The development of Digital Signal Processor (DSP) technology has made servo drives increasingly intelligent. The development of fieldbus has also made control designs based on distributed control structures more flexible and modular. It saves winding costs and simplifies machine construction. The intelligent CD1P servo drive is an independent signal axis controller, as shown in Figure 1, including: a control sequence program, a trajectory calculator, position, speed, and current servo loops, and a transformer in the same device. [align=center]Figure 1 Intelligent Servo Drive in Distributed Motion Control System[/align] Information transmission between the host control and each servo drive is based on the PROFIBUS DP standard. A highly reusable modular approach facilitates the creation of complex multi-axis control applications. Using bus devices, the control system can easily reconfigure parameters for new products without any hardware changes. This process can be edited by adding or removing control elements (servo drives, I/O modules, etc.) without editing or modifying the main control system. This flexibility helps survive in the long-term competition of automated production processes: it allows for easy editing and improvement of machines based on feedback from product downsizing and technology lifecycles. Meanwhile, the digital current, speed, and position servo loops are all included in the servo drive, further improving servo performance. Servo Controller Design Cascade control structures are well-suited for high-performance servo drives. The internal current control loop controls the motor torque and also affects the dynamic performance of the external speed and position control loops. Current Control Design The brushless servo drive consists of a permanent magnet synchronous motor (PMSM) with position sensors mounted on the shaft. A transformer converts a constant DC voltage to a three-phase AC voltage via a pulse width modulator (PWM). The constant DC voltage source is rectified from the AC mains power supply. To control the PMSM torque, the stator current amplitude and phase need to be controlled: the current amplitude is proportional to the set torque, and the current phase is related to the rotor position. Stator current control can be performed through either the stator or rotor coordinate system. When current control is executed in the stator coordinate system, the conventional control structure includes two to three independent phase current regulation loops based on the phase of the set current waveform. Current control in the rotor reference frame is based on the PMSM mathematical model, as shown in Figure 2. [align=center] Figure 2 PMSM mathematical model[/align] The direct and orthogonal decomposition values ​​of the current (id and iq) can be calculated through the current phase Park transformation. The id and iq adjustment loops are assigned to the direct and orthogonal voltage values ​​(vd and vq). The three-phase PWM commands are calculated using the inverse Park transformation with the values ​​of vd and vq. In current control in the rotor coordinate system, good torque performance can be obtained across the entire speed range, as shown in Figure 3. [align=center] (a) Influence curve of the current control method (b) Influence curve of the model control method Figure 3 Torque/speed curve of the servo drive[/align] Because the three-phase current phase loop is based on a sine wave in the stator coordinate system, the development of stator control performance in speed mode depends on the bandwidth of the loop. However, in the rotor coordinate system, the id and iq obtained by the Park transformation are based on DC variables. For this reason, current control in the rotor coordinate system is more suitable for the servo drive of CD1P. [align=center] (a) Symmetrical Triangular Adjustment Method for PWM (b) Space Vector Model Algorithm Figure 4 PWM Switching Frequency Calculator[/align] The commands for the three PWM pulse width modulators can be calculated using either the symmetric triangular adjustment method or space vector adjustment (SVM). In the symmetric triangular adjustment method, the number of PWM transitions is defined by the intersection of a symmetric high-frequency triangular wave (medium) and a sinusoidal voltage reference waveform, as shown in Figure 4(a). In the SVM model, the PWM transition sequence is obtained by vector decomposition of the eight possible current states in the converter from the Park vector, as shown in Figure 4(b). Compared to the symmetric triangular adjustment method, the SVM model allows the servo motor to operate at higher speeds (approximately 15% higher). This is attributed to the additional 3rd harmonic waveform within the SVM model. [align=center] (a) Symmetrical triangular modulation command (b) Space vector model command Figure 5 Output waveform of PWM at maximum speed[/align] As shown in Figure 5(b), the SVM command contains the third harmonic, which allows the use of the full output voltage range of the voltage converter with sinusoidal phase. As shown in Figure 5(a), in the symmetrical triangular modulation method, the sum of the three-phase commands is 0, so the output voltage range of the converter is reduced by 15%. Position control design In order to obtain a stable and fast response, the adjustment of speed and position servo loops must be optimized through mechanical load parameters. The position servo controller design is based on the RST polynomial controller structure and pole fixed-point tuning method. The RST polynomial controller structure is the most important and the most suitable controller structure for parameter tuning method. It is assumed that the drive can be described by the model transformation equations hmc and hmd, as shown in Figure 6(a). The three polynomials r, s and t included in the servo controller act on the servo loop error signal, servo loop feedback signal and servo loop reference signal, respectively. The conversion equations for closed-loop output/reference and output/disturbance are represented by HSR and HSD, respectively, in Figure 6(b). [align=center] (a) Servo closed-loop structure (b) Closed-loop transfer function Figure 6 Servo closed-loop structure and transfer function[/align] The controller tuning process includes pole and zero position tuning of the HSR and HSD conversion equations, which is to obtain the correspondence between the output/reference and output/disturbance of the position loop. This allows the servo loop adjustment and tracking behavior to be completely separated. The HMC and HMD conversion equations are derived from the mechanical equipment model. The equipment conversion function is obtained by executing the identification program under rated operating conditions. A conventional PID controller can conveniently execute the RST controller structure using the following polynomials: r(s) = r1s + r2s2 and t(s) = s(s) = s0 + s1s + s2s2 (s is the Laplace transform). The position loop tracking and adjustment performance in this scheme is shown in Figure 7. The RST controller structure allows for easy editing of servo loop tracking behavior by simply changing the t polynomial while maintaining the same adjustment performance. [align=center] (a) PID design (b) RST tracking design Figure 7 RST position controller waveform[/align] The flexibility of the RST polynomial controller structure also makes it easy to integrate special filters that meet specific application requirements. When the motor shaft is subjected to a large load inertia, elastic coupling can sense torsional vibration. The torsional vibration frequency is calculated through equipment parameters. Assume: jm = motor inertia value. ji = load inertia value, nr = motor/load coupling rate, ks = motor/load coupling strength; The torsional vibration frequency is obtained through the following formula: fr = (ks(nr²jm + jl)/(nr²jmjl))¹/². The effect of torsional vibration can significantly reduce servo loop current loss. Motion controller execution A distributed motion control system suitable for multi-axis applications is based on intelligent independent servo drives. In this scheme, the axis motion control task is distributed in each servo drive. Each servo drive can be independently programmed to execute the control task of a given axis. Bus communication between the host controller and the servo drive is used for servo drive presets and mechanical process control and monitoring. The servo drive's EEPROM can store more than 128 preset motion control sequences. There are five basic programmable motion sequences: axis homing sequence for axis reset addressing after servo drive startup; absolute positioning sequence for moving the axis to a given absolute address value via axis addressing index; relative positioning sequence for moving the axis to a given position relative to the current address; speed description sequence for describing the shape of an axis moving at a specific speed; and torque setpoint sequence for situations requiring a constant force to be provided to the load, such as gripper applications. Details of sequence execution can be defined by sequence parameters (acceleration and deceleration times, running speed, dwell time, initial state, trigger position, output signal, count, connection, etc.). All sequences can be chained together, and programmable time delays can be introduced into the sequence chain. The sequence counter also allows the same sequence to be executed multiple times or a group of sequences to be executed at once. These functions meet the requirements of integrated motion control of the servo drive, eliminating the need for traditional external motion control boards + servo drive solutions. PROFIBUS DP Communication Servo drives connected to a PROFIBUS serial link can be easily configured into industrial PC or PLC controller environments. PROFIBUS DP is an open, widely used bus protocol standard in industrial production, process control, and automation applications. The PROFIBUS DP version is specifically designed for distributed I/O communication at the automatic control system and field device level. Through PROFIBUS DP, the host PLC controller can communicate at high speed in a cyclic manner among its distributed bus devices (I/O, valves, equipment, etc.). PROFIBUS DP communication between the host PLC and the CD1P servo drive is based on PROFIBUS message exchange. The CD1P servo drive communication uses PPO1 to PPO4 message types. The PLC sends a message by writing a PPO and receives a message by reading a PPO. PPO read and write operations are cyclically converted through the PROFIBUS data exchange capabilities. The PLC can read and modify servo drive parameters in the parameter area. The PLC sends two types of execution data to the servo drive: "Control Statements" include the servo drive's actions: enable, run, push, return, move, etc.; "Input Statements" include the number of execution sequences and the sequence's initial state. The PLC receives two types of execution data from the servo drive: "Status" includes the servo drive's state: ready, stopped, running, waiting, error, etc.; "Feedback" includes the number of current sequences in the process, axis position, and motor speed values. CD1P communication is designed to configure the servo drive into the host PLC controller like a simple I/O device. This allows for high-level motion control directly in the servo drive without consuming the host PLC controller's computing resources. We also note that the bus cycle time value has no impact on servo motion control performance. Therefore, this solution is best suited for multi-axis position control applications. Conclusion Compared to traditional motion control based on CNC motion controllers and analog speed servo drives, the motion control structure based on intelligent servo drives is a highly efficient and flexible solution. The intelligent CD1P servo drive based on DSP technology can perform complete motion control for a given axis. This structure can improve the performance of the servo loop. Through space vector modulation technology, the torque of the servo motor is improved even at high speeds. The in-system self-tuning program optimizes the speed and position servo loops. Under the PROFIBUS DP communication protocol, the intelligent servo drive can be integrated into the host PLC controller like a simple I/O device. This motion control solution is particularly suitable for applications requiring multi-axis (over 100) control.