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Helping Stretcher Bearers

Assoc. Prof. John Raine

Over the past five years, Mechanical Engineering postgrad student Robert Henderson and Associate Professor John Raine have been developing a pneumatic stretcher suspension that offers a soft ride to ambulance patients.

Ambulances often have to transport ill or seriously injured patients at very low speeds to avoid aggravating the patient's condition. Around 10% of journeys may be expected to cause some patient deterioration. Patients lying down are very sensitive to road vibration between 2-6 Hz in frequency; much of the worst road vibration transmitted through the vehicle suspension occurs over this frequency range.

Purpose-built soft suspensions for ambulances are very costly and not necessarily much better for the patient. An alternative is to suspend the ambulance stretcher above the floor of the ambulance. In the past, a few expensive and heavy mechanical designs have appeared but little attention has been given to pneumatic suspensions, which offer the possibility of high levels of isolation in a lightweight low-cost unit.

Earlier project work with a simple degree-of-freedom scissor-linkage pneumatic suspension proved promising. This unit isolated only bounce motions. Henderson undertook to design a two degree-of-freedom unit that would isolate the patient from both bounce and fore-aft pitching motions of the ambulance floor. The research included analysis of the dynamics of different mechanisms and an investigation of optimal pneumatic damping.

The prototype uses an aluminium alloy framework and linkage, two pneumatic rams, and has automatic ride height adjustment. The suspension is designed for patients weighing 30-130 kg and achieves natural frequencies in bounce and pitch of less than 0.5 Hz. In ambulance road tests, the stretcher suspension has provided superb isolation from road induced vibrations. The sensation described by test "patients" is of an extremely soft and comfortable ride. The new system should allow much faster journeys to hospital for seriously injured patients.

Raine intends that the suspension be redesigned for manufacture before being handed over to industry. During further road tests it is also hoped to verify results of a computer simulation of active damping which further improves isolation by continuously varying the size of damping orifices in the pneumatic circuits. A commercial partner is being sought for the final development stage.

Associate Professor John Raine lectures in Mechanical Engineering Department at the University of Canterbury.