By using a miniaturized electromechanical sensor, makers of excavators, backhoes, and forklifts may soon be able to deliver programmable motion control.
The new sensing device, which is about half the size of a soda can, could be the missing link for designers who have long been trying to add position feedback to the hydraulic cylinders that do the lifting and pushing in tens of thousands of construction vehicles. With it, construction equipment could be endowed with motion control capabilities to help with the repetitive positioning of scoops, shovels, and lifting devices.
"Billions of dollars worth of construction machines have used the same method of manual control for the past 40 years," notes Richard Glasson, chief engineer for Control Products, Inc. (East Hanover, NJ). "But all of the necessary components for automation are now in place."
Glasson, inventor of the SL Series linear position transducer, says the new device solves a longstanding problem in the mobile equipment industry. Up to now, he says, the sensors used to measure the linear displacement of hydraulic cylinders have not been suitable for use in mobile equipment. The reason is that mobile equipment cylinders are distinctly different than those used in stationary hydraulic machinery, such as injection molders. Mobile cylinders typically have mounting eyes in back and front that prevent use of the long probe-type LDT (linear displacement transducers) sensors, which measure the linear movement of a piston, he says.
"The problem was that there was no sensing technology that could be deployed in a mobile cylinder without extensive and obtrusive modifications," Glasson says.
Glasson believes that his company has solved that problem, however, with the development of the SL Series. The key to the SL Series is that it does not depend on a probe-type LDT to measure long linear displacements of a piston. Instead, it combines an LVDT (linear variable displacement transducer) with a recoiling cable reel.
During operation, linear movement of the piston causes the reel to wind or unwind. As the reel winds, a micrometer connected to the reel moves in one direction. When the reel unwinds, the micrometer moves in the opposite direction. At the same time, the LVDT measures the micrometer's movement and sends a proportional signal to a controller that determines the cylinder's position.
The key to the device's success in mobile equipment is that it uses a non-contacting, short-range LVDT, instead of a long, probe-type sensor. Using compact technology is critical, Glasson says, because it eliminates the need for special drilling processes.
"With the recoil reel, we take the long linear motion of the cylinder and convert it to rotary motion," Glasson explains. "Then we use the micrometer to convert it back to linear motion, but on a much smaller scale."