Optimized Current Control

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 wave-forms caused by the motor's back-EMF, it also allows for resonance-free stepper motor operation with increased energy-efficiency.

 

 

Chopper Modes

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 a chopper classic chopper mode like constant TOFF, or an advanced chopper mode like Trinamic's SpreadCycle™ or StealthChop™.


What's a Chopper Mode?

Typical stepper motors for chopper operation are bipolar, 2-phase stepper motors. Bipolar means that both ends of each of the two coils are accessible to the motor driver and the motor has 4 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 means you can control the current of a stepper motor 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, a very high voltage is initially applied to the winding. This causes current in the winding to rise quickly. Controllers monitor current in each winding, usually by measuring the voltage across a small sense resistor in series with each winding. When the current exceeds specified limits, voltage is turned off, or chopped. When current drops below specified limits, voltage is turned back on. This approach is able to maintain relatively constant current particular step positions. Since modern microstepping drivers implement this control loop, additional controller interaction is eliminated.

 

 

Constant T_OFF Current Chopper

While still commonly used in many applications, constant TOFF PWM choppers cause a stepper motor to vibrate and make the typical buzzing or chirping stepper motor 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. When looking at the current with an oscilloscope, you will see a flat plateau at the zero-crossing where the motor has no torque, which leads to wobbling and vibrations.


SpreadCycle™ PWM Chopper

By adding a hysteresis function, SpreadCycle overcomes the issues that a constant TOFF chopper introduces to your mechanical system. It 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 gently drops the current so it doesn't fall too quick, leading to an average current matching the target current. Besides resulting in a near-perfect current sine wave, the cycle-by-cycle chopper mode SpreadCycle also reduces current ripple and torque ripple.

Even at high RPM, where classic constant TOFF chopper modes show excessive deformations caused by the back-EMF of the motor, SpreadCycle 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 SpreadCycle and setting the parameters, please see our app note: Parameterization of SpreadCycle™.


StealthChop™ Voltage Chopper

StealthChop 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. To understand how StealthChop allows for silent motor operation, it's important to know that 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 solution to silence stepper motors is the voltage-regulated chopper named StealthChop. It modulates the current based on the PWM duty cycle which minimizes current ripple due to the constant PWM frequency. By removing variations of the chopper frequency, or frequency jitter, only the commanded variations remain. At 50% PWM duty cycle, the current is actually zero. By adjusting the PWM duty cycle to control the current, StealthChop results in a perfect current sinewave with straight crossing of the zero-current level. Furthermore, minimizing the current ripple also minimizes the Eddy current in the stator, leading to less power loss and increased efficiency. The result is silent stepper motor operation at standstill and at low to moderate speeds.

Unlike other voltage-mode choppers, StealthChop2 doesn't require configuration. Instead, it automatically learns the best settings during 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. The result is whisper-quiet motion for applications like 3D printing, desktop manufacturing, conference cameras, and personal medical devices where audible noise is unacceptable. For more information on StealthChop, please see the application notes below.

 

 

 

 

Depending on the use case, a combination of SpreadCycle and StealthChop might be the best solution. While StealthChop eliminates motor noise, it's should only be used at low to moderate speeds and standstill. SpreadCycle works better for higher speeds.