Servo motor is a traditional motor. It is the executive part of the automatic device. The biggest feature of servo motor is controllable. When there is a control signal, the servo motor rotates, and the speed is proportional to the control voltage. When the control voltage is removed, the servo motor immediately stops rotating. Servo motors are widely used in almost all automatic control systems. In household appliances, such as video recorders and CD players, they are indispensable and important parts.

1. Working principle of simple servo motor

Figure 22 shows the simplest application of servo motor. Potentiometer RV 1 is driven by servo motor. The motor can be any servo motor with a current not exceeding 700mA and a voltage of 12 ~ 24V. In the figure, RV 1 and RV2 are connected into a Wheatstone bridge. The integrated circuit LM378 is a dual-channel 4-watt power amplifier, which also constitutes a differential amplifier driven by a motor in a bridging manner.

Any change of RV2 will destroy the balance of the bridge, and a differential voltage will be generated between RV 1-RV2, which will be amplified and sent to the motor. The motor will rotate and drag the potentiometer RV 1 to a new position, so that the bridge can reach a new balance again. Therefore, RV 1 tracks the motion of RV2.

Fig. 23 is a schematic diagram showing the servo motor system of the speed measuring sensor in block diagram form, which can be used for precise speed control of the turntable of the record player. The motor drives the turntable through the traditional belt mechanism. At the edge of the turntable, the graphic structure of reflective objects is used at equal intervals. Monitor with photoelectric velocimeter. The output signal of the photoelectric velocimeter is proportional to the rotating speed of the turntable. Compare the phase and frequency of the output signal of photoelectric velocimeter with the output signal of standard oscillator, and control the motor drive circuit with its error signal. Therefore, the rotating speed of the turntable is accurately maintained at the rated rotating speed. The conversion of rated speed can be controlled by operating the switch. These control circuits have been made into application specific integrated circuits by manufacturers.

2. Digital proportional servo motor

One of the best types of servo motor is digital proportional remote control system. In fact, these devices are composed of three parts: integrated circuit, servo motor and potentiometer mechanism of reduction gear box. Fig. 24 is a block diagram of the system. The driving input of the circuit is driven by a pulse signal with a period of 1.5 ms and a pulse width of 1 ~ 2 ms, and the width of the input pulse controls the position of the servo mechanical output. For example: 1ms pulse width, located at the far left; 1.5ms is the middle position, and 2ms is the rightmost position.

Each input pulse is transmitted simultaneously in three ways. One path of fixed pulse generator with trigger pulse width of 1.5 ms, one path of input to trigger pulse generator, and the third path of input to pulse width comparison circuit. The output from the gearbox to RV 1 controls the pulse generator with variable width. After these three pulses are sent to the pulse width comparator at the same time, there is a way to determine the direction of the motor drive circuit. The other path is sent to the pulse width expander to control the speed of the servo motor, so that RV 1 can quickly drive the mechanical position output to follow any change of the input pulse width.

The above servo motor types are commonly used in multi-channel remote control systems. Fig. 25 shows a waveform diagram of a four-way digital proportional control system.

As can be seen from the figure, it is a serial data input, and the control signal of each channel is separated by the decoder. Each frame contains a synchronization pulse of 4ms, followed by four "road" pulses with variable width (1 ~ 2 ms). The decoder converts the four pulses into parallel form, which can be used to control the servo motor.

3. Digital servo motor circuit

Digital servo motor control unit, you can buy ready-made integrated circuits. Such as ZN409CE or NE544N servo motor amplifier integrated circuit. Figures 26 and 27 show typical applications of these two integrated circuits.

The element values in the figure are applicable to the case that the input pulse width is 1 ~ 2 ms and the frame pulse width is about18 ms.

Fig. 28 is a general test circuit suitable for the above servo motor type. The servo power supply battery is usually 5V. Input pulse is sent to servo circuit through standard servo socket. The width of the frame pulse is13-28 ms; Adjust the control with RV 1 The pulse width of RV2 regulation control is between1-2 ms. The median value is1.5 ms. Use RV4. The output level is regulated by RV3.

Both integrated circuits are CMOS7555 time base circuits, and the power supply voltage can be as low as 3V and still work. IC 1 is an unstable multivibrator, which generates frame time pulses and its output triggers IC2. IC2 is a monostable circuit, which generates output test pulses.

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