TMC260 and TMC261 DATASHEET (Rev. 2.05 / 2012-NOV-05)
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4.1.2 Small Motors with High Torque Ripple and Resonance
Motors with a high detent torque show an increased variation of the stallGuard2 measurement value
SG with varying motor currents, especially at low currents. For these motors, the current dependency
might need correction in a similar manner to velocity correction for obtaining the highest accuracy.
4.1.3 Temperature Dependence of Motor Coil Resistance
Motors working over a wide temperature range may require temperature correction, because motor
coil resistance increases with rising temperature. This can be corrected as a linear reduction of SG at
increasing temperature, as motor efficiency is reduced.
4.1.4 Accuracy and Reproducibility of stallGuard2 Measurement
In a production environment, it may be desirable to use a fixed SGT value within an application for
one motor type. Most of the unit-to-unit variation in stallGuard2 measurements results from
manufacturing tolerances in motor construction. The measurement error of stallGuard2 – provided that
all other parameters remain stable – can be as low as:
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4.2 stallGuard2 Measurement Frequency and Filtering
The stallGuard2 measurement value SG 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 only be disabled when rapid response to increasing load is
required, such as for stall detection at high velocity.
4.3 Detecting a Motor Stall
To safely detect a motor stall, a stall threshold must be determined using a specific SGT setting.
Therefore, you need to determine the maximum load the motor can drive without stalling and to
monitor the SG value at this load, for example some value within the range 0 to 400. The stall
threshold should be a value safely within the operating limits, to allow for parameter stray. So, your
microcontroller software should set a stall threshold which is slightly higher than the minimum value
seen before an actual motor stall occurs. 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. These values
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 so that a reading of 0 occurs at maximum motor load, an active high stall
output signal will be available at SG_TST output.
4.4 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.). Other conditions will also lead
to extreme settings of SGT and poor response of the measurement value SG to the motor load.
Very high motor velocities, in which the full sinusoidal current is not driven into the motor coils also
lead to poor response. These velocities are typically characterized by the motor back EMF reaching the
supply voltage.
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