Stepper motors have primarily been favored for their high reliability and low cost. The price, however, was a significant amount of noise. And although the days in which stepper motors were operated at full & half-step are long over, the stepper motor is still tainted by its noise level. With modern control processes and a careful layout, stepper motors can now operate virtually silently. This opens up new fields of application.
The stepper motor is still one of the most common motor types. Stricter EMC requirements in simpler device enclosures are driving their expansion because classic, cheap DC motors are no longer considered on account of their brush commutators. At the same time, more and more formerly industrial machines are being miniaturized to desktop devices. Only a few years ago, dental inlays and implants were milled in a central laboratory operated by laboratory staff; now, the trend has shifted to small, decentralized equipment that every dentist can operate in their own practice, thereby providing e.g. inlays for patients with little to no waiting time and eliminating any need for complex temporaries. A similar trend can be seen with the currently ubiquitous 3D-Printer. The devices are constantly getting smaller and smaller.
One thing all of these devices have in common is that they are being operated ever-closer to the user – often for extended periods of time. This development would never have been possible if stepper motors were operated at full & half-step like in the days of dot-matrix printers. Since Trinamic introduced the TMC262 series stepper motor driver with a maximum resolution of 256 microsteps per full step in 2010, the motors can be driven with semi-sinusoidal current.
Avoiding Vibration with Microstepping
To better understand the sources of a stepper motor’s noise, it’s helpful to view a simplified model of the motor which describes its components and their physical properties as well as their functions. The model makes the motor easier to understand mathematically, and still accurately describes its behavior. Resonances occur as they do in e.g. a real motor; only their frequencies are shifted.
The model behaves as an oscillator, similar to a physical pendulum. In the simplified model, an electrical rotation equals one mechanical rotation. In a real stepper motor, an electrical rotation corresponds to four full steps, and in a typical hybrid stepper motor with 200 full steps, an angle of 7.2.° .
If every step is interpreted as the deflection of a damped pendulum, transient behavior as a reaction to each step is observed. In full step operation, the pendulum is deflected 90 electrical degrees, or 1.8 mechanical degrees. Accordingly, transient behavior, much like a pendulum, is found in a real stepper motor. Increasing the microstep resolution thereby reduces the deflection amplitude of the pendulum model. As a result, the vibrations caused by the transient effect are reduced.
Perfect Current Control is the Key
In the old days of full & half-step operation, the phases of the stepper motors were switched directly against the supply voltage – therefore, they were fully limited by their inductance, while in static situations, they were limited by their own internal resistance. With microstepping, constant power control is the standard. Typical bipolar control of the motor requires a full MOSFET bridge per motor phase.
As the coils of the motor, on account of the energy stored within them, tend to maintain current after switching off the supply voltage – especially at higher speeds – simply cutting the power will not generate a sufficient current in the opposite direction in order to reverse the magnetic polarity.
The full bridge allows three fundamental switching states: in the powered-on state, power is directed through the coil in the direction of the target current. For powered-off, two different modes are available depending on the need. If the current is quickly stopped in “Fast-Decay” mode, the voltage is switched in the opposite direction of the target current. That way, the descending branch of the sine wave can be quickly reduced. However, the ascending branch leads to an increased current ripple in the process. For this operational state, “Slow-Decay”, in which the power is recirculated, is better suited.
Modern stepper motor drivers combine the advantages of these switching states through what’s known as Mixed-Decay operation. In this mode, the ascending half of the sine wave is countered with Slow-Decay, while the descending half is countered with Fast-Decay. Allocation of both modes can normally be applied in multiple steps.
The integrated drivers utilize both powered-off switching states in sequence in order to reach the target current. Therein lies another problem. When switching off once the target current has been reached, the effective current in the middle always lags slightly behind the target current. A plateau is formed by the zero-crossing of the sine wave, corresponding to a single, slightly extended step. This plateau can be considerably noisy.
Since 2010, Trinamic has offered an advanced “Mixed-Decay” mode with the spreadCycle process; the ideal conditions of both power-off states are automatically calculated and combined for every chopper cycle, and hysteresis-based processing is applied to ensure that the determined target current is effectively met. This method is more representative of a pulse density than a pulse-width modulation. The range of switching frequencies is expanded and can be adjusted very quickly and dynamically to adapt to changing loads.
This method allows for the best commutation for stepper motors currently available on the market – especially at higher speeds. However, this fast and flexible control still operates with switching frequencies within the audible range. Through magnetostriction i.e. the mechanical deformation of the coils in changing magnetic fields, these switching frequencies are converted into audible mechanical vibration. This is the same effect which causes the hum of transformers and the high frequency noise of some switched-mode power supplies. The advantages of such control and the reduced mechanical vibrations and resonances are therefore partly gained at the cost of audibly perceptible motor coil deformation.
Constant Voltage Control for Low Speeds
The latest series of stepper motor drivers, introduced by Trinamic in 2014, addresses and counters this noise with a technology known as stealthChop. The core of this technology is a completely modified and optimized current regulation process at low to medium RPM.
The stealthChop method is based on constant voltage control and, as before, utilizes a sine wave with a full-step resolution of 256 microsteps. To allow an increased RPM range for e.g. a purely voltage-controlled motor, the current is constantly measured, evaluated and adjusted accordingly. Yet unlike other voltage-mode controls, there’s no need for the parameterization of speed and voltage dependency curves. stealthChop technology readjusts these parameters independently, thereby allowing the driver to be implemented extremely easily and sometimes even parameterized without a bus interface.
As the switching frequencies are above the audible range, the stepper motor has been silenced for the first time; even with the highest quality power control systems, the characteristic “music” inevitably produced by a CNC machine’s changing speeds has finally been muted – without any need for labor-intensive tuning.
Designing Stepper Motor Controls
Stepper motor driver stages typically pack significant currents into relatively small spaces. Therefore, power dissipation is generally the focus of layout design. Should the nominal currents of the driver modules be exhausted, care should be taken to allow for a sufficient amount of distribution area for thermal dissipation among the power switches and current measurement resistors.
To simplify the layout process, Trinamic has developed a modular evaluation and reference design system for the last few generations of motor drivers, which has been thermally optimized and well tested. Both the schematics and the layout are available as PDF and ECAD files. The board itself is marked to show the minimal wiring of the modules.
Good Current Control Needs Proper Current Measurement
As high-quality power control is only possible with accurate current measurement, special attention should be given to the placement and routing of the sensing resistors. Only a few millivolts are measured against the entire system; accordingly, this measurement is sensitive to crosstalk from other signals and to other fluctuations within the system itself.
A low-impedance and solid ground connection of the measurement resistors should therefore be provided by the layout, e.g. via a short connection to an ideally dedicated grounding plane. Above all else, signals placed under or near the measurement resistors can cause undesired noise that will be amplified by the current control system and thereby made detectable.
The proven layout of the test boards can also be used to determine the proper placement of measurement resistors.