-Shweta Dubey

Robotic Arms

What is a stepper motor? A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence.

What is it made up of??

Stator Basically it is the stationary part of the motor. It consists of magnetic poles with small teeth, each pole being provided with windings.

Rotor The rotor is the movable part which has no windings, brushes and a commutator. Usually the rotors are either variable reluctance or permanent magnet kind. 

Animation of a simplified stepper motor (unipolar) Frame 1: The top electromagnet (1) is turned on, attracting the nearest teeth of the gear-shaped iron rotor. With the teeth aligned to electromagnet 1, they will be slightly offset from right electromagnet (2). Frame 2: The top electromagnet (1) is turned off, and the right electromagnet (2) is energized, pulling the teeth into alignment with it. This results in a rotation of 3.6° in this example. Frame 3: The bottom electromagnet (3) is energized; another 3.6° rotation occurs. Frame 4: The left electromagnet (4) is energized, rotating again by 3.6°. When the top electromagnet (1) is again enabled, the rotor will have rotated by one tooth position; since there are 25 teeth, it will take 100 steps to make a full rotation in this example.

Types of Stepper Motor Variable Reluctance Permanent Magnet Hybrid

Variable Reluctance It is the basic type of stepper motor that has been in existence for a long time and it ensures easiest way to understand principle of operation from a structural point of view. As the name suggests, the angular position of the rotor depends on the reluctance of the magnetic circuit formed between the stator poles (teeth) and rotor teeth.

The rotor is made of soft iron, and performs a teethed disk like a gear. The stator has more than 4 coils. The coils are energized in opposite pairs, and will attract the rotor. The lack of a permanent magnet has a negative affect on the torque that is significantly decreased. But it has a great advantage.

These motors have no detent torque These motors have no detent torque. The detent torque, is the torque generated by the rotor permanent magnets that are magnetized to the stator's armature, when no current flows within the coils. You can easily understand what this torque is, if you try to rotate an unconnected stepper motor by hand (NOT a VR stepper). You will feel the distinctive "clicks" of each step of the motor. Actually, what you feel is the detent torque that pulls the magnets against the armature of the stator.

Single Stack VR The stator and rotor are made of high-quality magnetic materials having very high permeability. Thus, a very small exciting current is required. When a DC source is applied to the stator phase with the help of a semiconductor switch, a magnetic field is produced. The axis of the rotor aligns with the axis of the stator

Multi Stack VR A Multi Stack or m stack variable reluctance stepper motor is made up of m identical single stack variable reluctance motor. The rotor is mounted on the single shaft. The stator and rotor of the Multi Stack Variable motor have the same number of poles and hence, the same pole pitch.

Permanent Magnet Permanent Magnet (PM) motors use permanent magnet rotors and are commanded by electrical pulses. They are widely used in printers, copies, and scanners, among other applications. They are also used to operate valves in household water and gas systems as well as drive actuators in automotive applications.

One of the main benefits of the PM stepper motor is that, in addition to being electronically commutated like a brushless DC motor or any other type of stepper motor, the PM stepper motor requires no “teeth” as are typically found in the variable reluctance (VR) stepper motor. This makes the permanent magnet stepper an extremely popular choice for many motor applications. PM stepper motors have usually step angle from 45o to 90o.

Hybrid The Hybrid type motor, as the name suggests is a mixture of both above types. This consists a rotor which is magnetic and as well as teethed. The rotor of this type of motor is made up of two rotors joining like a shaft of motor. One of them is for north and other is for south pole.

Hybrid stepper motors provide excellent performance in areas of torque, speed, and step resolution. Typically, step angles for a hybrid stepper motor range from 200 to 400 steps per revolution. This type of motor provides a combination of the best features available on both the PM and VR types of stepper motors.

Advantages Stable. Inexpensive related to other motion control systems. Safe. If anything breaks, the motor stops. Long life. Bearings are the only wear-out mechanism. Excellent low speed torque. Can drive many loads without gearing. Excellent repeatability. Returns to the same location accurately. Overload safe. Motor cannot be damaged by mechanical overload.

Disadvantages Low efficiency. Motor draws substantial power regardless of load. Torque drops rapidly with speed (torque is the inverse of speed). Low accuracy. Motor gets very hot in high performance configurations. Motor will not "pick up" after momentary overload. Motor is audibly very noisy at moderate to high speeds.

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