A multi-joint robot, also known as an articulated robot, is a type of industrial robot that has two or more rotary joints. These joints allow the robot to move and operate in a way that resembles a human arm or other limbed system. Multi-joint robots provide more flexibility and dexterity than standard single-joint robots.
How a Multi-joint Robot Works
The key feature of a multi-joint robot that sets it apart from other robot designs is the presence of rotary joints. These are joints that allow motion around one or more axes of rotation. Common rotary joints used in robotics include:
-Revolute joints: Allow rotation about a single axis. Revolute joints enable bending and folding movements.
-Prismatic joints: Allow linear motion along a single axis. Prismatic joints enable telescoping and sliding movements.
-Spherical joints: Allow rotation about three perpendicular axes. Spherical joints enable twisting movements.
By connecting multiple rotary joints via rigid links, multi-joint robot arms can be constructed that mimic the range of motion of a human arm. The joints are powered by servo motors that rotate or slide the connected links based on control signals. Multi-joint robots also incorporate sensors to provide feedback on the current configuration of the robot to the control system.
Common Configurations
There are several common configurations of multi-joint or articulated robots:
- Cartesian robots: Used for pick-and-place and other linear motions, Cartesian robots have three prismatic joints whose axes are coincident with a Cartesian coordinate system. This provides movement in the X, Y, and Z dimensions.
- SCARA robots: Selective compliance assembly robotic arm (SCARA) robots are very common. They have two parallel rotary joints to provide compliance, plus a prismatic joint for vertical movement. The parallel joints allow SCARA robots to move efficiently in a horizontal plane.
- Anthropomorphic robots: Designed to mimic the motions of a human arm, these multi-joint robots have rotary shoulder, elbow, and wrist joints. Some even incorporate multi-jointed hands. Their human-like form allows them to tackle tasks meant for humans.
- Cylindrical robots: Built around a central "spine" with rotary joints, cylindrical robots can cover a large work envelope because the "shoulder" joint allows rotation around the cylinder.
Applications
Thanks to their rotational flexibility, multi-joint robots are very versatile and can automate a diverse range of industrial applications, including:
-Pick and place tasks
-Packaging and palletizing operations
-Assembly of parts
-Injection molding
-CNC machine tending
-Welding car bodies and appliances
-Painting curved surfaces
-Quality inspection through motion and scanning
In essence, multi-joint robots can tackle almost any task that requires moving tools or end effectors through a range of positions and orientations. Importantly, their ability to extend their work envelope and avoid collisions automatically makes them very useful for interacting with complex or crowded environments in a dynamic fashion.
Advantages Over Single-Joint Robots
While single-joint robots can achieve very high rates of speed and precision, they suffer from limited flexibility. For example, a simple Cartesian gantry robot can only achieve linear movement in three axes.
In contrast, multi-joint robot arms can:
- Reach Around Obstacles: By bending or rotating out of the way, they can operate in confined spaces.
- Reorient Tools: Tools mounted on the robot wrist or flange can be spun and tilted to almost any orientation during a task.
- Self-Collision Avoidance: Multi-joint arms have software that lets them effectively curl out of the way of themselves and other items in their workspace automatically.
- Continuous-Path Motion: They can "slew" smoothly via intermediate positions, enabling operations like welding large curved surfaces.
In essence, properly implemented multi-joint robots offer greater autonomy, efficiency, and production flexibility compared to more rigid robots.
Control Systems for Multi-joint Robots
Sophisticated computer control systems are needed to coordinate motion across numerous joints and axes present in articulated robots:
- Kinematics: Mathematical coordinate transformations must continuously track the 6D position of the robot's tooltip based on angular positions of multiple rotary joints.
- Path Planning: To avoid collisions and maximize efficiency, optimal motion pathways must be precalculated or dynamically generated as tasks are completed.
- Compensation Algorithms: Deviation from target paths due to arm flexibility and backlash in joints/gears must be sensed and counteracted via compensation signals sent to actuators.
- Multi-variable Control: Multiple coupled partial differential equations govern the complex dynamics of multi-joint arms, requiring very responsive, real-time control of multiple actuators for precision and speed.
As a result, multi-joint robot programming relies on software that simplifies control down to basic coordinate inputs, movement presets, tool orientations, and end effector actions. This relieves engineers from having to manually resolve complex mathematics and dynamics computations.
The Advantages of Collaborative Robots
A relatively new area in multi-joint robotics is that of collaborative robots, or "cobots". Cobots are designed to operate safely while physically interacting with humans in a shared workspace, without danger or need for safety guarding. Built-in sensors cause the robot arm or hand to automatically stop on human contact.
This collaborative approach offers numerous benefits:
- Intuitive Programming: Cobots can be trained manually via guided teaching instead of coding, making them very user-friendly.
- Easy Deployment: Lightweight designs enable cobots to be quickly setup almost anywhere. No need for rigid fencing or foundations.
- Safer Interaction: Sensitive tactile skins and/or torque sensing at each joint make cobots react appropriately to human contact.
- Flexible Automation: Shared workcell spaces mean operators can alternate tasks with robots in efficient coordination.
- More Affordable Solutions: Eliminating safety barriers greatly reduces overall system costs and footprint.
- Augmented Workforces: Humans handle complex cognitive tasks while cobots relieve them of dull, ergonomically taxing activities.
Cobots thereby lower the barriers to robot adoption for small businesses while also promoting safer human-machine collaboration in existing facilities. This new paradigm in intuitive and affordable automation continues to drive innovative cobot solutions across many industries.
The Future of Multi-joint Robots
As computer processing power keeps improving, so too will the capabilities of articulated robots and their control interfaces. Expanding applications combined with easier programming will lead multi-joint robots to become increasingly prevalent across more industries.
And by merging the flexibility of multi-joint designs with artificial intelligence and expanded sensory perception, tomorrow's robots will achieve far greater autonomy. Large-workspace robots are also transitioning from rigid masses of metal to more compliant and responsive "soft robots" through use of polymers, expanded kinematics, and even cables.
Ultimately, as their physical capabilities and intelligent responsiveness advances, multi-joint robots will play expanding collaborative roles alongside human partners – making them a key driver of the Fourth Industrial Revolution.
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