Introduction to the internal structure of servo motor 2

** Introduction of Servo Motor **

Servo motors (or actuator motors) are widely used actuators in automatic control systems and computing devices. Their function is to convert the received electrical signals into the angular displacement or angular velocity of the motor shaft. According to the type of current, servo motors can be divided into two categories: DC and AC.

1. AC servo motor

1. Structure and Principle The stator winding of the AC servo motor is similar to that of a single-phase asynchronous motor. Its stator is equipped with two windings with a 90° electrical angle difference in space, namely the excitation winding and the control winding. During operation, a certain AC excitation voltage is always applied to the excitation winding, and a control voltage whose magnitude or phase changes with the signal is applied to the control winding. The rotor has two structural forms: cage rotor and hollow cup rotor.

The structure of the cage rotor is the same as that of the general cage asynchronous motor, but the rotor is made slender and the rotor conductor is made of high resistivity material. The purpose is to reduce the rotor's rotational inertia, increase the starting torque's rapid response to the input signal and overcome the self-rotation phenomenon. The stator of the hollow cup rotor AC servo motor is divided into two parts: the outer stator and the inner stator. The structure of the outer stator is the same as that of the cage AC servo motor, and two-phase windings are placed in the core slot.

The hollow cup-shaped rotor is made of conductive non-magnetic material (such as aluminum) into a thin-walled cylinder and placed between the inner and outer stators. The bottom of the cup is fixed on the shaft, and the cup arm is thin and light, with a thickness of generally 0.2-0.8mm, so the moment of inertia is small, the action is fast and sensitive. The working principle of the AC servo motor is similar to that of the single-phase asynchronous motor. LL is the excitation winding with fixed voltage excitation, and LK is the control winding powered by the servo amplifier. The two-phase windings differ by 90° electrical angle in space.

If the phase difference between IL and Ik is 90°, and the amplitude of the magnetomotive force of the two-phase winding is equal, this state is called a symmetrical state. Like a single-phase asynchronous motor, the synthetic magnetic field generated in the air gap is a rotating magnetic field, and its speed is called the synchronous speed. The rotating magnetic field cuts against the rotor conductor, generating an induced current in the rotor.

The rotor current interacts with the rotating magnetic field to generate torque, causing the rotor to rotate. If the magnitude or phase difference of the current applied to the control winding is changed, the symmetry state is destroyed, the rotating magnetic field is weakened, and the motor speed decreases. The more asymmetrical the motor's working state is, the smaller the total electromagnetic torque is. When the signal voltage on the control winding is removed, the motor stops rotating immediately. This is the difference between the operation of AC servo motors and ordinary asynchronous motors.

AC servo motors have the following three speed control methods:

(1) Amplitude control: The phase difference between the control current and the excitation current remains unchanged at 90°, and the magnitude of the control voltage is changed.

(2) Phase control: The magnitude of the control voltage and the excitation voltage is controlled, the rated value is kept unchanged, and the phase of the control voltage is changed.

(3) Amplitude-phase control changes the amplitude and phase of the control voltage simultaneously. The direction of rotation of the AC servo motor shaft changes with the reversal of the control voltage phase.

2 Working characteristics and uses The working characteristics of servo motors are characterized by mechanical characteristics and adjustment characteristics. When the control voltage is constant, the load increases and the speed decreases; its adjustment characteristic is that when the load is constant, the higher the control voltage, the higher the speed. Servo motors have three significant characteristics:

(1) Large starting torque: Since the rotor conductor resistance is large, the critical slip rate Sm>1. Once the control voltage is applied to the stator, the rotor starts running immediately.

(2) Wide operating range: It can operate stably within the slip range of 0 to 1.

(3) No self-rotation phenomenon After the control signal disappears, the phenomenon of the motor rotating continuously is called "self-rotation". The self-rotation phenomenon destroys the servo performance and should be avoided. As long as the control voltage is lost, the servo motor will be in single-phase operation. Since the rotor conductor resistance is large enough, the total electromagnetic torque is always a braking torque. When the motor loses Uk (control voltage) during forward rotation, the torque generated is negative (0 < S < 1). When UK is lost during reverse rotation, the torque generated is positive (1 < S < 2). There will be no self-rotation phenomenon, and it can brake by itself and stop running quickly. This is also an important difference between AC servo motors and asynchronous motors.

Different types of AC servo motors have different characteristics. The cage rotor AC servo motor has the characteristics of small excitation current, small size, high mechanical strength, etc. However, it is not stable enough when running at low speed and has jitter.

Hollow cup rotor AC servo motor has the advantages of simple structure, easy maintenance, small moment of inertia, smooth operation, low noise, no radio interference, no jitter, etc. However, the excitation current is large, the volume is also large, the rotor is easy to deform, and the performance is not as good as that of DC servo motor. AC servo motor is suitable for 0.1-100W low-power automatic control system, with a variety of frequencies such as 50Hz, 400Hz, etc.

The cage rotor AC servo motor products are SL series. The hollow cup rotor AC servo motor is SK series, which is used in systems that require smooth operation. 2. DC servo motor The basic structure of DC servo motor is the same as that of ordinary separately excited DC motor. The difference is that the armature current of DC servo motor is very small, and commutation is not difficult, so there is no need to install commutation poles, and the rotor is made slender, the air gap is small, the magnetic circuit is not saturated, and the armature resistance is large.

According to the different excitation methods, it can be divided into electromagnetic type and permanent magnet type. The magnetic field of electromagnetic DC servo motor is generated by the excitation winding, which is generally separately excited; the magnetic field of permanent magnet DC servo motor is generated by permanent magnets, which does not require excitation winding and excitation current, and can reduce volume and loss. In order to meet the needs of various systems, many improvements have been made in structure, and low-inertia slotless armature, hollow cup armature, printed winding armature and brushless DC servo motor have been developed.

The working principle of electromagnetic DC servo motor is the same as that of separately excited DC motor, so electromagnetic DC servo motor has two speed control methods: armature control and magnetic field control. For permanent magnet DC servo motor, of course, there is only one speed control method, armature control. Since the performance of magnetic field control speed control method is not as good as armature control speed control method, DC servo motor generally adopts armature control speed control.

The direction of the DC servo motor shaft changes with the change of the polarity of the control voltage. The mechanical characteristics of the DC servo motor are similar to those of the separately excited DC motor, that is, n=n0-αT. When the excitation remains unchanged, there is a set of descending parallel straight lines for different voltages Ua. DC servo motors are suitable for automatic control systems with slightly larger power (1-600W).

Compared with AC servo motors, it has good speed regulation linearity, small size, light weight, large starting torque and large output power. However, its structure is complex, especially its low-speed stability is poor, and sparks can cause radio interference. In recent years, low-inertia slotless armature motors, hollow cup armature motors, printed winding armature motors and brushless DC servo motors have been developed to improve the rapid response capability and meet the development needs of automatic control systems, such as television cameras, recorders, X-Y function recorders, etc.

Permanent Magnet AC Servo Motor

Since the 1980s, with the development of integrated circuits, power electronics technology and AC variable speed drive technology, permanent magnet AC servo drive technology has made outstanding progress. Well-known electrical manufacturers in various countries have successively launched their own AC servo motors and servo drive product series and continuously improved and updated them.

AC servo system has become the main development direction of contemporary high-performance servo system, making the original DC servo face the crisis of being eliminated. After the 1990s, the commercialized AC servo system in various countries around the world uses a fully digitally controlled sine wave motor servo drive. The development of AC servo drive devices in the transmission field is changing with each passing day. Compared with DC servo motors, permanent magnet AC servo motors have the following main advantages:

⑴ There are no brushes and commutators, so it works reliably and requires low maintenance.