The noise level in a motor depends on the motor type, environmental conditions, and specific application. Permanent magnet motors and hybrid stepper motors are generally quieter because their rotation is more stable. Variable reluctance stepper motors, however, are the noisiest regardless of their application.
To better understand the source of noise, we need to consider how rotational motion is generated. When a stepper motor performs a step, it doesn't stop immediately; instead, it continues to move slightly back and forth before coming to a complete stop. This problem can be overcome by employing specific control logic in the motor driver. During motor operation, at the moment before the motor completes the previous step but stops, the driver issues a command to begin the next step. This continuous and regular motor movement reduces noise and vibration.
It's also important to note that every stepper motor has a resonant frequency, typically occurring when the motor moves at a speed of 150 to 300 steps per second. Many designers tend to avoid this operating frequency range to minimize noise and vibration. However, inserting a properly designed and sized gearbox can also reduce vibration. Another traditional solution is to install a rear-mounted vibration damper on the crankshaft, which also reduces vibration.
Noise reduction technology
Most stepper motors are controlled by pulse width modulation (PWM) signals, which continuously force the H- bridge to switch between switching states, thereby regulating the current fed into the motor. Driver circuits based on this technology are often called chopper drivers because they chop the output voltage following the PWM waveform, thus providing a constant current to the motor windings.
In contrast, L/R technology aims to keep the voltage applied to the winding constant, while the advantage of current chopping is that it is a very efficient, compact, and economical solution that generates very little heat.
It's important to note that the modulation signal applied to a stepper motor generates an audio signal, which is exacerbated when the PWM frequency falls within the audio frequency band. Experiments easily demonstrate how stepper motors produce noise, even when the motor is stopped or held in a position. This phenomenon primarily occurs at switching frequencies below 20kHz . Therefore, it's reasonable to deduce that the first way to reduce noise is to increase the switching frequency. Most chopper drivers allow increasing the switching frequency by modifying the values of external resistors or capacitors. The aim is to alter the duration of the PWM signal used for current regulation in the off state; a shorter duration corresponds to a higher switching frequency.
However, the frequency doesn't need to be too high, as exceeding a certain limit will increase switching losses. A suitable switching frequency is between 30 and 50 kHz . If this isn't enough, the current applied to the motor windings can be reduced. In fact, lower current means less vibration, and of course, less noise.
However, this method also has a side effect: reduced torque. If the torque is too low, it can lead to loss of synchronization. Because the motor is open-loop controlled, it must be supplied with sufficient current to cover all its operating conditions, even under the most severe conditions. A good compromise is to reduce the current when the motor is stopped.
Typically, the current required to hold a motor in position is much lower than the current required to accelerate or rotate the motor at a constant rate. In fact, all stepper motor drivers allow the current value to be set by modifying the analog reference voltage VREF. The trip current ITrip is a function of the external RSENSE resistor and the VREF reference voltage. Once the RSENSE resistor is selected by the designer , its value is fixed during operation, so ITrip can be modified by dynamically changing VREF .
If further noise reduction is required, the motor can be operated in slow decay mode (instead of fast or mixed decay mode). This mode minimizes drive current ripple, thereby reducing noise and improving driver efficiency. However, slow decay mode is not always the optimal solution, especially when using microstepping technology.
Stepper driver
Integrated drivers are designed to provide simple configuration and advanced control for a wide range of applications. The integrated encoder option makes stepper motors the best choice for synchronous position applications. Simply connect the coil to the power transistor, and the transistor to the control circuit, and you can drive the stepper motor.
Figure 1 : Block diagram of A3982 . (Image source: AllegroMicroSystems )
Allegro MicroSystems is a leader in the design and manufacture of brushed DC motor and stepper motor drivers, offering a range of safe and reliable solutions with integrated MOSFET gate drivers. The company's A3982 is a complete stepper motor driver with a built-in converter, easy to operate, and suitable for both low-power and high-power applications.
The A3982 is used to operate bipolar stepper motors in full-step and half-step modes, providing an output signal of up to 35V and ± 2A . The current decay mode (slow decay or mixed decay) can be selected by applying a signal to the STEP input pin, as shown in the block diagram in Figure 1 .
In hybrid mode, the chopper control is initially set to a fast decay mode for 31.25% of the fixed off time , and then to slow decay for the remaining off time. This current decay control mechanism reduces audible motor noise, improves stepping accuracy, and reduces power consumption.
The converter's features greatly simplify the design of motor control systems. A single pulse applied to the STEP input pin drives the motor one step, eliminating the need for phase sequence tables or high-frequency control lines. Therefore, the A3982 is the optimal choice for applications where the host microcontroller is unavailable or the load is too heavy .
in conclusion
Stepper motors have a simple structure and are easy to control. As a digital electronic component, stepper motors are widely used in various open-loop control systems. However, the noise generated by their drive circuits and resonant mechanical structures can affect overall performance. Most stepper motor applications require smooth operation. To achieve smooth operation, designers can modify the voltage, current, or more commonly, the microstep settings.
