During the operation of a stepper motor, characteristic current and voltage profiles occur with each succeeding step, allowing a conclusion about the motor’s operating status. By recording and analyzing these signals, sensorless detection of a motor stall is possible. ELMOS has the competence to integrate this function into ICs.
A stall condition of a stepper motor may occur if the load torque applied to the motor exceeds the maximum torque that the motor can generate. There are two different situations where this may happen. The first situation is that the load torque exceeds the maximum hold torque of the stopped motor and makes the motor rotate unintentionally. This case shall not be considered here. The most relevant stall situation for a stepper controller is that load torque exceeds the minimum transitional torque that occurs while the motor moves from one-step position to the adjacent position. Such situations may happen if e.g. the mechanical system driven by the stepper motor is blocked or reaches a mechanical end stop. By detecting this stall situation looking at the electrical behaviour of the motor, external limit switches can be superseded. In order to detect the stall condition, the commutation phase of the coil currents is observed. The following figures show the three phases that such a current commutation consists of.
In phase 1 the full bridge is driving a current from terminal A0 to terminal A1 through the motor coil. The coil current has reached its static value. The commutation starts in phase 2, when the switches previously in ON state are switched OFF and the low side switch of half bridge A0 is activated and terminal A0 will be pulled to ground. In this phase the inductance of the motor coil will try to keep its current constant and will generate an electromechanical force (BEMF). As a consequence the voltage on terminal A1, which is not controlled by its half bridge switches, will rise rapidly and reaches a final value of about one diode voltage above the supply voltage. In this state the reverse diode of the high side switch of half bridge A1 will carry the coil current. As the voltage seen over the inductance of the motor coil has reversed its sign when phase 2 is reached, the coil current will start to decrease. The next figure phase 3 shows the simplified timing diagram of the full bridge during the commutation process.