The Secret to Silent Stepper Motor Control
Stepper motors are actuated by applying a voltage to the motor coils with proper sequencing. To achieve the desired motor operation, this current must be chopped appropriately by a current chopper or voltage chopper. The constant off-time PWM chopper remains a popular and effective method for some applications; however, these classic choppers can cause stepper motors to vibrate and emit the hallmark buzzing or chirping stepper motor noise. This noise can interfere with performance in applications where audible noise is unacceptable such as 3D printing, desktop manufacturing, conference and surveillance cameras, personal medical devices, and more. It’s also a sign that the motor is not running as smoothly as it could be.
ADI Trinamic™ technologies enable smoother, more efficient stepper motor function via current chopper and voltage chopper control modes , which can be leveraged to improve efficiency and reduce or eliminate stepper motor noise.
What’s a Chopper Mode?
In a typical bipolar 2-phase stepper motor, both ends of each of the two coils are accessible to the motor driver and the motor has four wires. To control the current, each phase connects to one MOSFET half-bridge, which can switch either end to supply voltage or ground. Switching the MOSFETs in certain ways enables control of the stepper motor current in the following modes: ON-phase, where the current flows from supply voltage to ground; fast decay, where the current flows from ground to supply voltage; and slow decay, where the current is recirculated. The currents through both motor coils are controlled with choppers.
For each chopper cycle, the motor supply voltage is initially applied to the winding. This causes the current in the winding to rise quickly (ON phase). Controllers monitor the current in each winding, for example, by measuring the voltage across a sense resistor in series with each winding. When the current exceeds specified limits, voltage is supplied to the other end of the winding (fast decay) for a short time to unload the current from the winding. Then, the windings are shortened, and the current circulates and unloads slowly via the coil resistance (slow decay). When the current drops below specified limits, the voltage is turned back on to load the motor again.
With the correct chopper scheme, a current sine wave can be achieved with a smooth zero-crossing. This not only eliminates the typical spike in current waveforms caused by the motor’s back-EMF; it also allows for resonance-free stepper motor operation with increased energy efficiency. The coil of an electric motor like a stepper motor has a certain inductance, meaning it can save energy for a limited time. It also means a certain current can be held in the motor coil without feeding new current into it. This characteristic means you can save energy when driving a stepper motor by using either a classic chopper mode like constant TOFF, or an advanced chopper mode like SpreadCycle™ or StealthChop™.
Constant TOFF Current Chopper
The classic, constant TOFF current chopper approach can maintain a relatively constant current. The current pairs through both motor phases result in a particular step position. Since modern microstepping drivers implement this control loop, additional controller interaction is eliminated.
However, this classic chopper approach can invite vibrations that manifest as inefficiencies and unwanted noise. This is caused by the fixed relationship between fast decay and slow decay phase. As a result of this fixed relationship, the specified target current is reached but the average current is lower than the desired target. An oscilloscope will reveal a flat plateau at the zero-crossing where the motor has no torque, which leads to wobbling and vibrations.
SpreadCycle PWM Chopper
The ADI Trinamic SpreadCycle chopper overcomes the issues that a constant TOFF chopper introduces to mechanical systems by adding a hysteresis function. The SpreadCycle chopper automatically applies a fitting relation between slow decay and fast decay to create the optimal fast decay for that cycle. The hysteresis function acts as a parachute that gradually drops the current, leading to an average current matching the target current. Besides resulting in a perfect current sine wave, this cycle-by-cycle chopper mode also reduces current ripple and torque ripple.
Even at high RPM, where classic current chopper modes show excessive deformations caused by the back-EMF of the motor, the SpreadCycle chopper remains highly effective. The motor control technology measures the current during each chopper cycle and automatically adjusts the hysteresis function to optimize the fast-decay phase. For more info on the SpreadCycle chopper and setting the parameters, please see our app note: Parameterization of SpreadCycle.
StealthChop Voltage Chopper
StealthChop voltage chopper completely silences stepper motors by eliminating the noise caused by unsynchronized motor coil chopper operation, PWM jitter, and regulation noise of a few millivolts at the sense resistors.
A current-regulated chopper always reacts to the coil current measurement on a cycle-by-cycle basis. This leads to noise and electric and magnetic coupling between both motor coils. The coupling causes small variations of the resulting motor currents, thereby influencing the current chopper. The StealthChop chopper modulates the current based on the PWM duty cycle, resulting in a perfect current sinewave with straight crossing of the zero-current level. The constant PWM frequency minimizes current ripple and, by extension, any Eddy current that may be found in the stator, reducing power loss, and increasing efficiency. The StealthChop chopper also removes variations of the chopper frequency, or frequency jitter, so that only the commanded variations remain. Thus, at a 50% PWM duty cycle, the current is actually zero. The result of all this is silent stepper motor operation at a standstill and at low to moderate speeds.
For ease of integration, the StealthChop2 chopper automatically learns the best settings during the first motion following power-up and optimizes settings further in subsequent motions. An initial homing sequence is sufficient for this learning process. Both StealthChop and StealthChop2 applications have achieved noise levels of 10 dB below classical current control.