When I first heard about the stepper motor adjustment, I was really confused. How can the motor be out of order?
I began to recall the preparation for the high school band concert and wanted to know how system designers did similar things for their motors.
After watching the video of colleague Matt Hein, I learned three things about stepping motor adjustment: stepping motor adjustment involves the ability of current adjustment scheme to maintain current in the coil of stepping motor.
Poor tuning occurs when the stepper driver does not correctly adjust the current in the coil to an appropriate level.
Poor tuning of stepping motors will lead to jitter and severe vibration.
Poor adjustment will also have a negative impact on the efficiency of the motor drive system.
Adjusting the stepping motor can be time consuming because finding the best fixed attenuation settings for a given motor voltage current and speed is a trial and error process.
Stepping Motor adjustment is especially important for micro-stepping.
Micro stepping enables the stepping motor to have good position control.
The current waveform of the stepping motor after tuning should be close to sine wave, as shown in Fig. 1.
Figure 1: 25V power supply, 2,500 pulses/sec (PPS)
And 1/8 micro-stepping stepper motor coil current using ripple control on TI DRV8886AT, however, engineers using stepping motors do not always see the waveform shown in Figure 1 when connecting the motors for the first time.
They may see waveforms as shown in Figures 2, 3 or 4.
These figures are examples of poor tuning of stepping motors and can lead to excessive vibration and noise of stepping motors.
Figure 2: stepping motor waveform with 60% fast attenuation Figure 3: stepping motor waveform with slow attenuation Figure 4: slow speed (50% mixed attenuation)Slow (200PPS)
Stepping Motor waveform in each case, the attenuation setting selected for the current regulation scheme (Also known as current chopping)
Not applicable to selected motors, supply voltage, full-scale current and speed.
Stepping Motor adjustment is at the turn-off time of PWM current chopping cycle (t OFF)The process of selecting the appropriate attenuation settings in.
Fig. 5 shows the different ways in which current can flow through the H bridge of the stepping driver.
During rapid attenuation, the field effect transistor (FET)Conduction, current flows back from the motor to the power supply. In this case, the current decreases rapidly.
During slow attenuation, two low-end FETs are turned on and current is recirculated through these FETs.
In this case, the current attenuation is much slower. Figure 5: In normal operation (Driver), Dead time (All FETs are closed)
During rapid attenuation and slow attenuation, the current path through the H bridge, during the adjustment of the stepping motor, engineers search for the best'Mixed'Attenuation settings.
When the driver does not drive current into the coil, mixed attenuation includes a combination of fast attenuation and slow attenuation.
Selecting the mixed attenuation setting means that the percentage of fast attenuation and slow attenuation is set at the off time.
Fig. 6 shows how mixed attenuation helps regulate current.
Figure 6: using mixed attenuation to adjust the current TAG for each microstep