A spinal joint endurance testing machine evaluates how artificial spinal joints will function in human bodies. The machine from Datum Industrial Design Inc., North Bergen, New Jersey, can be programmed for a range of spinal joint tests and scenarios. One key to the programmable machine's success is its ability to quickly set-up or change parameters for different tests. Air cylinders and valves, rather than servomotors, simplify this process.

Servomotor technology, typically used for such functions, would have presented too many difficulties to overcome in the alotted time frame, say designers.

Engineer Chris Shatteman at distributor Progresive Hydraulics recommended letting an input voltage control a variable-pressure output valve. A requirement was to vary cylinder air pressure to control load on the spinal implant joints during testing. Control had to be easy and work without large fluctuations.

A push rod/control assembly delivers controllable power to simulate conditions under which a spinal joint is expected to function. “The system is based around two Parker P3P-R Series Regulators,” says Shatteman. The regulators control the air cylinder pressure from a low of 7 psi to a maximum 100, and do so proportionately to an analog electrical signal. Inlet regulators control 100-psi maximum input air pressure to the testing machine where electropneumatic regulators or I/P (input-to-pressure) valves use 0 to 10 Vdc input signals to set pressure.

Datum's Salvesen says the Parker air valve is easily controlled by the main PLC across the entire pressure range. “The actual downward force on the spinal joint test specimen was plus or minus four pounds on a controlled force of 270 lb,” he says.