GS-C200 datasheet(27/31 Pages) STMICROELECTRONICS

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Figure 15. Torque/Frequency Characteristic.

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 16. The GS-C200 Acceleration Ramp.

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.

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.

GS-C200 / GS-C200S

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