Abstract

This paper presents an assessment of the performance of a real time controller using an IP network to link the controller, the sensor and the actuator. This control system is part of a anthropomorphic robot manipulator with two arms and a pan-and-tilt stereo vision head, totalizing 19 joints. Each joint has an actuator, an incremental encoder, an electromagnetic break and an inductive contact sensor. There are also two six-axis force sensors. In order to cope with all devices an distributed control system was implemented. The devices are all connected to an IP network and the real time control loops are configured by software and run over the IP network. The real time requirements of the control loops, which must run at 100Hz rate, are enforced by using RTAI in LXRT hard real time mode. RTAI is also used for monitoring the time behavior of the network packets, in order to deal with the best-effort of assumption of the IP network. Indeed, an IP network assures that it will do its best effort to deliver each data packet, but there is no guarantee that the packet will be delivered. However, a real time control loop must run at an accurate rate or the system performance or stability will be impacted. Therefore, sensor data must be feed to the controller at the same rate, or it will not be able to compute the control action. Hence, the ability of the IP network to deliver the packets with reliability and rate accuracy is very important. Of course, IP networks were not designed with such applications in mind, but it is very difficult to neglect the low-costs due to mass-production. The time behavior and the performance of a controller implemented over an IP network is characterized and compared with those obtained with a local controller. A method to deal with lost packages is also proposed. The key to this method is to note that a control network does not carry digital data in the same sense a generic computer network. A control network carries control signals in a digital format, hence the corruption of the digital data can be tolerated as long as the control signal can be recovered. This paper proposes a method to recover the control signal even in the presence of lost or late packets.

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