Living tissue is absolutely dependent on an adequate blood supply for oxygen and nutrient delivery. Insufficient blood flow is therefore always a sign of suboptimal conditions in the body. Often, a superficial examination of the skin or tissue is sufficient for diagnosis. A new blood flow measuring device now takes advantage of this fact: A laser beam scans the blood flow through the tissue. A stepper motor drive provides the necessary mechanical deflection of the laser beam. This allows the sensor to obtain reproducible measurement results in a small space. Precise step size, suitable low-backlash reduction gears, easy control, long service life, and above all, small motor dimensions are essential for this application.
Modern miniature drives are indispensable, especially in compact devices. Electronic components and optoelectronic parts are shrinking daily. However, many applications still require very small mechanical components. Significant advancements have been made in this area. Mini and micro motors in a wide variety of designs now meet virtually all user needs.
Medical devices are often characterized by two things: they are large and they are expensive. A newly developed blood flow scanner proves that there is another way. It combines modern electronics, laser technology, and micromechanics with an evaluation PC to create a small, practical unit. Since good blood flow is essential for all bodily functions, it allows for quick, well-founded diagnoses and trends in certain diseases. Consider, for example, circulatory disorders in diabetes, peripheral artery disease, dermatological problems, or wound healing, burns, and transplants. The operating principle is simple: A diode laser beam penetrates near-surface tissue (approximately 0.5 to 1 mm deep, depending on the tissue type) and is partially reflected by the red blood cells. This allows both the speed and quantity of the blood cells to be determined. Several factors are crucial for the actual measurement: A sufficiently large area must be scanned in the shortest possible time to obtain a kind of "still image." At the same time, as many measurement points as possible should be located within this area. However, if the individual step length is smaller than the diameter of the laser beam, the measurements overlap. This wastes time – without any significant gain in information. With a laser beam diameter of approximately 1 mm, the following medically desirable properties result: The largest scannable area is approximately 50 x 50 cm², allowing, for example, the capture of angled limbs, with up to 255 x 255 measurement points. The adjustable resolution allows for step sizes of approximately 3 mm, 2 mm, and 1 mm for low, medium, and high resolution at a distance of 25 cm from the tissue. The highest-resolution mode operates with a step size of only 0.5 mm, resulting in overlapping measurement points. The reflected light is captured by a CCD camera and analyzed using specialized software.
The requirements for the scanner's mechanics are multifaceted. Firstly, it must be transportable as a mobile device without risk of breakage; secondly, reliability and EMC compatibility are crucial, especially in the medical field. Since the scanner can operate in different modes, an easily adjustable yet reproducible optical step size is essential. To save time and money, the drive control should be easy to integrate into the existing electronics. A stepper motor drive offers the best prerequisites for this. Stepper motors can be used without additional sensors at a defined step size; each control pulse advances them by a defined step angle. Thanks to the high step count, the resolution is very good and can be further improved by a subsequent reduction gear. The scanner drive also takes advantage of this: a low-backlash gearbox enables the high resolution and precision required for laser scanning. The gearbox has been thoroughly tested and certified for this application. The limits for service life, resolution, repeatability, etc., significantly exceeded the application requirements. This was also a key reason for choosing the FAULHABER PRECIstep motor-gearbox combination. The fact that stepper motors can maintain their position even when de-energized, or even deliver comparatively high torque when energized, is another advantage. Small stepper motors up to 15 mm in diameter now allow up to 24 steps per revolution or can reach speeds of up to 15,000 rpm. The evaluation is quite simple; an electrical pulse always corresponds to a defined angle of rotation, which...
