Scientists Build Wireless Bicycle Brakes

Most of the responses so far are people getting hung-up on the example and responding "wireless brakes on a bike are teh shitz." If you read the PDF, they're pretty clear that the experiment is about real-time control systems using wireless communications links. The wireless bike brake is a convenient structure to do some real-world prototyping, and provides some environmental bounds (response time, lag, bandwidth, etc.) that ratchet this up one level above being a purely academic exercise.

That said, the authors are faced with the horrible reality of wireless links - they are completely unreliable. Fundamentally. Period. The aether is a shared medium, and as such, you have to deal with collisions from other transmitters and interference from unintentional radiators (microwave oven, I'm looking at you.) The objective response time in this experiment is 150mS in the wireless link, and 100mS in the physical actuator. Ignoring the actuator time, 150mS is an abstract number without context. If you're brewing coffee wirelessly, 150mS to close the loop on the temperature control is effectively "instant." [no pun intended] However, if you're measuring RPM feedback on a turbine shaft, 150mS may be an eternity.

If you're placing the wireless link in the feedback path of a control loop, which these guys are doing, you have to account for the characteristics of the wireless link as part of the control loop stability analysis. Modeling packet loss and transmission delay as the equivalent phase shift and frequency characteristic of a classical analog component can be quite challenging. Further, the characteristics of interference sources may place you squarely in the "doomed from the start" category. If the above mentioned microwave oven can impair your wireless link for the duration of a bag of popcorn, your 150mS response time is irrelevant.

Wireless links and hard real-time control systems go together like fish and bicycles do.

Source: http://rss.slashdot.org/~r/Slashdot/slashdotScience/~3/lmjRUB_T1O0/scientists-build-wireless-bicycle-brakes

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