Cheshire, CT--Speed sensing is supposed to be non-contacting. Some applications, however, will not accept textbook solutions. An example: Euclid Hitachi's wheel slip system.
Designed for 60- to 85-ton off-road trucks, the system relies on right- and left-wheel speed sensors to initiate traction control (see DN 1/9/95, page 80.) Not only must these sensors generate a pulse for each and every tooth of the rear axle's 20-pitch, torque-carrying spline, they must accommodate substantial shaft movement as the truck grinds through quarries, construction sites, and timbered terrain.
"We were able to meet Hitachi's pulse train requirements with a product from our Hall effect sensor line," says John Lord, senior marketing engineer with Airpax Instruments, "but there was a problem with the air gap."
Lord explains that the ability to count a 20-pitch spline, which may rotate at a speed as fast as 600 rpm, dictates an air gap no greater than 0.030 inch. With the shaft moving as much as 1/8 inch off center, a conventional non-contacting Hall sensor was not feasible.
What about other sensor types? Capacitive inductive sensors, Lord claims, are better suited for gross motion control. "The technology that allows you to work over that size air gap," he says, "would not see this fine a pitch or give enough data points per revolution--even in ideal conditions."
Ruggedized packaging provided the solution. The sensor's spring-loaded hardware, developed for continual exposure to harsh environments, rides against the shaft. Contacting pressure keeps the sensor's magnets and electronics, potted in a DelrinTM cap, within the required tolerance.
• Shafts with large runouts
Operation is standard. As the gear tooth moves in front of the sensor, the sensor changes state. When the target leaves, the sensor returns to its original state, and this sequence repeats for each passing tooth. The sensor produces a square pulse wave of constant magnitude, regardless of speed.
"The design feature of loading the sensor against the tooth was straightforward," Lord says. "Making sure the unit would stand up to thousands of impacts per minute was the bigger issue." Initial units incorporated a cap made of beryllium-copper alloy. Airpax engineers subsequently determined that the lower-cost Delrin cap worked just as well, lasting more than 8,000 hours in accelerated life tests.
Additional details…Contact John Lord, Airpax Instruments, Philips Technologies, Cheshire Industrial Park, Cheshire, CT 06410, (203) 271-6027.