Inside a small servo motor, there is a small micro DC motor, a set of speed - changing gear sets, a feedback adjustable potentiometer, and an electronic control board. Among them, the high - speed rotating DC micro motor provides the original power to drive the speed - changing (decelerating) gear sets, generating a high - torque output. The larger the speed - change ratio of the gear sets, the greater the output torque of the servo motor. That is to say, it can bear a greater weight, but the rotating speed is lower.

A small servo motor is a typical closed - loop feedback system:

The reduction gear set is driven by a micro motor, and its terminal (output end) drives a linear proportional potentiometer for position detection. This potentiometer converts the angular coordinate into a proportional voltage and feeds it back to the control circuit board. The control circuit board compares it with the input control pulse signal, generates a correction pulse, and drives the micro motor to rotate forward or backward, making the output position of the gear set consistent with the desired value, and making the correction pulse tend to be 0, so as to achieve the purpose of accurately positioning the servo motor.

A standard micro servo motor has three control lines, namely: power supply, ground, and control. The power supply line and the ground line are used to provide the energy required by the internal DC micro motor and the control circuit. The voltage usually ranges from 4V to 6V. This power supply should be isolated from the power supply of the processing system as much as possible (because servo motors can generate noise). Even small servo motors can pull down the voltage of the amplifier when under heavy load, so the proportion of the power supply of the entire system must be reasonable.

Input a periodic positive - going pulse signal. The high - level time of this periodic pulse signal usually ranges from 1ms to 2ms, and the low - level time should be between 5ms and 20ms, not very strict. The following table shows the relationship between the positive pulse width of a typical 20 - ms periodic pulse and the position of the output arm of the micro servo motor:

There are three power leads. The white wire among the three wires is the control line, which is connected to the control chip. The middle one is the SERVO working power supply line (red), and the normal working power supply is 5V. The third one is the ground wire.

The instantaneous movement speed of a servo motor is determined by the cooperation of its internal DC micro motor and the reduction gear set. Under a constant voltage drive, its value is unique. However, its average movement speed can be changed by the control method of segmented stops. For example, we can divide the 90° rotation into 128 stop points, and control the length of time at each stop point to achieve an average speed change from 0° to 90°. For most servo motors, the unit of speed is determined by "degrees/second".

Unless you are using a digital servo motor, the position of the above - mentioned servo motor output arm is only an approximate value. Ordinary analog small servo motors are not precise positioning devices. Even for small servo motor products of the same brand and model, the differences between them are very large. It is normal for different servo motors to have a deviation of ±10° when driven by the same pulse.

For the above - mentioned reasons, it is not recommended to use pulses less than 1ms and greater than 2ms as drive signals. In fact, the initial design table of servo motors is only in the range of ±45°. Moreover, when beyond this range, the linear relationship between the pulse width and the rotation angle will also deteriorate.

It should be noted especially that never load a pulse signal that makes the output position of the servo motor exceed ±90°, otherwise, it will damage the output limit mechanism or mechanical components such as the gear set of the servo motor.