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BASICS OF STEPPER MOTOR Of all motors, step motor is the easiest to control. It's handling simplicity is really hard to deny - all there is to do is to bring the sequence of rectangle impulses to one input of step controller and direction information to another input. Direction information is very simple and comes down to "left" for logical one on that pin and "right" for logical zero. Motor control is also very simple - every impulse makes the motor operating for one step and if there is no impulse the motor won't start. Pause between impulses can be shorter or longer and it defines revolution rate. This rate cannot be infinite because the motor won't be able to "catch up" with all the impulses (documentation on specific motor should contain such information). The picture below represents the scheme for connecting the step motor to microcontroller and appropriate program code follows.
The key to driving a stepper is
realizing how
the motor is constructed. A diagram shows the representation of a 4 coil
motor, so named because 4 coils are used to cause the revolution of the
drive shaft. Each coil must be energized in the correct order for the motor
to spin.
In Figure 3.3, as in Figure 3.1, boxes are used to represent switches; a control unit, not shown, is responsible for providing the control signals to open and close the switches at the appropriate times in order to spin the motors. The control unit is commonly a computer or programmable interface controller, with software directly generating the outputs needed to control the switches. As with drive circuitry for variable reluctance motors, we must deal with the inductive kick produced when each of these switches is turned off. Again, we may shunt the inductive kick using diodes, but now, 4 diodes are required, as shown in Figure 3.4:
Step angle It is angle through which motor shaft rotates in one step. step angle is different for different motor . selection of motor according to step angle depends on the application , simply if you require small increments in rottion choose motor having smaller step angle. No of steps require to rotate one complete rotation = 360 deg. / step angle in deg. Steps/second The relation between RPM and steps per sec. is given by , steps or impulses /sec. =(RPM X Steps /revolution ) /60
Pause between impulses can be shorter or longer and it defines revolution rate. This rate cannot be infinite because the motor won't be able to "catch up" with all the impulses (documentation on specific motor should contain such information). So referring to RPM value in datasheet you can calculate steps/sec and from it delay or pause between impulses
INTERFACING TO 8051. To cause the stepper to rotate, we have to send a pulse to each coil in turn. The 8051 does not have sufficient drive capability on its output to drive each coil, so there are a number of ways to drive a stepper,
Stepper motors are usually controlled by
transistor or driver IC like
ULN2003.
Coil A
Coil B
Coil C
Coil D
Step
0
1
1
0
1
0
0
1
1
2
1
0
0
1
3
1
1
0
0
4
CODE EXAMPLE To move motor in forward direction continuously Connection -P1.0 -P1.3 connected to Coils A -D.
CONTROLLING STEPPER MOTOR WITH TWO PORT PINS ONLY
For Forward
D0 D0 Coil
energized 0 0 AB 0 1 BC 1 0 CD 1 1 DA
To reverse the motor just reverse the above sequence viz. 11,10,01,00.
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