For the rotational machines used to gather dental x-ray images, motion chatter can produce a fuzzy, which is not suitable for diagnostic purposes.
- Panoramic radiography defined
- Device innovation
- Gear design enables to precision control
Panoramic radiography is a branch of rotational tomography that creates images through the movement of a source and receptor in such a way as to cause the foreground and background structures to blur, leaving a defined focal trough.
As a panoramic radiographic device, the Vantage Panoramic X-ray System, designed and manufactured by Progeny, Lincolnshire, IL, incorporates a DC X-ray source, CCD digital receptor, distributed processing circuitry, and an LCD touch screen control panel for ease of use. What makes the Vantage system unique is that it is adjustable to the patient’s height via a motorized, 3-speed, telescoping column. Multiple lasers are used to locate the patient and configure the device to the patient’s morphology. Plus, a workstation is used to coordinate the individual processors.
The system incorporates an overhead, swing arm (lateral Y-axis) that supports a C-arm, which is the rotating member that moves around the patient’s head. The C-arm includes a tube-head, which produces the X-ray beam, and a removable CCD sensor, which is the digital image receptor. If this arm does not operate smoothly, a distorted image results.
The swing arm pivots on bearings located in the mounting casting fastened at the top of the column. Its motion is produced by a ball screw drive, one end of which is connected to the mounting casting and the other end to the swing arm. A step motor is mounted at the column end. Both mechanical connections of the drive assembly are through ball bearing assemblies.
The C-arm is suspended on a pair of bearings mounted to the underside of the X-axis translation plate. The C-arm casting incorporates a 10-in. ID internal tooth ring gear that meshes with a pinion gear on a step motor mounted on the stationary X-axis translation plate. The motor is spring mounted to maintain positive mesh and to minimize slop. The internal ring gear and the pinion are sized and shaped to engage on the inside surface of the C-arm. When the motor is activated, the stationary pinion engages the teeth in the internal ring gear causing rotation of the C-arm.
The engineering team at Progeny drew on the gear-design expertise of Closter, NJ-based Intech Corp, to help design the C-arm casting and its interface with the gear drive for the C-arm’s rotation. The company’s Power-Core products are specifically designed to reduce noise and vibration and run without lubrication, an important factor (a must) for medical equipment designers. Intech components are far lighter in weight than metal parts and offer longer life (less wear) and lower maintenance costs. Intech engineers used a proprietary gear load / life calculation to verify that the gears designed into the dimensionally restricted place, will last at least 8,000 hours of operation or about 15 years in field use.
The challenge was to design a backlash free gear to produce a steady rotational movement of the image producing components. There was no room to employ the traditional split gear design. To eliminate backlash, a spring attached to the pinion was installed on a slight angle relative to the axis connecting the gear centers, to pull the pinion toward the 10 in. ID internal ring gear. The spring arrangement did eliminate the backlash but caused the gear teeth to bottom out, resulting in chatter. The chatter registered on the x-ray image.
Intech designed and precision machined the pinion and the internal gear to incorporate a special contact surface which allows the components to control the center-to-center distance between the inner tooth gear and the motor pinion. Adding the center-to-center distance management element presented a method for precise gear positioning in the mesh, and drastically reduced system vibration generated by the spring force and the resulting bottoming out of the gear teeth in the earlier design.
This configuration provided precise control over gear mesh vibration and backlash, which resulted in highly increased image quality in both CW and CCW C-arm rotations. It also added a robust design element, which helped to increase product life so that image quality didn’t degrade with component wear and tear. With no fuzzy imaging due to chatter, dentists can make better diagnoses and provide better service to their customers.