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Stepper Motors Bring Precision to Medical Phototherapy Device
Skin diseases such as psoriasis, vitiligo, and eczema can be treated by one of two basic methods: medication and phototherapy. Sunlight, which contains therapeutically valuable UVA and UVB forms of ultraviolet (UV) radiation, has been used as a skin treatment for over 3000 years. Two basic phototherapy methods are currently available to treat skin diseases: irradiating the diseased skin with a floodlight and using digital phototherapy.

"During treatment, it is important that as little healthy skin as possible is exposed to minimize the risk of skin cancer," explains Friedrich Lullau, considered the inventor of digital phototherapy. Unlike normal phototherapy, the UV radiation is transmitted on a pixel matrix. The individual beams can be independently controlled to ensure that only afflicted skin is irradiated. Healthy skin remains unaffected.

A development goal for the new skintrek® PT5 digital phototherapy UV exposure device from Lullau Engineering (www.skintrek.com) was to avoid exposing areas of the skin without lesions, even if a patient moved. This required an exposure head capable of following even the slightest patient movements. It resulted in a further benefit that all parts of the body could be easily treated without requiring a patient to assume new positions.

Such four-dimensional (4D) mobility was realized by using advanced, high-precision sine-commutated stepper-motor technology. The UV exposure device contains a total of seven sine-commutated stepper motors of various sizes from Nanotec Electronic (www.nanotec.com) as well as the corresponding controllers. These stepper motors are primarily the firm's Plug & Drive motors, which integrate the controller and encoder within their housing. Plug & Drive motors were favored over the combination of a motor and separate controller due to their considerably reduced cabling requirements and improved performance. Dr. Matthias Kock, Head of Research & Development for Lullau Engineering GmbH, explains, "Plug & Drive motors only require supply and interface cables. The motor and encoder lines are already integrated."

Small Size, Large Torque
A model PD4-N6018 motor with a flange size of 60mm, length of 113mm, and holding torque of 3.54Nm is located in the crossbeam (Y2 axis) of the skintrek® device where the exposure head is housed. During product development, it became apparent that desired performance would require a deviation from the original design. Real-life testing showed that the holding torque of the originally specified model PD4-N5918 motor variant with a 56mm flange and 101mm length would be insufficient. A torque of 3Nm was needed instead of the planned 1.5Nm. Without requiring extensive redesign, the device now provides almost twice the holding torque with a size increase of only 4 x 4mm over the initial design. The motor functions as a swivel drive that can rotate the exposure crossbeam by 20° on each side. The additional swiveling of the exposure unit is necessary to cover side areas of the body more precisely.

Two additional drives control the adjustment of the entire exposure unit (Y1 axis) around a pivot point at the height of the bed so that the exposure head can cover all areas of the body, even the sides. Plug & Drive motors were also used in this case, where the initially calculated torque of 1.5Nm was sufficient and the initially specified model PD4-N5918 motor could be used. A motor is located on each side; they are synchronous, but run entirely autonomously due to their integrated controllers.

The exposure head is stored in the cross member as the central component of the skintrek® PT5. The UV light source, a color filter disk, the light modulator, and a camera are also located in the device. Continuous radiation from a UV light source is first bundled by a collimator. It is then filtered, as needed or as adjusted, so that UVA radiation with a 320 to 400nm spectrum or UVB radiation with a 300 to 320nm spectrum reaches the digital light modulator. The beam of light is digitized into approximately 800,000 individual (pixel) beams. The pixels projected onto the skin measure 0.14 x 0.14mm.

Based on image recognition data combined with a dosage calculation, only those pixel beams that impinge on the afflicted areas of the skin (lesions) are switched on. The pixel beams are digitally modulated by means of digital-light-processing (DLP) technology from Texas Instruments (www.ti.com). The light modulator is a micromechanical semiconductor component consisting of a matrix of 768 x 1024 = 786,432 tiny mirrors with a size of about 11 x 11µm that can be mechanically tilted by ±12°.

A color filter disk is situated between the light source and the light modulator. The integrated color filter disk positions the color spectrum required for exposure of the particular skin disease being treated. Filter positioning is accomplished by means of Nema 8 stepper motors, controlled with an external controller developed in-house and designed for small installation space.

The entire exposure unit can be moved independently in the crossbeam, back and forth (X axis) and vertically (Z axis). For a uniform drive architecture, a PD4-N5918 motor was chosen for the X and Z axes, with a holding torque requirement of 0.4Nm.

Closed Loop, Smooth Operation
The height of the exposure unit is set by means of two ST4118 stepper motors and an external SMCI47 controller from Nanotec, combined with a linear slide. Positioning is accomplished by means of field-oriented control with sinusoidal commutation, known as a high pole DC servo motor. This enables the motors to run with true torque control, high precision, and low resonance. Position errors are minimized during the run, with corrections made continuously to minimize load angle errors during a move.

A special camera is integrated in the exposure head for automatic detection of afflicted skin. The image data from the camera is processed in a computer and used to ensure that lesions are irradiated within their exact contours. The 4D movement of the entire exposure unit enables highly precise treatment. Patients need not be immobilized when using skintrek technology. Small patient movements are recorded by the camera and the UV radiation is automatically adjusted within fractions of a second. This ensures that healthy skin is not irradiated with UV light even if the patient moves.

A physician can freely set the exposure dosage on the phototherapy device. During therapy, the device output is controlled automatically and the required dosage is computed precisely. After full dosage has been applied, irradiation is switched off automatically. The correct dosage is also applied to skin areas that are oblique to the irradiation direction of the exposure head. In addition, a physician can establish dosage profiles that continuously decrease UV light dosages at the lesion edges. In case of psoriasis, for example, a higher dosage can be set to shorten the duration of the therapy.

The skintrek® PT5 has been on the market since the start of 2012. In looking back on two years of development, Dr. Kock draws the following conclusion: "It was the right decision to use the high-torque stepper motors including the motor controllers from Nanotec. Cooperating with Nanotec has been very productive and effective. In addition, the motors exhibit very quiet operating behavior in closed-loop servo mode."


Contact: Lullau Engineering GmbH, Zeppelinstrausse 23, 21337 Luneburg, Germany +49-4131-709799-0 E-mail: info@luellau-engineering.de Web:
http://www.skintrek.com

 
 
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