Servopneumatic actuators, which have spent the past decade toiling in factory-floor applications such as robotic welding and automotive assembly, are now playing a new role in the testing of human spinal cords.
Engineers at EnduraTec (www.enduratec.com) in Minnetonka, MN are using the new breed of smart air cylinders to apply axial and torsional forces to the spinal cords of cadavers, to test new biomedical devices.
"With our fixtures, we can simulate vibration or spinal column trauma," notes Lito Mejia, vice president of strategic market development for EnduraTec. "We can even program them to do step functions and simulate accident conditions that cause bone breakage or disk trauma."
The company, which specializes in the design of fixtures for fatigue testing of materials, is making the fixtures for biomedical companies wanting to test the effectiveness of spinal trauma devices implanted by surgeons. Typically, doctors perform the surgical procedure on blocks of human bone and disk, then hand off the spinal cord segments to researchers who run tests using EnduraTec's fixtures.
The company's fixtures employ a combination of technologies to perform axial and torsional force simulations. Axial force is applied by a conventional air cylinder from IMI Norgren Ltd. (www.norgren.com/usa), used in conjunction with a servopneumatic valve from HR Textron (www.systems.textron.com). After modifying the cylinder's seals, the company's engineers connect the valve to the cylinder, then use their own custom-made electronic controller to command the valve's spool movement. Position feedback to the controller is provided by a linear variable displacement transducer (LVDT). Operating at pressures of approximately 110-120 psi and at air flow rates of 18-35 cfm, the linear unit can produce axial forces of up to 1,250 lbs.
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Spine Tingler: EnduraTec engineers designed a system that uses pneumatic actuators to apply axial and torsional forces to the spinal column.
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Similarly, the company applies torsional forces to spinal cords by employing a Rotac rotary pneumatic actuator from Micromatic, a Textron Co. (www.micromatictextron.com) and coupling it with a custom-made controller. EnduraTec engineers design the circuit boards and software for the controller. During operation, they connect it to an encoder or to an angular displacement transducer, which provides position feedback.
With either system, engineers apply controlled loads and measure resulting displacements, or vice versa.
Because servopneumatic systems offer precise positioning accuracy, engineers typically use the systems to create repeated forces with a precision of less than 1mm , at frequencies of about 1-2 Hz, Mejia says.
"If you're testing a spinal fusion, you have to mimic a 'micro-motion,' and it has to be applied within 100-200 microns," he explains. "And if you need a precision of a couple hundred microns, then your ultimate resolution has to be about one-hundredth of that."
Servopneumatic systems such as EnduraTec's have begun to gain favor with engineers in recent years because they employ microprocessors to overcome the complexities of air compressibility, therefore achieve positioning resolutions measured in thousandths of an inch or less.
Mejia says that EnduraTec selected servopneumatic technology, rather than servohydraulic or electric technology, because of concerns over packaging and cleanliness.
The company says that pneumatics provide a good fit for biomedical customers, most of whom have easy access to 100-psi air supplies. Furthermore, they add that most biomedical customers shy away from hydraulic systems due to fear of oil leakage. "Leaking hydraulic fluid is seen as a problem in these kinds of applications," Mejia says. "In contrast, leaking air is never an issue."
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