Scanner with hand

X-rays aren't the only way to get a good picture of biologic detail. A recent blood-circulation scanner lets small stepper motors quickly position a laser to sweep over a grid of adjustable size. The device lets doctors quickly confirm diagnoses and assess the progress of illnesses.

A diode-generated laser in the device penetrates upper layers of tissue (about 0.5 to 1-mm deep) and is partially reflected by red blood cells. Reflected light is picked up by a CCD camera and analyzed by software that determines the cells' speed and quantity. A few monitoring applications include diabetes, occluded arteries, dermatological problems, and burns.

One design goal is to scan a sufficiently large area in as little time as possible. If the width of individual steps is smaller than the diameter of the laser beam, measurements overlap and take longer to finish, so precise positioning is essential.

Inside the scanner

PreciStep stepper motors from Faulhaber Group, Lackland, Fla. (micromo.com) proved best at meeting the positioning requirements of moving a mirror along x-y axes. The motors need no additional sensors to handle different scan areas. Each control pulse signals the motor to turn a defined step angle which allows calculating the total angle or distance traveled. Small resolutions are possible and can be further improved with low-backlash reduction gearing. Another stepper plus: they hold position without power, or with power when loads are sufficiently high. Stepper motors with diameters of up to 15 mm allow up to 24 steps/rotation, and hit up to 15,000 rpm. The machine's life expectancy, resolution, and accuracy have exceeded minimum requirements.