Servo motor system analysis

The servo motor is a conventional motor. It is the actuator of the automatic device. The biggest feature of servo motors is controllable. When there is a control signal, the servo motor rotates and the speed is proportional to the magnitude of the control voltage. When the control voltage is removed, the servo motor stops rotating immediately. Servo motors are used in a wide range of applications and are used in almost all automatic control systems. In home appliances, such as video cameras and laser players, they are an indispensable part.

1. The working principle of simple servo motor

The potentiometer RV1 is driven by a servo motor. The motor can be used with any servo motor with a current of no more than 700mA and a voltage of 12~24V. In the figure, RV1 and RV2 are connected to the Wheatstone bridge. The integrated circuit LM378 is a dual 4-watt power amplifier that also bridges the motor-driven differential amplifier.

Any change in RV2 will destroy the balance of the bridge, generate a differential voltage between RV1 - RV2, and amplify it and send it to the motor. The motor will rotate and drag the potentiometer RV1 to the new position to bring the bridge back to a new balance. So, RV1 is tracking the movement of RV2.

The motor drives the turntable with a conventional belt mechanism. The edge of the turntable reflects the graphic structure at equal intervals. Monitoring and testing with a photoelectric tachometer. The output signal of the photoelectric tachometer is proportional to the rotational speed of the turntable. The phase and frequency of the output signal of the photoelectric tachometer are compared with the phase and frequency of the standard oscillator, and the motor drive circuit is controlled by its error signal. Therefore, the rotational speed of the turntable is accurately maintained at the rated rotational speed. The shift of the rated speed can be controlled by the operation switch. These control circuits have been made into dedicated integrated circuits by manufacturers.

2. Digital proportional servo motor

One of the best types of servo motors is a digital proportional remote control system. In fact, these devices are composed of three parts: an integrated circuit, a servo motor, and a reduction gear box potentiometer mechanism. Figure 24 is a block diagram of such a system. The drive input of the circuit is driven by a pulse signal with a period of 15 ms and a pulse width of 1 to 2 ms. Enter the width of the pulse to control the position of the servo machine output. For example: 1ms pulse width, position at the far left; 1.5ms in the middle position, 2ms in the rightmost position.

Each input pulse is transmitted simultaneously in three ways. A fixed pulse generator that triggers a 1.5ms pulse width. One input trigger pulse generator, and the third path is sent to the pulse width comparison circuit. The gear box is output to RV1 to control the variable width pulse generator. After these three pulses are simultaneously sent to the pulse width comparator, the direction of the motor drive circuit is determined all the way. The other way is to the pulse width extender to control the speed of the servo motor so that RV1 quickly drives the mechanical position output to follow any changes in the input pulse width.

The above servo motor type is commonly used in multi-channel remote control systems.

Each frame contains a 4ms sync pulse, followed by four "variable" (1~2ms) sequential "road" pulses. The decoder converts the four pulses into a parallel form that can be used to control the servo motor.

3. Digital servo motor circuit

The digital servo motor control unit can be purchased as an off-the-shelf integrated circuit. For example, ZN409CE or NE544N servo motor amplifier integrated circuits.

The servo power battery is usually 5V. The input pulse is sent to the servo circuit via a standard servo jack. The width of the frame pulse is 13-28 ms; the control is adjusted with RV1. The RV2 adjustment control pulse width is between 1-2ms. Use RV4 to fine tune the middle value to 1.5ms. The output level is adjusted by RV3.

The two integrated circuits are time-based circuit CMOS7555 type, and the power supply voltage can be operated as low as 3V. IC1 is an unstable multivibrator that produces a frame time pulse whose output triggers IC2. IC2 is a one-shot circuit that produces an output test pulse.

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