Appliance Design - October 2008 - (Page 28)

MOTORS electric crystals. Nanomotion uses two AC sine waves to generate the ultrasonic standing wave. An electrical field is applied in pulses to the crystal in the direction of its main axis. The pulse changes the shape of the crystal and results in a quick elliptical movement as the voltage is pulsed on and off. The ellipse is proportional to the voltage that is applied. The larger the ellipse, the faster the mechanical structure will go. At a maximum velocity of 300 mm per second the motor operates at 270 Vrms. The piezo elements used by Nanomotion are of a standard size and generate 16 oz. of linear thrust. The company offers a singlebodied motor that will incorporate up to 8 piezo elements, all of which are the same size, and which generates 8 lbs. of thrust. It also offers a driver that can handle up to 32 elements, or four of the 8-element motors. This allows for a possible 32 lbs. of thrust. Because the motor pushes against a mechanical structure, Alan Feinstein, president of Nanomotion, says that the structure itself must be stiff enough to withstand the This diagram shows the open-loop control of the PneuStep motor from Johns Hopkins University. pressure. The motor applies a force to one side of the structure that is five times greater than the driving force. Normally, the motor needs a stiffness of 40 N/micron, but that is not always possible. Recently, Nanomotion’s HR-8 series of motors were used on the neuroArm robot for use in MRI applications. The neuroArm is a robotic arm that allows a surgeon to remotely operate on a patient’s head while the patient lies inside an MRI machine. The robot uses 16 Nanomotion ceramic motors per robot, spread over the robots’ six rotational axes and one linear axis. The largest of these actuators, which is used in the base of the robot, exerts about 12 lbs. of force. The smallest motor, those for the robot’s wrist joint, exert about 1 lb. of force. Where the designers could, the structure was made of materials stiff enough so that there wasn’t any play or bending when force was applied. In other areas, designers used ceramic braces to counterbalance the pressure and eliminate any slip in the structure. While the way that New Scale Technologies and Nanomotion use the reverse piezoelectric effect differs, both companies were able to overcome a problem that affects quality when a traditional piezo motor is used in an imager. Traditional piezoelectric motors use DC voltage to expand and contract the piezo crystal and the DC voltage levels can interfere with the electromagnetic field of the MRI’s imager and can cause artifacts in images. Traditionally, this meant that the piezomotors couldn’t be operated any closer than 0.5 m from the image isocenter, says Dan Stoianovici, associate professor of urology and Director of the Robotics Laboratory at Johns Hopkins University, Baltimore. 28 applianceDESIGN AD10084ELCO.indd 1 October 2008 9/12/08 10:50:35 AM www.applianceDESIGN.com http://www.elcomotors.com http://www.elcomotors.com http://www.applianceDESIGN.com

Table of Contents Feed for the Digital Edition of Appliance Design - October 2008

Appliance Design - October 2008 Contents Editorial Shipments/Forecasts News Watch Prototyping - Materials Play the Part Prototyping - Mix & Match Motors Electronics Coatings Design Marts Association Report: AHRI Advertiser's Index

Appliance Design - October 2008

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