Three-loop control of servo drive

Three-loop control of servo drive Current loop Speed loop Position loop

Motion servo is generally a three-loop control system, from the inside to the outside is the current loop, speed loop, position loop.

1. Current loop: The input of the current loop is the output after the speed loop PID adjustment. Let’s call it the “current loop setting”, and then the current loop setting is compared with the “current loop feedback” value. The difference value is PID adjusted in the current loop and output to the motor. The "current loop output" is the phase current of each phase of the motor. The "current loop feedback" is not the feedback of the encoder but is installed in each phase inside the drive The Hall element (magnetic field induction becomes current and voltage signal) is fed back to the current loop.

2. Speed loop: The input of the speed loop is the output of the position loop PID adjustment and the feedforward value of the position setting. We call it "speed setting". This "speed setting" is compared with the "speed loop feedback" value. After the difference value is PID adjusted in the speed loop (mainly proportional gain and integral processing), the output is the "current loop setting" mentioned above. The feedback of the speed loop comes from the feedback value of the encoder through the "speed calculator".

3. Position loop: The input of the position loop is an external pulse (usually, the servo that writes data directly to the drive address is an exception), and the external pulse is processed by smooth filtering and electronic gear calculation as the "position loop setting". The set value and the pulse signal feedback from the encoder are calculated by the deviation counter after the PID adjustment of the position loop (proportional gain adjustment, no integral and differential link), and the combined value of the output and the feedforward signal of the position setting is constituted The above mentioned speed loop setting. The feedback of the position loop also comes from the encoder.

　　 The encoder is installed at the tail of the servo motor. It has no connection with the current loop. It samples the rotation of the motor instead of the motor current, and has no connection with the input, output, and feedback of the current loop. The current loop is formed inside the driver. Even if there is no motor, the current loop can form a feedback operation by installing an analog load (such as a light bulb) on each phase.

The influence of each PID adjustment on the system:

1. A single P (proportion) is to calculate the difference in proportion. Its distinguishing feature is the difference adjustment. The meaning of the difference is that after the adjustment process is over, the adjusted amount cannot be exactly equal to the set value. They There must be a residual between them, and you can calculate the specific value of the residual through a proportional relationship.

2. A single I (integral) is to make the change speed of the output signal of the regulator proportional to the difference signal. If the difference is large, the change speed of the integral link is large. The proportional constant of this link is the reciprocal of the proportional constant. We are in the servo system It is usually called the integral time constant here. The smaller the integral time constant means the faster the system changes. PI (Proportional Integral) is the combination of the advantages of P and I. Use P to adjust quickly to offset the impact of interference, and at the same time to use I to adjust to eliminate Residual.

4. A single D (differential) is adjusted according to the direction and size of the difference. The output of the regulator is proportional to the derivative of the difference with respect to time. The differential link can only play an auxiliary adjustment role, and it can be adjusted with other Combined into PD and PID regulation.

5. The integrated function of PID can make the system more accurately and stably reach the control expectations.