With the development of digital signal processing (DSP) technology, many complex control tasks can be handled within the servo drive, and the servo drive is becoming increasingly intelligent [1]. At the same time, the increasingly mature fieldbus communication technology in the industrial control field has also provided the possibility for the implementation of flexible and modular control design based on distributed control system. Figure 1 is a simplified diagram of a distributed control system. [align=center] Figure 1 Distributed motion control system based on CANopen bus [/align] The communication between the host controller (PLC or motion controller) and the servo drive is based on the CANopen standard. This solution can be used to easily build complex multi-axis control applications in a high-performance modular way. Bus communication devices do not require hardware editing. They can be applied to new products by simply resetting the parameters of the existing control system [2]. The process can be edited by adding or deleting control elements (servo drives, I/O modules, etc.) without making major edits and changes to the control system. The flexibility of this solution is the biggest advantage in maintaining a competitive position in the process of industrial production automation: it can easily edit and upgrade the equipment in the feedback of reducing product and technology life cycle. Since digital current, speed and position servo loops are integrated into the servo drive, the servo performance is also greatly improved. Whether using interpolated or independent axis trajectories, the final motion control application must be able to know the bus communication parameters and the allocation of control tasks. This issue will be discussed in detail in the first part of this article. The second part focuses on the forced synchronization commands often required when maintaining interpolation axis alignment. The final part discusses how to achieve the best accuracy and dynamic performance in servo drive design. Distributed Motion Control System In a distributed motion control system, intelligent servo drives must provide bus communication devices and be able to perform high-level axis control tasks. The design of the bus device and the allocation of motion control tasks are key to building a high-performance motion control system. The interpolation motion control program calculates multi-axis trajectories through the host motion controller and then sends the axis position setpoints to each servo drive via the bus. The application and control process are still set in the host PLC or motion controller. However, the servo drive takes on more responsibility for motion control, such as software and hardware limit monitoring, motor braking mode control, and safety during low-speed operation during machine debugging. The trajectory generator can also be integrated into the servo drive through the final application. This makes the difference between interpolated axis applications and independent applications more significant. Many robots and machine tools require interpolated motion control (which must be able to continuously adjust a subset of axes). In this approach, axis trajectories must be calculated by processors at the same high frequency to maintain inter-axis coordination. [align=center]Figure 2 Interpolated Coordination Motion Control Structure[/align] As shown in Figure 2, a distributed motion control system suitable for multi-axis interpolation is based on the intelligence of servo drives, capable of performing complete servo control tasks including position and velocity, and current loops with power conversion. The host motion controller performs multi-axis trajectory calculations and sends digital signals of the position setpoints to each servo drive via a series of bus communications. Cubic interpolation is used to generate contours within the servo loop sampling period, obtained from two adjacent setpoints in the servo drive. This technique significantly reduces the resources required by the host controller (setpoint generation frequency) when maintaining smooth motion control. This allows for the control of a large number of axes within a given bus performance. Figure 3 illustrates the effect of cubic interpolation. In this application, the setpoint generation frequency is 100 Hz, and the servo loop sampling period is 0.5 ms. [align=center] a) No interpolation b) Cubic interpolation Figure 3 Effect of cubic interpolation in servo drive[/align] In most motion control in the field of automation, interpolation between axes (axis trajectories are independent) is not allowed. This method eliminates the need for centralized trajectory calculation, which allows trajectory calculation to be distributed to each servo drive. Figure 4 shows the corresponding control system. The host PLC controller is well-suited to provide application sequence control. This solution is based on intelligent servo drives that can provide high-performance motion control and are fully integrated into the PLC environment. Therefore, high-level motion control parameters can be obtained in the IEC 1131-3 standard programming language. [align=center] Figure 4 PLC-based distributed control system[/align] CAN Open communication Fieldbus is an important component and key feature, similar to synchronization, update rate, or communication parameters that determine the performance of the entire system. CAN was chosen because of its high speed, stability, and low cost[3][4]. The transmission rate can reach 1 Mbps within 40m, but it will decrease as the distance increases. CAN series buses are widely used in automation and automation industries, reducing the cost of hardware installation. CAN is a broadcast of information based on information priority selection on a public bus. Synchronous and asynchronous conversion models are distinguished in CAN. Asynchronous information focuses on servo drive parameter settings, while synchronous information focuses on motion control and axis trajectory adjustment. CANopen DS402 has been implemented for servo drive applications. In this scheme, the target position is sent to the servo drive by the host PLC controller via the bus, and then the servo drive performs trajectory adjustment calculations and completes the setting. The "interpolated position" model is used for interpolated axis applications. In this scheme, in order to maintain the inter-axis coordination, the position setpoint calculated by the multi-axis trajectory generator must be switched frequently between the motion controller and the servo drive. Therefore, it occupies more bus resources than the previous application. At the same time, any synchronization misalignment will significantly reduce the accuracy of the control path. Synchronization misalignment is caused by the difference in sampling time between the main motion controller and the servo drive. Vibration caused by bus transmission delay will also cause synchronization misalignment. If a position setpoint can be taken twice in the servo drive, especially when high dynamic performance is required, the zero speed reference signal will exceed one sampling period and will also strongly stimulate the servo motor [5]. The impact of bus vibration on the servo motor under constant speed operation is shown in Figure 5(a). To solve this problem, a forced synchronization command must be executed in the servo driver. Special attention must be paid to motor position and speed measurements to obtain accurate calculations of the servo loop error. Figure 5(b) shows an improvement in forced servo execution within the servo driver. [align=center] (a) No interpolation (b) Interpolation within the servo drive Figure 5 Impact of fieldbus nodes in position interpolation mode[/align] Servo Driver Design The servo driver control section is a single-chip motor processor (ADMC401), including motor current sensing, motor position acquisition, and a PWM pulse generator suitable for the power level. This design represents the optimal choice for performance in an integrated servo driver with very few components. Cascade control is best suited for high-performance servo drivers; the internal current control loop controls the motor torque, and the dynamic performance of the external speed and position control loops directly depends on the performance of the internal current loop. To achieve the shortest possible motor current response time and meet the maximum speed range of the servo motor, the voltage range of the external power converter must be maximized. The current controller is based on the space vector modeling (SVM) technique in the rotor reference coordinate system. A third-party modulation demodulator is added to the SVM class function, which gives the servo motor a higher speed range (more than 15%) compared to the traditional symmetrical triangular model [6], see Figure 6. [align=center] Figure 6 Torque/speed curves under the servo drive model method a) Servo closed-loop control b) Transfer function Figure 7 Speed ​​and position servo control structure a) Servo loop position application b) Envelope application Figure 8 Speed ​​and position servo loop tuning[/align] The speed and position servo loop tuning must be optimized through mechanical load parameters to achieve a stable and fast response state. The position servo controller design is based on the polynomial control system and the pole positioning and tracking method. The polynomial control system is the most important controller system and is very suitable for parameter tuning. It assumes that the drive can be expressed by the transfer function equations of hmc and hmd in Figure 7a. The servo controller includes two transfer equations hfb and hfw, which mainly act on the servo loop error signal and the servo loop reference signal. In Figure 7b, HSR and HSD are the transfer functions for the closed-loop output/reference and output/disturbance, respectively. The controller tuning process involves setting the positions of the poles and zeros in the HSR and HSD transfer functions to allow the position servo loop to set the output/reference and output/disturbance values. Therefore, servo loop tuning and tracking behavior can be completely separated. The HMC and HMD transfer functions are set by the mechanical device itself. The device transfer functions are obtained by performing a validation procedure under rated load. The actuator only selects the required bandwidth; aside from requiring precise technology, the self-tuning procedure does not require any special servo system knowledge. In polynomial control systems, the servo loop feedback can also be easily edited through the servo driver application. In Figure 8a), the servo loop response is optimized for the axis position application. This scheme requires rapid and accurate arrival at the target position. In Figure 8b), the servo loop response is optimized for the envelope application; the position error must be close to zero during axis displacement. Compared to previous tuning work, now only the controller HFW transfer function needs to be modified to meet the new requirements. The motor position measurement is obtained from the resolver feedback sensing via software resolver-to-digital (RDC) conversion. The sine and cosine feedback signals from the resolver enter the ADMC401 12-bit digital-to-analog converter (ADC) channel, and then the motor speed and position values ​​are calculated through a second-order tracking filter. When smooth motor motion is required, an external 16-bit ADC can be added to enhance the position solution. As shown in Figure 9, the optional 16-bit ADC can significantly reduce speed fluctuations and motor noise caused by position quantization errors. [align=center] a) 12-bit resolver signal b) 16-bit resolver signal Figure 9 The effect of ADC on resolver signal conversion[/align] Conclusion Motion control and intelligent servo drives based on CANopen bus communication are an efficient and flexible solution. The CD1K intelligent servo drive mentioned in this article can perform high-level multi-axis control tasks, providing a high-performance motion control solution for a wide variety of applications. The "interpolated position" mode is used for applications where axis trajectories must be continuously coordinated. In this solution, the host motion controller's resource usage is significantly reduced due to the cubic interpolation provided within the servo drive. The speed and position servo loop can be adjusted and optimized by running the target application's self-tuning program online within the machine.