Ten in the bed: ten motors drive medical imager's print mechanism. (Application Ideas).

The Horizon Multi-Media Dry Imager from Codonics, Middleburg Heights, OH, is capable of developing printed images and reports for MRI, CT, ultrasound, fluoroscopy, oncology, PACS and nuclear medicine units. It handles three, easy-to-load, sealed, disposable media cassettes at once, allowing the Horizon to switch from color to grayscale without manual intervention. (The Horizon is said to be the first imager on the market capable of this feat.) This diagnostic imager creates dry images on film and paper. Grayscale images are direct thermal, while the color images use dye sublimation technology.

Print speed is considered an essential asset. But in order to fulfill this time-sensitive operation, all components must work in unison. The image must be received and processed; the mechanism in turn must actually produce a copy from this operation. The components carry media through the imager, bring printing components into contact with the media, or drive components into positions from which they can drive the media. Often several of these operations are working simultaneously and must be coordinated with speed and accuracy. Since not everything moves at once, the actual rate of motion is very important. According to Kevin Roach, mechanical engineer for the Horizon product, "The faster our mechanisms can get into position, and the faster they can physically move the media through the system, the better off we are. We needed motors with a higher power output, allowing us to move everything faster, but with the same load capacities. The result is faster print times with the same image quality and reliabilit y."

By the time the motor decision was made, almost all the gear and part placements had been selected. Documentation provided all the necessary technical information to allow the company to predict which motor! gearbox combinations would be best suited to each of the applications within the imager. For the purposes of making the equipment rugged enough to handle repeated use, the company elected to use metal gearboxes. "The gearboxes were available with planetary geartrains, which also made for a more rugged product," Kevin adds.

Codonics performed extensive testing of the motors and their mechanisms. "A lifetime's worth of testing for specific mechanisms that have a hard stop, and a lifetime's worth of normal operations," Kevin said. "Using encoders allowed engineers to always know how fast the motor rotated, so that they could squeeze all the speed possible Out of the system," according to Kevin.

Seven 22 mm and three 16 mm DC motors from Maxon Precision Motors, Burlingame, CA, are used in the imager. A variety of gearbox ratios are used in conjunction with Maxon-supplied rotary encoders, which are installed on all but two of the motors. The encoders provide the home and feedback signals necessary for the hardware/software controls to pick, feed, print, and select a receive tray. In order to understand the motor applications within the unit, it's best to know a little more about the imager operation. There are three supply slots on the imager for the variety of media needed for different outputs. Each supply slot accepts a sealed, disposable media cassette that carries a barcode on its side. One of the 22 mm motors is used to run a scanner along the barcode so that the imager can detect which supply cassette carries which medium.

Another 22 mm motor moves the media from the supply cassettes to the print area. In the print area, the media are captured and positioned for imaging by a Pinch/Capstan roller assembly. For grayscale imaging on either film or paper, the medium is pulled past a thermal printhead for a single pass while heat is applied to imaging surface.

The color printing is done using a dye sublimation process where the media go through the printing function three times prior to completion. A three-color media/ribbon is presented over the media and a heat-producing print head puts down the first color. Because the imager has scanned the media information, it knows when to use the dye sublimation process. The first layer of color is printed onto the media, which is sent to a hidden receive tray. The medium is pulled back into the print area and a second color is put down. At the end of the second pass, the medium is pulled into the print area for the last time and the third color is applied. During completion, the medium is diverted into the appropriate receive tray.

The 22 mm diverter motor has a default setting that places top supply cassette materials into the top receive tray, the middle supply cassette materials into the middle receive tray, and bottom to bottom. Because the diverter is part of the overall motion system, it can be programmed differently. For example, one person can set all jobs to output to the middle receive tray. For applications where several people are connected to the same imager, each can have an individual receive tray.

One challenge Kevin met while designing the DC motors into the imager was the stall torque of the motors. Although the motor gearboxes incorporated metal parts, some of the other mechanisms in the imager had plastic components. The higher stall torques of these motors occasionally resulted in component breakage. The continuous torque is used to size the motors, so the stall torque couldn't be changed. "Knowing what we were up against, we easily programmed the system to reduce the voltage at the near-stall point, so that the movement could be more gentle," Kevin says. The encoder feedback was already present. These data are monitored easily through software, to alert the system when the voltage needs to be reduced.

The final product was so rugged, with the motor and gearbox components, that Codonics provides the first year service plan free, another first in the medical imager industry.

For more information:

Circle 401--Codonics, Inc., or connect directly to their website via the Online Reader Service Program at www.rsleads.com/207md--401

Circle 402--Maxon Precision Motors, or connect directly at www.rsleads.com/ 207md-402.