principle
As a special motor for control, stepper motor is an actuator that converts electric pulses into angular displacement. When the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (called the "step angle") in a set direction, and its rotation is step by step at a fixed step angle. . The angular displacement can be controlled by controlling the number of pulses to achieve the purpose of accurate positioning. At the same time, the speed and acceleration of the motor rotation can be controlled by controlling the pulse frequency, thereby achieving the purpose of speed regulation, changing the order of energization of the windings, and the motor will Reverse.
Drive principle
The stepping motor needs to be driven by a dedicated stepping motor driver, and the driver is composed of a pulse generation control unit, a power driving unit, a protection unit and the like. The power driving unit amplifies the pulse generated by the pulse generation control unit and directly couples with the stepping motor, and belongs to the power interface of the stepping motor and the microcontroller.
The control command unit receives the pulse and the direction signal, and the corresponding pulse generation control unit generates a corresponding set of phase pulses, which are sent to the stepping motor after passing through the power driving unit, and the stepping motor rotates through a step angle in the corresponding direction. . The pulse reference mode of the drive determines the operation mode of the stepper motor as follows:
(1) m phase single m shot operation
(2) m phase double m beat operation
(3) m phase single, double m beat operation
(4) subdivision drive, the driver needs to give different amplitude drive signals
Stepper motors have some important technical data, such as maximum static torque, starting frequency, and operating frequency. Generally speaking, the smaller the step angle is, the larger the maximum static torque of the motor is, the higher the starting frequency and the running frequency are. Therefore, the operation mode emphasizes the subdivision driving technology, which improves the rotating torque and resolution of the stepping motor. , completely eliminates the low frequency oscillation of the motor. So subdivided drive performance is superior to other types of drives.
The rotor inside the servo motor is a permanent magnet. The U/V/W three-phase electric motor controlled by the driver forms an electromagnetic field. The rotor rotates under the action of the magnetic field. At the same time, the encoder feedback signal from the motor is supplied to the driver. The driver according to the feedback value and target. The values ​​are compared to adjust the angle at which the rotor rotates.
ä¼ºæœ Servo motor
Servo motor principle
A servo motor, also known as an actuator motor, is used as an actuator in an automatic control system to convert an electrical signal received into an angular displacement or angular velocity output on a motor shaft. Divided into two major categories of DC and AC servo motors.
When the servo motor receives one pulse, it will rotate the angle corresponding to one pulse to realize the displacement. Because the servo motor itself has the function of emitting a pulse, the servo motor will emit a corresponding number of pulses every time it rotates by one angle. The pulse received by the servo motor forms a closed loop, and the system knows how many pulses are sent to the servo motor, and at the same time, how many pulses are returned, so that the rotation of the motor can be accurately controlled to achieve accurate positioning.
In terms of performance comparison, AC servo motor is better than DC servo motor, AC servo motor adopts sine wave control, torque ripple is small, and capacity can be relatively large. DC servo motors are controlled by trapezoidal waves and are relatively poor. Brushless servo motors in DC servo motors perform better than brushed servo motors.
Servo motor driver
The rotor inside the servo motor is a permanent magnet. The U/V/W three-phase electric motor controlled by the driver forms an electromagnetic field. The rotor rotates under the action of the magnetic field. At the same time, the encoder feedback signal from the motor is supplied to the driver. The driver according to the feedback value and target. The values ​​are compared to adjust the angle at which the rotor rotates.
Brushed DC servo motor driver: The working principle of the motor is exactly the same as that of the ordinary DC motor. The driver has a three-closed loop structure, which is a current loop, a speed loop and a position loop from the inside to the outside. The output of the current loop controls the armature voltage of the motor. The input of the current loop is the output of the speed loop PID. The input of the speed loop is the PID output of the position loop. The input of the position loop is the given input. The control schematic is as shown above.
Brushless DC servo motor driver: The power supply is DC, which is inverted into U/V/W AC through the internal three-phase inverter to supply the motor. The drive also adopts three closed-loop control structure (current loop, speed loop, position loop). ), the principle of drive control is the same as above.
AC servo motor driver: It can be divided into two modules: the power board and the control board with independent functions. The control board outputs the PWM signal through the corresponding algorithm as the driving signal of the driving circuit to change the output power of the inverter. The purpose of controlling a three-phase permanent magnet synchronous AC servo motor.
The power driving unit first rectifies the input three-phase electric power or the commercial power through a three-phase full-bridge rectifying circuit to obtain a corresponding direct current power. After rectifying the three-phase electric or commercial power, the three-phase permanent-magnet synchronous AC servo motor is driven by the three-phase sinusoidal PWM voltage inverter frequency conversion, which is simply the AC-DC-AC current conversion process.
The control unit is the core of the entire AC servo system, enabling system position control, speed control, torque and current control.
性能Comparison of performance between servo motor and stepper motor
Control accuracy: The more the phase number and the number of beats of the stepping motor, the higher its accuracy. The servo motor takes the block in its own encoder. The more the scale of the encoder, the higher the accuracy.
Low-frequency characteristics: Stepper motor is prone to low-frequency vibration at low speed. When it works at low speed, it usually adopts damping technology or subdivision technology to overcome low-frequency vibration. The servo motor runs very smoothly, and vibration does not occur even at low speed. phenomenon;
Moment frequency characteristics: The output torque of the stepping motor decreases with the increase of the rotational speed, and will drop sharply at high speed. The servo motor is the constant torque output at the rated speed and the constant power output at the rated speed;
Overload capability: The stepper motor does not have overload capability, and the servo motor has strong overload capability;
Operating performance: The control of the stepping motor is open-loop control, the starting frequency is too high or the load is too large, and it is easy to lose or step. The over-speed is easy to occur when the speed is too high, and the AC servo drive system is closed-loop control. The motor encoder feedback signal can be directly sampled, and the position loop and the speed loop are internally formed. Generally, the stepping motor is lost or overshooted, and the control performance is more reliable;
Speed ​​response performance: It takes hundreds of milliseconds for the stepper motor to accelerate from standstill to the working speed. The AC servo system has better acceleration performance, generally only a few milliseconds, and can be used in control situations requiring fast start and stop.
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