Final Design

The system combines a compact mechanical structure with a brushless DC (BLDC) motor-driven actuation mechanism that fits entirely within the scanner bore and is constructed from CT-compatible materials to minimize imaging artifacts.

At the core of the design is a dual crank-slider mechanism that converts continuous motor rotation into oscillatory motion at the specimen joint. This architecture enables smooth and repeatable articulation while allowing the range of motion to be adjusted through interchangeable settings. The mechanism was selected for its simplicity, reliability, and ability to generate consistent motion in a compact package.

The device also incorporates adjustable mounting features to accommodate a range of ex-vivo rat hind limb sizes while maintaining proper alignment between the knee joint and the mechanism’s pivot. By integrating actuation, mounting, and imaging compatibility into a single platform, the design provides a practical solution for studying joint mechanics under motion rather than in static positions.

Product Demo

ROM Adjustment

Instead of aiming for a continuous range of adjustment. We opted for 5 discrete ranges of motion. By changing which hole the pin of the input arm is inserted into the output pulley, the user can choose a range of motion of 40°, 55°, 70°, 85°, or 100°. This allows for a simple and easy to use way to adjust the range of motion.

Mounting

Thigh Mount Platform

Calf Mount Platform

Due to the uncertain geometry of a rat's leg, various design solutions were explored to ensure that the rat's leg could be rigidly secured to the device throughout operation. The final design used two distinct approaches to secure the rat's thigh and calf. For the thigh, we explored a sliding clamp idea. To provide clamping force, we used rubber bands to pull the clamp down against the thigh. The sliding clamp could also be removed to mount the leg using only rubber bands. For the calf, we opted to use a velcro strap and clamp pad to secure it to the output crank arm. By cinching the velcro strap and pulling the clamp pad against the calf, the calf is securely held against the output crank arm and does not slide around. Our sponsor also expressed the desire to use medical tape directly to secure the rat's leg later on in the design process. To accommodate this, a slot was added in the mounting platform to allow tape to be fed through to mount the thigh. For the calf, instead of using velcro and the clamp pad, medical tape can be used directly.

Encoding and Powertrain

The Powertrain is a 4:1 compound belt-driven pulley system that allows for the back-and-forth motion of the linear slider. The driving pulley is attached to a small BLDC Drone Motor that can spin up 120 RPM. The output pulley acts as part of a crank-rocker, allowing the pure rotational motion of the output pulley to be turned into back-and-forth linear motion.

While the motor used for the device has an encoder, we also want to directly measure the crank arm's output position, as various sources of error within the powertrain can lead to inaccurate position measurements when using the motor's encoder. To directly measure the output position, we used a pulley setup to transmit the crank arm position out of the CT scanner to a rotary optical encoder located away from the scanning area. To achieve this, a pulley profile is added to the bottom of the crank arm shaft so that we can run a pulley belt out of the CT scanner to the encoder. This is done in two stages, as seen in the diagram, to avoid hitting the powertrain. The system also utilizes a 1:1 ratio for direct angular translation between the crank arm and the encoder.

MATLAB Interface

User-inputted Parameters

Once the user inputs the type of motion, a separate window pops up that prompts users to input their research parameters; for the "Continous Motion" option, the user will input the frequency or speed of motion, the range of motion (ROM) and the number of cycles. Once these parameters are input, the user sends them back to the main app, as seen on the right.

Real-time Motion Profile Tracking

Once connected and the user has input parameters in the separate window, as seen on the left, the user will send these parameters over to the device. Once this step is done, the user may begin the device using the "Start Device" button; upon initiation, encoder output data will be plotted in real-time on the graph on the left. In the lower right text box, updates on the number of cycles completed is updated and once the number of input cycles is reached, the device automatically turns off and finishes the plot. The user may then save the created plot or save the data themselves for replotting/analysis.

Electronics

Electronics Housing

All electronics were centralized into a housing unit with designated wire exits for user friendliness, as seen on the right. A block diagram of the connections between the electronics can be seen below.

Performance

The final prototype successfully met the project’s core functional requirements and demonstrated reliable operation within a real micro-CT environment. The device fit within the scanner’s 10 cm bore, operated continuously for over 30 minutes without motor failure, accurately tracked motion through encoder feedback, and successfully actuated an actual rat leg specimen.

Prior to scanner testing, the system underwent extensive validation across a range of motion settings and operating speeds. The device consistently completed hundreds of flexion-extension cycles while maintaining synchronization between the control interface and encoder feedback. Testing also confirmed stable operation during extended runtime, although performance limitations emerged at the most demanding conditions of 2 Hz and 100° range of motion, where motor heating, occasional stalling, and minor motion inconsistencies were observed.

Device fits inside the micro-CT scanner bore

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