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Partnering March, 2014
Stepper Motors Bring Precision to Medical Phototherapy Device
By Stephan Huber, CEO, Eng. Mgr., Nanotec Electronic S
kin diseases such as psoriasis, vi- tiligo, and eczema can be treated by one of two basic methods: med-
ication and phototherapy. Sunlight, which contains therapeutically valuable UVA and UVB forms of ultraviolet (UV) radiation, has been used as a skin treat- ment for over 3000 years. Two basic phototherapy methods are currently available to treat skin diseases: irradi- ating the diseased skin with a floodlight and using digital phototherapy. “During treatment, it is impor-
tant that as little healthy skin as possi- ble is exposed to minimize the risk of skin cancer,” explains Friedrich Lul- lau, considered the inventor of digital phototherapy. Unlike normal pho- totherapy, the UV radiation is trans- mitted on a pixel matrix. The individ- ual beams can be independently con- trolled to ensure that only afflicted skin is irradiated. Healthy skin re- mains unaffected. A development goal for the new
neering (
www.skintrek.com) was to avoid exposing areas of the skin with- out lesions, even if a patient moved. This required an exposure head capa- ble of following even the slightest pa- tient movements. It resulted in a fur- ther benefit that all parts of the body could be easily treated without requir- ing a patient to assume new positions. Such four-dimensional (4D) mo-
skintrek® PT5 digital phototherapy UV exposure device from Lullau Engi-
bility was realized by using advanced, high-precision sine-commutated step- per-motor technology. The UV expo- sure device contains a total of seven sine-commutated stepper motors of various sizes from Nanotec Electronic (
www.nanotec.com) as well as the cor- responding controllers. These stepper motors are primarily the firm’s Plug & Drive motors, which integrate the con- troller and encoder within their hous- ing. Plug & Drive motors were favored over the combination of a motor and separate controller due to their consid- erably reduced cabling requirements and improved performance. Dr. Matthias Kock, Head of Research &
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Development for Lullau Engineering GmbH, explains, “Plug & Drive motors only require supply and interface ca- bles. 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 locat- ed in the crossbeam (Y2 axis) of the skintrek® device where the exposure head is housed. During product devel- opment, it became apparent that de- sired performance would require a de- viation 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 neces- sary 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 speci- fied 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
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the cross member as the central compo- nent of the skintrek® PT5. The UV light source, a color filter disk, the light modulator, and a camera are also locat- ed 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 ra- diation with a 320 to 400nm spectrum or UVB radiation with a 300 to 320nm spectrum reaches the digital light mod- ulator. 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
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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 digi- tal-light-processing (DLP) technology from Texas Instruments (
www.ti.com). The light modulator is a micromechan- ical semiconductor component consist- ing 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 be-
tween the light source and the light modulator. The integrated color filter disk positions the color spectrum re-
quired for exposure of the particular skin disease being treated. Filter posi- tioning is accomplished by means of Nema 8 stepper motors, controlled with an external controller developed in-house and designed for small instal- lation space. The entire exposure unit can be
moved independently in the cross- beam, back and forth (X axis) and ver- tically (Z axis). For a uniform drive ar- chitecture, 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 con- troller from Nanotec, combined with a linear slide. Positioning is accom- plished by means of field-oriented con- trol 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 min- imized 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 detec- tion of afflicted skin. The image data from the camera is processed in a com- puter and used to ensure that lesions are irradiated within their exact con- tours. The 4D movement of the entire exposure unit enables highly precise treatment. Patients need not be immo- bilized when using skintrek technolo- gy. Small patient movements are recorded by the camera and the UV ra- diation is automatically adjusted with- in 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 ex-
posure dosage on the phototherapy de- vice. During therapy, the device output is controlled automatically and the re- quired dosage is computed precisely. After full dosage has been applied, ir- radiation is switched off automatically. The correct dosage is also applied to skin areas that are oblique to the irra- diation direction of the exposure head. In addition, a physician can establish dosage profiles that continuously de- crease 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 develop- ment, Dr. Kock draws the following conclusion: “It was the right decision to use the high-torque stepper motors in- cluding 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:
www.skintrek.com r
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