How to solve common stall detection problems of stepper motors
Time:2023-09-09
Views:704
Stepping motor drivers such as DRV8434S, DRV8434A, and DRV8889-Q1 include a stall detection function that can detect overload or stall situations during motor stepping. When the motor steps, due to the magnetic field of the rotor magnet, each phase winding will generate back electromotive force. The phase difference between the back electromotive force and the driving current waveform is proportional to the reserve torque of the stepper motor. Therefore, for an unloaded stepper motor, the phase difference of the back electromotive force is the maximum value, and when there is no reserve torque, that is, when the stepper motor is fully loaded and stalled, it will approach zero.
When stall detection is enabled, the device calculates the torque meter value as the moving average of the last four electrical half cycles of the rotating motor. The torque meter value is proportional to the phase difference of the back electromotive force described in the previous paragraph. Subsequently, during each electrical half cycle of the phase current, a new torque meter value is updated and compared with the programmed stall threshold to determine the stall situation.
The stall threshold is programmed into the stall detection module of the driver using one of the two methods described in the device data sheet. The first method is to use an automatic stall learning process. The second method is to manually enter a user-defined stall threshold. When DRVOFF=0 or EN_ When STL=0, the maximum torque meter value will be reported.
Common issues that may lead to unreliable stall detection
1. The stall detection function requires the use of intelligent tuning ripple control attenuation mode. On devices with stall detection function, this is the default attenuation mode.
2.In order for stall detection to work properly, not only must the stall threshold be correctly defined, but the amplitude of the back electromotive force should also meet the SNR required for the stall detector to calculate the torque count. The higher the phase winding resistance of a stepper motor, the smaller the amplitude of the back electromotive force. Therefore, stepper motors with high winding resistance of about tens of ohms may not work well with stall detectors.
3. Small size and/or low torque stepper motors may have weak permanent magnets and may not provide sufficient SNR (to generate back electromotive force), and may not work well with stall detectors.
4.Under extremely low step rate conditions, the amplitude of the back electromotive force may not be sufficient to be within the detectable range of the stall detector. Under low step rate conditions, stall detection may not function properly. The minimum step rate that can be used for stall detection can be determined by evaluating different motors under various VM and current settings using device specific TI EVM.
5. The stall threshold previously obtained at one speed may not be suitable for detecting stall at another speed. Every time you change the step speed, you must complete a new learning process.
6.Similarly, manually entering a stall threshold at one speed may not be suitable for detecting stall at another speed. Every time the step speed changes, a new stall threshold must be entered.
7.If implemented in applications, stall detection using stall threshold learning may not be reliable during the acceleration phase. In this case, the appropriate stall threshold must be manually set.
8. The torque count calculation used for stall detection relies on observing the limited TOFF duration during each current regulation chopper cycle. If the voltage VM of the motor driving power supply is not sufficient to push enough current through the phase winding inductance to maintain voltage stabilization at a given step rate, not only will the current regulation function be lost, but stall detection will also not be effective. If the torque meter value suddenly jumps to an abnormally high value when the step speed increases, it indicates that the current regulation and stall detection capabilities have been lost at that speed. When the current regulation function is lost, the motor may or may not stall, depending on the inertia and acceleration curves of the system.t process.
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