Stepper motors are a form of a brushless motors that has a very high pole count, 100 poles or 50 pole pairs. Stepper motors are considered a low-cost simple motion control system where a 100 pole motor can rotate the rotor in 200 unique positions (steps) during one rotation of the motor shaft. Combined with a simple step and direction input drive, stepper motors are very simple motion control building blocks used extensively in lower end motion control systems. Traditional stepper motor systems operate in an “open loop” mode where the motor shaft rotation is assumed to occur, given the number of steps provided to the motor drive. This results in “over specifying” the output torque (essentially motor size) to ensure the motor does not stall or fail to rotate the number of input steps.
With the addition of an encoder (absolute or incremental) to the tail shaft of a stepper motor, the motor’s shaft rotation can be verified. The use of an encoder with a stepper motor results in a motion control system called “move and verify”. In this case the stepper motor no longer has to be oversized to guarantee the motor will rotate the desired number of steps. The feedback encoder position information can tell the system if motor did or did not move the desired number of steps. Therefore resulting in a more efficient motion control system in terms of current consumption and physical size.
Designed for Holding Torque and Speed
Recently, stepper motors are being applied as Brushless DC, BLDC, motors with a high pole count (100 poles). Stepper motor operation in this mode (stepper servo or step servo) provides a lower cost servo motor system, due to the lower cost of a stepper motor comparatively to a servo motor, but with lower performance than traditional BLDC servo systems. Stepper motors are designed for holding torque first and speed, RPM, is secondary.
To achieve high holding torque, stepper motors have lots of windings that create strong magnetic fields. But, with the increased number of windings comes an increased back EMF which reduces the motor shaft speed per unit of voltage. Stepper servo motors always require encoder feedback to allow the drive electronics to switch the motor phase currents at the correct time in order to have the motor shaft rotate in a controlled manner. The advantage of using a stepper as a servo motor is that in a closed loop control, the amount of current to the motor is proportional to the load on the motor. No load is very low current and likewise high load is high motor current. This is in contrast to a stepper motor running in open loop mode where the motor always operates at maximum current regardless of the load on the motor.
POSITAL’s Multiturn Kit Encoders without battery are a very economic and compact option to upgrade stepper motors to advanced stepper servo systems. They are using Wiegand Technology as energy harvesting system and cover a large number of turn as for example required in linear actuator applications.