Over the past few years, semiconductor companies have increasingly focused on the medical device market. Here, an emerging trend is the increase of home-managed care versus inpatient therapy — in turn, fueling the need for medical device manufacturers to develop products that are portable, affordable, and user friendly. More specifically, diagnostic and therapeutic devices such as dialysis equipment, portable insulin pumps, insulin inhalers, and diabetes management systems are increasingly accepted self-care medical devices, and are driving device manufacturer innovation.

To meet the challenge, product designers are integrating specialized components that suit small medical devices. Case in point: Miniature reflective encoders are proliferating in portable medical devices because, among other reasons we will explore, these motion feedback devices consume less power than traditional magnetic encoders.

Motion feedback objectives

Portable medical device design usually begins with building a list of product requirements based on market needs and conditions. In most circumstances, tradeoffs must be made between component sophistication and cost.

Weight and size - Medical devices labeled “portable” imply that they are easily carried — so size and weight specifications are critical. Therefore, portable medical devices requiring precise mechanical positioning benefit from lightweight reflective encoders for accurate position tracking.

Most commonly, a rotary version of such an encoder is attached to the back of the motor used to drive a series of mechanical systems. Alternatively, a linear encoder is used with a codestrip that has a series of black and white tracks on it. Here, the encoder is mounted on the system's moving part for motion tracking.

Resolution, frequency, and accuracy - Another set of criteria for selecting the right motion feedback device is the sensor's ability to provide suitable resolution, frequency, and accuracy. For a rotary system, resolution is the total number of steps representing one revolution of motion — often measured as counts per revolution (CPR) — or, for linear applications, as lines per inch (LPI). Higher CPR does not necessarily imply better accuracy in the design or indicate any detailed information about potential cycle error. A rotary encoder's final output resolution is determined by its module's count density and the size of the matching media or codewheel. The relationship between encoder output resolution (CPR), codewheel size (often expressed as optical radius, or Rop), and encoder count density (measured in LPI) is:

LPI = CPR/2 • π • Rop

A 6 mm-diameter housed optical encoder can offer at least 50 CPR pre-quadrature resolution or higher — or this can be quadrupled with external electronics to obtain post-quadrature output.

An encoder's frequency rating determines how fast the motor can spin without the encoder losing count. A typical miniature dc motor is rated around 20,000 rpm or lower at no-load conditions, with typical applications running around 6,000 to 10,000 rpm. At a rated motor speed, a typical 50 CPR encoder will need to have a frequency rating of at least 16.7 Khz or higher.