Festo MTRE−ST42−48S Séries Notice D'utilisation page 28

MTRE−ST42−48S−..
Accuracy and resolution
The step angle depends on the structural form; for the two−phase Festo stepping
motors, the angle is 1.8° (+/−5%) in full step. A single step takes place when the
two coils receive full alternating current. Here, the stepping motor performs a step
of 1.8° each time the current to the coils alternates.
But it is also possible to supply the two coils simultaneously with different current
shares. The result is a half, quarter, fifth, eighth or tenth of a step, depending on
the current share. This allows considerable refinement in the maximum resolution
of positions to which a stepping motor can run.
The smallest incremental path (resolution) on a positioning axis is determined by
the motor's step angle (number of steps per revolution) and the feed constant of
the positioning axis (determined by the diameter of the input pinion or the slope of
the spindle).
This can be calculated as follows:
Number of steps per revolution
Resolution
Example
Motor step angle:
Feed constant of the positioning axis:120mm/revolution
Gear multiplication:
The above example results in a resolution of 0.075 mm per step.
All positions that this motor−gear−axis combination can run to are divisible by
0.075.
In contrast, if the motor runs in quarter step, the resolution is 0.0375 mm; in one−
tenth step, even a resolution of 0.00375 mm per step would be possible. Axis play,
circumferential backlash of any gears and the tolerance of the stepping motor it
self have to be calculated in. The stepping motor's tolerance must always be taken
into account as the absolute value of the full step.
This must be considered when very precise positioning assignments are to be ex
ecuted.
28
Feed constant of the axis Gear multiplication
Steps Revolution
360°
Motor step .angle
0.9° (+/− 5%) at half−step
1/4 ( gear reduction i = 4 )
Festo MTRE−ST42−48S−.. 0503b English