Machine Design

Encoder helps drag bike hook up and go

Horsepower isn't a problem for the Top Fuel drag bike from Salakazi Racing in Finland.

Encoder helps drag bike hook up and go

Removing the clutch housing reveals the transmission input shaft. A Renishaw RM22 encoder mounts on the shaft end to measure rpm. Another RM22 goes on the crankshaft end to monitor crank position. The encoders contain a small, diametrically polarized magnet and a custom ASIC with an array of Hall-effect sensors. The sensors generate a voltage when exposed to a magnetic flux field. The ASIC detects the changing magnetic field and cancels any magnetic interference. The encoders come in seven to 13-bit models and industry-standard absolute, incremental, and linear-output formats. They provide up to 8,192 counts/rev at accuracies to 0.4° and speeds exceeding 30,000 rpm. RM22s withstand temperatures from –25 to 125°C, as well as high levels of shock, vibration, acceleration, and pressure.

The crew works on the clutch of their KTMpowered Salakazi Racing drag bike.

The nitromethane-fired 1,995-cc KTM engine makes 1,500 hp at 7,500 rpm, enough to propel the 800-lb dragster to nearly 200 mph in about 6.7 sec on a quarter-mile track. The trick is getting that much power to the ground from a dead stop without timerobbing wheel spin, an impossible task for a rider to manage alone.

That is why Salakazi opted for a Prowork digitally controlled, three-disc, four-stage clutch. The controller progressively engages the clutch when the rider snaps open the throttle. Properly adjusted, the clutch transmits just enough power to keep the rear wheel at the threshold of breaking traction.

Feedback for controller programming comes from a pair of Renishaw Inc. RM22 encoders. One encoder monitors enginecrankshaft position; the other measures speed of the transmission output shaft. "The encoders aren't interactively controlling the clutch while the bike is underway," Salakazi Racing's Petri Mäkinen explains. Rather, "The data help technicians properly adjust the clutch firststage counterweights for track conditions."

The RM22's small, 22-mmdiameter body lets the device fit on the transmission-shaft end (inside the clutch housing) without compromising the strength of the housing itself. The crew removes the encoder immediately after a race so it escapes heat soak from the clutch, Mäkinen says.

The noncontact, two-part design eliminates bearings and seals, allowing for rapid, easy installation and removal. Next season Salakazi plans to use Renishaw encoders for controlling ignition advance and, in the future, to adjust cam timing.

Renishaw Inc.,

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