Silicon Laboratories Stepper Machine Manual de usuario descargar pdf (Pagina 6)

AN155

6 Rev. 1.1

Four of these states only have one transistor on at any

one time. The half-step pattern allows a positioning

accuracy of 0.9° for a 1.8° stepper motor. Note that the

holding current and consequently the holding torque will

be less for the states with only one transistor on at a

time.

The step pattern for a bipolar stepper motor is similar.

Normally the diagonally opposite transistors in each H-

bridge are turned on simultaneously. Thus the stepper

pattern can be readily understood by listing the upper

transistors first in the same order as the unipolar drive.

Then the lower transistors are listed in the order

corresponding to the same pattern (13246857). The end

result is that the upper and lower nibbles of the

hexadecimal stepping pattern are identical.

3.5. Stepping Algorithm

A Linear-Velocity Profile is shown in Figure 4. The

velocity ramps up and down in the shape of a trapezoid.

The three distinct phases are named the acceleration

phase, the constant velocity or slewing phase, and the

deceleration phase. The resulting angular step position

curve n is a smooth s-shaped curve. The acceleration is

constant in the acceleration and deceleration phases.

The acceleration is zero in the slewing phase.

We would like to derive a table or step periods that will

be used to commutate the motor. The step period T

defined as the difference in time between two adjacent

steps in Equation 1. The step period will be used to

control the microcontroller timer.

Equation 1.

The angular acceleration is defined in Equation 2 and

the step position is defined in Equation 3. These

equations are straightforward textbook definitions for

angular velocity and position. Since the stepper motor

position moves in discrete steps the step position is an

integer denoted by the letter n instead of θ.

Equation 2.

Equation 3.

Table 4. Unipolar Half-Step Pattern

A B Q1 Q3 Q2 Q4 Hex

z-00010x01

- -00110x03

-z00100x02

-+01100x06

z+01000x04

++11000x0C

+z10000x08

+-10010x09

Table 5. Bipolar Full-Step Pattern

A B Q1 Q3 Q2 Q4 Q6 Q8 Q5 Q7 Hex

0- 000100010x11

- - 001100110x33

- 0001000100x22

- +011001100x66

0+010001000x44

++110011000xCC

+0100010000x88

+- 100110010x99

Accelerating

Decelerating

Constant-Velocity

Figure 4. Linear Velocity Profile

n1+

–=

ωαt

---

αt

1 2 3 4 5 6 7 8 9 10 11 ... 35 36

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