EdGrasso

An Olympic Level of Precision Measurement

Blog Post created by EdGrasso Employee on Feb 22, 2018

In a sport where victory and defeat are often separated by 1/100th of a second, it was surprising when both the German and Canadian two-man bobsled teams won gold medals at the PyeongChang 2018 Winter Olympics after finishing with exactly the same times. In fact, the top five teams were separated by just 0.13 seconds. Which is roughly the blink of a human eye.

 

Precise measurement is absolutely essential for many Olympic events, including bobsled, skeleton, and luge. And luge pushes the measurement limits even further by scoring speed down to 1/1000 of a second.

 

We’re no strangers to precision measurement at Analog Devices. Or to doing so for Olympic sports. ADI associate design engineer Tom Westenburg was the Principal Engineer for the US Olympic Committee’s Sports Science division. He spent 18 years with the USOC before joining Linear Technology Corporation (LTC) and now ADI.

 

I had a chance to talk with Tom about some of his experience related to timing and scoring, as well as improving athletic performance.

 

You discovered a flaw with the timing systems used at many of the sliding tracks and came up with a solution. What was the flaw?

 

Bobsled and luge tracks use optical sensors at the start and finish. They use modulated light sources so they aren’t  affected by changes in the surrounding lighting, such as when a cloud passes in front of the sun.

 

So these lights are looking for a specific modulation rate, say 100 Hz. But when the athlete breaks that beam, it really matters as to where that light is in terms of its flash cycle. As a result, you can end up with a random 10 millisecond error at the start and the finish. Now, you might think, “Well 10 milliseconds at the start and the finish will just cancel each other out.” But because they were random, an unlucky athlete could have them both work against him, adding 10ms to his time. Each light can have an error of 0ms to +10ms, so the maximum is 10ms not 20ms. And luge is a sport that’s measured down to the millisecond, so it needed to be much better than this to fairly judge every athlete.

 

So how did you fix the issue?

 

We wanted to increase the modulation rate as high as we could. We found a few commercially available lights that would work, and did some lab testing. One had a modulation rate of 20 kHz and looked great in the lab, but it had too many false triggers on the track. Around ~9.4 kHz gave us the best overall performance. It was lower than I had planned, but it was still much better than the 200 to 700-Hz lights that most other tracks were using at that time.

 

As a side note, when a light beam is broken, typically three pulses must be missed before it counts as a valid break. The random part is when the athlete enters into the pulse cycle. The second and third pulses add a slight delay, which is equal at the start and finish, so it doesn’t affect accuracy.

 

Then after we had an acceptable timing light, I wanted to make sure it was accurate end-to-end. At this point I needed some expertise and some help. I got in touch with the time frequency expert at NIST and got access via satellite to the NIST Cesium Fountain atomic clock, one of the most accurate clocks in the world. We then built a system that had super high-speed beryllium shutters used for pulsing lasers in surgical applications. We had a set of shutters with a satellite receiver at the start and at the finish. These could be programmed to break the timing light beam with 100ns resolution. The error, including the shutters, was around 50-100us. Without the satellite setup it would have been difficult to accurately test a track that is almost a mile long. In the past, the system timer would be verified in a calibration lab, but not with the timing-lights and a mile of cabling attached. That is a lot easier than an end-to-end test. As far as I know the 2002 Salt Lake games were the only ones ever tested to this level.

 

You had an interesting experience with luger sliders taking advantage of the timing system, didn’t you?

 

Yes, that’s kind of how I got involved in all of this. Many of the older tracks were using retroreflective timing lights. The transmitter and receiver were on the same side of the track with a reflector on the other side. So the light from the transmitter would reflect back to the receiver. Once the beam was broken by the feet of a luge slider passing through, the timer would start.

 

As it turned out, some athletes had suits made of a highly reflective material and a matte black helmet. The suit would reflect the beam back to the receiver and the sensor would not record a break in the beam until the athlete’s head passed through. So the slider was getting basically a full body-length head start, which could be over 200ms (i.e. -250ms + 50ms = -200ms). In a sport won and lost in thousandths of seconds, this was a huge advantage.

 

Of course, they had to be completely flat on the luge for the helmet to trip the light, and they weren’t always doing that. So there’d be these weird instances when the timer never started, and that raised some eyebrows within the sport.

 

So they came to us and we looked into it, and with some time and head-scratching we figured out what was happening. Now, nearly all the tracks use a transmitter on one side and the receiver on the other.

 

You were also involved in helping athletes improve their performance as well.

 

Yes. One example was the U.S. bobsled team. Like luge, you’re looking for any way to shave a tenth of a second or more off the run. The start is a very important and it can win or lose the race. We focused on how the two-person and four-man team members pushed and loaded into the sled. The goal was to get the sled going as fast as possible with a clean load going into the first timing light, which is where the timing of the run begins.

 

We had a real sled, but it was a dry-land sled with wheels instead of runners. We used photo-electric sensors on the wheels to measure distance and velocity, and strain-gauges in each of the push handles to measure force. In fact, we used AD626 amps to amplify the strain gauges.

 

An athlete’s excitement, especially at an event such as the Olympics, can cause him or her to push a bit longer than they should. If the first three athletes in a 4-man sled team do that and delay their load-in, it can cause the brakeman to have to run beyond the point where he/she is applying propulsive force to the sled. They then have to pull themselves into the sled. All of which can cause a poor load-in and slow the sled going into the first timing light.

 

Using that system, we could calculate where they were on the track and when they were loading. The system transmitted the sled data and mixed in a live video of the athlete to a coach’s laptop. We’d display a force profile on top of that and calculated other parameters which indicated the quality of the push and load. So teams knew how well they were pushing and loading. We wanted each team to have the optimal start burned into memory and not deviate from it. This real-time feedback enabled athletes to find that optimal point by making corrections when what they just did was fresh in their minds. Previously, it would take days to process the data, but by then, it was hard for an athlete to remember what they did, and so it would be almost useless in terms of making an effective correction.

 

The US four-man team is competing in a few days. Thanks for giving us a unique look at some of what goes on behind the scenes to enhance the precision of their performance.

 

You’re welcome.

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