Understanding the Gait of a Quadruped Robot

Boston Dynamics Spot Robot

Quadruped robots, inspired by four-legged animals, have garnered significant attention due to their versatility and adaptability in navigating diverse terrains. A critical aspect of their locomotion is the gait—the pattern of limb movements during locomotion. Understanding and optimizing gait patterns are paramount for enhancing the stability, efficiency, and adaptability of these robots.

1. What is a Gait?

In biological terms, gait refers to the sequence and timing of limb movements. Common gaits observed in quadrupeds include:

• Walk: Each leg moves independently in a sequence, ensuring that at least three legs are in contact with the ground, providing maximum stability.

• Trot: Diagonal pairs of legs move together, offering a balance between speed and stability.

• Pace: Lateral pairs of legs move in unison, often used for faster movement but can be less stable.

• Gallop: High-speed gaits where fore and hind limbs move in synchrony, suitable for rapid locomotion but require advanced balance control.

Roboticists have adopted these natural gait patterns to design locomotion strategies for quadruped robots, aiming to replicate the efficiency and adaptability observed in animals.

2. Gait Generation Techniques

i. Central Pattern Generators (CPGs)

CPGs are neural network models that produce rhythmic patterned outputs without sensory feedback. In robotics, they are employed to generate cyclic limb movements, facilitating smooth and coordinated gaits. For instance, a study by Li et al. demonstrated the use of CPGs to control hip and knee joints, achieving stable walking and trotting gaits in quadruped robots.

ii. Trajectory Planning

Trajectory planning involves defining the path each limb should follow during movement. Cycloidal trajectories are often used due to their smoothness and natural motion characteristics. These trajectories ensure minimal impact forces during foot-ground contact, enhancing stability and energy efficiency.

iii. Inverse Kinematics (IK)

IK is utilized to calculate the joint angles required to place the robot's foot at a desired position. By solving IK problems, robots can adapt their limb positions in real-time, allowing for precise foot placements and adjustments to uneven terrains.

3. Gait Transition and Stability

Efficient gait transitions are crucial for robots operating in dynamic environments. Zhang et al. proposed a multi-gait strategy that considers both the cost of transport and stability, allowing quadruped robots to switch gaits based on terrain and speed requirements .

Moreover, incorporating flexible spines in quadruped robots has shown to enhance locomotion. Research indicates that a flexible spine can increase average speed, reduce body pitch angle, and lower the center of mass, contributing to faster and more stable movement.

4. Gait and Trajectory Details of my custom quadruped CERBRUS

CERBRUS adopts the trot gait as its default walking strategy, where diagonal pairs of legs move together alternately. This was chosen after careful comparison of alternative gaits such as rectangular, sinusoidal, and trapezoidal trajectories.

Final gait trajectory used was Cycloidal. Each leg’s motion follows a cycloidal curve, ensuring a smooth lift-swing-plant cycle. This minimizes impact and jerky transitions, especially during diagonal motion.

• Step Height Function: 
  
  
  
• Step Length Function:
  
  
  
• Sidestep Function:

  

Where steph, stepz and stepy are constants that define the maximum limits of each. t is time varying from 0 to 2pi seconds.

5. Conclusion

Understanding and implementing effective gait strategies are fundamental for the advancement of quadruped robotics. By drawing inspiration from biological systems and leveraging modern computational techniques, researchers continue to enhance the locomotion capabilities of quadruped robots, paving the way for their deployment in complex and dynamic environments.