Servo motor system circuit

Servo motor is a traditional motor. It is the actuating element of the automatic device. The biggest feature of servo motors is controllability. When there is a control signal, the servo motor rotates, and the speed is proportional to the control voltage. After the control voltage is removed, the servo motor stops rotating immediately. Servo motors are widely used and are used in almost all automatic control systems. In home appliances, such as video cameras, compact disc players, etc., they are all indispensable and important components. 1. How a simple servo motor works 　　Figure 22 shows the simplest application of a servo motor. Potentiometer RV1 is driven by a servo motor. The motor can be any servo motor with a current not exceeding 700mA and a voltage of 12~24V. In the figure, RV1 and RV2 are connected to form a Wheatstone bridge. The integrated circuit LM378 is a dual-channel 4-watt power amplifier that also forms a motor drive differential amplifier in a bridge connection. Any change in RV2 will destroy the balance of the bridge, causing a differential voltage to be generated between RV1 and RV2, which will be amplified and sent to the motor. The motor will rotate and drag the potentiometer RV1 to a new position, causing the bridge to reach a new balance again. Therefore, RV1 tracks the movement of RV2. 　　Figure 23 is a schematic diagram of a speed sensor servo motor system in block diagram form, which can be used for precise speed control of a turntable. The motor drives the turntable using a traditional belt mechanism. The edge of the turntable reflects the graphic structure of strips at equal intervals. Use photoelectric tachometer for monitoring and detection. The output signal of the photoelectric tachometer is proportional to the rotation speed of the turntable. Compare the phase and frequency of the photoelectric tachometer output signal with the phase and frequency of the standard oscillator, and use its error signal to control the motor drive circuit. Therefore, the rotation speed of the turntable is accurately maintained at the rated speed. The gear shifting at rated speed can be controlled by the operating switch. These control circuits have been made into dedicated integrated circuits by manufacturers. 2. Digital Proportional Servo Motors 　　One of the best types of servo motors is the one with a digital proportional remote control system. In fact, these devices are composed of three parts: integrated circuits, servo motors, and reduction gear boxes and potentiometer mechanisms. Figure 24 is a block diagram of such a system. The driving input of the circuit is driven by a pulse signal with a period of 15ms and a pulse width of 1 to 2ms. The width of the input pulse controls the position of the servo mechanical output. For example: 1ms pulse width, the position is at the far left; 1.5ms is at the middle position, and 2ms is at the far right position. Each input pulse is transmitted simultaneously in three channels. One way triggers a fixed pulse generator with a pulse width of 1.5ms. One input triggers the pulse generator, and the third input is sent to the pulse width comparison circuit. Use the gear box output to RV1 to control the variable width pulse generator. After these three types of pulses are sent to the pulse width comparator at the same time, they determine the direction of the motor drive circuit. The other channel is sent to the pulse width expander to control the speed of the servo motor, so that RV1 can quickly drive the mechanical position output to follow any changes in the input pulse width. The above servo motor types are commonly used in multi-channel remote control systems. Figure 25 shows the waveform diagram of the four-way digital proportional control system. 　　It can be seen from the figure that it is a serial data input, and the control signals of each channel are separated through the decoder. Each frame contains a 4ms sync pulse, followed by four sequential "channel" pulses of variable width (1 to 2ms). The decoder converts the four pulses into a parallel form, which can be used to control the servo motor. 3. Digital Servo Motor Circuits Digital servo motor control units are available as off-the-shelf integrated circuits. For example, ZN409CE or NE544N type servo motor amplifier integrated circuit. Figures 26 and 27 illustrate typical applications of these two integrated circuits. 　　The component values ​​in the figure are suitable for the case where the input pulse width is 1~2ms and the frame pulse width is about 18ms. Figure 28 is a general test circuit suitable for the above servo motor type. Servo power batteries are usually 5V. Input pulses are sent to the servo circuit via a standard servo socket. The width of the frame pulse is 13-28ms; use RV1 to adjust and control. RV2 adjusts and controls the pulse width between 1-2ms. Use RV4 to fine-tune the intermediate value to 1.5ms. Output level is adjusted by RV3. 　　The two integrated circuits are time base circuit CMOS7555 type, and the power supply voltage can be as low as 3V and still work. IC1 is an astable multivibrator that generates frame time pulses, and its output triggers IC2. And IC2 is a monostable circuit that generates output test pulses.