Final Design

The guidance system needs to track the position of the transducer tip as it glides horizontally across the patient’s gumline, while maintaining a minimal measurement accuracy of at 100 microns. In order to fulfill this requirement, one rotary encoder and one linear encoder were deemed necessary.

Assuming the final design of the guidance system will have a maximum radius of 150 mm, the accuracy and precision of the rotary encoder cannot be greater than 0.038. This assumption was made based on the upper limit of an average human oral cavity radius. This value was determined using the following;

tan = a/b

a = 0.100 mm

b = 150 mm

𝛉 = tan-1(a/b)=0.038

The accuracy and precision of the linear encoder alone cannot exceed 100 microns due to the required accuracy of at least 100 microns for the overall system. To ensure precision and repeatability of the guidance system, the absolute encoders were deemed necessary as opposed to incremental encoders.

The digital encoders chosen for this device are both RLS manufactured electronics for their high precision and accuracy, competitive prices, adequate customer support, and transparency in lead time and price. For the device's axial encoder, the RLS Aks-IM2 Rotary Encoder was chosen. This encoder has an accuracy of 0.05 degrees before installation errors. For the device's linear motion encoder, the RLS LA11 Linear Encoder was chosen. This specific model has anaccuracy of 10 micrometers for short travel distances. The scale chosen for this encoder is approximately 80mm long.

Both encoders use an absolute scale as to eliminate the need for a predetermined home position for the device. They are both magnetic to reduce physical wear and to decrease the likelihood of dirt and debris affecting the positional readout.

To reduce the patient's head movement during the transducer scanning process, an opthalmic chin rest is used. The head positioner is a heavy duty chin rest that was purchased from a company that specializes in optometry products called Good-lite. This device is constructed of machined chrome uprights and cast metal cross bars. The materials used in the construction of the device help in maintaining a patient's head position due to its rigidity. A forhead rest is also included in the purchased system, allowing for secure positioning. A height adjustable knob located on one upright allows for easy adjustment of the chin height. The overall height of the system is 20 in. The chin rest is adjustable from 10.75 in to 14.25 in. Another key feature of this product is the table clamps that accomodate for tabletops up to 2 in. The table clamps are necessary for fixing the overall system to a lab table to further prevent any unnecessary movement during the ultrasound scanning process.

The transducer mount design consists of a 3D printed saddle that's designed to fit the organic contours of the transduce handle. The transducer is placed within the mount while the tip of the transducer is pressed against the removable alignment surface. With the transducer in place, two set screws on the side of the mount is screwed in to pushing a separate plate against the transducer's body. This clamping mechanism ensures the security of the transducer and prevents it from moving out of place during operation. The open end of the mount allows for the transducer to be easily interchanged.

For the linear motion of the transducer guidance system, a Thomson Linear 500 Series Ball Rail and Carriage were chosen. lThis specific ball rail and carriage system were chosen for their high accuracy class. The linear motion carriage is preloaded to minimize deflection along the rail. The linear rail has a travel length of 80 mm to adjust the transducer's position in the radial direction.

The rotary joint component consists of a CNC machined arm made of 6061 aluminum. Stiffness is important not only for housing the rotary encoder but also for reducing deformation during operation in order to meet the accuray requirements of the project. This arm housing for the rotary encoder also prevent dirt and debris from affecting the encoder's performance. The shaft is a machined piece of stainless steel. To reduce friction when rotating the system about its axis, tapered roller bearings were chosen for their ability to withstand large axial and radial loads.

For the elctronics components of the transducer guidance system, the device uses Arduino IDE to read and save positional data of the transducer. The communication interface is a part of our risk reduction to ensure that our components are compatible with each other. We are using SSI, synchronous serial interface, and it is a method to transmit information to the microcontroller as binary strings of data. In a basic parallel protocol, each wire connection equates to a bit of data. As you can imagine, this is difficult to achieve with larger strings of data so there are different types of protocols that reduce the amount of wire connections needed.

Although not synchronized, the images to the left show the rotational motion of the device along with the position output by the interface. The encoder's indicator light glows green when the encoder can properly read the magnetic ring. This green light can be seen towards the base of the rotational component.