Applying Zero Backlash Brakes in Magnetic Therapy

Major depression is a serious medical disorder affecting more than 14million American adults every year. Often a debilitating condition, depressionresults in a persistent state of sadness which interferes with an individual'sthoughts, behavior, mood and physical health.

Common treatment for sufferersinvolves pharmaceutical products prescribed by physicians that are sometimesrequired for extended periods of time. Medications and professional counselingwill usually help patients manage the negative effects of depression but forsome, anti-depressants fail to provide improvement. Extreme cases can result incomplete incapacitation of the patient and hospitalization.

As an alternative to thepharmaceutical approach to depressiontreatment, Neuronetics Corp. of Malvern, PA, has designed the NeuroStar TMS(transcranial magnetic stimulation) Therapy System. TMS is a non-invasive,non-systemic treatment for depression that applies a highly focused, pulsedmagnetic field to stimulate function in targeted brain regions. NeuroStar TMSTherapy is performed in a physician's office. Each treatment lasts about 40minutes and takes place daily for four to six weeks. To date, NeuroStar reportsthat there are have been none of the physical effects reported that are commonplacewith drug treatment. Patients can immediately return to normal dailyactivities.

A recent article in The Wall Street Journal titled, "Using Electricity, Magnets forMental Illness," detailed increasing research into techniques to stimulate orcalm the brain with electricity, magnets, ultrasound and infrared waves. Thearticle notes that "of the new brain-stimulation therapies, the most developedis transcranial magnetic stimulation, (which was) cleared by the FDA in 2008."

Designing the TMS Device
NeuroStar engineers developingthe TMS treatment needed a reliable means of holding the treatment coil inplace during therapy sessions and turned to Ogura Industrial Corp. forsuggestions. This was a major design consideration as it is necessary for the magnetictreatment coil to be able to be positioned in different axes of motion toaccommodate the different physical characteristics of individual patients.

For operatorconvenience, depressing one button releases all the Ogura brakes, providingfreedom of movement to the system. The counter-balanced gantry can be extendedor retracted as well as rotated around the mast. The therapist manually movesthe coil into the most effective location. Once the treatment coil ispositioned and directed at the targeted brain region, the button is releasedand the Ogura Power-Off Brakes lock and hold the coil in position for theduration of the procedure. Should the position need to be readjusted, theone-button operation provides for simple repositioning.

Once the patient is reclined inthe treatment chair, the treatment coil is positioned so that magnetic pulsesare focused on specific regions of the brain. It is critical that once the coilis positioned and therapy commences, no slipping or wandering of focus locationoccurs. The magnetic pulses generated by the treatment coil must remain on thetargeted area of the brain.

RNB electromagneticspring-applied brakes from Ogura were selected to hold the mast and gantry armof the TMS securely during the therapy sessions. These brakes, designed forholding and emergency braking, are considered power-off brakes in that no poweris needed to hold the brake, due to the use of a spring to keep it in place. Inaddition, the zero backlash feature of the RNB design eliminated thepossibility of flutter or drift of the gantry arm and mast during operation.

Furthermore, because the RNB is apower-off brake, there is also no electrical signal to interfere with the TMSsystem during use. Power is only applied to release the brake when thetreatment coil is adjusted to the individual patient.

Although it was not a primarydesign consideration, cost of operation of the RNB is negligible. Powerconsumption is minimized since the RNB is only energized while adjusting thesystem to the individual patient. During treatment the RNB is static, stable,non-interfering and power-free.

Spring Set Brake Details
The Ogura RNB-1.6Z Zero backlashbrake used in the TMS system is a self-contained electromagnetic system forbraking and holding rotary shafts with high angular accuracy.

Power off (or spring set) brakesuse coil springs to apply a force to stationary plates to squeeze or clamp arotating disc. When the rotating disc needs to spin freely again, a dcelectromagnetic field is used to act against the spring forces, releasing thebrake.

Most spring set brakes contain abit of radial air gap or looseness between the rotating disc and the customersshaft. This is called backlash, and in most applications is not an issue.However, in exact positioning applications, like Neuronetics TMS, backlash isdetrimental to system performance. To eliminate backlash and make the brakessuitable for Neuronetics' positioning requirements, Ogura designed a uniquehub/spring assembly.

For Neuronetics' TMS, Ogura useda machined steel drive hub (affixed to the customer's shaft using a key and twosets screws). This drive hub is fixed to Ogura's low inertia flat spring steelfriction disc. The resulting strong, yet flexible disc allows for stiff (zerobacklash) radial torque while smoothly flexing (axially) during braking.

Ogura's high coefficient frictionmaterial is bonded to the brakes pressure plate and backing plate, both ofwhich will make contact with the rotating steel friction disc during braking.On most zero backlash brakes the friction material is bonded to the steel disc,which means the friction material would rotate during disengagement. SinceOgura bonds the friction material to the non-rotating components, only the thinspring steel disc rotates so higher operating speeds can be achieved. Moreimportantly, there is less vibration because the thin steel disc is inherentlybalanced. To ensure smooth braking and high torque, the pressure plate/frictiondisc and the backing plate/friction disc assemblies are precision ground forflatness before assembly.

Constant torque is created by theuse of three coil springs captured in precision trepanned cavities. Thesesprings apply even pressure to the pressure plate to contact and compress thefriction disc against the stationary backing plate.

Once thebrake is engaged (and shaft stopped) the Ogura spring steel friction discassembly holds the load firmly and will not flex radially. This provides a firmposition for the equipment to apply the proper magnetic pulses to the patient.

To release the brake, a dcvoltage is applied to the coil, creating a magnetic field in opposition to (andstronger than) the springs clamping forces. This attracts (pulls) the pressureplate back though its air gap, releasing the brake from the friction disc/drivehub assembly and allows the shaft to turn freely with no drag or noise.

This was an importantconsideration in the TMS design, since quiet operation is essential in atherapeutic environment. Since the RNB power-off brakes are only energized whenthe treatment coil is being positioned, the treatment coil, once put intoplace, is held securely with no electrical noise or "hum" to distract thepatient.

Another feature of the RNB-Zseries brake is that, because of its unique hub arrangement, it can operate inany orientation (shaft verticle for example) and not produce any residual dragwhen the brake is disengaged. A traditional splined hub design would haveproduced both unwanted drag and noise in Neuronetics' sensitive application.
Frank Flemming is president of Ogura IndustrialCorp.
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