Closed- Loop Stepper Control System

- Mar 09, 2019-

Closed- loop stepper control system


The stepper motor is basically driven by an open loop circuit for position control. In other words, motors other than stepper motors, especially high-precision stepper motors, do not have open-loop control positioning, while motors driven by open-loop circuits have only stepper motors. For example, a brushless motor, first of all for switching phases, needs to measure the rotor position, and requires a position closed loop circuit with a position sensor. Moreover, if driving at a certain speed, the speed of the rotor needs to be measured, which is a speed closed loop circuit; if it is desired to position control, a closed loop circuit of a sensor such as an encoder including a rotor position signal is required. Closed-loop circuits with sensors are more expensive than open-loop driven stepper motors. Therefore, stepper motors are called low-cost drive systems for speed control or position control.


The open-loop circuit of the stepping motor drives problems such as out-of-step, vibration (noise), and high-speed operation when rotating at high speed. To compensate for these shortcomings, the stepper motor mounts an angle sensor that forms a closed loop control to detect and avoid out of step. The closed-loop control of stepper motors is roughly divided into two types:


1. The phase relationship between the excitation magnetic flux and the current is kept consistent, so that the electromagnetic torque capable of driving the load torque is generated. The method for controlling the motor current is the same as that of the brushless DC motor, and is called a brushless driving method or Current closed loop control method.

2. The motor current is kept constant, and the way of controlling the phase flux of the excitation flux and current is called the power angle closed-loop control method. The power angle is the phase angle at which the rotor pole and the stator excitation phase (or the stator rotating magnetic field axis of the synchronous machine are also considered to be attracted). This power angle is small at low speeds or at light loads, and is large at high speeds or high loads. Referring to the following figure in the principle part of the open-loop control, the "bar A" phase attracts the rotor pole, and the second "bar B" phase has an angle of π/2 when the magnet is excited, and the rotor pole is located at the leading edge of the "bar A" phase ( When the S pole of the rotor is located on the left side of the A phase, the magnetic pole "bar B" phase is excited.

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3. Why? Because of the influence of the inductance of the coil, the turn-off time of the A-phase current is prolonged, and the rise time of the B-phase current is also prolonged. Therefore, the angle at which the maximum torque is accelerated is generated, and the value becomes larger as the speed becomes faster.


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