For children who use walkers or wheelchairs, the elevated activity platforms on many playgrounds remain sadly out of reach. To remedy this, three Johns Hopkins undergraduate engineering students invented a portable, air-powered device that can lift a disabled child and his or her caretaker more than 3 feet above the ground, giving them easy access to play structures.

The lifting device was designed and built by seniors Christian Callaghan, Denice Koh and Nate Kruis during a two-semester course in which students tackle real-world engineering assignments, working within a budget of no more than $8,000. For the lifter project, the three students prepared a budget of $6,100, which they exceeded by only $400. They completed the device shortly before graduating in May. Callaghan and Kruis were mechanical engineering majors; Koh, a biomedical engineering major.

The playground lifting device that Christian Callaghan, Denice Koh and Nate Kruis invented for children with disabilities can support 500 pounds. A scuba tank provides the power.

The project began as an assignment proposed by the Baltimore-based Volunteers for Medical Engineering Inc. The organization had contacted area schools to ask how the group could help make playgrounds more accessible to children with limited mobility. Educators reported that children who require leg braces, walkers and wheelchairs had trouble getting onto the elevated playground structures outside their schools. On most of these structures, a child must be able to climb steps or a ladder in order to crawl through tubes or use the play station features mounted above the platform. Ramps are often impractical because of space limitations.

To solve this problem, the Hopkins students designed an aluminum lifting device featuring a 3-foot-square passenger compartment capable of supporting 500 pounds. Their biggest challenge was figuring out the safest and most efficient power source for raising and lowering the enclosure. Because the lifter was to be used outside on a busy playground, an electric motor requiring a long extension cord was ruled out. The engineering students considered water power, a weight and pulley system and other options before settling on a scuba tank. "It turned out that compressed air was the cheapest and cleanest power supply," Callaghan said.

Air pressure from the tank activates a pneumatic piston and pulley system that gently moves the passenger compartment up and down. The students estimate the device can be used for 100 lifts and descents before the air tank requires a $5 refill. To help prevent accidents, the students added several safety features. A brake holds the passenger compartment in place even if air pressure is lost, and a lock box restricts access to the controls.

The students installed two pairs of wheels on one end so that the lifter can be tilted back like an appliance dolly and rolled from one place to another.

The finished unit will be turned over this week to Volunteers for Medical Engineering, whose members plan to demonstrate it at area schools attended by children with

disabilities. "I was very impressed by what the Johns Hopkins students built," said Jan Hoffberger, the organization's executive director. "I was impressed by how smoothly it worked and how easy it was to wheel the lifter from one place to another. We're hoping this is a design that can be replicated and used wherever it's needed."

The playground lifting device was one of 12 projects completed this year by undergraduates in the Department of Mechanical Engineering's Senior Design Project course. The class is taught by Andrew F. Conn, a Hopkins graduate with more than 25 years of experience in public and private research and development. Each team of two or three students had to design a device, purchase or fabricate the parts and assemble the final product within its predetermined budget. Corporations, government agencies and nonprofit groups provided the assignments and funding.

The course is traditionally well-received by students. Kruis said the practical engineering experience was invaluable. "It's a big jump from working something out on paper to making the real thing," he said. Added Koh, "It was exciting to be able to do a real hands-on project that will help children."