Generally, stabilizer/tracking arms require a minimum of three degrees of freedom to locate a fixed point in space. Adding the fourth degree of freedom to the stabilizer/tracking arm allows it to reflect the position of the arm more easily during the initial positioning/setup of Artemis. It also provides additional length to allow more clearance from the bedside. This type of setup was favored as the most cost-effective and simple method. Further cost savings were seen when it came to machining the components (because they are symmetric) and by reducing assembly time by several hours.
Precise control/sensing of the stabilizer/tracking arm is a key component to the device's success in fighting cancer. Although Artemis has image stabilization (i.e., motion compensation) and can account for minor movement, its arm also had to have the ability to move if the patient should shift. Since the Ogura brakes use a single-face friction area, the arm has the ability to slip when required to avoid patient injury.
Conventional power-on electromagnetic brakes have been utilized in a wide range of motion control applications for decades. The technology and application is well understood in various industries, wherever almost any form of linear or rotary motion needs to be dynamically arrested or statically held until released. These devices need the application of power to generate the magnetic field to give required braking or clutching action.
Although this is acceptable for many industrial applications, it is generally not the case for medical, safety-critical, or emergency-stop requirements. While there are various failsafe or power off alternatives available, including conventional spring-applied brakes, there are advantages to Ogura Permanent Magnet Brakes:
- There is no power/current required to achieve holding torque;
- They are backlash-free; a one-piece diaphragm spring connecting the armature to the hub is torsionally rigid;
- Existing drive components such as gears, pulleys, etc., can be easily incorporated into the brake assembly as needed;
- They tend to achieve a higher torque than a comparable spring-applied brake having the same diameter.
When there is no voltage/current going to the Ogura Permanent Magnet Brake, a series of permanent magnets produce a magnetic field that passes through the brake body and attracts the armature to the brake body. The brake body is mounted to a solid portion of the machine so whatever is attached to the armature/hub is held in place.
Conversely, when a current is applied to the brake, a magnetic field is generated. That field is equal in magnitude to that of the permanent magnets, thereby cancelling out the permanent magnets and allowing the brake to release. Once the magnetic field is no longer holding the armature, the sine-wave spring pulls the armature back creating an airgap between the armature and the brake. This allows whatever is attached to the hub/armature to rotate freely. There is no drag and no contact in this disengaged position.
With few exceptions, most medical equipment is designed to be transported relatively easily within hospitals or to other medical facilities. Therefore, size and weight are always important design considerations. Ogura's Permanent Magnet Brakes offered the Artemis device a small profile and a high torque-to-weight ratio. Also, since there was no backlash in the brake design, it allows for the key design feature of holding exact registration. Additionally, it provides less potential noise since there was no chance of the armatures and hub to rattle as might occur with a spline design.
Advancements continue to be made to Artemis based on feedback from physicians at many of its installed locations. The next-generation Artemis units will incorporate these enhancements to successfully aid physicians in diagnosing prostate cancer.
— Fred Cacace is industrial product manager for Ogura.