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Stepper motors are known for their vibration. It's a natural characteristic of their operation, but there are ways to minimize it. Understanding the source of these vibrations is crucial before attempting any corrective measures.
A stepper motor achieves precise positioning without an encoder by moving in small, incremental steps. However, this step-by-step movement can cause tiny vibrations between each step. While these vibrations might be imperceptible in most cases, they can translate into noise that may be amplified through connected mechanisms. Identifying the root causes of these vibrations is the first step toward effective solutions.
In the video above, you can see that the vibration is not noticeable at first glance. But when viewed under a microscope, it becomes clear that the motor shaft overshoots and undershoots with each step. The following video uses a 5-phase 0.72° stepper motor to demonstrate its 1-step vibration characteristics and how performance improves at higher speeds.
Inside a stepper motor, there's a small air gap between the rotor and stator where magnetic flux flows, and the only friction present is in the ball bearings. When the motor is commanded to move and stop, the ball bearing doesn't provide enough friction to prevent the shaft from overshooting and undershooting due to inertia. This period of oscillation is referred to as "settling time."
There are situations where the vibration caused by steps becomes significant, potentially leading to missed steps or excessive noise, especially when operating at resonant frequency. Resonant frequency is determined by the relationship between torque stiffness and inertia, and adjusting either parameter can shift the resonant frequency out of the operational range to enhance performance. Avoiding the resonant frequency for the motor is an effective way to eliminate this type of vibration.
The next video demonstrates how vibration is measured and how it changes at different speeds.
While it's challenging to completely eliminate vibration in a stepper motor, several mechanical and electrical approaches can significantly reduce it. In this post, we will explore the most common and effective methods to lower vibration.
Mechanical Methods to Reduce Vibration
First, let's look at some of the most common mechanical methods for reducing vibration in stepper motors.
1. Install a mechanical clean damper.
One simple method to reduce vibration is to install a clean damper on the rear shaft of the motor. Clean dampers consist of hermetically sealed plastic cases containing an inertia body filled with silicon gel. This extra inertia and silicon gel help absorb the vibration and provide a stable damping effect.
2. Use a high-resolution type stepper motor.
Some manufacturers offer stepper motors with an increased number of rotor teeth (100 instead of 50). This increase doubles the full-step resolution and helps reduce the amplitude of vibration since the motor moves a shorter distance with each step. For example, a standard 1.8° 2-phase motor becomes a 0.9° motor, and a standard 0.72° 5-phase motor becomes a 0.36° motor. Even finer resolutions can be achieved through gearing or microstepping.
3. Use a 5-phase stepper motor.
Similar to a high-resolution type motor, a 5-phase 0.72° stepper motor can achieve a finer full-step resolution and reduce vibration compared to a 2-phase 1.8° motor. A 5-phase motor has the same rotor but 10 stator poles instead of 8, increasing the full-step resolution from 200 to 500 steps per revolution. Stop position accuracy depends on the number of teeth, so high-resolution 2-phase or 5-phase motors still offer better accuracy than standard ones.
4. Use geared stepper motors.
Gearheads are typically used to increase torque while reducing speed, but they can also help suppress vibration transmission from the motor to the load. Gear friction can reduce overshooting and undershooting, and consistent torque output provides more stable operation.
5. Perform thorough motor sizing.
Proper motor sizing involves finding the optimal point where the motor operates comfortably. It's a mistake to size a motor for its maximum specifications. Oversizing the motor can lead to increased vibration. If oversizing is necessary, reducing the motor current can help decrease the excessive torque.
Electrical Methods to Reduce Vibration
There are also several electrical methods to reduce vibration in stepper motors.
1. Use a microstepping driver.
Microstepping divides the motor current between phases and is often the recommended method for reducing vibration. With microstepping, the driver's ability to precisely control torque and position greatly affects its ability to reduce torque ripple, vibration, and audible noise.
2. Use a driver that resolves vibrations independently.
Oriental Motor's CVD bipolar constant current chopper drivers are designed to provide low vibration performance across all speed ranges by treating each type of vibration independently. These drivers use individual phase current detectors to regulate current effectively and provide a uniform current waveform.
The CVD drivers treat each type of vibration differently. Here are three main types of vibrations that stepper motors exhibit:
- Low-speed vibration (about 0~50 RPM) caused by step movement
- Mid-speed vibration (about 50~200 RPM) caused by torque variance or resonant frequency
- High-speed vibration (about 500 RPM and above) caused by back EMF and instability of constant current control
For more information about the CVK Series CVD driver's vibration suppression technologies, please watch the following video and refer to the white paper for more details.