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Backhoes are widely used machines with hydraulic shovels and buckets operated by mechanical controls. That combination can give rise to some unexpected vibration problems, according to a recent study by °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Davis mechanical engineering professor Don Margolis and Taehyun Shim, assistant professor of mechanical engineering at the University of Michigan.

In some backhoe designs, a small movement of the mechanical controls leads to a rapid vibration that shakes the vehicle so hard the operator cannot hold the controls. The mechanical controls open valves in the hydraulic systems that move the bucket and shovel.

That shaking is caused by positive feedback between the hydraulic and mechanical controls, Margolis said.

The problem is made worse when the hydraulic controls are attached to the chassis but the mechanical controls are attached to a cab isolated from the chassis by dampening springs. Using numerical models, Margolis and Shim showed that simply stiffening the suspension of the cab can eliminate much of the problem. Automatic control systems could also be developed that would eliminate the feedback problem, Margolis said.

The study was published in the September issue of the Journal of Dynamic Systems Measurement and Control.