When I first heard about stepper motor tuning, I was really confused. How can a motor be out of tune? I started reminiscing about warming up for band concerts in high school and wondered how system designers could do something similar for their motors.
After watching a video from my colleague Matt Hein I learned these three things about stepper motor tuning:
Stepper motor tuning is especially important for microstepping. Microstepping gives stepper motors fine positional control. The current waveform of a tuned stepper motor should approximate a sine wave, as shown in Figure 1.
Figure 1: Stepper motor coil current for a 25V supply, 2,500 pulses per second (PPS) and one-eighth microstepping using ripple control on the TI DRV8886AT
However, engineers who work with stepper motors do not always see a waveform like Figure 1 the first time they connect their motor. They may see waveforms like the ones shown in Figures 2, 3, or 4. These figures are examples of poor stepper motor tuning, and will cause excessive vibration and noise from the stepper motor.
Figure 2: Stepper motor waveform with 60% fast decay
Figure 3: Stepper motor waveform with slow decay
Figure 4: Stepper motor waveform at a slow speed (200PPS) with slow decay/50% mixed decay
In each of these cases, the decay setting chosen for the current regulation scheme (also called current chopping) was not appropriate for the chosen motor, supply voltage, full-scale current, and speed.
Stepper motor tuning is the process of choosing the proper decay setting during the off time of the PWM current chopping cycle (tOFF). Figure 5 shows the different ways that current can flow through the H-bridges of a stepper driver. During fast decay, the field-effect transistors (FETs) opposite from the driving FETs turn on, and the current flows from the motor back to the power supply. For this condition, the current decreases quickly. During slow decay, the two low-side FETs turn on, and the current recirculates through these FETs. For this condition, the current decays much more slowly.
Figure 5: Current paths through the H-bridge during normal operation (driving), dead time (all FETs are off), fast decay and slow decay
During stepper motor tuning, engineers search for the optimal “mixed” decay setting for the stepper. Mixed decay consists of a combination of fast and slow decay when the driver is not driving current into the coils. Choosing the mixed-decay settings means setting the percentage of fast decay versus slow decay for the off time.
Figure 6 shows how mixed decay helps regulate the current.
Figure 6: Using mixed decay to regulate the current at each microstep level
Some customers have told us that it can take as long as a month to find the proper decay settings for their stepper motor. The combination of the supply voltage, back electromotive force (EMF) and microstepping current levels all impact the way the current in the stepper coils charges and discharges.
The following list provides some details as to why this is the case:
Table 1 provides explanations for why the decay modes chosen in Figures 2, 3, and 4 exhibit poor tuning.
25V supply, 2,500PPS, 1/8 microstepping, 60% fast decay for increasing and decreasing current
Too much fast decay percentage causes large ripple currents
25V supply, 2,500PPS, 1/8 microstepping, slow decay for increasing and decreasing current
Significant back EMF distorts the waveform when the current decreases using slow decay
25V supply, 200PPS, 1/8 microstepping, slow decay for increasing current and 50% fast decay for decreasing current
The blanking time for slow decay is too long, so the motor coils charge faster than the driver can regulate the current
Table 1: Explanations for examples of the poorly tuned stepper motor waveforms shown in Figures 2, 3 and 4
At this point, you may be thinking, “Instead of manually testing all of the fixed mixed-decay settings to find the best one, what if the stepper driver had some kind of smart tuning feature to choose the best mixed-decay setting while driving the motor in real time?”
Devices like the DRV8880, DRV8881 and DRV8886AT have smart tuning features that can help eliminate the time-consuming effort of stepper motor tuning. All of these devices have the option to select dynamic decay, which is a smart tuning feature that automatically adjusts the percentage of fast decay used in the total off time. The DRV8886AT has an additional smart tuning feature called ripple control that regulates the current at each microstep level between the upper and lower thresholds. Ripple control only uses slow decay, but because the driver continually senses and adjusts the current, the current does not lose regulation.
Other than achieving nice sinusoidal current waveforms like Figure 1, these smart tuning features have other benefits as well. When the stepper motor has a well-tuned current waveform, it runs more quietly and more efficiently. Interestingly, the examples of poor tuning in Figures 2, 3, and 4 all result in currents larger than the targeted stepper motor current at certain microstep levels. This excess current increases the temperature of the motor and the driver. With dynamic decay and ripple control, the driver does not use excess current at each microstep level. Figure 7 shows how a stepper motor tuned with dynamic decay reduces thermal dissipation of the stepper driver chip.
Figure 7: Dynamic decay vs. mixed decay at various tOFF times
Ultimately, smart tuning features like dynamic decay and ripple control can help you save time and reduce temperature and noise in your stepper motor applications.
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