Electricity and Control November 2022

SENSORS + SWITCHES

Maximising safety in working with cobots Human-centred manufacturing places high priority on safety in manufacturing facilities where people work closely with machines.Where cobots are used, they are designed to work side by side with people, unlike the typical industrial robots that are physically separated from human operators.This proximity leads to a significantly higher requirement for safety. Research institution IDTechEx, based in Cambridge in the UK, looks at the use of sensors to ensure safety in these operations.

T orque sensors can be used for collision detection and force control. Generally, human operators set the range of torque sensors in advance. When a colli sion occurs, the force/torque detected by the sensors will exceed the pre-set range, triggering the emergency stop function. The robustness and relatively low cost of torque sensors makes them the sensors most widely used with collaborative robots. However, torque sensors only detect the torque change once a collision has happened, which means they cannot make predictions and detect an up coming collision beforehand. Proximity sensors and visual sensors can provide a solu tion to this as they can detect and monitor the distance be tween human operators and machines. Proximity sensors are usually capacitive sensors. The capacitance of the air gap between sensors and people will change when the human operators are in proximity to the cobotic arm. How ever, proximity sensors are relatively expensive, and quite a number of proximity sensors are needed to enable the cobot to detect human operators from every direction. This leads to higher costs and limits affordability. Aside from proximity sensors, vision systems also play an important role in detecting people. However, as with capacitive proximity sensors, precisely detecting the distance between the sensors and the object is difficult

because the measurement value varies depending on the object’s characteristics. Another drawback of proximity sensors is that the maximum distance detected by the sensors is limited, typically to about several hundred millimetres. To mitigate this issue, distance sensors can be used to help improve the safety and functionality of human-robot interaction (HRI). A time-of-flight (ToF) sensor is one of the distance sensors that can be used, either on the robotic skin as a measurement for collision avoidance, or as end-of-arm tooling (EoAT). To detect distance, ToF sensors use photodiodes (single sensor elements or arrays) and active illumination. The reflected light waves from the obstruction are compared to the transmitted light waves, thus determining the distance. This information is then used to construct a 3D model of the object. In simple terms, the ToF sensors detect the distance to an object by measuring the time to receive light reflected from the object when it is irradiated with an infrared ray. The average error of distance measuring for ToF sensors is relatively small (less than 10%). However, despite its great promise in detecting distance, so far, research done by IDTechEx has not seen any commercialised ToF sensors being used for proximity sensing. There may be several reasons for this. - ToF sensors are susceptible to ambient light con ditions – since ToF sensors essentially use light to function, they can be affected by the ambient light conditions - Scattered light – in the event that the surfaces around the ToF sensors are very bright, they can scatter light waves and this results in unwanted reflections, which will affect the accuracy to the ToF sensors’ measure ment - Multiple reflections – when the ToF sensors are on corners or concave shapes, there might be unwant ed reflections because the lights could be reflected multiple times, which distorts measurements. IDTechEx has noted that torque sensors are still the most commonly used sensors on commercial cobots and the only sensor system used for collision detection. However, with the increasing complexity of tasks and safety require ments, some companies have begun to integrate different sensors and apply sensor fusion. Sensor fusion entails a

Cobots are designed to work side by side with people which raises requirements for safety.

20 Electricity + Control NOVEMBER 2022