TMC2208-LA Datasheet, PDF (39/79 Pages) TRINAMIC Motion Control GmbH & Co. KG. – POWER DRIVER FOR STEPPER MOTORS

TMC220X, TMC222X DATASHEET (Rev. 1.01 / 2016-Nov-16)
39
6.2 stealthChop Options
UART
In order to match the motor current to a certain level, the effective PWM voltage becomes scaled
depending on the actual motor velocity. Several additional factors influence the required voltage level
to drive the motor at the target current: The motor resistance, its back EMF (i.e. directly proportional
to its velocity) as well as the actual level of the supply voltage. Two modes of PWM regulation are
provided: The automatic tuning mode (AT) using current feedback (pwm_autoscale = 1, pwm_autograd
= 1) and a feed forward velocity controlled mode (pwm_autoscale = 0). The feed forward velocity
controlled mode will not react to a change of the supply voltage or to events like a motor stall, but it
provides very stable amplitude. It does not use nor require any means of current measurement. This
is perfect when motor type and supply voltage are well known. Therefore we recommend the
automatic mode, unless current regulation is not satisfying in the given operating conditions.
It is recommended to operate in automatic tuning mode.
Non-automatic mode (pwm_autoscale=0) should be taken into account only with well-known motor
and operating conditions. In this case, programming via the UART interface is required. The operating
parameters PWM_GRAD and PWM_OFS can be determined in automatic tuning mode initially.
Hint: In non-automatic mode the power supply current directly reflects mechanical load on the motor.
The stealthChop PWM frequency can be chosen in four steps in order to adapt the frequency divider
to the frequency of the clock source. A setting in the range of 20-50kHz is good for most applications.
It balances low current ripple and good higher velocity performance vs. dynamic power dissipation.
CHOICE OF PWM FREQUENCY FOR STEALTHCHOP
Clock frequency
f_CLK
18MHz
16MHz
12MHz (internal)
10MHz
8MHz
PWM_FREQ=%00
f_PWM=2/1024 f_CLK
35.2kHz
31.3kHz
23.4kHz
19.5kHz
15.6kHz
PWM_FREQ=%01
f_PWM=2/683 f_CLK
(default)
52.7kHz
46.9kHz
35.1kHz
29.3kHz
23.4kHz
PWM_FREQ=%10
f_PWM=2/512 f_CLK
(OTP option)
70.3kHz
62.5kHz
46.9kHz
39.1kHz
31.2kHz
Table 6.1 Choice of PWM frequency – green / light green: recommended
PWM_FREQ=%11
f_PWM=2/410 f_CLK
87.8kHz
78.0kHz
58.5kHz
48.8kHz
39.0kHz
6.3 stealthChop Current Regulator
In stealthChop voltage PWM mode, the autoscaling function (pwm_autoscale = 1, pwm_auto_grad = 1)
regulates the motor current to the desired current setting. Automatic scaling is used as part of the
automatic tuning process (AT), and for subsequent tracking of changes within the motor parameters.
The driver measures the motor current during the chopper on time and uses a proportional regulator
to regulate PWM_SCALE_AUTO in order match the motor current to the target current. PWM_REG is the
proportionality coefficient for this regulator. Basically, the proportionality coefficient should be as
small as possible in order to get a stable and soft regulation behavior, but it must be large enough to
allow the driver to quickly react to changes caused by variation of the motor target current (e.g.
change of VREF). During initial tuning step AT#2, PWM_REG also compensates for the change of motor
velocity. Therefore, a high acceleration during AT#2 will require a higher setting of PWM_REG. With
careful selection of homing velocity and acceleration, a minimum setting of the regulation gradient
often is sufficient (PWM_REG=1). PWM_REG setting should be optimized for the fastest required
acceleration and deceleration ramp (compare Figure 6.3 and Figure 6.4). The quality of the setting
PWM_REG in phase AT#2 and the finished automatic tuning procedure (or non-automatic settings for
PWM_OFS and PWM_GRAD) can be examined when monitoring motor current during an acceleration
phase Figure 6.5.
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