Conference Paper · October 013 doi: 10. 1109/cobep. 2013. 6785223 citations reads 2,638 authors
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DRIVES
A.
Driving Schemas Besides the mechanical design, the final resolution of a stepper motor is also dependent on how the stepper motor is driven. These machines are digital like actuators, in a sense that only one of its windings is energized at a time. But, if one drives the motor with smoother signals, a smoother motion is yielded. So, instead of driving a winding at a time to its nominal current, a discretized sine-squared (sin²) wave can be applied in order to achieve smoother motion. Traditional driving schema is known as full step. If one breaks the transitions in 2, it is known as half step. For more fractional driving schema, it is named n-th micro step, were n is the number of subdivisions on the signal. The Tables 1, 2 and 3 illustrates the three driving schemas, respectively. On those Tables, the symbols ½, ¼ and ¾ actually are shorted of , e . Table 1 – Lookup table used for full step driving schema. 1 1 0 0 0 2 0 1 0 0 3 0 0 1 0 4 0 0 0 1 Table 2 – Lookup table used for half step driving schema. 1 1 0 0 0 2 ½ ½ 0 0 3 0 1 0 0 4 0 ½ ½ 0 5 0 0 1 0 6 0 0 ½ ½ 7 0 0 0 1 8 ½ 0 0 ½ Table 3 – Lookup table used for micro step 1/4 driving schema. 1 1 0 0 0 9 0 0 1 0 2 ¾ ¼ 0 0 10 0 0 ¾ ¼ 3 ½ ½ 0 0 11 0 0 ½ ½ 4 ¼ ¾ 0 0 12 0 0 ¼ ¾ 5 0 1 0 0 13 0 0 0 1 6 0 ¾ ¼ 0 14 ¼ 0 0 ¾ 7 0 ½ ½ 0 15 ½ 0 0 ½ 8 0 ¼ ¾ 0 16 ¾ 0 0 ¼ Those tables are used as current reference for a controlled current source, which is better explained in the next subtopic. B. Controlled Current Source From Equation (1), the torque exerted by the motor is related by the displacement from the rest position and also to the current flowing on the winding. For a constant holding torque, a controllable constant current source must be implemented. In order to do so, H-Bridge circuit along with comparators and logic circuitry can be used. Such a circuit is depicted on Figure 3.Transistors Q1 through Q4 are power transistors and carry the load current. Diodes D1 through D4 are free- wheeling diodes. !"#$. is a low resistance resistor in series with load current, generating a small voltage signal, &('), which is filtered with a low pass filter, in order to remove high frequency noise, and compared to a current reference, finally switching on and off Q1 through Q4. The values applied to the inputs In1 and In2 are the values of the current index of the active index table, presented in last subsection. The active index table depends on which driving schema is being used. Figure 3 – Diagram illustrating a controllable current source. III. IMPLEMENTATION OF THE SYSTEM The circuit presented by Figure 3 relies on analog devices, such as comparators, and sources, such as the adjustable current reference. Pure analog circuits usually are harder to implement and are more expensive. The developed system employs digital, analog and power circuits implement the functionalities of the blocks previously described. Download 0.84 Mb. Do'stlaringiz bilan baham: |
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