GS-C200 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
Manufacturer
GS-C200 Datasheet PDF : 31 Pages
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GS-C200 / GS-C200S
The Slew speed is also limited by the motor electri-
cal and physical characteristics, as shown on Fig.
15 where the behaviour of the minimum available
torque versus the driving frequency is reported.
It can be noted that the torque decreases almost
linearly starting from a certain frequency, and this
frequency depends on the motor windings imped-
ance and the rotor inertia.
Figure 15. Torque/Frequency Characteristic.
RAMP LENGHT (R command) SELECTION
The acceleration and deceleration ramps are not
likely to be calculated and they shall be optimized
during the system debugging and testing phase.
The testing may start with very conservative ramp
gradients, i.e. very long ramps, that will be gradually
shortened until the first positioning error is detected.
The acceleration and deceleration ramps gener-
ated by the GS-C have the trend shown in fig. 16.
Figure 16. The GS-C200 Acceleration Ramp.
It is important to note that, when the number of
steps to be executed does not allow to reach the
Slew speed, the GS-C moves to the target position
performing a partial acceleration ramp linked to a
shortened deceleration ramp. This represents the
minimum time consuming way to reach the speci-
fied position.
CLOSED LOOP OPERATION
The stepper motor is a device normally driven in an
open loop mode and there is no direct control
between the cause and the effect. In adverse con-
ditions an issued step may not be performed me-
chanically because the driving conditions do not
match the required torque and speed. In addition,
the resonance phenomenon, common to all the
stepper motors, can also affect the correct position-
ing.
In some particular applications, when the load has
a very large spread of values and the torque margin
is limited, it is sometimes necessary to implement
an external electronic circuitry to guarantee the
correct system positioning
To this purpose three different methods can be
adopted:
a) Digital encoding of the absolute position.
b) Recognition that a step has been executed by
the usage of a slotted disk, two optocouplers
and some logic.
c) The same as above by the usage of velocity coils
and some logic.
The first solution is very expensive and the digital-
ized position value must be read by the computer
through a parallel port by using a specifically written
program. A further limitation arises from the fact
that every shaft encoder provides just the informa-
tion relative to the position but it does not take care
if more than one turn has been performed by the
motor shaft, and an external logic is also required
to detect and save this condition.
The second solution is less expensive but it requires
a tedious trimming of the mechanical positioning of
the optical sources and detectors to be effective.
The major drawback of this solution is its sensitivity
to dust, and the whole position sensing system
must be contained in a dust free box.
The last solution is probably the best under every
point of view because it does not require any me-
chanical positioning adjustement that has been
previously made by the motor manufacturer; more-
over it is dust insensitive beeing based on flux
variation across an air gap and finally no mechani-
cal hardware must be added to the system.
27/31
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