Monday, 10 November 2014

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STEPPER MOTOR

STEPPER MOTOR 
Stepper motors provide a means for precise positioning and speed control without the use of feedback sensors. The basic operation of a stepper motor allows the shaft to move a precise number f degrees each time a pulse of electricity is sent to the motor. Since the shaft of the motor o moves only the number of degrees that it was designed for when each pulse is delivered, you can control the pulses that are sent and control the positioning and speed. The rotor ofthe motor produces torque from the interaction between the magnetic field in the stator and rotor. The strength of the magnetic fields is proportional to the amount of current sent to the stator and the number of turns in the windings


The stepper motor uses the theory of operation for magnets to make the motor shaft turn a precise distance when a pulse of electricity is provided. You learned previously that like poles of a magnet repel and unlike poles attract.  typical cross-sectional view of the rotor and stator of a stepper motor. From this diagram you can see that the stator (stationary winding) has eight poles, and the rotor has six poles(three complete magnets). The rotor will require 24 pulses of electricity to move the 24 steps to make one complete revolution. Another way to say this is that the rotor will move precisely 15° for each pulse of electricity that the motor receives. The number of degrees the rotor will turn when a pulse of electricity is delivered to the motor can be calculated by dividing the number of degrees in one revolution of the shaft(360°) by the number of poles (north and south) in the rotor. In this stepper motor 360° is divided by 24 to get15°.When no power is applied to the motor, the residual magnetism in the rotor magnets will cause the rotor to detent or align one set of its magnetic poles with the magnetic poles of one of the stator magnets. This means that the rotor will have 24 possible detent positions. When the rotor is in a detent position, it will have enough magnetic force to keep the shaft from moving to the next position. This is what makes the rotor feel like it is clicking from one position to the next as you rotate the rotor by hand with no power applied


When power is applied, it is directed to only one of the stator pairs of windings, which will cause that winding pair to become a magnet. One of the coils for the pair will become the North Pole, and the other will become the South Pole. When this occurs, the stator coil that is the North Pole will attract the closest rotor tooth that has the opposite polarity, and the stator coil that is the South Pole will attract the closest rotor tooth that has the opposite polarity. When current is flowing through these poles, the rotor will now have a much stronger attraction to the stator winding, and the increased torque is called holding torque. 

By changing the current flow to the next stator winding, the magnetic field will be changed 45°. The rotor will only move 15° before its magnetic fields will again align with the change in the stator field. The magnetic field in the stator is continually changed as the rotor moves through the 24 steps to move a total of 360°. the position of the rotor changing as the current supplied to the stator changes

 Stepper Motor Switching Sequence
The stepper motor can be operated in three different stepping modes, namely, full-step, half-step, and micro step.
Full-Step
The stepper motor uses a four-step switching sequence, which is called a full-step switching sequence which is already described above.
Half-Step
Another switching sequence for the stepper motor is called an eight-step or half-step sequence.   The main feature of this switching  sequence is that you can double the resolution of the stepper motor by causing the rotor to move half the  distance it does when the full-step switching sequence is used. This means that a 200-step motor, which has a resolution of 1.8°, will have a resolution of 400 steps and 0.9°. The half-step switching sequence requires a special stepper motor controller, but it can be used with a standard hybrid motor. The way the controller gets the  motor to reach the half-step is to energize both phases at the same time with equal current
In this sequence the first step has SW1  is on, and SW2,SW3 and SW4 are off. The sequence for the first step is the same as the full-step sequence. The second step has SW1 and SW2 are on and all of the remaining switches are off. This configuration of switches causes the rotor to move an additional half-step because it is acted upon by two equal magnetic forces and the rotor turns to the equilibrium position which is half a step angle. The third step has SW2 is on, and SW1, SW4 and SW3 are off, which is the same as  step 2 of the full step sequence. The sequence continues for eight steps and then repeats. The main difference between this sequence and the full-step sequence is that the energizing sequence for half step is A A BB BC CC DD  DA.
Micro Step Mode
The full-step and half-step motors tend to be slightly jerky in their operation as the motor moves from step to step. The amount of resolution is also limited by the number of physical poles that the rotor can have. The amount of resolution (number of steps) can be in-creased by manipulating the current that the controller sends to the motor during each step. The current can be adjusted so that it looks similar to a sine  wave.sent to each of the four sets of windings is timed so that there is always a phase difference with each other.The fact that the current to each individual phase increases and decreases like a sine wave and that is always out of time with the other phase will allow the rotor to reach hundreds of intermediate steps. In fact it is possible for the controller to reach as many as 500 micro steps for a full-step sequence, which will provide 100,000 steps for each revolution.

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