Troubleshooting: Cobot Stops Working During Human Interaction

Why Does a Cobot Stop During Human Interaction?

A collaborative robot halting while a person is nearby is a normal safety response, but repeated stops disrupt workflow. The robot controller detects a condition that may indicate a collision, an unexpected obstacle, or a misconfigured boundary. The immediate reaction is to stop movement and wait for an operator or system reset. The robot does not judge whether the stop is annoying or convenient. It simply executes the logic defined in its safety architecture.

Understanding the specific trigger is the first step. A stop after direct contact is expected. A stop while a person is merely walking nearby suggests a different mechanism, such as an over-sensitivity in the proximity sensors or a software timer that expires too quickly. The control system must distinguish between a genuine safety event and a false positive. This distinction often lies in the data logged by the controller.

Operators often mistake a single stop for a malfunction. In many cases, the robot is working exactly as programmed. The goal of troubleshooting is to determine whether the stop is a correct safety function or a configuration error that reduces productivity. If the robot stops because a hand entered the workspace during a welding cycle, the system is performing its job. If it stops because a worker simply walked behind a pillar, the system configuration needs adjustment.

Common Symptoms and Their Causes

The symptoms of a stopping cobot vary based on the safety layer that triggered the halt. Some stops are immediate and forceful, while others are gradual. The table below summarizes the most frequent issues encountered in shared workspace operations.

Symptom	Likely cause	What to do
Robot stops instantly upon contact	Force or torque sensor threshold is set too low	Review the force limits in the safety settings and calibrate the sensors
Robot stops before physical contact	Proximity sensor detects a person too early	Adjust the safety zone boundary or reduce sensor sensitivity
Robot stops after a short delay	Software timeout or watchdog timer is misconfigured	Check the control software for timing parameters and reset the timer
Robot stops randomly with no contact	Electrical noise or a failing sensor wire	Inspect cable connections and shielding to reduce interference
Robot stops only during specific movements	A joint limit or workspace boundary was exceeded	Re-map the safe workspace and update the motion profile

These symptoms require different interventions. A contact stop requires mechanical and sensor checks. A no-contact stop often points to software or environmental factors. Misdiagnosing the cause leads to unnecessary recalibrations or missed safety risks. For example, if an engineer sees a contact stop and immediately adjusts the software speed limits, they may ignore a loose cable that is actually causing the erratic behavior. The symptom must match the diagnostic path.

Inspecting Safety Sensors and Calibration

The first physical check involves the safety sensors. Collaborative robots typically use a combination of force sensors, vision systems, and proximity detectors. Each layer contributes to the overall safety architecture. A fault in any one component can trigger a stop. The sensors are not standalone devices. They feed data into a central controller that evaluates the input against predefined safety criteria.

Start with the force sensors. These are often built into the joints or the end-effector. If the sensor reads a higher force than it should, the robot may stop during normal operation. This can happen if the sensor is misaligned after maintenance or if the end-effector is heavier than the calibrated value. Consider a pick-and-place task where the robot carries a pallet of metal parts. If the weight of the pallet changes, the torque on the joints changes. The force sensor detects this change. If the threshold was set for a lighter payload, the robot stops to prevent damage or injury.

Next, check the proximity sensors. These detect the presence of a person within a defined zone. If the sensor is dirty, blocked by dust, or misaligned, it may detect a person who is not actually in the danger area. Clean the sensors and verify their alignment against the safety zone map. In a paint shop, for instance, overspray can coat laser sensors or ultrasonic detectors. A thin layer of paint is enough to alter the return signal. The sensor sees a blockage and interprets it as an obstruction. The robot stops to protect the area.

Calibration is a critical step. After any hardware change, the robot must be recalibrated. This involves moving the robot through its full range of motion and recording the baseline values for force and position. Without this step, the safety limits remain outdated and unreliable. Calibration is not a one-time event. It is a recurring task that ensures the digital model of the robot matches the physical reality. If the robot arm has flexed slightly due to wear, the calibration data becomes inaccurate. The safety margins shrink. The robot stops more often.

Reviewing Software Configuration and Boundaries

Software settings control how the robot interprets sensor data. A misconfiguration here can cause stops that do not match physical reality. The control software contains parameters for force thresholds, speed limits, and safety zone boundaries. These parameters are not arbitrary. They are derived from risk assessments and manufacturer guidelines. Changing them requires a clear understanding of the consequences.

Review the force thresholds. These define the force level at which the robot stops. If the threshold is set too low, the robot will stop during normal task execution. If it is set too high, the robot may not stop during a collision. The correct value depends on the task and the surrounding environment. For a delicate assembly task involving electronics, the force limit must be low to prevent damage. For a heavy material handling task, the limit can be higher. The software must reflect the specific application. If the robot is used for multiple tasks without updating the thresholds, stops will occur.

Check the safety zone boundaries. These define the area where a person may be present. If the boundary is set too close to the robot, the robot will stop frequently. If it is too far, the robot may not stop in time during a collision. Adjust the boundary to match the actual workspace layout. The boundary is a virtual fence. It does not exist in the physical world. It exists only in the controller’s memory. If the physical layout changes, for example, a new machine is installed next to the robot, the virtual fence must move to match.

Examine the speed limits. A faster robot requires a larger safety zone to allow for stopping. If the speed limit is too high for the available space, the robot may stop to prevent a collision. Reduce the speed if the workspace is tight or the task is delicate. Speed is a trade-off between productivity and safety. A robot moving at 50% of its maximum speed can stop in a shorter distance. This allows a smaller safety zone. A robot moving at 100% of its maximum speed requires a larger zone. If the zone is fixed and the speed increases, the robot will stop to maintain the required stopping distance.

Checking Workspace Layout and Human Behavior

The physical layout of the workspace directly impacts how often the robot stops. A cluttered or poorly designed area increases the likelihood of false positives. Keep the area around the robot clear of tools, materials, and obstacles. The safety system is designed to protect people, not objects. However, many sensors cannot distinguish between a person and a stack of boxes. If a pallet is placed within the safety zone, the sensor may trigger a stop. The robot halts to check if a person is present. When no person is found, it waits for an operator to confirm the area is safe. This delay wastes time.

Define clear paths for human movement. If workers walk unpredictably, the robot may stop more often. Use floor markings or barriers to guide traffic. This reduces the chance of a person entering the safety zone unexpectedly. In a busy plant floor, workers often take shortcuts. A worker might step into the robot’s zone to retrieve a fallen part. The sensor detects the person. The robot stops. The worker retrieves the part and steps back. The robot resumes. This cycle repeats. The result is a fragmented workflow. Clear paths and barriers prevent this behavior. They create a predictable environment where the robot and people can coexist.

Train operators on safe interaction. Workers should understand the robot’s limits and how to reset it after a stop. Clear communication prevents confusion and reduces the risk of a second stop caused by operator error. Many stops are caused by incorrect reset procedures. An operator might press the emergency stop button to reset the robot. This puts the robot into a safe state that requires a specific sequence to restart. If the operator does not follow the sequence, the robot remains stopped. The operator waits. The line is idle. Training ensures that operators know the correct reset procedure. It also teaches them to observe the robot’s behavior before intervening.

Preventing Recurring Stops

Prevention is more effective than constant troubleshooting. A structured approach to maintenance and monitoring keeps the robot running smoothly.

1. Perform regular sensor calibration. Check force and proximity sensors at least once per month, or after any maintenance.
2. Keep the workspace clean. Dust and debris can interfere with sensors and cause false triggers.
3. Monitor stop logs. Review the system logs for patterns. A recurring stop at the same location or time indicates a specific issue.
4. Update software and firmware. Manufacturers release updates that improve sensor accuracy and safety logic.
5. Conduct periodic safety audits. Verify that the safety zone boundaries and force limits still match the current workspace.

These steps reduce the frequency of unexpected stops and maintain a safe environment. They also help operators trust the robot’s safety systems. Trust is built through consistent performance. If the robot stops only when it should, operators learn to rely on it. If the robot stops randomly, operators begin to distrust the system. They may start bypassing safety protocols to keep the line running. This is a dangerous trend. Prevention maintains the integrity of the safety system.

When to Escalate the Issue

Some stops cannot be resolved through basic troubleshooting. If the robot stops after a direct collision, inspect the mechanical components for damage. A bent arm or a cracked sensor housing requires professional repair. The mechanical structure of a cobot is designed to withstand certain forces. If a force exceeds these limits, the structure may deform. A deformed arm changes the kinematics of the robot. The software does not know about the physical change. The robot continues to operate based on the old model. This leads to erratic behavior and repeated stops. Professional repair restores the mechanical integrity.

If the stops persist after recalibration and software checks, there may be a deeper electrical issue. A failing power supply or a damaged control board can cause erratic behavior. Contact the manufacturer’s support team for advanced diagnostics. Electrical issues are hard to diagnose without specialized tools. A multimeter may show normal voltage, but a transient fault may only appear under load. A damaged control board may have intermittent connections that cause the safety logic to fail. The manufacturer’s support team has access to diagnostic software and spare parts. They can identify the root cause and provide a permanent fix.

Do not bypass safety features to restore operation. Disabling sensors or increasing force limits compromises safety. The robot is designed to stop, and that behavior should be preserved. Bypassing safety is a shortcut that creates a long-term risk. If a sensor is disabled, the robot relies on other layers of safety. If those layers are also compromised, the robot becomes a hazard. The cost of a repair is small compared to the cost of an injury or a shutdown.

Final Thoughts

A cobot stopping during human interaction is a signal, not a failure. It indicates that the safety system is active. The task is to identify why the system is reacting and whether the reaction is appropriate. By checking sensors, reviewing software, and adjusting the workspace, operators can restore smooth operations.

The goal is a balance between safety and productivity. A safe cobot must protect people without constantly interrupting work. Troubleshooting is the process of finding that balance. It requires a methodical approach and a clear understanding of the safety layers.

When the robot stops, pause and observe. Note the exact moment of the stop, the position of the person, and the status of the sensors. This information guides the next step. With the right checks, most stops can be resolved quickly and safely.