TMC249 / TMC249A DATA SHEET (V2.09 / 2009-Jul-30)
14
Stall Detection stallGuard™
Using the sensorless load measurement
The TMC249 provides a patented sensorless load measurement, which allows a digital read out of the
mechanical load on the motor via the serial interface. To get a readout value, just drive the motor using
sine commutation and mixed decay switched off. The load measurement then is available as a three
bit load indicator during normal motion of the motor. A higher mechanical load on the motor results in a
lower readout value. The value is updated once per fullstep.
The load detection is based on the motor’s back EMF, thus the level depends on several factors:
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-
Motor velocity: A higher velocity leads to a higher readout value
Motor resonance: Motor resonances cause a high dynamic load on the motor, and thus
measurement may give unsatisfactory results.
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-
Motor acceleration: Acceleration phases also produce dynamic load on the motor.
Mixed decay setting: For load measurement mixed decay has to be off for some time before
the zero crossing of the coil current. If mixed decay is used, and the mixed decay period is
extended towards the zero crossing, the load indicator value decreases.
Implementing sensorless stall detection
The sensorless stall detection typically is used, to detect the reference point without the usage of a
switch or photo interrupter. Therefore the actuator is driven to a mechanical stop, e.g. one end point in
a spindle type actuator. As soon as the stop is hit, the motor stalls. Without stall detection, this would
give an audible humming noise and vibrations, which could damage mechanics.
To get a reliable stall detection, follow these steps:
1. Choose a motor velocity for reference movement. Use a medium velocity which is far enough
away from mechanical resonance frequencies. In some applications even motor start / stop
frequency may be used. With this the motor can stop within one fullstep if a stall is detected.
2. Use a sine stepping pattern and switch off mixed decay (at least 1 to 3 microsteps before zero
crossing of the wave). Monitor the load indicator during movement. It should show a stable
readout value in the range 3 to 7 (LMOVE). If the readout is high (>5), the mixed decay portion
may be increased, if desired.
3. Choose a threshold value LSTALL between 0 and LMOVE - 1.
4. Monitor the load indicator during each reference search movement, as soon as the desired
velocity is reached. Readout is required at least once per fullstep. If the readout value at one
fullstep is below or equal to LSTALL, stop the motor. Attention: Do not read out the value within
one chopper period plus 8 microseconds after toggling one of the phase polarities!
5. If the motor stops during normal movement without hitting the mechanical stop, decrease
LSTALL. If the stall condition is not detected at once, when the motor stalls, increase LSTALL
.
v(t)
v_max
x
ma
_
a
t
load
indicator
acceleration
constant velocity
stall
max
LMOVE
LSTALL
stall threshold
min
t
acceleration
jerk
stall detected!
vibration
Copyright © 2005, TRINAMIC Motion Control GmbH & Co KG
TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
