Integrated Linear Motion in Medical Device Design

There are a number of linear motion technologies availableto machine and device designers, each with advantages and disadvantages.Medical device design in particular, in which applications typically requirehigh accuracy and repeatability, linear motion systems with belts and pulleysor rack and pinion are unlikely to meet performance specifications. Likewise, forclean and quiet operation, hydraulic and pneumatic systems are inappropriate.

Mechanical linear motiontechnologies, on the other hand, provide high performance and reliability,clean operation and low maintenance. The motion technologies in this group mostsuitable for medical device/machine applications include the linear motor,linear slide with rotary motor and stepper motor linear actuator.

A linear motor is essentiallya stepper motor where the stator has been "unrolled" so that instead ofproducing torque (rotation), it produces linear force along its length. Linearmotors allow direct coupling to the load and deliver high performance, but at acomparatively high cost. Pricing a linear motor system must include costs for acomplete stage, or enclosure, with linear bearings, limit switches, cabletrack/carrier, protective bellows and linear encoder.

A linear slide consists of aframe-mounted carriage that travels along a lead or ball screw. A rotary motoris mounted to one end of the frame where it is coupled to the screw that itturns to produce linear motion. Advantages of this technology are accuracy andrepeatability, with support provided by the frame already in place.Disadvantages are the high cost of components and locating or designing an adapterfor the motor interface.

A stepper motor linearactuator uses a threaded shaft inserted in a nut integral to the motor's rotor.Linear motion is produced by the rotation of the threaded shaft, which can becoupled to a load using a variety of methods. Versatile, low-cost stepper motorlinear actuators are available with different shaft styles, including:

For a complete linear motionsystem, each of the previous linear motion technologies listed requireadditional components including a compatible stand-alone controller, drive,encoder and cabling.

One technology, however, thestepper motor linear actuator, is available with all of these componentsintegrated into a single, compact linear motion product not much larger thanthe linear actuator alone. As a result, the integrated stepper motor linearactuator can significantly lower device cost and complexity.

Integrated Stepper Motor Linear Actuator

In response toincreased demand for smaller,low-cost machines, machine designers are trending toward motion componentsolutions with reduced size and price. As a result, integrated motion controlsolutions - all-in-one rotary motor with drive and/or controller and encoder -represent a rapidly growing market segment.

Integrated motion control products,both rotary and linear versions, reduce the factors that impact the cost ofcreating a device, which include:

Clark Hummel is managerof Applications Engineering, IMS Schneider Electric Motion USA.