TMC428_06 Datasheet, PDF(43/58 Page) List of Unclassifed Manufacturers – Intelligent Triple Stepper Motor Controller with Serial Peripheral Interfaces

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TMC428_06 Datasheet, PDF (43/58 Pages) List of Unclassifed Manufacturers – Intelligent Triple Stepper Motor Controller with Serial Peripheral Interfaces

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TMC428 DATASHEET (v. 2.02 / April 26th, 2006)

12.3 Micro Step Enhancement

Even for stepper motors optimized for sine-cosine control, it is possible to improve micro step behavior

by adapting the micro step look-up table (LUT). For different types of stepper motors, a periodic

trapezoidal or triangular function similar to a sine function or a superposition of these function as a

replacement of the pure sine wave function (Figure 12-1) might be a better choice. Taking the physics

of stepper motors into account, the choice of the function for microstepping can be determined by a

single shape parameter Ï as explained below. The programmability of the micro step look-up table

provides a simple and effective facility to enhance microstepping for a given type of two-phase stepper

motor. Enhanced microstepping requires accurate current control. So, stepper motor driver chips with

enabled and well tuned fast decay (resp. mixed decay) operational mode are need to be used, e.g.

TRINAMICs smart power TMC236 / TMC239 / TMC246 / TMC249 drivers.

Non-linearity resulting from magnetic field configuration determined by shapes of pole shoes,

ferromagnetic characteristics, and other stepper motor characteristics effect non-linearity in micro step

behavior of real stepper motors. The non-linearity of microstepping causes micro step positioning

displacements, vibrations and noise, which can be reduced dramatically with an adapted micro step

table. The best fitting micro step table can be determined by measuring the micro step motor behavior,

e.g. using a laser pointer based on the sine-cosine microstepping table.

Nevertheless sine-cosine microstepping is a good first order approach for microstepping. The micro

step enhancement possible with the TMC428 is based on replacement of the look-up table initialization

function sin(Ï ) used for sine-cosine microstepping by a function with the shape parameter Ï. A quarter

sine wave period is the basic approach for initialization of the micro step look-up-table . A quarter of a

trapezoidal function or a quarter of a triangular function is chosen depending on the shape parameter Ï

for a given stepper motor type.

f (Ï)

ï£±f

(Ï )

f =

ï£´ï£´

ï£²

ï£´

fï£´ï£³

box _ circle

(Ï )

circle

(Ï )

circle _ r hom b

for Ï > 0

for Ï = 0

for Ï < 0

with

â1.0 â¤ Ï â¤ +1.0

and

â¤

The look-up table (f(Ï ) ) of the TMC428 enfolds a quarter period (0 â¤ Ï < Ï /2) only. This quarter period

is expanded to a full period (0 â¤ Ï < 2Ï ) and the phase shifted companion function value (f(Ï - Ï /2 )) is

added automatically by the TMC428 during operation. So, to reach function value (f(Ï)), one

automatically gets a pair of function values {f(Ï ); f(Ï - Ï /2 )} respectively {sin(Ï ); cos(Ï )}. This

automatic expansion of the TMC428â primary provided for sine cosine microstepping (f(Ï ) = sin(Ï )) â

also works fine with other micro step wave forms f Ï.

f (Ï)

up to 64 micro steps

1 full step

3Ï

2Ï

cuiprctole641mficurollsstetpespwithin a quadrant

box

Figure 12-1: Microstep enhancemant by introduction of a shape function fÏ(Ï )

▷