Motors make medical imager fast

"Doctors don't like to wait for their diagnostic prints," says Kevin Roach, a mechanical engineer for the Horizon, a multi-media dry imager from Codonics. "They want the prints now."

For Roach, the design objective wasn't just physically moving paper and film through the imager faster. In fact, too much speed would prompt a loss of print resolution because the paper and film can be driven faster than the print head can move.

The Horizon imager uses seven 22-mm A-max dc motors with a speed torque gradient of 445 rpm/mNm (milli Newton meter) and three 16-mm A-max dc motors with a speed torque gradient of 2,700 rpm/mNm from Maxon Precision Motors (Burlingame, CA). 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 500 counts per turn encoders provide the home and feedback signals necessary for verifying position of pick, feed, print, and tray selection functions.

There are three supply slots on the imager for the different media. Each supply slot accepts a disposable media cassette. Each cassette has a barcode on its side. One of the 22-mm motors runs a scanner along the barcode that enables identification of the media in the supply cassette.

Another 22-mm motor moves the media from the supply cassettes to the print area. In the print area, a pinch/capstan roller assembly captures the media and positions it for imaging. For grayscale imaging, on either film or paper, another motor pulls media past a thermal print head while the imaging surface is heated.

The 22-mm diverter motor has a default setting that places the top supply-cassette materials into the top receive tray, the middle supply-cassette materials into the middle receive tray, and the bottom supply-cassette materials into the bottom receive tray.

During printing, the image itself must be received and processed. The printing mechanism in turn must produce an image. In the act of moving media through the system and printing on the media, many components are moving. The components either carry media through the imager, bring printing components into contact with the media, or drive components into positions. Often, several of these operations are simultaneously working and require coordination with speed and accuracy.

Every motion during the printing operation requires careful coordination. Because not everything can move at once, the rate of motion is important in each step. The process requires different combinations of movements, depending on such factors as which input tray to use, which output tray to use, which printhead mechanism to use, and proprietary motion relative to output requirements.

Roach reduced the printing time of a 14- x 17-inch sheet of film with multiple color images down to one minute-a process that requires approximately one and one-half to two minutes with other printers. "The key to shaving time is motors that handle load capacities ranging from inch-ounces to inch-pounds with a variety of steps," he explains. The motors had to fit within a small envelope that contained the previous motors. Maxon's motors were the only ones that fit within the envelope and met the variety of load requirements.

According to Roach, the faster the mechanisms get into position, the sooner they can move the media through the system. "We needed motors with a higher power output, allowing us to move everything faster, but with the same load capacities as our original motors," says Roach. "The result is faster printing with the same image quality and reliability."

Gaps in the magnetic path inside motors causes magnetic cogging. "The lack of magnetic cogging in our motors allows for very smooth rotation, which is beneficial in this application," explains Paul McGrath, an engineer at Maxon. "Our iron-less core motor construction, which is based upon the motor being built around a permanent magnet with the coil rotating through a tiny air gap between the housing and the magnet, is only the beginning." Finally, the housing closes the magnetic path. "This technology is still relatively rare in the motor industry and provides the advantages Codonics required."

The Horizon develops printed images and reports for MRI, CT, ultrasound, fluoroscopy, oncology, picture archiving and communications systems, and nuclear medicine units. The combination of proprietary algorithms and lack of magnetic cogging provided the higher speeds the company desired. Roach reports that Codonics was so confident in the imager, including the motor components, that the company began providing a free first-year service plan.