TMC428_06 Datasheet, PDF (17/58 Pages) List of Unclassifed Manufacturers – Intelligent Triple Stepper Motor Controller with Serial Peripheral Interfaces

TMC428 DATASHEET (v. 2.02 / April 26^th, 2006)
17
8.4 v_max (IDX=%0011)
This parameter sets the maximum motor velocity. The absolute value of the velocity will not exceed
this limit, except if the limit v_max is changed during motion to a value below the current velocity.
Note: To set target position x_target and current position x_actual to an equivalent value (e.g. to set
both to zero at a reference point), the assigned stepper motor should be stopped first, and the
parameter v_max should be set to zero to hold the assigned stepper motor at rest before writing into
the register x_target and x_actual.
8.5 v_target (IDX=%0100)
In modes RAMP_MODE and SOFT_MODE this register holds the current target velocity calculated
internally by the ramp generator. In mode VELOCITY_MODE a target velocity can be written into this
register. Then the associated stepper motor accelerates until it reaches the target velocity specified. In
VELOCITY_MODE the velocity is changed according to the motion parameter limits if the register
v_target is changed. In HOLD_MODE the register v_target is ignored.
8.6 v_actual (IDX=%0101)
This read-only register holds the current velocity of the associated stepper motor. Internally, the ramp
generator of the TMC428 processes with 20 bits while only 12 bits can be read out as v_actual. So, an
actual velocity of zero read out by the microcontroller means that the current velocity is in an interval
between zero and one. Because of this, the actual velocity should not be used to detect a stop of a
stepper motor. For stop detection there is a dedicated bit within the interrupt register, which can simply
be read out by the micro processor or generate an interrupt. Writing zero to register v_actual, which is
possible in HOLD_MODE only, immediately stops the associated stepper motor, because hidden bits
are set to zero with each write access to the register v_actual. In HOLD_MODE only, this register is a
read-write register. In HOLD_MODE, motion parameters are ignored and the microcontroller has the
full control to generate a ramp. In that mode, the TMC428 only handles the microstepping and
datagram generation for the associated stepper motor of the daisy chain.
8.7 a_max (IDX=%0110)
The absolute value of the maximum acceleration is defined by this register. It ranges from 0 to 2047.
The unit of the acceleration is change of step frequency per time unit divided by 256. The scale of
acceleration parameters (a_max, a_actual, a_threshold) is defined by the parameter ramp_div (see
section 8.14, page 26 for details) and depends on the clock frequency of the TMC428. Setting a_max
to zero during motion of the stepper motor results in the inability of the stepper motor to stop, because
it cannot change its velocity.
8.7.1 a_max_lower_limit & a_max_upper_limit for ramp_div ≠ pulse_div
Under special conditions, the parameter a_max might have a lower limit (>1) and might an upper limit
(<2047) concerning deceleration in RAMP_MODE and SOFT_MODE if the difference between
ramp_div and pulse_div is more than one. This is because the deceleration ramp is internally limited
to 2^19 steps respectively micro steps, which is sufficient for most applications. The lower limit
concerning the deceleration is given by
a_max_lower_limit = 2^( ramp_div - pulse_div –1 )
With v_max set to 2048 / √2 (≈ 1448) or lower, the a_max_lower_limit is half of this value. If
ramp_div - pulse_div – 1 ≤ 0 the limit a_max_lower_limit is 1 and the parameter a_max may be set
to down to 1 and of course to 0. On the other side, the upper limit of a_max is given by
a_max_upper_limit = 2^( ramp_div - pulse_div + 12) – 1
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