TMC5161 DATASHEET (Rev. 1.01 / 2018-NOV-20)
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13.2 stallGuard2 Update Rate and Filter
The stallGuard2 measurement value SG_RESULT is updated with each full step of the motor. This is
enough to safely detect a stall, because a stall always means the loss of four full steps. In a practical
application, especially when using coolStep, a more precise measurement might be more important
than an update for each fullstep because the mechanical load never changes instantaneously from
one step to the next. For these applications, the sfilt bit enables a filtering function over four load
measurements. The filter should always be enabled when high-precision measurement is required. It
compensates for variations in motor construction, for example due to misalignment of the phase A to
phase B magnets. The filter should be disabled when rapid response to increasing load is required
and for best results of sensorless homing using stallGuard.
13.3 Detecting a Motor Stall
For best stall detection, work without stallGuard filtering (sfilt=0). To safely detect a motor stall the
stall threshold must be determined using a specific SGT setting. Therefore, the maximum load needs
to be determined, which the motor can drive without stalling. At the same time, monitor the
SG_RESULT value at this load, e.g. some value within the range 0 to 100. The stall threshold should be
a value safely within the operating limits, to allow for parameter stray. The response at an SGT
setting at or near 0 gives some idea on the quality of the signal: Check the SG value without load and
with maximum load. They should show a difference of at least 100 or a few 100, which shall be large
compared to the offset. If you set the SGT value in a way, that a reading of 0 occurs at maximum
motor load, the stall can be automatically detected by the motion controller to issue a motor stop. In
the moment of the step resulting in a step loss, the lowest reading will be visible. After the step loss,
the motor will vibrate and show a higher SG_RESULT reading.
13.4 Homing with stallGuard
The homing of a linear drive requires moving the motor into the direction of a hard stop. As
stallGuard needs a certain velocity to work (as set by TCOOLTHRS), make sure that the start point is far
enough away from the hard stop to provide the distance required for the acceleration phase. After
setting up SGT and the ramp generator registers, start a motion into the direction of the hard stop
and activate the stop on stall function (set sg_stop in SW_MODE). Once a stall is detected, the ramp
generator stops motion and sets VACTUAL zero, stopping the motor. The stop condition also is
indicated by the flag stallGuard in DRV_STATUS. After setting up new motion parameters in order to
prevent the motor from restarting right away, stallGuard can be disabled, or the motor can be re-
enabled by reading and writing back RAMP_STAT. The write and clear function of the event_stop_sg
flag in RAMP_STAT restarts the motor after expiration of TZEROWAIT in case the motion parameters
have not been modified. Best results are yielded at 30% to 70% of nominal motor current and
typically 1 to 5 RPS (motors smaller than NEMA17 may require higher velocities).
13.5 Limits of stallGuard2 Operation
stallGuard2 does not operate reliably at extreme motor velocities: Very low motor velocities (for many
motors, less than one revolution per second) generate a low back EMF and make the measurement
unstable and dependent on environment conditions (temperature, etc.). The automatic tuning
procedure described above will compensate for this. Other conditions will also lead to extreme
settings of SGT and poor response of the measurement value SG_RESULT to the motor load.
Very high motor velocities, in which the full sinusoidal current is not driven into the motor coils also
leads to poor response. These velocities are typically characterized by the motor back EMF reaching
the supply voltage.
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