode plot Table 2 shows the difference in stabilization time according to the difference in coupling types and servomotor gain. If the servomotor gain is the same, there is no difference in the stabilization time due to the difference in couplings. However, when comparing the servomotor limit gain, the stabilization time is 12 ms for the disc type limit gain (25) and 3 ms for the high-gain rubber type limit gain (32). The high-gain rubber type which suppresses hunting and improves servomotor gain can more effectively reduce the stabilization time.
Formatted: Indent: First line: 1 ch
Table 2 Difference in stabilization time according to coupling type and servomotor gain Servomotor gain*
Disc type
High-gain rubber type
25
12 ms
12 ms
32
Occurrence of hunting
3 ms
* Values with all gains, such as position control gain and speed control gain, adjusted
Additional testing results in an actual system were completed and are shown below as an example. Ball screw shaft diameter: φ15 Servomotor: 100 W Motor revolution: 3,000 min-1 Acceleration/deceleration time: 50 ms Workpiece load: 3.0 kg Ratio of moment of inertia of load: 3.5
Rubber Durability Rubber is used as a component to impart damping properties and data on the durability has been accumulated since the advent of high-gain couplings in 2007. Fig. 6 shows that even after 108 drive tests, there is no drop in performance due to rubber deterioration. 5.0
Torque [N.m]
Torque [N.m]
5.0
0.0
-5.0 -0.05
0.00 Angle [rad]
0.05
0.0
-5.0 -0.05
0.00 Angle [rad]
0.05
After
Before Fig. 6 Results of 108 drive tests
Future Issues Servomotors are expected to display even higher frequency response in the future paired with demands for higher precision and higher speed. Couplings technology will continue to require a meld of high torsional rigidity and high-level damping properties to help maintain peek servo system performance.
References Atsushi Matsubara: Design and Control of Precision Positioning and Feed Drive
Systems, 2008 Nabeya Bi-tech Kaisha: NBK General Catalog, 2017
NBK America LLC, Hiroki Goto, Engineering Specialist Paulo Castelo, Technical Sales Solution Surpervisor
Formatted: Left, Indent: First line: 0 ch
