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STEP-BY-STEP MOTORS

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STEP-BY-STEP MOTORS

Stepper motors are a form of brushless motors with a very high pole count, 100 poles, or 50 pole pairs. Stepper motors are considered to be a simple and inexpensive motion control system where a 100 pole motor can turn the rotor in 200 unique positions (stepping) over one full rotation of the motor shaft. Combined with a simple step and direction input drive, stepper motors are such simple building blocks that they are widely used in low-end motion control systems. Systems incorporating traditional stepper motors operate in an “open loop” mode where rotation of the motor shaft is expected to occur, given the number of steps supplied to the motor drive. This results in an “over-specification” of the output torque (essentially the motor size) to ensure that the motor does not lock up or stick to the desired number of steps.

Adding an encoder (absolute or incremental) to the rear of a stepper motor allows you to check the rotation of the motor shaft. The use of an encoder with a stepping motor makes it possible to obtain a movement control system called “move and verify”. In this case, the stepper motor no longer needs to be oversized to ensure that the motor rotates the desired number of steps. The encoder position information can tell the system whether or not the motor has moved the desired number of steps. The result is a more efficient motion control system in terms of power consumption and footprint.

Designed to Maintain Torque and Rotation Speed

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Recently, stepper motors have been used as brushless DC, BLDC motors, with a high number of poles (100 poles). Running the stepper motor in this mode (stepper servo motor) can lower the cost of the system due to the lower cost of a stepper motor compared to a servo motor, but with lower performance than servo BLDC systems traditional. Stepper motors are designed to maintain torque first and speed, in rpm, is secondary.

To achieve a high holding torque, stepper motors have a lot of coils, thus creating strong magnetic fields. However, the increase in the number of coils is accompanied by an increase in the rear electromagnetic field, at the same time reducing the speed of the motor shaft. Stepper servo motors always require feedback from the encoder to allow the drive electronics to switch motor phase currents at the right time so that the motor shaft rotates in a controlled manner. The advantage of using a stepper motor as a servo motor is that in closed loop control the amount of current to be supplied to the motor is proportional to the motor load. A low load requires a very low current, just as a high load implies a high motor current. This contrasts with a stepper motor operating in an open loop where the motor always operates at maximum current regardless of the load on the motor.

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POSITAL’s battery-less multiturn kit encoders are a very economical and compact option for upgrading stepper motors to advanced servo systems. They use Wiegand technology as an energy harvesting system and cover a large number of turns, for example in linear actuator applications.

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