Collaborative Robots (Cobots): The Engineering of Human-Robot Collaboration in Manufacturing
What makes a cobot safe is not the robot itself, but a properly engineered application. The four collaborative operating methods, how power and force limiting works, and why a risk assessment is mandatory — a measured technical perspective for manufacturing engineers.
A collaborative robot (cobot) is often summarized as a "fenceless robot that stops when you touch it." This description is incomplete and misleading. What makes a robot collaborative is not the label on its box; it is an application design in which the robot, the tool, the workpiece, the speed and the environment are all assessed together. In this article we examine which engineering principles actually make cobots safe, the four collaborative operating methods, and why a risk assessment is non-negotiable.
The real difference between a cobot and a conventional industrial robot
Mechanically, a cobot and a conventional industrial robot arm consist of similar components: axes, servo motors, gearboxes, a controller. The difference is that the cobot is designed to be suitable for collaborative operation. Typically it features force/torque sensing in its joints, a rounded body with reduced pinch points, limitable speed, and a safety-rated control architecture.
The critical distinction here is this: a robot can be "suitable for collaborative operation," but an installation is collaborative only when the risk assessment confirms it. When a sharp blade, a hot end effector or a heavy part with pointed edges is mounted on a cobot arm, the resulting hazard is no longer limited by the robot's own mass; such an application may require additional protection even when a cobot is used. In short: it is not the robot that is collaborative, but the application.
The four collaborative operating methods
ISO 10218-1 and ISO 10218-2 form the foundation of industrial robot and robot system safety; the ISO/TS 15066 technical specification details collaborative operation and the associated biomechanical limits. The four methods defined within this framework determine how a cell is made to "work together." In an application they can be used on their own or in combination.
| Method | How it works | Main hardware required | Typical use |
|---|---|---|---|
| Safety-rated monitored stop | When the operator enters the shared space, the robot stops safely; when the space is cleared, the cycle continues. The motors can remain energized. | Safety-rated stop function, area monitoring | Cells where the operator occasionally enters to load parts |
| Hand guiding | The operator moves the robot by hand using a guiding device. | Hand-held control unit/force sensor, safe speed | Teaching, assisted handling of heavy parts |
| Speed and separation monitoring (SSM) | A protective separation distance is maintained between the robot and the human; as the distance decreases the robot slows down and, if necessary, stops. | Laser area scanner, light curtain or vision system | Cells where high speed is to be maintained and contact is not intended |
| Power and force limiting (PFL) | In the event of possible contact, force and pressure are limited through the robot's design and speed so that they remain below the biomechanical limits. | Force/torque sensing, collision detection, limited speed | Genuine side-by-side, fenceless operation with humans |
In common parlance, "cobot" usually brings to mind the fourth method, power and force limiting. However, many real-world cells use SSM together with PFL in order to preserve speed: while the human is far away the robot runs fast, and as they approach it slows down and brings contact into the safe range.
How does power and force limiting become safe?
PFL is the combination of three engineering layers:
- Sensing: Joint torque sensors or a wrist force/torque sensor recognize an unexpected contact from the increase in force and bring the robot into a protective stop.
- Design: Rounded edges, reduced pinch/shear points and a limited moving mass reduce the severity of a possible contact from the outset.
- Speed management: The permitted speed is chosen so that even the worst-case contact remains below the limits.
ISO/TS 15066 distinguishes between two types of contact, and this distinction directly affects the design. Transient (dynamic) contact is a brief impact from which the body region can recoil. Quasi-static (clamping) contact is the more dangerous situation in which the body region is trapped between the robot and a fixed surface and cannot escape; for this reason, lower limits apply in clamping scenarios. These limits are derived from pain thresholds specific to each body region; in an application, the permitted robot speed is calculated backwards from these limits. In this article we do not give specific force/pressure figures, because the correct values must be taken from the relevant specification and from the application's own measurement.
There is no "safe cobot" without a risk assessment
The safety of a cobot cell is proven not on paper, but by that installation's risk assessment. Within the framework of the general risk assessment principles of machinery safety (ISO 12100) and robot system integration (ISO 10218-2), the tool's geometry, the workpiece's edges, the mass being handled, the cycle and the operator's actual movements are each addressed individually. In a power- and force-limited cell, the result is usually confirmed by a validation measurement: the contact force/pressure is measured with biomechanical measuring devices and shown to remain below the limits.
The safety functions themselves must also carry a reliability level. For example, in the ROKAE CR series cobot cells built by Elektrodeha, the manufacturer's safety controller is TÜV certified and rated at PL d / Category 3 according to ISO 13849-1:2015; this level is the precondition for integrating the robot into the line without a separate fence. The safety level is a far more concrete and measurable concept than the phrase "it stops when you touch it."
Where is it useful in manufacturing, and what are its limits?
Cobots create value in tasks such as machine tending, assembly, transducer-based screwdriving, pick-and-place, poka-yoke verification and palletizing. Their key advantages:
- Space savings and flexibility: In suitable applications no fence is required; the robot can be integrated into an existing workbench or a tight station and reassigned to another task when needed.
- Ergonomics: Repetitive, tiring or posture-straining tasks are taken away from the operator; the human remains on the steps that require judgment and control.
- Fast commissioning: Teaching by hand guiding shortens the setup time in some applications.
To be honest, it also has its limits. Power and force limiting inherently restricts speed; therefore the cycle time of a PFL cell can be longer than that of a fenced high-speed robot. When sharp tools, hot surfaces or heavy/pointed parts are involved, the cell often has to be supported with SSM or physical protection. The right engineering decision resolves the "cobot or conventional robot" dilemma on the basis of the application's real risk and cycle target.
At Elektrodeha, we combine ROKAE's SR and CR families (cobot series spanning a 3–45 kg payload band) with gripper, safety and PLC components to build turnkey robot cells. Our approach is measured: we first carry out the application's risk assessment, then choose between power and force limiting and speed and separation monitoring according to the force/mass and cycle requirements, combining the two when necessary. We engineer the place of collaborative robots in manufacturing according to these principles in our assembly and screwdriving cells.