Definition and main characteristics of a rectangular coordinate robot
A robot, as defined by ISO 8373, is an industrial automation device capable of being fixed or movable, automatically controlled, reprogrammable, multifunctional, and having an end effector with programmable position in three or more degrees of freedom. Here, degrees of freedom refer to axes that can move or rotate. Cartesian robots primarily use linear motion axes, which typically correspond to the X, Y, and Z axes in a Cartesian coordinate system. In most cases, the angles between the linear motion axes of a Cartesian robot are right angles.
Cartesian robots mainly consist of linear motion units and their combinations, drive motors , control systems, and end effectors. They can be easily and quickly assembled into Cartesian robots of different structures, dimensions, strokes, and load capacities for various applications. In automated production lines across industries ranging from cosmetics manufacturing to automotive and mobile phone assembly, multiple Cartesian robots and other equipment work in strict synchronous coordination. We have successfully completed hundreds of application cases with different structural forms. It can be said that Cartesian robots can handle almost all industrial automation tasks. Below are their main features:
1. Robots that can be arbitrarily combined into various structural styles, with different load capacities and strokes.
2. By using multiple cascaded linear motion units and gear and rack transmission, a robot with an ultra-large stroke of tens of meters can be formed.
3. By employing multiple linear motion units connected to a multi-slider structure, its load capacity can be increased to several tons.
4. Theoretically, the maximum operating speed can reach 8 meters per second, and the acceleration can reach 20 meters per second.
5. The repeatability accuracy can reach 0.05mm～0.01mm.
6. Five-axis or higher CNC systems with RTCP functionality can complete tasks with very complex trajectories.
Shenyang Baige Robotics Co., Ltd. has been an authorized distributor of Cartesian coordinate robots from the German company Baige since 1995, successfully assisting users in various industries to solve their automation tasks. Over the past decade, the company has also sent personnel to Baige for training and German experts to provide on-site training and programming services to users in China. This has enabled us to continuously accumulate experience and improve our service capabilities, from understanding and analyzing tasks, selecting and designing robots, to installation and commissioning at user sites until design requirements are met. Below, we will introduce how to select a suitable Cartesian coordinate robot.
II. Selection of Cartesian Coordinate Robots
1. Usage Requirements Analysis
For those selecting a robot, a foundation in physical kinematics, mechanics of materials, and experience in servo drive usage and CNC system applications is essential. However, the most crucial element is a clear understanding of the user's problems and requirements. For simple tasks, experienced engineers can communicate effectively via phone and email. For complex tasks, on-site collaboration is necessary to analyze and develop a task description, providing specific and reasonable requirements. The following key information is required: the robot's working process, the total weight of the gripper and load, the length of a complete work cycle, the possible sub-motions and their corresponding times, synchronization/handshake requirements with other devices during movement and gripping, the effective travel length of each axis, the maximum permissible speed and acceleration/deceleration, spatial limitations around the robot's working area, and special protection requirements for the operating environment, including those related to powder, high temperatures, water, and humidity.
2. Selection of Robot Structure
The robot's structural form should be selected based on the information obtained in the "Usage Requirements Analysis" section above. In principle, a gantry-type Cartesian coordinate robot should be chosen whenever possible, but sometimes workspace limitations necessitate the use of cantilevered or similar types. In projects involving food handling and glass cutting, which generate large amounts of powder that can damage the guide rails inside the motion axes, a suspended robot is preferable. Sometimes, a cantilevered robot must be selected due to load, travel distance, and space constraints. The required motion position accuracy of the load should be determined based on the robot's task, taking into account positional errors caused by swaying during deceleration. The number of motion axes and their respective strokes should be determined based on the robot's task and workspace limitations.
3. Plan the movement trajectory and speed
Plan the motion trajectory based on the robot's task and space constraints. Minimize the movement distance; for applications with strict work cycle requirements, utilize multi-axis simultaneous motion as much as possible to reduce movement time and speed. Keep the movement speed low after grasping the load and fast when returning to the starting point without load. Keep acceleration and deceleration low under heavy loads to avoid generating large impact forces. Based on the above principles, specify the speed, acceleration, and deceleration for each motion segment. Ensure smooth speed changes between motion segments to guarantee work cycle time and reduce impact forces and operating noise. When allocating motion speeds, fully consider the synchronization and coordination time between each motion process and other equipment; the planned motion time should be shorter than the user-required time.
4. Force Analysis
Based on velocity analysis, the maximum acceleration and deceleration of each axis are obtained. Then, the combined maximum deceleration generated when multiple axes move simultaneously is calculated. The larger of the deceleration of independent motion and the combined deceleration during simultaneous motion is selected. Based on this maximum deceleration, the maximum impact forces Fx, Fy, and Fz in the XYZ directions, and the resulting maximum torsional moments Mx, My, and Mz are calculated. When calculating the torsional moments Mx, My, and Mz on different axes, the center of gravity of the equivalent load, total gravity, and the impact force generated during deceleration must be considered.
The structural form and model of each motion axis are selected based on Fx, Fy, and Fz, as well as the maximum torsional moments Mx, My, and Mz generated. The connection method between the motion axes must also be considered to ensure their strength and sufficient impact resistance, enabling them to operate stably, at high speed, and efficiently for extended periods.
5. Deformation Analysis: Deflection deformation only occurs in large-span suspended configurations and under significant stress. The calculation method for its deflection deformation is shown in the formula below.
However, the mounting method of the Z-axis and X-axis ensures that our company's robots do not experience any deflection deformation or the deflection deformation is extremely small and negligible.
f = (F×L3)/(E×I×192) f: Deflection (mm) f≤ 1 mm F: Load pressure (N) L: Guide rail length (mm) E: Elastic modulus (70,000 N/mm2) I: Area squared (mm4)
While a certain amount of deformation is permissible in many tasks, it is unacceptable in CNC equipment applications such as glass cutting machines. Therefore, we need to examine the deformation curves of various motion axis models based on the maximum force calculated earlier. If necessary, reinforced types or additional reinforcing plates can be selected. For details, please contact Zhou Wenbao at Shenyang Baige Robotics Co., Ltd.
6. Select the drive motor
The drive motor should be selected based on the maximum speed of the linear positioning unit's drive shaft. When the maximum speed of the drive shaft is below 600 rpm, a stepper motor is typically used; otherwise, an AC servo motor should be used. However, the maximum speed of the AC servo motor should not exceed 3000 rpm, otherwise its lifespan will be affected. When using a stepper motor as the drive shaft, the ratio of the load's moment of inertia to the stepper motor's moment of inertia should be less than 12. When using a servo motor as the drive shaft, the ratio should be less than 8; otherwise, its high dynamic characteristics will be affected. If the moment of inertia ratio is greater than the above values, a precision planetary gearbox from Neutraltec (Germany) should be added. Within the maximum speed limit of the drive motor, a gearbox with a large reduction ratio should be selected whenever possible. To ensure high dynamic characteristics and to guarantee task completion within the agreed time, the maximum output of the drive motor should be at least 85% higher than the theoretically calculated value. We typically select drive motors with a maximum output at least 100% higher than the theoretically calculated value, and a moment of inertia ratio less than 5. It's also important to consider whether the selected servo motor is compatible with the precision planetary gearboxes from Neutraltec (Germany), whether the gearboxes can be mounted on the drive shaft, and whether the motor's control method is compatible with the CNC system. For detailed technical data and usage precautions regarding drive motors and gearboxes, please contact Shenyang Baige Robotics Co., Ltd.
7. Determine the robot's structure and each motion axis.
Based on the information and data from the above six aspects, the final robot structure and the specific model and length of each motion axis can be selected. We can usually find photos of similar structures in image libraries; these photos refer to CAD drawings or photos of previous user robots. The connecting plates between each axis also need to be designed, considering not only the mechanical assembly precision, material strength, and tensile strength of the connecting screws, but also reinforcing the connecting plates in the main impact directions, adding additional connecting plates if necessary. The main screws and nuts should be glued to prevent loosening after long-term vibration.
Robots generate powerful impacts during acceleration and deceleration, and typically operate 24 hours a day, so they must be securely mounted on a support frame. The frame must be sufficiently impact-resistant and have feet to ensure no swaying under prolonged high-speed, high-dynamic motion. Furthermore, during installation, the parallelism, flatness, and perpendicularity between the motion axes must be guaranteed.
8. Connecting plate design
The design of the connecting plate is crucial and should be undertaken by a master craftsman with years of practical experience. The design and quality of the connecting plate directly affect the long-term, high-speed, efficient, and stable operation of the designed and manufactured robot. Therefore, it is necessary to calculate Fx, Fy, and Fz for each motion axis, as well as the maximum torsional torques Mx, My, and Mz generated. Here, the calculation of Fx, Fy, and Fz does not refer to the force generated by each axis during its own movement, but rather to the Fx, Fy, and Fz corresponding to the maximum resultant velocity and acceleration/deceleration values generated by that axis during the robot's high-speed movement. The calculation of Mx, My, and Mz must also consider the equivalent center of gravity position and the slider center position. After determining Fx, Fy, and Fz and the maximum torsional torques Mx, My, and Mz, the designed connecting method should have a margin of at least three times the required torque.
9. Select the end effector – gripper system
Depending on the specific application, the gripper system may include pneumatic suction cups, pneumatic grippers, electric grippers, electromagnetic grippers, welding guns, glue guns, special tools, and testing instruments. In many situations, it can grip multiple workpieces at once.
10. Select Control System
The robot needs to complete specific tasks within a certain time frame, such as performing a transport operation every 10 seconds. While performing tasks like grasping, acceleration/deceleration, high-speed movement, and workpiece release, it also needs to coordinate and synchronize with related equipment via communication or I/O ports. In adhesive application, each motion axis needs to perform linear and circular interpolation movements. Therefore, the number of control axes, I/O ports, and software functions of the CNC system must be selected according to the specific application requirements. Typically, a CNC system, PLC, industrial computer with motion control cards and drive motors with axis card functionality and I/O ports are selected. For many tasks, we choose the CNC system from Engelhardt (Germany), and for heavy-duty tasks, we choose the motion control card from Movtec (Germany). Detailed information on these two CNC systems can be obtained from Shenyang Baige Robotics Co., Ltd.
11 Technical Agreement
Each robot has technical requirements such as operating speed, repositioning accuracy, and various specific functions. When signing a contract, it is essential to include a technical agreement specifying the acceptance criteria and methods. You must also learn how to maintain the robot. For robot maintenance information, please contact Shenyang Baige Robotics Company.
III. Conclusion
In the design process of Cartesian coordinate robots, design drawings and technical analysis calculation results must be continuously sent to the user for repeated communication and improvement. The designed Cartesian coordinate robot is naturally related to the experience of the technical personnel and the company's technical staff, as well as their diligence and responsibility; the most suitable is the best. It is also important that the user has experienced operators to correctly operate and maintain the Cartesian coordinate robot in a timely manner. Shenyang Baige Robotics Company has hired three retired senior engineers from machine tool factories to design the robots and connecting plates. For over 10 years, our company has also sold German-made stepper motors, servo motors, planetary gearboxes, machine vision systems, robot gripper systems, and CNC systems, accumulating rich practical experience to ensure that the Cartesian coordinate robots we sell can work efficiently and stably for extended periods. This is also the foundation for our long-term customer base. The above content represents some of my experience accumulated over 12 years in the field of Cartesian coordinate robots. We also have hundreds of successful application cases. We welcome interested parties to analyze and discuss with Shenyang Baige Robotics Company to jointly promote the application of Cartesian coordinate robots in various industries in China.
Author: Zhou Wenbao
Position: Technical Department Manager, Shenyang Baige Robotics Co., Ltd.
Telephone: 024-25369850/25375028 Fax: 024-25817012
Website: www.germanytek.com
Email: [email protected]
Products: Exclusive distributor in China for BergerLahr (Germany) and Roboworker (Germany)
