Industrial robots excel at moving through space with remarkable precision. They can place a component within fractions of a millimeter, repeat the same trajectory thousands of times, and do it faster than any human operator. Yet ask that same robot to polish a surface, insert a peg into a hole, or hand someone a cup of coffee, and it struggles—or worse, causes damage.

The problem isn't mechanical. It's conceptual. Traditional robot control assumes the world is perfectly known and infinitely rigid. Contact tasks shatter that assumption. When a robot touches something, forces appear that position controllers weren't designed to handle. The solutions involve a fundamental shift in how we think about robot behavior.

This article examines the engineering principles behind force control—the strategies that enable robots to interact safely and effectively with their environment. We'll explore why position control alone isn't enough, how impedance control creates programmable mechanical behavior, and the practical considerations for integrating force sensors into robotic systems.

Consider a robot arm programmed to push a button. A position controller would command the end-effector to move to coordinates that happen to be 2mm behind the button's surface. If the button is exactly where expected, this works. If the button is 1mm closer than anticipated—perhaps due to manufacturing tolerance or mounting variation—the robot attempts to occupy space the button already claims.

Something has to give. Either the robot's motors stall, the button breaks, the mounting fails, or the robot's structure deflects. None of these outcomes are acceptable. The fundamental issue is that position control treats the environment as empty space to move through, not as something to interact with.

Force control inverts this paradigm. Instead of commanding where the robot should be, we command how hard it should push. A force-controlled robot approaching that button would apply, say, 5 Newtons regardless of the button's exact position. If the button is closer, the robot stops sooner. If it's farther, the robot travels farther. The robot adapts to reality rather than demanding reality conform to its model.

The practical implications extend far beyond button-pushing. Assembly tasks often involve sliding parts against each other until they align. Grinding and polishing require consistent pressure despite surface irregularities. Collaborative robots working alongside humans must yield when contact occurs unexpectedly. All these applications share a common requirement: the robot must respond to forces, not just positions.

Takeaway

Position control asks 'where should I be?' while force control asks 'how should I interact?' Contact tasks demand the second question.

Rather than choosing between position and force control, impedance control offers a unified framework. The core idea is elegant: make the robot behave like a mechanical system with programmable stiffness, damping, and inertia. When the environment pushes back, the robot responds as if it were connected to virtual springs and dampers.

The mathematics describe a target relationship between position error and force. If the robot deviates from its desired position by a certain amount, it generates a corresponding force—just like a spring. The proportionality constant is the virtual stiffness, and engineers can set it to any value that suits the task. High stiffness for precision positioning in free space. Low stiffness for compliant contact behavior.

Damping enters the equation to prevent oscillation. A pure spring would bounce forever; damping absorbs energy and smooths the response. Virtual inertia—the third component—determines how quickly the robot accelerates in response to force changes. Together, these three parameters define an impedance: the dynamic relationship between motion and force.

The practical value becomes clear in contact transitions. A robot reaching toward a surface doesn't know exactly when contact will occur. With impedance control, the robot moves toward the nominal position, but the moment it touches the surface, forces appear. Those forces, filtered through the impedance relationship, produce natural yielding behavior. The robot settles into equilibrium where its virtual spring force matches the contact force—no special contact detection required, no mode switching, just smooth physics-like behavior emerging from the control law.

Takeaway

Impedance control doesn't choose between position and force—it defines the relationship between them, letting physics-like behavior emerge naturally during contact.

Impedance control can be implemented without direct force measurement—computing forces from motor currents and robot dynamics. But explicit force sensing offers superior accuracy, especially for detecting small forces and compensating for friction and other non-idealities. The engineering challenge lies in extracting useful force information from real sensors in real environments.

Six-axis force/torque sensors are the gold standard for robotic manipulation. Mounted between the robot wrist and the end-effector, they measure three force components and three torque components simultaneously. This complete information enables sophisticated control strategies. However, placement matters critically—mounting the sensor closer to the contact point reduces the influence of tool dynamics but may not always be practical.

Signal processing presents its own challenges. Force sensors pick up vibrations, motor cogging, and structural resonances. Raw signals often require filtering, but aggressive filtering introduces phase lag that can destabilize force control loops. Engineers typically implement cascaded control architectures: a fast inner position loop handles the robot dynamics while a slower outer force loop shapes the interaction behavior. This separation of timescales keeps the system stable while achieving responsive force regulation.

Practical implementation also demands attention to sensor specifications. Resolution determines the smallest detectable force change. Accuracy affects absolute force measurement. Temperature drift can shift readings over hours of operation. Overload protection prevents permanent damage during unexpected impacts. These specifications must match application requirements—a robot for precision assembly needs different sensor characteristics than one for heavy-duty grinding.

Takeaway

Force sensors don't just measure—they enable. But their value depends entirely on proper selection, mounting, and signal processing that respects both physics and control stability.

Force control transforms robots from blind position-followers into adaptive systems capable of genuine physical interaction. The shift from commanding positions to shaping force relationships opens entire categories of applications—assembly, polishing, human collaboration—that pure position control cannot address.

Impedance control provides the conceptual framework, letting engineers program how robots should feel to the environment rather than where they should be. Force sensors provide the feedback that makes this behavior precise and reliable.

The fundamental insight applies beyond any specific algorithm: robots that work with people and objects must respect the physics of contact. Engineering that understanding into control systems is what makes gentle, effective interaction possible.