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2022.10.05 motor

Stepping Motors: Microstep Operation Principles

Stepping Motors

Stepping motors are capable of finer step angle control through a driving method called microstepping. Here, the operation principles of microstepping are explained.

Stepping Motors: Microstep Operation Principles

In "Basic Operating Principles of Stepping Motors" in the previous article, an example was presented in which the two-phase bipolar type coils are excited one phase (one set of coils) at a time. In this example, it was stated that the motor rotates 90° for each step in which a current is made to flow in one of two sets of coils and no current is made to flow in the other coil set. In contrast to this, in microstep driving, the motor can be rotated through smaller (finer) step angles.

There are two main advantages of microstep driving. One is that position control in increments of very small angles is possible. The other is that vibrations and noise at low motor speeds can be reduced. A stepping motor stops at a prescribed position with each step accompanied by damped oscillation. That is, the stopping position is overshot, the return movement again overshoots the stopping position, and this is repeated a number of times before stopping completely. When the stepping motor rotation is at low speed, this damped oscillation may cause vibrations and noise. But the damped oscillation can be reduced by using finer step angles, and through the use of microstep driving, vibrations and noise at lower speeds can be decreased.

The diagrams below are used to explain the principles of microstep operation. The diagrams are for a two-phase bipolar configuration, and for simple coil switching, the step angle would be 90°, but in this example, the steps are divided by four in 1/4 microstep driving. The currents and magnetic fields of the coils when rotating the motor by 1/4 of 90° or 22.5° each time are indicated in the order ① to ⑤.

  • ・Current is caused to flow into coils 1 from the left and flow out of coils 1 from the right.
  • ・No current is made to flow in coils 2.
  • ・At this time, the inside of coil 1 on the left is the N pole, and the inside of coil 1 on the right is the S pole.
  • ・The permanent magnet in the center * is attracted by the magnetic field of coils 1 such that the S pole is on the left and the N pole is on the right, and the magnet stops.
  • ・Suppose that the magnetic field magnitude is M0 when the coil current is Io.
    * Not shown in the diagrams ① to ⑤ above in order to simplify the presentation; see the diagram on the right.
  • ・Starting from state ①, in order to rotate the motor clockwise through 22.5° (1/4 of 90°), the magnetic field intensity M0 is maintained, and a magnetic field is generated to cause the permanent magnet to stop at the relevant position.
  • ・In order to do so, a magnetic field equal to M0×cos (22.5°) ≒ M0×0.924 is generated using coils 1, and a magnetic field equal to M0×sin (22.5°) ≒ M0×0.383 is generated using coils 2.
  • ・In order to do so, the current in coils 1 is set to Io×cos (22.5°) ≒ Io×0.924, and the current in coils 2 is set to Io×sin (22.5°) ≒ Io×0.383.
  • ・In order to advance another 22.5° so as to rotate clockwise 45° from the state in ①, the relevant magnetic field M0 is generated.
  • ・In order to do so, a magnetic field equal to M0×cos (45°) ≒ M0×0.707 is generated using coils 1, and a magnetic field equal to M0×sin (45°) ≒ M0×0.707 is generated using coils 2.
  • ・In order to do so, the current in coils 1 is set to Io×cos (45°) ≒ Io×0.707, and the current in coils 2 is set to Io×sin (45°) ≒ Io×0.707.
  • ・In order to advance another 22.5° from the state ③ to rotate 67.5°, the relevant M0 is similarly generated.
  • ・In order to do so, a magnetic field equal to M0×cos (67.5°) ≒ M0×0.383 is generated using coils 1, and a magnetic field equal to M0×sin (67.5°) ≒ M0×0.924 is generated using coils 2.
  • ・In order to do so, the current in coils 1 is set to Io×cos (67.5°) ≒ Io×0.383, and the current in coils 2 is set to Io×sin (67.5°) ≒ Io×0.924.
  • ・In order to advance another 22.5° so as to rotate 90° from the state ①, the current Io is passed in the coils 2, and the current in the coils 1 is set to zero.

Such an operation in which, with the magnetic field intensity held constant, the currents flowing in each of the coil sets are controlled according to the angle to form magnetic fields that cause the rotor to rotate and stop in prescribed steps is called a microstep operation.

In the diagrams, an example is shown of quarter-step driving in which 90° rotation is divided into four steps; at present, driving as finely as in 1/32 steps is possible. As noted above, through the use of microstep driving it is possible to control the motor position in fine angular increments while reducing noise and vibrations.

Key Points:

・In a stepping motor, fine step angle control is possible through microstep driving.

・The advantages of microstep driving are the ability to control the motor position in fine angle increments, and the ability to reduce vibration and noise at low speeds.

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