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| Solenoids in Medical Laser Controls |
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Saia-Burgessā has introduced a new version of its solenoid for medical applications that require reliable control of lasers. The Ledex rotary solenoid offers exceptional speed for limited stroke applications, the ability to achieve high peak torque and a relatively small package size (less than 30mm diameter by 28mm length).
Lasers help surgeons perform operations with less blood loss. Lasers are used either directly or indirectly. A laser's energy is used energy to develop heat which causes blood clotting. Indirectly, a laser is used to sear the skin during an incision. Laser power can range from milliwatts for eye surgery to hundreds of watts for major operations, such as open heart surgery. A high degree of safety must be observed because stray laser energy can cause damage to the body. Ledex solenoids are used for control of highly powered lasers because of their size and consistent performance.
Laser surgery apparatus includes the laser beam, mirrors and a laser cavity. Normally, the laser cavity is small, such that the solenoid must not exceed 1" in any direction. The best way to control the laser is to break the beam in the small laser cavity. Typically, laser cavity mirrors reflect 90% of the laser's energy, releasing only 10% for work. If the laser delivers 100 watts for work to be done, there are 900 watts of remaining power in the laser cavity which translates to heat. Special Ledex solenoids are constructed for this environment from low outgassing materials. Some materials, under heat and vacuum, give off minute amounts of contaminants such as water or oil. If these contaminants migrate to the mirrors or lens, localized heating and a loss of efficiency occurs.
To control the laser's output power, the solenoids have a shutter attached to the armature plate or the shaft. Some systems use up to three solenoids, one in front of each mirror in the laser cavity and one on the output. All three must operate in order to allow the laser beam to be passed to the surgeon's tool. Since solenoids are inherently fast, the surgeon has "instant" control of the energy, usually through a foot pedal. Because the laser can operate on kilowatts, sometimes the laser and the controls are on separate power lines. If the control line fails, the solenoids return to their fail-safe mode with the laser blocked and cannot be opened until power is restored to the shutter.
The safety system consists of both a software watchdog system that controls the functions of the microprocessor 100 times per second and a redundant solenoid operating synchronously with the first one. If one solenoid fails, neither the operator nor the patient can be endangered by unintentionally applied power.
In low power laser applications, the beam is obstructed when the solenoid moves a shutter into its path. The shutter is massive enough to absorb the laser's energy with deforming. Usually, the shutter becomes quite hot. As a result, the solenoid must be protected against outgassing.
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