DN Staff

September 27, 2004

9 Min Read
Engineering Feat

No downloading data, don't worry about changing batteries--just remember to turn it on when you go out running.

In March 2001, three engineers at Adidas with no experience using electronics set out to pursue an idea considered to be more than a little out there: Design a computer-based, dynamic cushioning system to fit into the sole of a high-performance running shoe. Following the approach of Lockheed's now classic Skunk Works team that put the XP-80, the U.S.' first jet fighter aircraft, into production, the team worked in isolation and under a cloak of secrecy to bring the world's first smart running shoe into production.

Consisting of an 8-bit microcontroller (MCU) operating at 20 MHz, motor-controlled lead screw, Hall-effect sensor, and specially-designed plastic cushioning element, the system maintains the desired cushioning level by measuring the compression of the cushioning in the midsole and automatically adjusting it on the fly. The motor drives a series of gears with a 50:1 ratio, turning the lead screw. It expands or contracts a cable, altering the available space for the movement of the cushioning element, which features two concentric walls on each side to withstand shear force and provide stability. Typical compression in a 24-mm-high running shoe is 10 mm-or almost 50 percent.

Power management key

Innovation Team Leader Christian DiBenedetto says that the overall design was driven by a desire to retain the runner's experience (not add weight or height to the shoe, for example). Battery life was another issue. "Ideally, we wanted the battery to last the life of the running shoe, says DiBenedetto. "The target life was 100 hours."

First, the design team had to figure out a way to accurately measure the amount of compression in the midsole cushioning-virtually in real time. Just 25 milliseconds from the instant a runner's foot hits the ground, the cushioning will achieve maximum compression. They eventually hit on the idea of using a linear Hall-effect sensor from Allegro on top of the cushioning and a magnet located below it. The voltage output of this contactless sensing device accurately tracks the changes in magnetic flux density based on its distance from the magnet. The sensor is not only able to measure the distance that the cushion element has compressed, it also measures the time to achieve full compression-allowing the software to identify particular surface conditions.

DiBenedetto says that Hall-effect technology made the most sense based on size and cost, and most importantly from the standpoint of power consumption . "We looked at other ways to do it, but the Hall-effect solution allowed us to tweak the power budget. An inductive sensor, for example, would have required that the coil be powered continuously," says DiBenedetto. The Hall-effect sensor also met weight requirements.

Pulsing technique key

But even with only 5.6 mA current draw, the sensor still posed a battery-life problem. Engineers had chosen a readily available 2450 lithium watch battery with a nominal 3V cell, and nominal life of 550 mAh for the power source. (The sensor requires 5V, so the battery's nominal 3V is boosted to 5V in the system.) "It's a standard battery and that was a key consideration for us," says DiBenedetto. "We did not to want to create a product that required the consumer to have to hunt around for a specialty store to replace a dead battery."

Test Rig: Not exactly the place you'd expect to find a running shoe--Adidas engineers used an Instron test machine to determine comfort level for cushioning and ruggedness requirements for motor housing protection.

Engineers implemented several strategies to reduce the sensor and MCU's appetite for power. One technique they use is to power the sensor only when needed. For example, it has to be on and stabilized to perform the A to D conversion. But once the A to D conversion is complete, the MCU turns off the power to the Hall-effect sensor and the voltage reference. After the calculations and storage, the MCU itself goes into the sleep mode until the next reading.

Engineers also investigated ways to reduce the power required to adjust the compression over the targeted range of 1 to 10 mm-namely making sure that the system could react fast enough to make adjustments between strides. "Since we have a compression measurement system we know when the foot is going to leave the ground," says DiBenedetto. "Because the system is in its at-rest position at this point, it takes minimal power to make a change, so that's the only time we alter the cushioning level."

Race to the End: Although they had no previous design experience with putting electronics into a running shoe, Adidas engineers beat their original project timetable by many months.

Just like other shoes

Equally important to the timing issue was making sure that any changes would be transparent to the runner. So engineers designed the system to make small changes over a series of steps, evaluating four ground strikes and disregarding any anomalies. Changes are made incrementally over a series of steps. By sampling the sensor 1,000 times a second, engineers obtain sufficient points on the curve to make an accurate-enough measurement and eliminate noise-influenced spikes. The software algorithm helps determine a change in surface conditions. "We are able to extract from compression data when a runner leaves the road and goes onto a trail or grass," says DiBenedetto. The shoe adapts to the new conditions and then back again-a discovery that allows the shoe to do more than engineers originally intended it to do.

Some seemingly simple design issues proved more complicated than expected for the team: For example, the criteria for viewing the LEDs in the user interface called for a clean, crisp, and non-dispersed light that the runner could easily read from the side and above. "We went through 15 iterations before settling on light pipes behind the part that tunnels down to the LEDs," says DiBenedetto. The light is pumped through five pipes molded out of the same material as the user interface, which is a clear thermoplastic urethane (TPU).

Power Management: To save on power requirements, the microcontroller pulses the Hall-effect sensor, which is one of the most power-hungry devices in the system. The black line represents the actual raw data the shoe received. The green line indicates when the shoe goes into the sleep mode to conserve power. The red line is critical data the system processed for this particular experiment. The important information is the rise and fall of the peaks, which helps the MCU determine when to change the cushioning.

Packaging was another challenge. In a worst-case scenario, a 150-lb runner lands on a curb, resulting in a 450-lb impact load directly over the motor box. The team had to construct a housing that could withstand this maximum force, but would not transfer it to the runner's mid-foot. They came up with a two-part solution: A rigid, high-impact polycarbonate housing around the motor box protects it and maintains alignment for the parts inside it. This housing is then placed in a flexible TPU. The shape and ribbing of this housing distributes the load around the motor and up into the shoe, providing a cradling effect for the foot.

Like any consumer electronics devices, the Adidas 1 comes with a user's manual that will be in digital form and it even comes in a special box. "We wanted to move away from the cardboard and tissue experience so we created a whole new shoe box," says DiBenedetto.

With all the attention given to the details in the world's first computerized running shoe, users won't have to worry about rebooting. "No downloading data, don't worry about changing batteries," reassures DiBenedetto. "Just remember to turn it on before you start running."

How It Works: To adjust compressive force on the cushioning element, the motor drives a lead screw that expands or contracts a cable. This alters the space (d) available for movement of the cushioning element. As the screw turns, it expands or contracts a cable connected to a cushioning element in the heel of the shoe.

Minimum Impact: The dynamic cushioning system in the Adidas 1 running shoe consists of a microcontroller, brush dc motor and lead screw, cables, and Hall-effect sensor to automatically adjust the degree of cushioning.

Contributing writer Randy Frank can be reached at [email protected] .

Web Resources

//Check out the links below for more info//

Video of the Adidas 1 dynamic cushioning system:

Lithium coin battery from Panasonic:

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