How does pcl control a servo motor to return to zero

Servo motors are electrical-mechanical converters that can rotate continuously. As a hydraulic valve controller servo motor, belongs to a very small power micro motor, permanent magnet DC servo motor and parallel excitation DC servo motor is most commonly used.　　DC servo motor output speed is proportional to the input voltage, and can realize forward and reverse speed control. It has the advantages of large starting torque, wide speed range, good linearity of mechanical characteristics and regulation characteristics, and convenient control, etc. However, the wear and tear of commutation brushes and easy to generate sparks will affect its service life. In recent years, the brushless DC servo motor avoids brush friction and commutation interference, so it has high sensitivity, small dead zone, low noise, long service life, and small interference to the surrounding electronic equipment.　　The transfer function of output speed/input voltage of DC servo motor can be approximated as a first-order delayed link, and its electromechanical time constant is generally about ten milliseconds to tens of milliseconds. The time constants of certain low-inertia DC servomotors (e.g., hollow cup rotor type, printed winding type, and slotless type) are only a few milliseconds to twenty milliseconds.　　The rated speed of DC servo motors with small power specifications is above 3000 r/min and even greater than 10000 r/min. Therefore, as a controller for hydraulic valves, it needs to be equipped with a high-speed ratio reducer. The DC torque servo motor (i.e., low-speed DC servo motor) can be in tens of rpm at low speeds, and even in the long-term blocking conditions, so it can be directly driven by the control without deceleration servo motor, also known as the implementation of the motor, in the automated control system, used as an actuating element, the received electrical signals are converted into an angular displacement or angular velocity output on the motor shaft. Divided into two categories of DC and AC servo motors, its main feature is that when the signal voltage is zero no self-rotating phenomenon, the speed of the torque increases with a uniform decrease in speed, servo and stepping have their own advantages and disadvantages, are irreplaceable with each other: 2, servo in the micro-motion or positioning to maintain a dynamic balance is indeed, it is the system through the detected positional signals of the negative feedback PID regulation, which is lower than a Encoder resolution when the micro-motion does not respond to the positioning hold is also a dynamic response to external loads and change the torque at any time to achieve dynamic static balance, keep the accuracy is worse than the stepper.　　3, due to the stepper motor drive usually with subdivision, and stop usually stop at the subdivision point is not the magnetic pole point, then after the power outage again when the drive will not be in accordance with the current distribution of the phases of the stop, then there is a stepper motor re-powered usually appear a strong small vibration, that is, the rotor quickly with the initial stator magnetic field corresponds to the servo does not have the phenomenon.　　4, on the response time, stepping in its start frequency and acceleration allows the conditions can indeed be faster than the servo more frequent forward and reverse start and stop, but it has a strict start frequency and acceleration requirements, if it is a high-frequency start, for example: a single 0 to 1,000 rpm (ordinary stepping can only be a few hundred rpm, for example, according to the ability to achieve high speed 3-phase hybrid stepping count), servo from the receipt of a pulse to the end of the setup time will be faster than the acceleration time of the stepper.　　5, on the highest speed and the stepper has lost steps on the problem: servo advantage is obvious.　　6, servo as a result of PID regulation, there will be the problem of setting time, the time will vary with the speed of the high and low and load changes, the setting time is poorly controllable. Setting time and acceleration and deceleration time is different, setting time by the system PID gain, integral time constant, set the speed value and other factors. Stepping but there is no concept of setting time, acceleration and deceleration time is simple and controllable.　　7, torque control, stepper motor torque will change with the speed of the obvious changes in the high-speed region will produce a strong decline with the speed of change; servo in the rated speed of the maximum torque for the constant torque output, and servo can be torque control, which is stepping can not be done down.　　8, the price, the stepper advantage is obvious.