A servo motor, also called an actuator motor or control motor, is an actuator in automatic control systems. Its function is to convert signals (control voltage or phase) into mechanical displacement, that is, to transform received electrical signals into a certain speed or angular displacement of the motor. Its capacity is generally between 0.1-100W, with 30W and below being commonly used. Servo motors are available in both DC and AC versions.
The rotor inside a servo motor is a permanent magnet. The three-phase electricity (U/V/W) controlled by the driver creates an electromagnetic field, causing the rotor to rotate under the influence of this magnetic field. Simultaneously, the motor's built-in encoder feeds back signals to the driver, which compares the feedback value with the target value and adjusts the rotor's rotation angle. The accuracy of a servo motor depends on the accuracy (line count) of the encoder. A servo motor is the engine that controls the operation of mechanical components in a servo system. It is a type of auxiliary motor with indirect speed control. Also known as an actuator motor, it is used as an actuating element in automatic control systems, converting received electrical signals into angular displacement or angular velocity output on the motor shaft. Servo motors are broadly classified into DC and AC servo motors.
Servo motor debugging method
1. Initialize parameters
Initialize the parameters before wiring.
On the control card: Select the control mode; clear the PID parameters; ensure the enable signal is off by default when the control card is powered on; save this state to ensure that the control card is in this state when it is powered on again.
On the servo motor : Set the control mode; enable external control; set the gear ratio of the encoder signal output; set the ratio between the control signal and the motor speed. Generally, it is recommended that the maximum design speed of the servo motor correspond to a control voltage of 9V. For example, Sanyo sets the speed to 1V, with a factory value of 500. If you only intend for the motor to operate below 1000 RPM, then set this parameter to 111.
2. Wiring
Power off the control card and connect the signal lines between the control card and the servo. The following lines are mandatory: the analog output line of the control card, the enable signal line, and the encoder signal line output by the servo. After verifying that the wiring is correct, power on the motor and the control card (and PC). The motor should not move at this point and can be easily rotated with external force. If not, check the enable signal settings and wiring. Rotate the motor with external force to check if the control card can correctly detect changes in motor position; otherwise, check the encoder signal wiring and settings.
3. Try the direction
For a closed-loop control system, if the direction of the feedback signal is incorrect, the consequences will be disastrous. Enable the servo via the control card. The servo should then rotate at a low speed; this is the so-called "zero drift." Control cards typically have instructions or parameters to suppress zero drift. Use these instructions or parameters to see if the motor's speed and direction can be controlled. If not, check the analog wiring and control mode parameter settings. Confirm that a positive value results in the motor rotating forward and the encoder count increasing; a negative value results in the motor rotating backward and the encoder count decreasing. Do not use this method if the motor is under load and has limited travel. Do not apply excessive voltage during testing; below 1V is recommended. If the directions are inconsistent, modify the parameters on the control card or motor to make them consistent.
4. Suppress zero drift
In closed-loop control, zero drift can negatively impact control performance, and it's best to suppress it. Carefully adjust the zero drift suppression parameters on the control card or servo motor to bring the motor speed close to zero. Since zero drift itself has a degree of randomness, it's not necessary to require the motor speed to be absolutely zero.
5. Establish closed-loop control
Re-enable the servo enable signal via the control card. Input a small proportional gain on the control card; what constitutes "small" is subjective and can be determined by feel. If unsure, input the minimum value allowed by the control card. Enable the control card and the servo again. At this point, the motor should be able to roughly perform movements according to motion commands.
6. Adjust closed-loop parameters
Fine-tuning the control parameters to ensure the motor moves according to the control card's instructions is a necessary task, and this part relies heavily on experience, so we'll skip the details here.
Servo motor selection
1. Traditional selection method
This discussion focuses solely on the motor's power. For linear motion, we use velocity v(t), acceleration a(t), and required external force F(t), all of which can be expressed as functions of time and are independent of other factors. Clearly, the motor's maximum power P<sub>motor</sub> should be greater than the peak power P<sub>peak</sub> required by the workload. However, this alone is insufficient. Physically, power includes both torque and speed, but these are constrained in actual transmission mechanisms.
Let T<sub>peak</sub> represent the maximum or peak value. The maximum speed of the motor determines the upper limit of the reduction ratio of the reducer, n<sub>upper limit</sub> = peak value / maximum / peak value. Similarly, the maximum torque of the motor determines the lower limit of the reduction ratio, n<sub>lower limit</sub> = T<sub>peak</sub> / T<sub>motor</sub> / maximum. If n<sub>lower limit</sub> is greater than n<sub>upper limit</sub>, the selected motor is unsuitable. Conversely, a feasible transmission ratio range between the upper and lower limits can be determined by extensive analogy for each type of motor. Using peak power alone as a principle for selecting a motor is insufficient, and the accurate calculation of the transmission ratio is very cumbersome.
2. New selection method
A new selection principle separates motor characteristics from load characteristics and represents them graphically. This representation makes it easier to check the feasibility of the drive unit and compare different systems. In addition, it provides a possible range of transmission ratios.
The advantages of this method are: it is applicable to various load conditions; it separates the characteristics of the load and the motor; all parameters related to power can be represented graphically and are applicable to various motors. Therefore, it is no longer necessary to use numerous analogies to check whether a motor can drive a specific load, as the transmission ratio between the motor and the load will change the power load parameters provided by the motor.
It is more convenient, and in addition, it provides a possible range of transmission ratios.
The advantages of this method are: it is applicable to various load conditions; it separates the characteristics of the load and the motor; all parameters related to power can be represented graphically and are applicable to various motors. Therefore, it is no longer necessary to use numerous analogies to check whether a motor can drive a specific load, as the transmission ratio between the motor and the load will change the power load parameters provided by the motor.
The following section will introduce some servo motor systems, including permanent magnet synchronous motors and induction asynchronous motors.
A servo system is not just a single motor. It is a closed-loop motion system that includes a controller, driver, electrical components, and feedback devices, and typically also includes an optical or magnetic encoder.
Servo systems can synchronize machinery using permanent magnet (PM) technology, equipped with brushed or brushless PM motors, or establish asynchronous mechanical systems on an AC induction motor.
Permanent magnet synchronous motors (PMSMs) offer high peak torque and sustained torque, making them suitable for drive servo systems in high-acceleration and rapid-deceleration applications requiring precise displacement. Torque is directly proportional to the input current. Motor shaft speed is related to the input voltage; the higher the input voltage, the higher the motor speed. The torque-speed ratio curve is linear.
The permanent magnet structure is related to the air gap of the motor. For example, the structure of a brushless PM motor includes two interacting magnetic structures: a moving rotor (connected to a permanent magnet) and stator coils that generate an electromagnetic reaction, resulting in the motor's torque and speed.
The three-phase stator field generates energy sequentially, and the PM rotor moves synchronously with the rotor field. A specific electronic compensation system is used to check the rotor position and energize the stator coils. The brushless PM motor is the preferred choice for precision displacement systems among all other motors, except for automotive applications and very large motor systems. The brushless PM motor is the only servo motor system that can be used for closed-loop torque, speed, or displacement systems.
Different rotors
AC induction motors possess the same physical characteristics as PM brushless motors.
The stator is the same as the rotor, but its rotor structure is completely different. The squirrel-cage induction motor contains a series of induction aluminum or copper bars placed in the rotor structure and connected to the end coils.
These short rotor bars are electromagnetically coupled to the rotating magnetic field of the stator, generating a new rotor field, which reacts with the stator field to form rotor motion.
There is a difference between the synchronous stator and the slower stator field and the actual speed. This speed difference is called slip. The input frequency determines the motor speed.
For example, a 60Hz, two-pole AC induction motor has a speed of nearly 3,600 rpm without load, while a four-pole AC motor operates at speeds below 1,800 rpm, depending on the slip value. As the motor begins to generate torque, the slip increases, and the speed decreases.
AC induction motors output more torque initially, but as the speed decreases until the load reaches the fault point, the motor speed will drop sharply to zero. An inherent characteristic of AC motors is their low initial torque, necessitating the removal of the load at startup.
With the advent of frequency converters and electronic drives in the late 1980s, the torque-speed performance curve unique to motors underwent significant changes. The performance of frequency converters lies in their ability to simultaneously change voltage and frequency, using adjustable or variable speed drives to reconstruct the torque-speed curve. AC induction motors are a key component of this speed system.
How to use
The continuous improvement in drive technology has brought brushless PM motors and AC induction motors into the competitive drive market, but brushless PM motors still dominate the control field. AC induction motors are not suitable for use at low and high speeds.
A brushless PM motor is typically used in servo displacement systems, usually in systems with a power output of 50kW (67hp) or higher. AC induction motors are commonly used in constant speed or variable speed systems. Hybrid systems are less common. Other motors can also partially achieve this, but few solutions outperform AC induction motors or brushless PM motors in terms of performance.
Brushless PM motors have impacted the speed control market for 1kW (1.37hp) DC brushed motors and applications with lower power outputs. Meanwhile, AC induction motors dominate most applications exceeding 1MW.
