Stepper Servo Motors

Electric Motors
Motors come with many motion types such as rotary and linear, and power sources such as electric, hydraulic, or pneumatic power. When most people think of motors, they think of an electrically powered device with a rotating shaft, and indeed this is the most common type of motor in use today.

Electric motors fundamentally operate on the principle of magnetism: opposite poles of a magnet attract, and like poles repel. To achieve motion, a motor must do three things: generate magnetic fields on the moving part of the motor (rotor or armature), generate a magnetic field in the stationary part (stator), and provide some means to keep one of the fields moving. As one field "chases" the other attempting to align, motion results.

Although these simple ideas are the basis of all electric motors, there are hundreds, perhaps thousands, of ways to generate and move the magnetic fields. For example, magnetic fields can be generated from permanent magnets or from coils carrying current (electromagnets), and in the latter case, direct connection can deliver current to the coils, through brushes, or by induction. Some motors have permanent magnets on the rotor and coils on the stator, others have permanent magnets on the stator and coils on the rotor, and still others have coils on both rotor and stator.

Given the variety, how do we begin to classify electric motors and select them for use? Fortunately, most electric motors fall in one of three classes: AC, DC, or stepper.

AC Servo Motors
AC motors operate from alternating current (AC) power sources. The magnetic fields typically are generated using coils on the rotor and stator, and the field movement occurs naturally in the stator due to the alternating nature of the input power. These motors are inexpensive to build and operate, reliable, and usually run from standard line power. ("Standard line power" is the power produced by the power generating utilities for a give region of the world. In the US, standard line frequency is 60 Hz, and standard voltages are around 120, 240, and 480 volts AC.) The power supply frequency determines the speed of an AC motor, so if operated from line power, the speed of rotation is always the same. Variable frequency power drives control the speed of AC motors, but such drives are expensive. Therefore, AC motors usually are not used for sophisticated motion control applications

DC Servo Motors
Another common type of electric motor is the DC motor. DC motors operate from a direct current power source. Movement of the magnetic field is achieved by switching current between coils within the motor. This action is called "commutation." Some DC motors (brush-type) have built-in commutation, meaning that as the motor rotates, mechanical brushes automatically commutate coils on the rotor. Other DC motors (brushless) rely on the external power drive to perform the commutation of coils on the stator. In general, users select brush-type DC motors when low system cost is a priority, and brushless motors to fulfill other requirements (such as maintenance-free operation, high speeds, and explosive environments where sparking could be hazardous). Because varying the voltage and current from the power supply are controlled by speed and torque (twisting force), DC motors work well in complex motion tasks.

Stepper Motors
All motors discussed to this point have produced continuous rotary motion. Steppers, on the other hand, produce motion in discrete steps. Similar to brushless DC motors, steppers usually have permanent magnets on the rotor and coils on the stator with field movement provided by commutation from the power supply. Stepper motors have a specified number of steps per revolution (typically around 200 steps/rev, or 1.8 degrees per step), but more advance stepper drives can provide microstepping - additional stops between the normal step locations. Microstepping greatly increases the resolution of a stepper motor. Stepper motors are usually controlled by digital signals from the controller to power drive, with one pulse corresponding to one step. Thus, the frequency of the digital signals controls the speed of the motor. Inexpensive counter/timer circuits control the digital signal. Stepper motors have characteristic holding torque (ability to hold the position) and pullout torque (ability to move to the next position). Other torques can be difficult to achieve. Therefore, precise torque control is difficult with steppers. Stepper systems are economical to implement, intuitive to control, and have good low speed torque, making them ideal for many low power, computer-controlled applications. However, steppers are not recommended for high-speed or high-power applications, or for applications requiring precise torque control.

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