The Stepper motor is a special motor used for control. Its rotation is performed at a fixed angle (called "step angle") step by step. It is characterized by no accumulated error (100% accuracy), so it is widely used. Applied to various open-loop control. The operation of the stepper motor needs to be driven by an electronic device. This device is the stepper motor driver, which converts the pulse signal sent by the control system into the angular displacement of the stepper motor, or: every time the control system sends a pulse signal, The stepper motor is rotated by a steep angle through the driver, so the speed of the stepper motor is proportional to the frequency of the pulse signal. Therefore, controlling the frequency of the stepping pulse signal can precisely adjust the speed of the motor; controlling the number of stepping pulses can precisely locate the motor. The existence of this linear relationship, coupled with the fact that the stepping motor has only periodic errors and no cumulative errors. It makes it very simple to use the stepper motor to control the speed, position, and other control fields.
1. The structure of the stepper motor (take a five-phase stepper motor as an example)
The structure of the stepper motor is shown in the figure below, which is roughly divided into two parts: the stator and the rotor. The rotor consists of rotor 1, rotor 2, and permanent magnet steel.
The stator has 10 tooth-shaped magnetic poles, all of which are wound with coils. The magnetic poles at the diagonal positions of the coil are connected to each other, and when the current flows, the coil will be magnetized to the same polarity. (For example, after a certain coil flows through the current, the magnetic poles on the diagonal will be assimilated into S poles or N poles.) The two magnetic poles on the diagonal form one phase, and since there are five phases equal to A phase to E, Therefore, it is called a 5-phase stepper motor.
The outer ring of the rotor consists of 50 small teeth, and the small teeth of rotor 1 and rotor 2 are staggered by 1/2 pitch from each other in structure. The rotor thus forms 100 small teeth. At present, there are high-resolution types of rotors that are individually machined to 100 teeth, so the high-resolution rotors have 200 small teeth. Therefore, it can mechanically achieve the resolution of a half-step of an ordinary stepping motor (a half-step of an ordinary stepping motor requires an electrical subdivision to achieve).
2. Working principle of stepper motor
When current flows through the stator windings, the stator windings generate a vector magnetic field. The magnetic field will drive the rotor to rotate by an angle so that the direction of a pair of magnetic fields of the rotor is consistent with the direction of the magnetic field of the stator. When the vector magnetic field of the stator rotates by an angle. The rotor also rotates an angle with this magnetic field. Each time an electrical pulse is input, the motor rotates an angle and moves forward one step. The angular displacement it outputs is proportional to the number of input pulses, and the rotational speed is proportional to the pulse frequency. Change the order in which the windings are energized and the motor will reverse. Therefore, the rotation of the stepper motor can be controlled by controlling the number of pulses, the frequency, and the power-on sequence of each phase winding of the motor.
3. Control of the stepper motor
There are three basic drive modes for stepper motor drive: full step, half step, and subdivision. The main difference is the control accuracy of the motor coil current (ie the excitation method). Usually, stepper motors have the characteristics of low-frequency vibration, and the balance of low-speed operation of the motor can be improved through subdivision and coordination. The following will give you a detailed introduction:
1. Full-step drive
In full-step operation, the same stepping motor can be equipped with both full/half-step drivers and subdivision drivers, but the operating effects are different. The stepper motor driver cyclically excites the two coils of the two-phase stepper motor according to the pulse/direction command (that is, the coil is charged to set the current), and each pulse of this driving method will move the motor by a basic step angle, that is, 1.80 degrees (a standard two-phase motor has a total of 200 step angles in one revolution).
2. Half-step drive
During single-phase excitation, the motor shaft stops at the full-step position. After the driver receives the next pulse, if it is excited to another phase and remains in the original excitation state, the motor shaft will move half a step angle and stop. in the middle of two adjacent full-step positions. A single-phase and then two-phase excitation of the two-phase coils in this manner will rotate the stepper motor in half steps of 0.90 degrees per pulse. Compared with the full-step mode, the half-step mode has the advantages of twice the accuracy and less vibration during low-speed operation, so the half-step mode is generally used when the full/half-step drive is actually used.
3. Segmentation driver
The subdivision drive mode has two advantages extremely small low-speed vibration and high positioning accuracy. For stepper applications that sometimes require low-speed operation (that is, the motor shaft sometimes works below 60rpm) or where the positioning accuracy is required to be less than 0.90 degrees, subdivision drives are widely used.
Fourth, the attention points in the application
1. The stepper motor is used on low-speed occasions---the speed should not exceed 1000 rpm, (6666PPS at 0.9 degrees), preferably between 1000-3000PPS (0.9 degrees), and it can be made to work here through the deceleration device, At this time, the motor has high working efficiency and low noise.
2. It is best not to use the full-step state of the stepper motor, and the vibration is large in the full-step state.
3. For loads with large moments of inertia, a large frame-size motor should be selected.
4. When the motor is at a high speed or a large inertia load, it generally does not start at the working speed, but gradually increases the frequency and speed, so that the motor does not lose steps, and the noise can be reduced and the positioning accuracy of the stop can be improved.
5. When high precision is required, it should be solved by mechanical deceleration, increasing the motor speed, or using a driver with a high subdivision number. A 5-phase motor can also be used, but the price of the entire system is more expensive and there are fewer manufacturers.
Five, stepper motor test
With the advancement of science and technology, the traditional test method has been eliminated, and now the comprehensive test system of stepper motors is gradually applied to the stepper motor industry.
The stepper motor comprehensive test system developed by AIP can quickly complete the detection of preset items in one clamping, and the test items can be customized according to customer requirements.