How to Reduce Audible Noise in Stepper Motors

Stepper motors are known to generate audible noise as they operate, which is undesirable for many applications. Traditional stepper motor drivers with fixed-percentage mixed decay current chopping and low levels of microstepping greatly contribute to the noise. But features such as smart tune technology and high microstepping can help lower the audible noise significantly. The goal of this application report is to explain how specific motor driver features can reduce the audible noise for a wide variety of stepper motor systems and applications.

Trademarks

All trademarks are the property of their respective owners.

1 Introduction

Stepper motors are used in a wide array of applications such as printers, projectors, textile machines, stage lighting, industrial automation, electronic point of sale, automotive headlight, and head-up display. A stepper motor moves in discrete steps as defined by a step angle during its rotation. It has two electrical current windings and each winding can be controlled with an H-Bridge. As shown in Figure 1, the stepper motor driver applies current waveforms approximating a sine wave (blue) into one coil and a cosine wave (red) into the other. One quadrant (90°) of the current waveforms corresponds to the stepper motor rotation by one step angle, which is 1.8° for most hybrid stepper motors used today.

Figure 1. Microstepping Current Waveform

Most stepper motor drivers limit current by "chopping" their drive output at some frequency, called the PWM frequency. During PWM current chopping, the H-bridge is enabled to drive through the motor winding until the PWM current chopping threshold is reached. This is shown in Figure 2, Item 1.

Once the chopping current threshold is reached, the H-bridge can operate in two different states, fast decay or slow decay. In fast decay mode, once the PWM chopping current level has been reached, the H-bridge reverses state to allow winding current to flow in a reverse direction. The opposite FETs are turned on; as the winding current approaches zero, the bridge is disabled to prevent any reverse current flow. Fast decay mode is shown in Figure 2, item 2. In slow decay mode, winding current is re-circulated by enabling both of the low-side FETs in the bridge. This is shown in Figure 2, Item 3. Most legacy stepper drivers feature a fixed-percentage mixed decay scheme, where the decay mode is fast decay for a fixed percentage of the OFF time, and slow decay for the rest of the OFF time.

Figure 2. Decay Modes

With traditional slow, fast and mixed decay modes, stepper motors can be known to hum and whine as they operate. In most applications, the audible noise can annoy users and might warrant expensive noise suppression methods, such as rubber isolators, which do not completely eliminate the noise. For example, household or office printers and 3D printers have multiple stepper motors within them, and the noise from the motors can break concentration of their users. Higher quality stepper motor drivers that can reduce the motor noise go a long way towards user satisfaction. Another example is electric vehicles, which are generally quiet due to the absence of an internal combustion engine, and lowering the noise level from stepper motors used in headlight leveling helps to achieve a overall quiet cabin.

In the following sections, we will discuss the various sources of noise in stepper motors and how different stepper motor driver features can alleviate most of that noise.

2 Details of the Test Setup

In this application report, several measurements were taken to demonstrate the effectiveness of different settings in stepper motor drivers in reducing the motor noise. Sound pressure level (SPL) was used as one method of measurement. SPL is the measurement of local pressure deviation from the ambient pressure caused by a sound wave. SPL is represented graphically with sound in decibels (dB) versus frequency. To obtain the SPL plots shown in this application report, a microphone and a stepper motor driven by a DRV8424 driver were placed inside an acoustic chamber. The audible noise from the stepper motor was recorded at various operating conditions to obtain SPL plots. Figure 3 shows a picture of the microphone inside the chamber. The details of the setup are as follows:

The motor was securely mounted on rubber to minimize extra vibrations.
The stepper motor is rated for 2.3 A, has 1.8° step angle, 1.9 mH inductance and 0.93 Ω resistance.
A calibrated USB microphone was used for recording the noise.
First, a few seconds ( approximately 5 sec) of ambient noise was captured to use as reference.
Then, a few seconds (approximately 5-10 secs) of audio was recorded while the motor was spinning.
The files from the audio recording were imported to a software to obtain the SPL plots.

Figure 3. Audible Noise Measurement Setup

3 Sources of Noise in a Stepper Motor

The amount of audible noise from a stepper motor depends on the type of motor and the operating conditions. Stepper motor noise has been described as a high-pitched whine, a hissing noise or even a deflating tire. Human audible frequency range is generally considered as 20 Hz to 20 kHz, but the human ear is most sensitive to frequencies between 2 kHz and 15 kHz. Permanent magnet and hybrid stepper motors are generally quieter, whereas variable reluctance stepper motors are the noisiest.

The sources of noise coming out of a stepper motor can be broken into three main categories: magnetic, mechanical, and electrical.

3.1 Magnetic Noise

A property of ferromagnetic materials is magnetostriction, which causes magnetic materials to expand or contract in response to a magnetic field. The molecular dipoles and magnetic field boundaries shift in response to the magnetic field, changing length slightly in the direction of the applied field, as shown in Figure 4. In a stepper motor, magnetostriction deforms the iron and pulls the rotor and stator teeth toward each other in the air gap, causing audible noise.

Figure 4. Molecular Dipoles Changing With Applied Magnetic Field

For stepper motors, the audible sound of magnetostriction manifests itself as an intense low pitch hum. Stepper motors operating at low speeds show the worst effects of magnetostriction. Thus, magnetostriction has the worst noise effects for motors used in laser printers and copiers due to generally low motor operating speeds.

Noise resulting from magnetostriction can not be completely eliminated, but it is known that certain types of metals are more prone to magnetostriction than others. There are special materials that compromise between magnetostriction and core losses to achieve the best performance for a given application.

3.2 Mechanical Noise

Mechanical noise is caused by the physical components in the structure of the stepper motor. Common examples contributing to noise include unsecured mounting structures, bent shafts, and loose or no bearings. All of these examples cause unnecessary vibrations and resonant frequencies to appear. Other mechanical noise factors include motor housing, balance of rotor, and bearing choice.

The motor housing structure has a significant effect on high speed motor applications. If the rotor is out of balance, there will be a spike in frequency directly related to speed of the motor. Electric motors use serval types of bearings: sliding sleeve bearings or rolling bearings. Sleeve bearings are generally considered to be quiet bearings. A properly lubricated sleeve bearing will only produce very high noise frequencies due to the bearing and shaft finish. Rolling bearings are generally considered to be noisy and have many factors that could lead to a noisy outcome.

Most mechanical noise can be minimized by stiffening the mounting structures and choosing noise dampening materials - such as mounting the motors in sound absorbing materials like rubber, balancing the rotor, and using properly maintained bearings.

3.3 Electrical Noise

There are various electrical sources of audible noise for a stepper motor, detailed in the subsequent sections. Unlike magnetic and mechanical noise, proper selection of the stepper motor driver can lead to the reduction of the electrical noise.