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The Principle of Servo Motor
- May 07, 2018 -

Servo motor theory: the servo is a kind of servo motor. It can give the angle information of the rotation angle through the PWM wave signal, rotate to the specified angle, and can keep at the specified angle and bear certain external torque.

The control signal enters the signal modulation chip from the receiver's channel to obtain the DC bias voltage. It has a reference circuit internally, generates a reference signal with a period of 20ms and a width of 1.5ms, and compares the obtained DC bias voltage (dc servo motor) with the voltage of the potentiometer to obtain a voltage difference output. Finally, the positive and negative voltage difference is output to the motor driver chip to determine the positive and negative motor rotation. When the motor rotates at a certain speed, the potentiometer is rotated through the cascade reduction gear so that the voltage difference is 0 and the motor stops rotating.

The servo motor controller generally needs a time base pulse of about 20ms. The high level part of the pulse is generally an angle control pulse part within the range of 0.5ms-2.5ms, and the total interval is 2ms. Take 180 degree angle servo as an example, then the corresponding control relationship is this:

0.5ms--------------0 degrees;

1.0ms------------45 degrees;

1.5ms------------90 degrees;

2.0ms-----------135 degrees;

2.5ms-----------180 degrees;

 

Follower Characteristics

Assume now that the servo is stable at point A. At this time, the CPU sends out a PWM signal and the servo rotates from point A to point B at full speed. In this process, it takes a while for the servo to move to point B.

Hold time Tw

When Tw ≥ T, the servo can reach the target and there is time remaining;

When Tw ≤ T, the servo cannot reach the target;

Theoretically: When Tw = T, the system is the most consistent, and the servo moves fastest.

In the actual process, w is not the same, and the limit ΔT during continuous motion is difficult to calculate.

When the PWM signal changes in order of minimum change (1DIV = 8us), the servo has the highest resolution, but the speed will slow down.