Page 30 - EE Times Europe Magazine | March 2020

30 EE|Times EUROPE

MOTOR CONTROL DESIGN
Noise Reduction in Stepper Motors

By Maurizio Di Paolo Emilio

he amount of noise in a motor depends on the type of motor, Normally, the current required by the motor to maintain the position
environmental conditions, and the specific application. Perma- is considerably lower than the current required to accelerate or move
nent magnet and hybrid stepper motors are generally quieter, the motor at a constant speed. Virtually all stepper motor drivers allow
T as they have a more consistent rotation. Conversely, variable setting the current value by modifying the analog reference voltage
reluctance stepper motors are the noisiest, regardless of the applica- V REF . The trip current, I Trip , is a function of both the external R SENSE resis-
tion in which they are used. tor and the V REF reference voltage. Because the first, once chosen by the
To better understand the origins of noise, we need to think about designer, has a fixed value, at run time, it is possible to modify I Trip by
how the rotation movement takes place. When a stepper motor changing V REF on the fly.
performs a step, it does not stop immediately but continues to move If further noise reduction is required, the motor may be operated in
slightly forward and backward before stopping completely. This behav- slow-decay mode instead of fast- or mixed-decay modes. This mode
ior can be overcome by adopting specific control logic in the motor minimizes the driving current ripple, reducing the noise and increasing
driver. During motor operation, the driver commands the movement of the driver efficiency. Slow-decay mode, however, is not always the best
the next step one moment before the motor has stopped after complet- solution, especially if you want to use the microstepping technique.
ing the previous step. This continuous and regular motor advancement
helps to reduce both noise and vibration. STEPPER DRIVERS
It should also be noted that each stepper motor has a resonance Integrated drivers have been designed to offer easy configuration
frequency, which typically occurs when the motor moves at speeds and advanced control functions for every type of application. Inte-
between 150 and 300 steps per second. Many designers tend to avoid grated encoder options make stepper motors a suitable choice for
this operating speed range in order to minimize both noise and vibra- synchronized position applications. The stepper motors are driven
tion. The insertion of gearheads, appropriately designed and sized, can by connecting the coils to power transistors and the transistors to a
help to reduce vibration. Rear-mounting dampers positioned on the control circuit.
crankshaft are another traditional solution for reducing vibration. Allegro MicroSystems, a leader in the design and manufacturing of
brushed DC and stepper motor drivers, provides a broad portfolio of
NOISE-REDUCTION TECHNIQUES safe and robust solutions with integrated and MOSFET gate drives.
Most stepper motors are controlled by a pulse-width modulation Suitable for low- and high-power applications, Allegro’s A3982 is
(PWM) signal, which continuously forces the switching of the H bridge a complete stepper motor driver with a built-in translator for easy
between the on and off states, thus regulating the current that feeds operation. Designed to operate bipolar stepper motors in full- and
the motor. Driver circuits based on this technique are commonly called half-step modes, the driver can provide an output signal up to 35 V
chopper drivers because they supply the motor windings with a con- and ±2 A. The current decay mode (slow or mixed) can be selected by
stant current by chopping the output voltage following the application applying a signal at the STEP input pin, as shown in the schematic Focused on
of the PWM waveform. block diagram of Figure 1.
Unlike the L/R technique, which instead aims to keep the voltage In mixed mode, chopping control is initially set to a fast decay for a POWER ELECTRONICS?
applied to the windings constant, current chopping has the advantage period amounting to 31.25% of the fixed off time, then to a slow decay
of being a very efficient, compact, and economical solution, generating for the remainder of the off time. This current decay control scheme Exhibit at the PCIM Europe!
a small amount of heat.
One caveat is that the modulated signal applied to the stepper motor International Exhibition and Conference pcim-europe.com/exhibitors
can generate an audible signal, even more so if the PWM frequency falls for Power Electronics, Intelligent Motion,
within the audio band. Experimentally, it is indeed easy to verify how Renewable Energy and Energy Management
a stepper motor can generate noise even when it is stopped or when it
holds the position. This phenomenon occurs mainly at switching fre-
quencies below 20 kHz. Thus, it can be deduced that the first method to Nuremberg, 5 – 7 May 2020
reduce noise is to increase the switching frequency. Most chopper driv-
ers offer the capability to increase the switching frequency by modifying
the value of an external resistor or capacitor. The effect is to change the
duration in the off state of the PWM signal used for current regulation.
The shorter this duration, the higher the switching frequency.
It is not necessary to exceed the frequency value, however, because
beyond a certain limit, the switching losses also increase. An appropri-
ate switching frequency value could be between 30 and 50 kHz. If this
technique is not sufficient, the current applied to the motor windings
can be reduced. Lower current means, in fact, a reduction of vibration
and, therefore, of noise.
However, a side effect is a reduction in torque, which, if too low, can
cause the loss of steps during operation. Because the motor is con-
trolled in an open loop, the motor must be supplied with the amount
of current sufficient to cover all operating conditions, even the most
severe. A good compromise is to reduce the current during periods
when the motor is stopped. Figure 1: A3982 block diagram (Image: Allegro MicroSystems)

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