Servo systems primarily rely on pulses for positioning. Essentially, a servo motor receives one pulse and rotates by the angle corresponding to that pulse, thus achieving displacement. Because the servo motor itself has the function of emitting pulses, it emits a corresponding number of pulses for each rotation angle. This creates a feedback loop with the pulses received by the servo motor, or a closed loop. In this way, the system knows how many pulses were sent to the servo motor and how many were received, enabling precise control of the motor's rotation and achieving accurate positioning down to 0.001mm.
So what exactly is a servo motor? "Servo motor" is a colloquial term coined by model airplane and boat enthusiasts, as this type of motor is commonly used for steering wheel control. Essentially, a servo motor is a low-end servo motor system, and it's the most common type of servo motor system, hence the English name "Servo," short for Servomotor. It compares a PWM signal with the voltage of a sliding rheostat, achieving fixed-gain position control through hardware circuitry. In other words, it includes a motor, sensor, and controller, forming a complete servo motor (system). It's inexpensive and compact, but has low precision and poor position stability, suitable for many low-end applications.
Servo type
Marine steering gears are currently mostly electro-hydraulic, meaning the hydraulic system is remotely controlled by an electric device. There are two types: one is the reciprocating piston steering gear, which works by switching between high and low pressure oil to generate linear motion, which is then converted into rotary motion by the rudder handle. The other is the vane steering gear, which works by directly applying high and low pressure oil to the rotor; it is smaller and more efficient, but more expensive.
Steering mechanism
A servo motor mainly consists of a housing, circuit board, drive motor, reducer, and position detection element. Its working principle is as follows: a receiver sends a signal to the servo motor, which is then driven by an IC on the circuit board to start the coreless motor rotating. Power is transmitted to the swing arm through a reduction gear, while a position detector sends back a signal to determine if the desired position has been reached. The position detector is essentially a variable resistor; its resistance changes as the servo motor rotates, and the angle of rotation is determined by detecting the resistance value. A typical servo motor winds thin copper wire around a three-pole rotor. When current flows through the coil, a magnetic field is generated, which repels the magnet surrounding the rotor, thus generating a rotational force. According to the principles of physics, the moment of inertia of an object is proportional to its mass; therefore, the greater the mass of an object, the greater the force required to rotate it. To achieve high speed and low power consumption, servo motors wind thin copper wire into an extremely thin hollow cylinder, forming a very lightweight, stepless hollow rotor, and place the magnet inside the cylinder—this is the hollow cup motor.
To suit different working environments, servos are available with waterproof and dustproof designs. Furthermore, to meet varying load requirements, servo gears are available in both plastic and metal. Servos with metal gears are generally high-torque and high-speed types, with the advantage of not having their gears break under excessive load. Higher-end servos utilize ball bearings, resulting in smoother and more precise rotation. Ball bearings are available with one or two bearings; two bearings are naturally preferred. Newly released FET servos primarily utilize FET (Field Effect Transistor) transistors. FETs have the advantage of low internal resistance, thus resulting in less current loss compared to conventional transistors.
Servo motor working principle
1. A servo mechanism is an automatic control system that enables the output controlled variables, such as position, orientation, and state, of an object to follow any changes in the input target (or given value). Servos primarily rely on pulses for positioning. Essentially, a servo motor receives one pulse and rotates by the angle corresponding to that pulse, thus achieving displacement. Because the servo motor itself has the function of generating pulses, it generates a corresponding number of pulses for each rotation angle. This forms a feedback loop, or closed loop, between the pulses sent to and received by the servo motor. In this way, the system knows how many pulses were sent to and received by the servo motor, allowing for very precise control of the motor's rotation and achieving precise positioning down to 0.001mm. DC servo motors are divided into brushed and brushless motors. Brushed motors are low-cost, simple in structure, have high starting torque, wide speed range, and are easy to control. However, they require maintenance, which is inconvenient (replacing carbon brushes), generates electromagnetic interference, and has environmental requirements. Therefore, they are suitable for cost-sensitive general industrial and civilian applications.
Brushless motors are small in size, lightweight, powerful, fast-responding, high-speed, low-inertia, smooth-rotating, and stable in torque. While their control is complex, they are easily made intelligent. Their electronic commutation is flexible, allowing for either square wave or sine wave commutation. The motors are maintenance-free, highly efficient, operate at low temperatures, have minimal electromagnetic radiation, and a long lifespan, making them suitable for various environments.
2. AC servo motors are also brushless motors, and they are divided into synchronous and asynchronous motors. Currently, synchronous motors are generally used in motion control because they have a wide power range and can achieve very high power. They have high inertia, low maximum rotational speed, and their speed decreases rapidly as power increases. Therefore, they are suitable for applications requiring low-speed, stable operation.
3. The rotor inside the servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver forms an electromagnetic field, and the rotor rotates under the influence of this magnetic field. At the same time, the encoder built into the motor feeds back signals to the driver. The driver compares the feedback value with the target value and adjusts the rotor's rotation angle accordingly. The accuracy of the servo motor depends on the accuracy (line count) of the encoder.
The functional differences between AC servo motors and brushless DC servo motors: AC servo motors are generally better because they use sinusoidal wave control, resulting in less torque ripple. DC servo motors use trapezoidal wave control. However, DC servo motors are simpler and cheaper.
Servo motor function
The main function of a servo motor is to control the rotation speed evenly and stably according to the voltage change. The servo motor mainly relies on pulses for positioning. When it receives a pulse current, it will rotate by the corresponding angle of the pulse, thus achieving uniqueness. Because the servo motor itself also has the function of emitting pulse current, it will emit a corresponding number of pulses every time it rotates by an angle, forming a response with the pulses received by the servo motor, or a closed loop. In this way, the system will know how many pulses were sent to the servo motor and how many pulses were received, so it can accurately control the rotation of the motor. The accuracy of positioning can reach 0.001mm.
