Q: We've been using stepper motors in the CNC machines we build. However, we sometimes lose steps because of the heavy table. We’ve considered putting a feedback device on the stepper application to optimize torque, but is there a better solution? Would a servo motor solve this? What are the tradeoffs? Question posed by a reader of Control Design (http://www.controldesign.com)
A: The ideal motor choice ultimately depends on your application requirements. If the motor is not properly sized, synchronization may be lost regardless of servo motors or stepper motors. Oriental Motor offers both stepper and servo options. Generally, servo motors feature high speed, high efficiency, and less noise due to lower winding inductance, lower current utilization, and less pole counts respectively. However, servo systems are more complex and costly since encoder feedback and a PID loop for velocity, torque, and position control are required. A stepper motor system can be open-loop or closed-loop. Both stepper systems would be less complex than a servo motor system. Since stepper motors typically utilize full current regardless of load, they feature the highest torque per volume but at a cost of operating temperature and therefore duty cycle. If a high duty cycle is necessary, stepper motors often need to be oversized in order to allow a lower current setting to reduce operating temperature. Servo motors only utilize the current necessary for the load, and since current is proportional to temperature, servo motors typically run cooler. Adding a position feedback device to the stepper motor to close the loop itself does not help optimize torque. The primary task of an encoder on a stepper motor is to provide pulses for position verification. Something else is necessary in order to optimize the torque. We offer a patented technology on our Alphastep AR closed-loop stepper motor and driver series called "Keep-In-Step (KIS)" that optimizes torque on stepper motors based on rotor position feedback as well as self-correction on the fly to keep synchronization with input pulses. The Alphastep AR series also offers patented motor technology to enable higher efficiency, lower heat generation, and continuous duty operation. With products like the AR series, we have resolved the disadvantage of limited duty cycle for stepper motors.
Traditionally, servo motors are better for very fast and long moves while a step motor is better for quick and short moves. For applications such as CNC machines where positioning accuracy, synchronization between axes, and timing are critical, step motors may be a better choice because its response time is quicker than servo motors. The high pole count toothed rotor from the step motor allows the operator to command it to rotate to a very specific target position and immediately stop, settle, and hold. The structure of a servo motor is similar to a brushless DC motor based on a 3-phase winding and a permanent magnet rotor. It does not have a high pole count toothed rotor like a stepper motor, so it has to move to a position close to the target position, then "hunt" until the positioning error is minimized to an acceptable level. This effect can potentially increase delays in between moves in the same axis or multiple axes. Because a servo motor has a built-in encoder which it relies on for positioning accuracy, the servo motor driver (or amplifier) will also need to be more complex in order to quickly interpret the encoder signals and work with a PID loop; whereas a stepper motor relies on rotor construction and winding tolerance for positioning accuracy. A typical step motor system has much less components than a step motor system and does not require a PID loop. It's important to point out that if a step motor is sized properly for the application requirements, even without an encoder to close the loop, the step motor would not miss steps. We offer closed-loop stepper motor systems for users who want the peace of mind of verifying if a move has completed or not.
Here's a comparison between typical open-loop stepper motor system versus a typical servo motor system:
Even if you install encoders to make a closed-loop step motor system, it will still be simpler than a servo motor system. Please see below.
Below shows the system configuration of an Alphastep AR series stepper system with built-in position feedback which is used for torque optimization as well as a self-correcting function within the driver (our own KIS technology):
In comparison with set up/commissioning time, the typical servo motor system would take longer than step motor systems since there's typically a PID loop that requires tuning for the specific load attached to the motor. This tuning process could be quite complex and time-consuming. Many servo systems now offer auto-tuning which shortens setup time; including our own NX series. However, if you decide to change the load after you have tuned the servo motor system, you will have to tune it again to get the best performance out of the motor. You do not have to do the same with a step motor system.
From the performance side, the more compliant your mechanical set up is, the worse the servo motor will perform. I will show a few graphs below that depict this performance difference between servo motors and step motors in both compliant (belt & pulley), and rigid (ball screw) systems. We have decided to use one of our closed-loop Alphastep AR series motor systems to do a fair comparison with several servo motor systems.
The above graph shows a comparison of our AR motor system with 4 different servo motor systems on a ballscrew application. All five systems were timed for different move distances and their values plotted. Because of the lack of gain setting, high torque and low rotor inertia, the AR motor system outperforms servos for short distance moves.
Long distance moves prove more difficult for the AR system. Because of the high pole count, the torque of the Alphastep AR motor drops off as speed increases, and we can’t rotate as fast as a servo motor with load. Servo systems are very good for applications that require a high speed.
Highly compliant systems such as belt drives are a problem for servos. Because of the compliance, the extra ringing or overshooting/undershooting has to be tuned out and that results in delays. The AR system is great for highly compliant systems as evidenced by the short move test.
Even in long moves, the AR system can outperform servo systems in a compliant system. This is due to the fact that the gain settings have to be set low for a servo system, and it results in longer delays.
For motor sizing assistance, we offer the following application-specific motor sizing forms to help calculate your motor requirements such as speed, torque, and load inertia. With this information, we can make sure that your next motor is sized properly in order to eliminate missed steps.
For an overview of all of our step motor options including the AR series, please click here:
http://www.orientalmotor.com/products/s ... index.html
For an overview of our tuning-free servo motor systems, please click here:
http://www.orientalmotor.com/products/s ... index.html
If you have any questions, please contact our knowledgeable technical support group.
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