LAM Technologies DS10 Series, User Manual

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DS10xx Functionality and Setting
39
The left figure shows the current wave form with a resolution of 800
steps/rev (1/4 step) while the right one shows the current wave form with a
resolution of 25600 steps/rev (1/128). In both case the motor rotates at the
same speed.
When minor resolution is set (left figure) the motor movement is noisy
while when the resolution is greater (right figure) the motor moves
smoothly and with less vibrations.
In order to maintain the same motor rotation speed, in the example above,
the pulses frequency applied to the STEP input is changed from 1KHz,
which stands for the 800 steps/rev resolution (left figure), to 32KHz,
required by the 25600 steps/rev resolution (right figure).
5.2.3 Automatic current reduction
Stepper motor drivers works at impressed current, i.e. they always supply
the motor at the set nominal current independently from the motor giving
torque to the load or not.
In other types of motor, as for example in DC or Brushless motors, the
driver supplies the motor with the current strictly necessary to maintain the
required position or rotation speed. In these cases the driver is able to
obtain the information on the position or speed errors through transducers
such as tachogenerator, encoder, resolver, etc. Therefore, such a system
disposes of a feedback and it is a reactioned or closed loop kind.
On the contrary, the step motors can be used at open-loop without the help
of any transducer, also granting constant speeds and precise and repeatable
positioning.
What makes this operating method possible is the particular motor and
driver conformation. As being without feed-back, it always delivers
nominal phase current to the motor which therefore always produces the
maximum torque.