Silicon Laboratories Stepper Machine Manual de usuario descargar pdf (Pagina 5)

AN155

Rev. 1.1 5

The performance differences between unipolar and

bipolar drives are subtle. The unipolar drive only uses

half of the actual motor windings at any one time. Thus,

the bipolar stepper motor should theoretically have

much better performance for a given motor volume. In

practice, this is not always the case. Often the six-wire

stepper motors have a lower phase resistance and

consequently a higher holding torque for a particular

motor size. The trade-offs of bipolar versus unipolar are

summarized in Table 2.

3.4. Stepping Patterns

The two possible stepping patterns for stepper motors

are full-step and half-step. A full-step pattern has four

states and moves the motor one full step for each state.

A 1.8° stepper motor will move 1.8° for each state and

7.2° for the full pattern. A full step pattern is shown in

Table 3. In the full-step pattern, two transistors are

always on. The first two columns indicate whether the A

and B phase voltages are positive +, negative -, or high

impedance z. The next four columns indicate the state

of the four transistors for the unipolar stepper motor

shown in Figure . The last column is the state of all four

transistors expressed in hexadecimal for use with

microcontrollers.

Note that the transistor order in Table 3 has been

rearranged listing Q3 before Q2 to yield a clear pattern.

The polarity of the A and B windings is only important in

determining if the rotation of the motor is clockwise or

counter clockwise. Swapping the polarity of either

phase will change the direction of the motor. Swapping

A and B windings will result in no change of rotation.

Table 4 is the stepping pattern for a half step stepper

motor. The half step stepping pattern has eight states.

Q2 Q3 Q4Q1

Q5 Q6 Q7 Q8

Figure 3. Bipolar Stepper Motor Drive

Table 2. Bipolar vs. Unipolar Trade-offs

Bipolar Unipolar

number of transistors 8 4

number of high-side drivers 4 0

number of clamps 0 4

transistor voltage 1 x Vs 2.5 x Vs

winding usage 100% 50%

motor wires 4 6

Table 3. Unipolar Full-Step Pattern

A B Q1 Q3 Q2 Q4 Hex

--00110x03

- +01100x06

++11000x0C

+- 10010x09

1 2 3 4 5 6 7 8 9 10 ... 35 36

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