Controlling Creatures with Linux

How embedded Linux satisfies the various real-time needs of The Jim Henson Company's animatronics and 3-D computer graphic puppets.
Sequencing and Timing

The timing requirements of the system are varied. The Motion Engine has to have an accurate 60Hz invocation frequency in order to update animatronic motors smoothly. Time-domain jitter in motor position data sent to a puppet adds high-frequency accelerations to that data and could cause a puppet limb to jitter. Our ultimate goal was to have a precise 60Hz Motion Engine frequency, but we planned to arrive there in three architectural iterations, as the system came on-line.

In order to prototype the system, periodic software interrupts were used to invoke the Motion Engine as a handler. Additionally, POSIX.1b SCHED_FIFO prioritization was used to make sure that once the Motion Engine was invoked, it didn't get preempted by the kernel scheduler. This allowed the Motion Engine to run in user space easily, and most importantly, in a debugger. The downsides of alarm handlers are twofold: 1) they can have jitter the magnitude of a timeslice or more, caused by a busy kernel scheduler, and 2) one can't specify their period very accurately, since it is in quantums of kernel timer ticks. We recompiled our kernels to increase the number of timer ticks per second for two reasons: to round the Motion Engine period closer to 1/60Hz and to help insure that lower-priority control system processes got descheduled more frequently, helping them all keep current with the Motion Engine state.

For the second architectural iteration, we created an RTLinux hard real-time periodic thread at a frequency of 60Hz. This thread, because it runs in RTLinux space, is about three orders of magnitude more precise than the kernel scheduler. We refer to this thread as our Hard Real Time Pacer. When it wakes up, it puts a flag into an RTLinux FIFO from RTLinux space, and our Motion Engine, blocking on this FIFO in user space, wakes up when the flag arrives. Although the Motion Engine still relies on the the Linux kernel for invocation, this architecture proved to be more accurate than we anticipated, as I/O latency is of a higher priority in the kernel than signal handling. Typical latencies of the FIFO-blocker pacer are less than 40 microseconds when there is no other heavy system activity, which means that the Motion Engine does have a true 60Hz invocation frequency, as accurate as the CPU timers can provide.

Under heavy load, the kernel may not unblock the Motion Engine on time, so this is not a deterministic hard real-time solution, but it served us well as we started using the system for production work. A dual-processor Athlon motherboard can maintain Motion Engine invocation accuracy while running the GUI, Tool Server and the Viewer, rendering OpenGL scenes in a busy loop!

For the third and final architecture we contracted FSMLabs to add an extension to RTLinux that allows deterministic scheduling of Linux user-space processes. The mechanism, called PSC, allows a sort of “jump” from a RTLinux periodic thread to user space, where we run our Motion Engine, then we fall back to RTLinux and finish. Part of our contract was that the code be donated back to open-source RTLinux for all to use.

Input Devices

The input devices used in the control system are a special combination of linear input potentiometers designed for both ergonomics and flexible use. They are uniquely suited for producing the types of motion needed by a puppeteer. We started with the rather aggressive design goal of running the whole control system on a Linux laptop, for maximum portability “on location”. Only when servicing CGI did we start creating 19" rackmount Control Systems.

Puppeteer at work on the HDPS digital puppeteering station.

One of the challenges of laptops was getting 64 channels of analog data into the machine. No A/D converter (ADC) drivers were available, so Steve Rosenbluth wrote one for the Computer Boards DAS16s/16. To date, no PCMCIA ADC card provides more than 16 channels, so Steve also designed an external analog multiplexer. In order to switch the multiplexer through four banks within one motion control frame (16.6msec) we relied on RTLinux, which gave us the determinacy we needed with sub-20-microsecond accuracy.

While writing low-level ADC drivers can be rewarding, it isn't the best way to spend our available R&D resources. We were elated to find during 2000 that United Electronics Industries offered both Linux and RTLinux drivers for its Powerdaq ADC cards, which we used in our PCI bus systems. Their 64-channel PD2-MF-64-333/16L worked like a charm, and UEI was responsive to our needs as the driver developed.