What is a Closed Loop Stepper Motor?

The control method of stepper motor is generally divided into two control methods: open-loop control and closed-loop control. Among them, the simplest control method for open-loop control of stepper motor is the ring control system. Under this control method, the stepper motor controls pulses. The input of the motor does not depend on the position of the rotor, but sends out its control pulses according to a fixed rule. The stepper motor works only by this series of predetermined pulses. This control method is more suitable for controlling stepper motors.

Closed-loop control is a basic concept of cybernetics. Refers to a control relationship in which the controlled output returns to the control input in a certain way, and exerts a control influence on the input. The closed-loop control of stepper motor is to use position feedback and (or) speed feedback to determine the phase transition suitable for the rotor position, which can greatly improve the performance of the stepper motor.

In the closed-loop control stepper motor system, the working speed range can be expanded when tracking and feedback with a given accuracy, or the tracking and positioning accuracy can be improved at a given speed, or the limit speed index and limit can be obtained.

Control method of closed-loop stepper motor

The closed-loop control methods of stepper motors are roughly divided into two types:

  • Keep the phase relationship between the excitation magnetic flux and the current consistent, so that it can generate an electromagnetic torque that can drive the load torque. This method of controlling the stepper motor current is the same as the brushless DC motor control method, which is called the brushless drive method or Current closed-loop control method.
  • The method of controlling the phase angle of the excitation magnetic flux and the current when the motor current is kept constant is called the power angle closed-loop control method. The power angle is the phase angle formed by the mutual attraction between the rotor magnetic pole and the stator excitation phase (or the axis of the stator rotating magnetic field considered as a synchronous motor). This power angle is smaller at low speed or light load, and larger at high speed or high load. Referring to the following figure in the open-loop control principle section above, the “bar A” phase attracts the rotor magnetic pole, and the angle of the “bar B” phase excitation is π/2, and the rotor magnetic pole is located at the leading edge of the “bar A” phase (When the S pole of the rotor in the figure is located on the left side of the A phase), the magnetic pole “bar B” phase starts to be excited.
Figure 1: Static torque of stepper motor at all angles

Why? Because at high speed, due to the influence of coil inductance, the turn-off time of phase A current is prolonged, and the rise time of phase B current is also prolonged. Therefore, the angle at which the maximum torque acceleration is generated increases as the speed increases.

Working principle diagram of closed-loop stepper motor

Figure 2: Schematic diagram of closed loop control

This control method directly or indirectly detects the position or speed of the transfer (or load), and then automatically gives the drive pulse sequence of the stepper motor through feedback and appropriate processing. This drive pulse sequence is based on the load or load. There are many implementation methods of this control method that the position of the rotor changes at any time. In the occasions where high precision is required, combined with micro-step driving technology and microcomputer control technology, high position precision requirements can be achieved.

Advantages of closed-loop stepper motors

  • As the output torque increases, the speed of both decreases in a nonlinear form, but the closed-loop control improves the torque-frequency characteristic.
  • Under the closed-loop control, the output power/torque curve is improved. The reason is that under the closed-loop, the motor excitation conversion is based on the rotor position information, and the current value is determined by the motor load. Therefore, even in the low speed range, It can also be fully converted into torque.
  • Under closed-loop control, the efficiency-torque curve is improved.
  • Adopting closed-loop control can obtain higher running speed, more stable and smoother speed than open-loop control.
  • Using closed-loop control, the stepping motor can be accelerated and decelerated automatically and efficiently.
  • The quantitative evaluation of the improvement in rapidity of closed-loop control relative to open-loop control can be obtained by comparing the time passing through a certain path interval in step IV: Topen-loop / Tclosed-loop = 0.625 √ N / n
  • Using closed-loop drive, the efficiency can be increased to 7.8 times, the output power can be increased to 3.3 times, and the speed can be increased to 3.6 times. The performance of a closed-loop driven stepper motor is better than that of an open-loop driven stepper motor in all respects. The stepper motor closed-loop drive has the advantages of the stepper motor open-loop drive and the DC brushless servo motor. Therefore, in the position control system with high reliability requirements, the closed-loop controlled stepping motor will be widely used.

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