Control Loops in RTLinux

Control loops have the schematic form while (active) {wait for trigger; do something to a device;} and they are the building blocks of most realtime programs. Control loops in RTLinux can be just a few lines of code or can involve the synchronized performance of thousands of loops. People have used RTLinux for some of the most demanding real-time applications as well as for quick experiments. Whether you are developing a 100-microsecond duty cycle magnetic bearing controller [3], a jet engine control and hardware-in-loop simulation (as did Pratt & Whitney), or a simple robotic controller using a sound card as an improvised analog-to-digital (A/D) device (several Japanese universities), you need the same ingredients, the same principles, and a good understanding of the device or plant you are controlling.

This chapter covers basic control loop design and emphasizes issues of moving data and control information between the real-time loop and the outside world. We provide abbreviated treatment of scheduling and synchroniza- tion. Our experience is that pure priority scheduling1 satisfies 90% of all requirements and slot schedulers2 satisfy most of the rest. Both are built into RTLinux. Synchronization is not hard if a few basic rules are followed [8] and proper attention is paid to making the design robust. The RTLinux programming model is most effective when engineers can properly modularize their programs and reuse the powerful software found in Linux or BSD.

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