Cloud-based Active Disturbance Rejection Control for Industrial Robots

The classical design of industrial robot controllers relies strongly on constant and stable cycle times for any closed loop operation. Meanwhile, decentralizing robot control into independent services and rolling them out to virtual machines and cloud instances is an upcoming trend. The advantage of the distributed robot control lies in achieving full flexibility of location independent service execution. In contrast, the internet connectivity leads to unknown latency conditions and resulting unpredictable cycle times. That is not well handled by standard feedback controllers like PID or P-PI cascades. In addition to the time-critical aspect, the performance of a closed -loop control relies on the accuracy of the underlying model. Modeling is often labor-intensive and time-consuming. This paper presents four different implementations of a non-model based modified Switching Active Disturbance Rejection Control (ADRC) in a robotic use case. The comparison of these four concepts reveals the strengths and weaknesses of the ADRC approach regarding controller performance under the assumption of full dynamics compensation with tough communication conditions. By extending the Switching-ADRC approach by a gain-scheduler to smooth the switching between the linear and non-linear ADRC description and a static adjustment of control-parameters related to the measured round-trip time, the presented control-concept is able to stabilize the high-dynamic, partly unstable robot-system without model-information. The stability of the modified Switching-ADRC is demonstrated empirically by extensive experiments with a distributed industrial robot joint position control.