Due to the limitation of mechanical structure, in the application of adaptive headlamp system (AFS), the stepper motor may sometimes be blocked. Once the motor stalls, the electronic control unit (ECU) will lose the tracking information of the headlight position and make an inappropriate response, which breeds a very serious safety problem. Therefore, stall detection in AFS applications is essential.
Generally, the motor's back electromotive force (BEMF) can be used to determine whether the motor is locked or not. BEMF varies with motor speed, load and supply voltage. The traditional stepper motor driver chip has no BEMF output, but includes a built-in locked-rotation detection algorithm. The customer can only set a fixed lock-up threshold value in the register, which means that under real road conditions, all set values ​​must be preset “offline†before work, and cannot be adapted to real working conditions.
ONV Semiconductor's NCV70522 microstepping motor driver provides BEMF output through the SLA pin, which means that it can perform stall detection calculation in real time and adjust the detection level according to different conditions.
Algorithm description
Because the recirculation current during the coil current decay is relatively large, the coil voltage Vcoil exhibits transient characteristics. Since transient state is not always wanted in the application software, two operating modes can be selected by the bit setting value. At high, the SLA pin shows a fully visible voltage transient. If the bit is cleared, then only voltage samples at the zero-crossing end of each coil current are visible on the SLA pin. Since the transient characteristics of the coil voltage are no longer visible, this mode produces a more common BEMF input for subsequent processing by software, etc.
In order to adapt the sampled BEMF output level to the (0 V to 5 V) range, the sampled coil voltage Vcoil can be optionally divided by 2 or 4. This setting is controlled by the SPI bit. The following figure shows the operation of the SLA pin. The transparent bits "PWMsh" and "Icoil = 0" are internal signals, which together with SLAT define the sampling and maintenance instants of the coil voltage.
This BEMF voltage is sampled during each so-called "coil current zero crossing". Each coil has 2 zero current positions in each electrical cycle, so there are 4 zero crossing observation points in each electrical cycle, so it can measure 4 times of BEMF. If the microstep position is at the "coil current zero crossing", the BEMF voltage will only be sampled by the motor driver. The microstep position can be read through the SPI interface.
Through the software, we can flexibly determine whether the stall is based on the four SLA values ​​measured in one electrical cycle.
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