What are singularities in a six-axis robot arm?

What is singularity in robotics?

Singularity is one of those phenomena that can make even experienced robot programmers go cross-eyed. Here, we’ll explain what this robotic quirk is all about.

A singularity is a specific arm configuration where the robot loses one or more degrees of freedom. In simple terms, it means the robot arm gets locked in a particular posture and can't move in certain directions anymore. 

This happens because the math that controls the arm's movements breaks down at these singular points.

While singularities are unavoidable, they can cause issues like unpredictable arm motions, loss of control, and damage.

Skilled programmers use clever tricks like singularity avoidance algorithms, joint re-ordering, and optimal path planning to steer clear of these singular configurations as much as possible.

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Types of singularity robots

We’ve already established that singularities are bad news for robots. But did you also know there are different types of singularities? 

Let's explore the three main categories:

• Wrist singularities: This type of singularity comes up when two axes within the robot's wrist become aligned, resulting in a loss of one degree of freedom.
  
  In a desperate attempt to follow its programmed path, the robot may try to spin these joints at an infinite speed, leading to potential errors, spasm-like movements, or even damage to the robot's delicate wrist components.
  
  Out of all of the singularities, this is by far the most common. 
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• Elbow singularities: This type of singularity occurs when the robot's arm is fully extended, causing the wrist to lie on the same plane as the second and third joints.
  
  A common situation is a robot trying to reach for something just beyond its grasp — its arm locks up, and it can't move any further without contorting itself into an unnatural position.
  
  This can lead to inaccurate positioning, unexpected movements, or strain on the robotic components. 
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• Shoulder singularities: Shoulder singularities occur when the robot's wrist aligns with the base, forcing the first and fourth joints to attempt an impossible feat — rotating 180 degrees on the fly.
  
  This can result in a robotic equivalent of a pulled muscle, causing jerky movements, inaccurate positioning, and potential damage to the robot's shoulder joint.
  
  You don’t want that kind of trouble in your life, trust us. 

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Impact of singularity on robotic arm performance

Singularity positions go far beyond being annoying: They can cause expensive, complex, and frustrating stoppages. This is particularly problematic in tasks requiring high precision, as the robot's inability to maintain stable motion typically leads to many more significant errors.

Now, we’ll explain what it is about robotic singularities that can cause you so much grief in more detail below. 

Here’s the lowdown on singularities and robotic arm performance: 

• Singularities reduce overall skill. At certain configurations, the robotic arm loses degrees of freedom, reducing its dexterity and making it harder to position the end-effector precisely.
  
  This loss of mobility can seriously hamper complex tasks.
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• Singularities also amplify errors. Small variations in joint angles near singularities can cause large movements of the end-effector, amplifying positioning errors.
  
  Precision operations become really tricky in these zones.
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• Wrist singularities are the usual suspects. The wrist joint is especially prone to singularity issues due to its kinematic structure.
  
  Careful path planning is needed to avoid getting stuck in awkward, uncontrollable postures.

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How to detect singularities in robot arms

Singularities tend to come up at the worst moments, completely messing with your workflow — and causing costly delays. 

 Here's how to detect and deal with these flies in the ointment:  

• Know your enemy. We’ve established that a singularity occurs when two or more axes of the robot arm become aligned, causing it to lose one or more degrees of freedom.
  
  You need to know which angles are prone to singularities to nip concerns in the bud.
  
  Understanding these limitations can help you design tasks and workflows that minimize the risk of encountering these troublesome configurations.
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• Map it out. Each robot arm has specific singularity configurations based on its geometry and design. Study the arm's kinematics and identify potential singularity points.
  
  This knowledge will help you plan trajectories that avoid these danger zones, ensuring smooth and uninterrupted operation.
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• Monitor joint angles and velocities. As the arm approaches a singularity, certain joint angles or velocities may exhibit unusual behavior, such as sudden spikes or erratic fluctuations.
  
  Set up real-time monitoring to detect these telltale signs and take preventive action, such as adjusting the robot's path or speed.

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Use singularity avoidance strategies

The best offense is a good defense when it comes to singularities. 

Give some thought to these proactive measures:

• Path planning know-how. Develop algorithms that plan paths around singularity configuration for smooth and controlled movements throughout the arm's workspace.
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• Introduce software limits. Implement software constraints that prevent the arm from entering singularity zones, essentially creating "no-fly" zones in its workspace.
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• Robust control techniques. Explore advanced control methods like damped least-squares or singularity-robust inverse kinematics solvers, which can handle singularities more gracefully.

Real-world applications affected by singularities

Singularity situations aren't just theoretical problems; they can throw a real wrench into practical applications of robots across various industries.

 Let's delve deeper into some scenarios where singularities can cause trouble:

• Precision assembly: Singularities can wreak havoc when it comes to intricate assembly processes.
  
  An example: A robotic arm, programmed to meticulously place tiny components on a circuit board, suddenly jerks uncontrollably due to a wrist singularity.
  
  This mistake can lead to misaligned parts, damaged circuitry, and, ultimately, a faulty product.
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• Welding: Welding robots are a common sight, but they, too, are susceptible to bothersome elbow or shoulder singularities.
  
  When a robot's arm gets stuck in a fully extended position, or its wrist aligns with the base, the ensuing erratic movements and inaccurate positioning can result in uneven welds, incomplete joints, or even structural damage to the workpiece itself.
  
  An example: A robotic welder that has to join critical components of a car chassis falters due to a singularity, compromising the vehicle's safety and leading to costly repairs or recalls.
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• Material handling: Robots are increasingly taking on heavy loads in warehouse environments. However, an unexpected encounter with a singularity can turn this efficiency into a potential disaster.
  
  A singularity can cause a sudden loss of control, leading the robot to drop its load, potentially damaging valuable goods and posing a significant risk to nearby workers. 
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An example: A fully loaded robotic forklift abruptly tips over due to an elbow singularity, resulting in not only financial losses — but also a serious threat to workers. 

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Advanced ways to handle robot singularity

So, are there any ways to mitigate the many woes a robot singularity can bring? We’re talking erratic motion, loss of precision, and potential damage.

Fortunately, yes there are advanced strategies to tame this issue:

• Real-time monitoring: Think of this as a robotic health check. Sensors and software constantly monitor the robot's movements and joint configurations, detecting potential singularities before they become a problem.
  
  If a singularity is imminent, the system can issue a warning or even automatically adjust the robot's path to avoid it.
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• Adaptive algorithms: These algorithms are a robot's built-in reflexes. They can dynamically adjust the robot's movements on the fly to avoid singularities, ensuring smooth and continuous operation.
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• Collaborative robot applications: Cobots are designed to work alongside humans, and their collaborative nature can help mitigate singularities.
  
  For example, a human operator can manually guide a cobot away from a singularity position, preventing it from getting stuck.
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• Software solutions: Specialized software tools can help you identify and analyze potential singularities during the programming phase.
  
  By simulating different scenarios and optimizing robot paths, you can cut down the risk of encountering singularities in real-world operations.
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• Redundancy resolution: Some robots have extra joints or degrees of freedom, which can be used to avoid singularities.
  
  These redundant robots can find alternative solutions to achieve the same goal, even if one joint configuration leads to a singularity.
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• Path planning adjustments: By carefully planning the robot's path and avoiding configurations that are known to cause singularities, you can minimize the risk of encountering these issues during operation.
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• Hardware and design modifications: In some cases, physical modifications to the robot's design, such as adding joint limits or adjusting the robot's workspace, can help prevent singularities.
  
  However, this should only be done by qualified professionals.

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Summing up

And that’s the long and the short of the robot singularity!

Singularities in a six-axis robot arm are important to understand for effective robotic control and motion planning. 

With this understanding, you can avoid and manage singular configurations as you program and control multi-axis robots to do whatever you need.

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Next steps

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