Q: We currently have a 3-axis controller with 3 servo motors and drivers in my machine. We are designing a new machine to increase the throughput by 4 times. Since keeping a compact design is also critical, I can only add 3 more axes to achieve 4 times the throughput. Therefore I will need to reduce the positioning time in half. Each motion will only require just a few revolutions on the motor. All axes are belt-pulley systems and they move independently.
Can you suggest some products capable of such quick move and high duty cycle? Also, I am considering using network to achieve multi axes control. What are the benefits of using network compared to a 6-axis controller?
A: Depends on the application and mechanical setup, each type of motor has its advantages and disadvantages. In general, servo motors have the advantage in long distance moves with high rigidity mechanisms. Because the servo motors are structured with fewer poles, they can reach a higher speed for long distance positioning. However, the gain loops are sensitive for load/inertia fluctuation with servo system: the gains need to be set low to reduce hunting with the less rigid mechanisms such as belt drive systems.
Stepping motors, also known as steppers, have the advantage in short distance and quick moves due to its higher output torque in the middle and low speed range. They run synchronously with the command pulses, and have high response characteristics to the command. Unlike the servo motors, stepping motor system do not need to wait until it settles before the next move, thus minimizing the settling time. This is critical for high duty cycle operation because settling time can be up to 100 msec depending on the load inertia. The mechanical rigidity does not affect the stepping motor system because there is no gain tuning. And it operates as an open-loop system under normal situation thus the system process delay is also minimized. See the sample data of positioning time between servo motors and stepper below.
Using network products is getting more popular in the industry. Because the network products have built-in motion control function, there are many advantages of using the network products. Some common advantages are: 1) reducing cost, 2) reducing wiring and associated labor time, 3) improving system reliability and 4) reducing overall spacing.
1) Reducing Cost
Network products are designed for easy communication between devices with a master controller. Since one master controller can communicate with more than 30 axes. Devices are arranged in a multi-drop configuration so it is easier to add or remove axis. Both Servo motors and Steppers require a motion controller for pulse and direction signals to create motion. A network product has its own built-in motion controller function thus there is no need to for external motion controller to generate pulse and direction commands.
2) Reducing Wiring
The network products use the serial communication. The number of wiring can be significantly reduced compared with I/O control. Reducing wiring also reduce the overall set up time as well as the chance of mis-wiring. See the samples below.
3) Improve system reliability
Serial communications are considered more reliable communication because they use differential signals while most I/Os use photocouplers. Although photocouplers are electrically insulated, the recommended cable length should be less than 2m. On the other hand, the serial communications use differential, also known as line driver signal, they are much more noise resistant thus the cable length can be extended significantly longer. See below.
4) Reducing overall space
As discussed above, the external motion controllers can be eliminated because the network products have built-in controller function and the amount of wiring is significantly reduced. Using network products can also reduce overall spacing. In addition, stepping motors are known as the highest torque motors per volume and they do not use encoders on the back of the motors. Thus stepping motors are shorter than servo motors and take less space to allow for more compact design.
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