L9942 stall detection methods

AN3327

10/22

DocID18343 Rev 3

3

L9942 stall detection methods

3.1

Current sensing

The L9942 has as part of its feature set the ability to detect stall by looking at motor current

rise time.

During stall, the motor current rises because the BEMF is absent. Lack of BEMF results in

two effects. First, it increases the potential current in a winding at a given voltage per Ohm's

law. Secondly, it increases the rate of change of the current in the windings because the rate

of change of current in an inductor is proportional to the voltage across the inductor. With

little or no BEMF in a motor winding the current both raises and raises quickly.

Figure 6. Typical motor phase winding components

However, the L9942 regulates the motor phase current by turning off the phase when the

preprogrammed current threshold is reached. As a result the motor current does not spike

when the motor is stalled. The duty cycle reduces to something fairly small as the current

control algorithm compensates for the loss of BEMF. So then the L9942 detects a loss of

BEMF by observing and reporting an abnormally low duty cycle for a given commanded

current.

The difficulty with this method is that there are many motor current related parameters that

can move around in the normal operating space of a stepper motor. Things like temperature,

battery voltage and loading or torque can have a dramatic affect on the current regulation

duty cycle. Unfortunately, the operating point at one end of the normal spectrum can look

like a stalled motor at the other end. Overlapping parameters makes it difficult at best to

safely discern a stalled rotor.

To minimize the effects of motor resistance, battery voltage, and temperature the stall

detection algorithm can look directly at BEMF.