Servo systems must possess fundamental properties such as good controllability, high stability, and strong adaptability. The performance indicators of an AC servo system can be measured from aspects such as speed range, positioning accuracy, speed stability, torque ripple, dynamic response, and operational stability.
Speed range refers to the ratio of the motor's highest speed to its lowest stable speed. Low-end servo systems have a speed range of 1:1000 or higher, typical systems range from 1:5000 to 1:10000, and high-performance systems can reach 1:100000 or higher. Positioning accuracy depends on the encoder's pulse count and the driver's subdivision factor for the encoder. For example, a 2048-line encoder, with a subdivision factor of 2048, can theoretically achieve a positioning accuracy of 360°/(2048×2048). In the past, when measurement technology was less advanced, users took additional measures to improve system accuracy. For example, they connected the measuring axis of the measuring element (such as a synchro) to the rotating shaft via a reducer, amplifying the rotation angle of the shaft to improve relative measurement accuracy. Servo systems using this approach are called fine/coarse measurement systems or dual-channel systems.
The angle measurement circuit that meshes with the shaft through the reducer is called the fine reading channel, while the angle measurement circuit that takes the value directly from the shaft is called the coarse reading channel. Speed stability, especially at low speeds (e.g., when the speed is set to 1 r/min), is generally within ±0.1 r/min, with high-performance drives achieving within ±0.01 r/min. Typically, some drives also use their rated speed as a percentage of their speed accuracy; for example, the ideal speed accuracy of Siemens servo drives is usually 0.001% of their rated speed.
Torque ripple is also a performance indicator for servo systems, determined by the driver, motor, and load. A high-performance servo system should have torque ripple within ±3%. Typically, dynamic response is measured by the system's highest response frequency and rise time. Rise time refers to the time from a sudden change in the speed setpoint to the actual speed stabilizing within ±2% of the setpoint, including dead time. The highest response frequency is the output speed waveform of a given sinusoidal speed command with a phase lag of no more than 90° or an amplitude of no less than 71% (-3dB). As shown in the figure, X(t) and Y(t) signals are the system's input and output, respectively.
Signal curves in the time domain
Furthermore, from a frequency domain perspective, the system's bandwidth reflects the tracking speed of the servo system. A larger bandwidth indicates better speed. The system bandwidth can be obtained from the Bode plot of the speed closed-loop. The bandwidth of a servo system is mainly limited by the inertia of the controlled object and the actuator; the greater the inertia, the narrower the bandwidth. Generally, the bandwidth of a servo system decreases as the power range increases. Currently, most products have a speed bandwidth of 200–500 Hz, with a few products reaching higher levels, such as the Mitsubishi MR-J3 series servo motor, which claims a response frequency as high as 900 Hz.
Bode plot of velocity closed loop
Operational stability mainly refers to the system's ability to maintain stable operation and ensure certain performance indicators under conditions such as voltage fluctuations, load fluctuations, changes in motor parameters, changes in the output characteristics of the upper-level controller, electromagnetic interference, and other special operating conditions. In this respect, Chinese products (including some Taiwanese products) lag significantly behind the world's advanced levels.
Factors affecting servo system performance
(1) Motor
The motor is a crucial component of a servo system, and its performance directly determines the control characteristics of the entire system. Common servo motors can be categorized into DC speed-regulating motors and AC speed-regulating motors. Compared to DC motors, AC servo motors do not suffer from the drawbacks of commutators and brushes found in DC motors. However, the motor's moment of inertia, rotor impedance, brush structure, and heat dissipation all affect the performance of the servo system.
(2) Encoder
As a feedback element in control, the encoder is a crucial factor affecting system accuracy. Firstly, the number of encoder pulses directly impacts the positioning and speed control accuracy of the system; secondly, the encoder's maximum speed also limits the motor's maximum speed. Currently, encoders used in servo control systems are typically photoelectric encoders, categorized into incremental, absolute, sine/cosine, and rotary transformer types. The encoder's anti-interference capability directly affects system stability. For permanent magnet synchronous motors, accurate rotor position recognition is a prerequisite for control; therefore, the encoder's ability to provide the driver with the correct rotor position is critical to control performance.
(3) Driver
The driver is the core of servo control. Depending on the type of motor, drivers are categorized into different types, such as transistor amplifier drivers, DC drivers, and AC drivers. Currently, AC drivers are more common in the industrial control industry. For example, the Siemens Sinamics S120 driver actually controls the motor using SPWM (Spatial Width Modulation), a space vector control method. Typically, the current loop and speed loop are implemented in the driver, while position control can be performed in the motion controller or the driver. The closed-loop characteristics of the current and speed loops are a standard for measuring the performance of a control system. Factors such as the sampling period of the current and speed loops, the bandwidth of the speed and current loops, and various filters and delays in the control loop all affect the system's accuracy and dynamic response capability.
(4) Motion controller
Motion control is a control method that adds position control, gear synchronization, cam, interpolation, and other motion control functions to the speed loop of the driver. There are three ways for the motion controller to control the driver: digital communication, analog signal, and pulse signal.
① Digital Communication Methods: These methods offer high resolution, fast and reliable signal transmission, and enable high-performance, flexible control. They require a communication protocol. For example, Siemens' Simotion uses the Profidrive protocol based on Profibus or Profinet for data exchange with its drives. Other European companies use the CAN bus, while Yaskawa, a Japanese company, offers drive products based on the MECHATROLINK bus. These bus methods enable data transmission and control between the drive and motion controller, making them particularly suitable for applications requiring coordinated synchronization and interpolation control between axes. Besides providing essential torque, position, and speed control functions, they can also achieve highly precise phase coordination control.
② Analog signal method: Low resolution, poor signal reliability and anti-interference performance, but good compatibility. For example, the control between Siemens' Simotion motion controller and third-party drivers can be achieved through analog signals.
③ Pulse method: High reliability, but poor speed and flexibility. The driven object is a stepper motor.
During system selection and configuration, the control method of the motion controller over the drive is a crucial factor that designers need to consider. Communication is the most stable and fastest control method, and the transmission speed must also be taken into account. The communication cycle is constrained by the communication rate and the amount of data. Taking Siemens' Simotion motion controller as an example, with a transmission rate of 1.5 Mbit/s, when controlling more than six axes, the system's communication cycle is 3 ms by default. Furthermore, due to the limitation of the communication cycle, the interpolation cycle and position loop sampling cycle of the motion controller are usually integer multiples of the communication cycle. For the motion controller, its interpolation cycle and position loop sampling cycle are key metrics for evaluating system performance.
(5) Mechanical transmission
Motors typically connect to the load via mechanical transmission structures (such as couplings, gearboxes, lead screws, conveyor belts, and mechanical cams). Therefore, the rigidity of the coupling, gear backlash, and the tension of the conveyor belt all affect the system's control accuracy. For example, for linear motion actuators, motors are usually connected via synchronous pulleys or lead screws. The meshing clearance of the synchronous pulley or the clearance between the balls and raceways of the lead screw nut can affect the accuracy of linear motion displacement. For mechanical cams, speed or acceleration boundary conditions must be guaranteed to prevent the system from developing mechanical resonance.
(6) Load
As the ultimate object of control, the load's impact on system performance cannot be ignored. The magnitude of the load's moment of inertia affects the system's dynamic characteristics. For example, a large moment of inertia requires a large output torque during acceleration and stopping, demanding high driving capability from the driver. Furthermore, the ratio of the load's moment of inertia to the motor's moment of inertia also affects system performance. A smaller ratio makes control easier but reduces motor efficiency; a larger ratio introduces high-frequency resonance points, increasing control complexity. Regarding the allocation of the load-motor moment of inertia ratio, refer to Bosch Rexroth's "fitting standard": fast positioning < 2:1, corrective positioning < 5:1, high-speed conversion < 10:1.
Moment of inertia
concept
Moment of inertia is a physical quantity that characterizes the magnitude of a rigid body's rotational inertia and measures its resistance to rotational motion. It is analogous to the mass of a rigid body in translational motion, and it is related to the body's mass and its distribution relative to the axis of rotation.
Physical meaning
Understanding moment of inertia directly is rather abstract, but we can use mass, which is the most common and intuitive thing in our lives, as an analogy.
If we apply the same force to two objects of different masses, the heavier object will change velocity more slowly. Therefore, the physical meaning of mass is that it reflects the inertia of an object in translational motion: the greater the mass, the greater the inertia, that is, the more difficult it is to change its state of motion in translational motion (starting from rest, a larger mass object is more difficult to accelerate than a smaller mass object).
Similarly, if we apply the same torque (a force that causes an object to translate is called a force, and a torque that causes an object to rotate) to make it rotate, different objects will have different rates of change in angular velocity (similar to acceleration in translation). This factor affecting the rate of change of angular velocity is the moment of inertia. In other words, the moment of inertia reflects the inertia of an object under rotation: objects with a large moment of inertia are more difficult to change in angular velocity.
formula
Where: is the moment of inertia of the rigid body, is the mass of each infinitesimal element, and is the distance from each infinitesimal element to the axis of rotation.
1. For objects of the same mass, if the mass distribution (the distance of each infinitesimal element from the axis of rotation) is different, their moment of inertia will also be different.
2. For objects with uniform mass distribution, the moment of inertia varies depending on the mass.
Relationships of physical quantities in translation and rotation
(7) Installation
Once the aforementioned objects are confirmed, the installation of field devices will bring new challenges to the entire system. These include ensuring proper system grounding, avoiding EMC interference, and using suitable shielded cables – all crucial aspects of system design. For instance, if the encoder cable shield is not properly grounded, the feedback signal will be mixed with noise, which significantly impacts control accuracy and may even cause the device to shut down.
(8) System completeness
In the design of the entire motion control system, it is recommended that users use products from the same manufacturer as much as possible, including motion controllers, drivers, servo motors, etc., to ensure system integrity. This avoids problems such as wiring, configuration, and communication issues. Purchasing individual components leads to several problems, primarily the complexity of the connection sequence. Motors, drive terminals, and feedback devices (including encoders, resolvers, Hall sensors, etc.) can have multiple different connection orders. Using motors and drivers from the same supplier also has the advantage of better software installation and debugging, ensuring compatibility. Furthermore, each motor has different parameters, and the drivers that match them have their default parameters. Drivers also have proprietary methods for recognizing motor parameters. For third-party motors, the driver's recognition program may not be accurate enough; in a sophisticated motion control system, a difference in a single parameter can affect the motor's drive performance, thus impacting control accuracy. Therefore, it is recommended that users use compatible products from the same manufacturer whenever possible.
