11.11. Force and Impedance Control

Ensures that the force applied by the robot on the environment stays within desired bounds.

Force control is a crucial aspect of robotics that ensures the robot does not exert too much force on its environment. This is important in applications where robots interact with delicate objects, as excessive force can cause damage. By managing the amount of force applied, robots can perform tasks more safely and effectively.

Imagine a person carefully holding a soft fruit, like a ripe peach. If they grip it too tightly, they might crush it. Similarly, a robot using force control applies just the right amount of pressure to handle the peach without causing harm.

• Hybrid Position/Force Control: Applies position control along some directions and force control along others.

Hybrid position/force control is a strategy that combines both position and force control. In this approach, the robot moves to a desired position while also managing the force it exerts in different directions. For instance, when gripping an object, the robot might need to move to a specific position but also control the pressure applied to the object to avoid damaging it.

Think about a person pushing a shopping cart. They need to guide it (position control) while also being careful not to push too hard against items in the cart (force control). This balance helps prevent accidents and ensures smooth operation.

• Force Feedback: Uses force/torque sensors to regulate contact forces in real-time.

Force feedback is an essential component in force control systems. It involves the use of sensors that detect the forces or torques acting on the robot. By continuously measuring these forces, the robot can adjust its actions in real-time to maintain the appropriate level of force during interaction with the environment.

Consider a robot vacuum cleaner that encounters a barrier. Its force sensors detect when it hits something, allowing it to reverse or change direction to avoid damage, similar to how a person might step back when they bump into an object.

Introduced by Hogan (1985), impedance control regulates the mechanical impedance (relationship between force and motion):

Impedance control is an advanced technique designed to manage how a robot interacts with its environment. It focuses on the mechanical impedance, which is the relationship between the force a robot exerts and its resulting motion. By controlling parameters like mass, stiffness, and damping, robots can adapt their behavior depending on how they contact different surfaces or objects.

Think of a trampoline. When you jump on it, the trampoline's material deforms and pushes back, regulating how high you bounce. Similar to how your body adapts when you land, impedance control helps robots adjust their forces based on interactions.

F = M x¨ + B x˙ + K x

This formula shows how impedance control works, where F represents the force exerted, M is the desired mass, B is the desired damping, and K is the desired stiffness. Each of these parameters allows the robot to behave differently in response to the forces it encounters. For instance, a robot with high stiffness will resist motion more than one with low stiffness.

Imagine a child on a swing. If they push against the ground (force), the swing will either resist their push (high stiffness) or allow them to sway gently (low stiffness). This analogy helps illustrate how robots can be programmed to respond differently depending on their control settings.

This control mode is compliant and widely used in human-robot interaction, rehabilitation robots, and contact-rich tasks.

Impedance control is particularly useful in scenarios involving direct interaction between humans and robots or where delicate manipulation is required. Its compliance allows robots to adjust their force output based on feedback from their environment, making it ideal for rehabilitation robots that assist people in physical therapy or collaborative robots in factories.

Consider how a personal trainer might guide someone through an exercise. The trainer adjusts their support based on the trainee's strength and comfort level, similar to how a robot with impedance control adapts its resistance based on how much force it feels from the object it is interacting with.

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