The word servo comes from the Greek word for slave. "Servo motor" can be understood as a motor that absolutely obeys the command of the control signal: before the control signal is sent, the rotor is stationary; When the control signal is sent, the rotor rotates immediately; When the control signal is lost, the rotor can stop immediately.
Servo motor is a micro motor used as an actuator in automatic control device, whose function is to convert electrical signals into angular displacement or angular velocity of the rotating shaft. The servo motor, also known as the executive motor, is used as the executive element in the automatic control system to convert the electrical signal received into the angular displacement or angular velocity output on the motor shaft.
Classification of servomotors
Servo motor is divided into AC servo and DC servo two categories.
The basic structure of AC servo motor is similar to that of AC induction motor (asynchronous motor). Two exciting windings Wf and control windings WcoWf with 90° phase space displacement on the stator are connected with constant AC voltage. The purpose of controlling motor operation is achieved by using the change of AC voltage or phase applied to Wc. Ac servo motor has the characteristics of stable operation, good controllability, fast response, high sensitivity, and strict nonlinearity index of mechanical characteristics and adjusting characteristics (less than 10% ~ 15% and less than 15% ~ 25%, respectively).
Advantages and disadvantages of DC servo motor
Advantages: precise speed control, torque speed characteristics is very hard, simple control principle, easy to use, cheap price.
Disadvantages: brush reversal, speed limit, additional resistance, wear particles (not suitable for dust-free explosive environment).
The basic structure of DC servo motor is similar to that of general DC motor. Motor speed n=E/K1j=(Ua-iara)/K1j, where E is armature back electromotive force, K is constant, j is magnetic flux per pole, Ua and Ia are armature voltage and armature current, Ra is armature resistance, change Ua or change φ, can control the speed of DC servo motor, but generally use the method of controlling armature voltage. In permanent magnet DC servomotor, the field winding is replaced by permanent magnet, and the magnetic flux φ is constant. Dc servo motor has good linear regulation characteristics and fast time response.
The advantages and disadvantages of AC servo motor
Advantages: Good speed control characteristics, smooth control can be achieved in the whole speed zone, almost no oscillation, more than 90% high efficiency, less heat, high speed control, high precision position control (depending on the encoder accuracy), rated operation area, can achieve constant torque, low inertia, low noise, no brush wear, free of maintenance (suitable for dust-free, explosive environment).
Disadvantages: The control is more complex, and the driver parameters need to be adjusted on site to determine the PID parameters, which requires more wiring.
Dc servo motors are divided into brushless and brushless motors.
Brush motor has the advantages of low cost, simple structure, large starting torque, wide speed regulating range, easy control, the need for maintenance, but convenient maintenance (carbon brush), electromagnetic interference, requirements for the use of the environment, usually used for cost sensitive common industrial and civil occasions.
Brushless motor small volume, light weight, large output response, high speed, small inertia, torque stable rotation smooth, complex control, intelligent, electronic commutation mode flexible, square wave or sine wave commutation, motor maintenance free, high efficiency and energy saving, electromagnetic radiation, low temperature rise long life, suitable for all kinds of environment.
Ac servo motor is also brushless motor, divided into synchronous and asynchronous motor, synchronous motor is generally used in motion control at present, its power range is large, the power can be very large, large inertia, the highest speed is low, the speed decreases evenly with the increase of power, suitable for low speed and smooth operation
The rotor inside the servo motor is a permanent magnet, and the driver controls the U/V/W three-phase electricity to form an electromagnetic field. The rotor rotates under the action of this magnetic field. At the same time, the encoder of the motor sends feedback signals to the driver, and compares the feedback value with the target value, so as to adjust the rotation Angle of the rotor.
Q
What is the performance difference between AC servo motor and brushless DC servo motor?
A
Ac servo motor performance is better, because AC servo is sinusoidal control, torque ripple is small; And brushless DC servo is trapezoidal wave control. But brushless DC servo control is simpler and cheaper.
The rapid development of permanent magnet AC servo drive technology makes the DC servo system face the crisis of being eliminated [/p][p= 30,2, left] Since the 1980s, with the development of integrated circuit, power electronics technology and AC variable speed drive technology, permanent magnet AC servo drive technology has made outstanding development. Famous electrical manufacturers have introduced new AC servo motor and servo drive series products. Ac servo system has become the main development direction of contemporary high performance servo system, which makes DC servo system face the crisis of being eliminated.
Compared with DC servo motor, permanent magnet AC servo motor has the following advantages:
(1)No brush and commutator, more reliable operation, maintenance-free.
(2)The heating of stator winding is greatly reduced.
(3) The inertia is small and the system has good fast response.
(4) High speed and large torque working condition is good.
(5) Small volume and light weight under the same power.
Servo motor principle
The stator structure of AC servo motor is basically similar to that of capacitor split phase asynchronous motor. The stator is equipped with two windings with a position difference of 90°, one is the excitation winding Rf, which is always connected to the AC voltage Uf; The other is the control winding L, connected to the control signal voltage Uc. So AC servo motor is also called two servo motors.
The rotor of AC servo motor is usually made of squirrel cage type, but in order to make the servo motor has a wide speed range, linear mechanical characteristics, no "rotation" phenomenon and fast response performance, compared with ordinary motor, it should have large rotor resistance and small moment of inertia of these two characteristics. At present, there are two types of rotor structures widely used: one is the squirrel cage rotor with high resistivity guide bar made of conductive material with high resistivity. In order to reduce the moment of inertia of the rotor, the rotor is made slender; The other is made of aluminum alloy hollow cup rotor, cup wall is only 0.2-0.3mm, hollow cup rotor has a small moment of inertia, rapid reaction, and smooth operation, so it is widely used.
When the AC servo motor has no control voltage, the stator only has pulsating magnetic field generated by the excitation winding, and the rotor is stationary. When there is a control voltage, the stator will generate a rotating magnetic field, the rotor along the direction of the rotating magnetic field rotation, in the case of constant load, the motor speed with the size of the control voltage changes, when the phase of the control voltage is opposite, the servo motor will reverse.
Although the working principle of AC servo motor is similar to that of capacitor running single-phase asynchronous motor, the rotor resistance of the former is much larger than that of the latter. Therefore, compared with the capacitor running asynchronous motor, the servo motor has two significant characteristics:
1. Large starting torque: because the rotor resistance is large, the torque characteristics (mechanical characteristics) are closer to linear, and it has a large starting torque. Therefore, when the stator has a control voltage, the rotor rotates immediately, that is, it has the characteristics of fast starting and high sensitivity.
2. Wide operating range: smooth operation, low noise. [/p][p=30, 2, left]3, no rotation phenomenon: the servo motor in operation, as long as the control voltage is lost, the motor will stop running immediately.
Precision drive micro special motor
"Precision drive micro special motor" can quickly and correctly execute frequently changing instructions in the system, drive the servo mechanism to complete the expected work of the instructions, most of which can meet the following requirements:
1. Can frequently start, stop, brake, reverse and low speed operation, and high mechanical strength, high heat resistance grade, high insulation grade.
2. Good quick response ability, large torque, small moment of inertia, small time constant.
3. With drive and controller (such as servo motor, stepper motor), good control performance.
4. High reliability and precision.
The categories of precision drive micro motor and their structure and performance are compared as follows:
Ac servo motor
(1) Cage type two-phase AC servo motor (slender cage rotor, approximately linear mechanical characteristics, small volume and excitation current, low power servo, low speed operation is not smooth).
(2) Non-magnetic cup rotor two-phase AC servo motor (hollow cup rotor, approximately linear mechanical characteristics, large volume and excitation current, low power servo, smooth running at low speed).
(3) Ferromagnetic cup-rotor two-phase AC servo motor (ferromagnetic cup-rotor with approximately linear mechanical characteristics, large rotor moment of inertia, small tooth groove effect and stable operation).
(4) synchronous permanent magnet AC servo motor (consisting of permanent magnet synchronous motor, velocity measuring machine and position detection element coaxial unit, stator is 3 phase or 2 phase, magnetic material rotor, must be equipped with a driver; Wide speed range, mechanical characteristics by constant torque zone and constant power zone, continuous plugging, rapid corresponding performance is good, large output power, small torque fluctuation; Square wave drive and sine wave drive two ways, good control performance, for electromechanical integration products).
(5) asynchronous three-phase AC servo motor (rotor and cage asynchronous motor similar, must be equipped with a driver, vector control, expand the constant power speed control range, mostly used for machine tool spindle speed control system).
Dc servo motor
(1) Printed winding DC servo motor (disk-shaped rotor, disk-shaped stator axially bonded cylindrical magnetic steel, small rotor moment of inertia, no slot effect, no saturation effect, large output torque).
(2) wire-wound DC servo motor (disk-shaped rotor and stator are axially bonded with cylindrical magnetic steel, rotor moment of inertia is small, control performance is better than other DC servo motors, high efficiency, large output torque).
(3) cup-type armature permanent magnet DC motor (hollow cup rotor, small rotor inertia, suitable for incremental motion servo system).
(4) brushless DC servo motor (stator is polyphase winding, rotor is permanent magnet, rotor position sensor, no spark interference, long life, low noise).
Torque motor
(1) DC torque motor (flat structure, number of poles, number of slots, number of reversing plates, number of series conductors; Large output torque, continuous operation at low speed or blocked rotation, good mechanical and regulatory characteristics, small electromechanical time constant).
(2) brushless DC torque motor (similar to brushless DC servo motor structure, but flat, the number of poles and slots number of series conductors; Large output torque, good mechanical and adjusting characteristics, long life, no spark, low noise).
(3) Cage type AC torque motor (cage type rotor, flat structure, more pole slots, large starting torque, small electromechanical time constant, can run for a long time, soft mechanical characteristics).
(4) Solid rotor AC torque motor (ferromagnetic material solid rotor, flat structure, the number of poles and slots, can be blocked for a long time, smooth operation, soft mechanical characteristics).
Stepper motor
(1) Reaction stepper motor (stator rotor is made of silicon steel sheet, rotor core has no winding, stator has control winding; Small step Angle, high starting and running frequency, low step Angle accuracy, no self-locking torque).
(2) permanent magnet stepper motor (permanent magnet rotor, radial magnetization polarity; The step Angle is large, the starting and running frequency is low, the torque is maintained, the power consumption is smaller than the reaction formula, but the positive and negative pulse current must be provided.
(3) hybrid stepper motor (permanent magnet rotor, axial magnetization polarity; High precision of step Angle, keeping torque, small input current, both reaction type and permanent magnet type advantages).
Switched reluctance motor (fixed rotor is made of silicon steel sheet, convex pole type, and pole number close to the step distance reaction stepper motor structure is similar, with rotor position sensor, torque direction is independent of current direction, speed range is small, loud noise, mechanical characteristics by constant torque region, constant power region, series characteristic region of three parts).
Linear motor (simple structure, guide rail can be used as a secondary conductor, suitable for linear reciprocating motion; High speed servo performance, high power factor and efficiency, constant speed performance is excellent).