Traditional drives that generate circular motion in large medical machines, such as MRI and CT scanners, often use a motor and gear box driving a pinion that meshes with an appropriately sized ring gear to spin a turntable of some sort. This setup works but has drawbacks, such as critical gear alignment, routine maintenance, noise, and manufacturing costs. Manufacturability concerns also limit consistency from device to device.

Using linear-arc motors instead can eliminate most maintenance, much of the alignment issue, and produce a quieter machine, which is always a plus for patients. What's more, the part count drops from about six items to two.

Linear-arc motors are really a combination of existing technologies, and are newcomers to the general motion-control market. The motors have been used for years in small-scale limited-angle applications, such as a computer hard drives. Expanding the technology to multiple-phase construction with large arc segments makes it suitable for large diameter applications such as scanners. Another way to view the device is a curved linear motor.

There are no size limits to linear arc motors. Fifty-in. diameters are routine. The motors are wound for standard three-phase electromagnetic operation and can use just about any three-phase servo driver. Mechanical alignment is not as critical as it is with gears. Axial and radial alignment within normal mechanical tolerances is all that is required. However, better positioning precision always results in better performance with this type of motor. In addition, there are no mechanical wear items.

Building a linear-arc motor into a medical machine involves a little mechanical design work and the selection of feedback and bearing components. Many encoder kits use high resolution flexible-metal scales with more than 1 million counts/rev.

Bearings are also available in arc segments for limited angle or complete 360° applications. Other than bearings, no mechanical parts contact this type of motor.

The most important design characteristic to consider is to define the required torque. The motors are intended for relatively medium torque loads and work best where torque is only needed to overcome starting friction and during accelerations. Linear arc motors generally produce between 1 to 2 lb-in for every linear inch of radius.

The motors generally have less torque than a conventional rotational drive of the same size. For example, a conventional drive on a 50-in. dia. turntable may provide up to 5 to 10 times more continuous torque than a linear arc design. However, few applications need such excess torque.

Applying linear-arc motors is analogous to applying linear motors. Motor speeds can typically match traditional motors, while in general the applications for large diameter, don't typically require more than 1,000 rpm. The force they generate is small compared to that possible from conventional linear drives. But for tasks that don't require high torque, the speed, throughput, accuracy, lower complexity, lower maintenance, and other features make the technology a better pick.