This solution is applied to the hardware-in-the-loop simulation verification tests and measurements of research projects at Beihang University, capable of simulating motion across three translational and three rotational degrees of freedom. The system consists of a mechanical platform, electrical control system, and main simulator, utilizing digital servo drive and networked system technologies to achieve precise six-degree-of-freedom control (yaw, pitch, roll, longitudinal, lateral, vertical) of the mechanical platform.

Electrical Control System Composition

The electrical control system of the multi-degree-of-freedom motion platform consists of networked servo motor drivers and MBOX communication control cards, employing CAN bus and Ethernet bus technologies to enable multi-platform, multi-degree-of-freedom, high-speed, and large-capacity operational control.

The diagram above illustrates the typical architecture of a networked servo control system, where the networked servo drive system forms a network through CAN bus and MBOX communication controllers. The MBOX communicates with the host computer via an Ethernet switch, and with servo drivers via CAN bus. The MBOX can also establish MODBUS communication with local computers or touchscreens through RS232 interfaces for monitoring and debugging. Additionally, the MBOX provides 8 digital inputs, 12 digital outputs, and 2 analog outputs for cost-effective data acquisition and control via Ethernet bus.

Figure 2 HS Servo CAN Bus Networking with MBOX Communication Control Card

2.2 Electrical Control System Functional Features

(1) Networked Servo Drive System

  The networked servo drive system employs TI's 32-bit dual-core digital signal processing chip with floating-point capabilities, utilizing advanced servo control algorithms for precise control of permanent magnet synchronous servo motors' position, velocity, acceleration, and output torque. General motion control functions: position/velocity/torque control, mode switching;

Fast current loop floating-point CLA core processing with excellent dynamic tracking;

Integrated grid voltage compensation control for automatic adaptation to voltage fluctuations;

Built-in dual selectable resonance low-pass filters and dual-stage resonance notch filters to effectively address mechanical resonance;

Proprietary intelligent regenerative braking control technology;

Built-in torque observer technology for automatic load adaptation;

Configurable control gain switching or internal adaptive matching;

Integrated dynamic braking control for additional drive safety;

Overload auto-unload algorithm with parameterizable overload protection selection and smooth unload/recovery settings;

Supports MODBUS protocol via RS232/RS485 interfaces for direct communication with host computers and touchscreens;

Supports CAN bus interface with proprietary communication protocol for customized applications;

Software-configurable control ports with logic settings and programmable filters for flexible and reliable operation;

Pulse/analog interface with maximum pulse input frequency up to 1MHz;

Dual analog outputs for real-time driver status monitoring during field commissioning;

Integrated power device temperature monitoring with overcurrent, overvoltage, and overheat protection for reliable operation;

Reliability management functions including historical fault record tracking.

(2) MBOX Communication Controller

   MBOX is a multifunctional board card designed for networked servo control applications, enabling bidirectional data transfer between Ethernet and CAN bus. It features RS232/RS422 interfaces, 12 digital outputs, 2 analog outputs, and 8 digital inputs. The CAN bus employs electrical isolation technology to ensure reliable and safe bus operation.

(3) Networked Control System with HS Servo Drives and MBOX

The networked servo drive system provides three bus interfaces: RS232, RS485, and CAN. RS232 is standard, while RS485 and CAN are optional with electrical isolation and transient voltage suppression protection, ensuring excellent electromagnetic compatibility and reliability.

       The standard RS232 interface enables point-to-point communication between the networked servo system and host computer/touchscreen for status monitoring, parameter configuration, and bus mode control. For networked operation, RS485 or CAN bus can be selected.

      The networked servo drive system enhances bus mode functionality, supporting multiple bus control modes via RS232, RS485, and CAN interfaces, including:

Bus interface torque mode control

Bus interface speed mode control

Bus interface relative position mode control

Bus interface absolute position mode control

3. Beihang Six-Degree-of-Freedom Platform Design

All-Electric Multi-DOF Motion Platform Solution: Utilizes electric cylinders with AC servo motors and drivers, offering fast response, high control precision, user-friendly operation, and convenient maintenance. Our proprietary kernel-level optimization for electric cylinder control significantly outperforms hydraulic solutions in control response and environmental performance.

Full Networked Topology Communication Control: Achieves dual-layer Ethernet+CAN bus topology through Hollysys' self-developed MBOX networked motion controller, providing high communication capacity, speed, scalability, and CRC-verified absolute position signals with superior reliability. This hybrid topology offers better performance-to-cost ratio compared to single Ethernet or CAN bus architectures.

 Bi-directional Remote Monitoring: Host computer connects to MBOX via Ethernet, which is tightly integrated with HS servo drives. The host can control MBOX parameters and individual DOF positions, while monitoring real-time speed, position, load rate, and servo status data.

Automatic Overload Unloading & Recovery: Servo drives feature intelligent automatic overload management, eliminating downtime risks from temporary overloads.

 Reliability Management: System tracks electric cylinder/motor operating hours and lifespan for proactive maintenance scheduling.

Multiple Emergency Stop Options: Emergency stops can be triggered via host computer Ethernet communication or local digital inputs. Options include stopping at current position, leveling any axis, or returning to home position.

Intelligent Regenerative Braking: Manages regenerative energy during rapid motion platform descent.

Grid Voltage Compensation: HS servo drives automatically compensate for grid voltage fluctuations.

Electromagnetic & Dynamic Braking: Supports electromagnetic (brake holding) and dynamic braking during power failures or faults for safe shutdown.

Human-Centric PC Interface: Graphical LabWindows-based HMI software supports jogging, sine control, attitude control, and other 6-DOF operations with upgradable functionality.

Sensorless Initial Positioning: Utilizes Hollysys HS servo drives' proprietary technology to eliminate limit sensors, reducing system cost and failure points.

 Commissioning HMI: Real-time RS232 monitoring of DOF speed, position, load rate, and servo status via MBOX for efficient commissioning and diagnostics.