Final Design

Summary of Final Design

Annotated trimetric CAD image Annotated front CAD image

Key Components

Bridge Structure

Functional requirements:

• Structurally support the microscope and additional hardware such as the double rotisserie, linear slides, etc.

• Lightweight and rigid.

CAD image of bridge structure attached to tilting mechanism

The bridge was constructed entirely out of 5.08 cm x 5.08 cm (2" x 2"). The modularity of the extrusions was appealing since it would allow the sponsor to easily modify the design of the bridge if they desired to do so. Stress calculations revealed that the member sizes would be more than adequate, indicating that a smaller cross section could have been implemented. The main reason for using a larger cross section was to allow for greater stability of the design. Additionally, the larger members provided greater rigidity. This resulted in a higher natural frequency of the structure, which was desirable since typical O.R. excitations fall anywhere in the range of 2 - 15 Hz. Having a natural frequency above this range would limit the potential for the structure to oscillate due to typical excitations.

Double Rotisserie

Functional requirements:

• Provide pitch and roll rotation.

• Easy to adjust and lock.

• Safety feature that minimizes potential for operator error.

Roll and pitch two axis gimbal. Nicknamed the "Double Rotisserie"

Photo of double rotisserie assembly attached to top portion of bridge structure

The two axis gimbal, affectionately dubbed the double rotisserie eliminates the need for external counterbalancing mechanisms to easily rotate the hefty 30 kg microscope. This was accomplished by ensuring that the center of mass (COM) of the payload is coincident with both axes of rotation. From basic engineering principles, moment is the product of force and distance. The double rotisserie utilizes this concept to its advantage. Since both axes of rotation pass through the COM of the assembly, the lever arm distance is approximately zero. Therefore the resistive torque due to the mass of the assembly that the user would experience when attempting to tilt the mechanism is approximately zero.

COM of the double rotisserie passing the pitch axis COM of the double rotisserie passing the roll axis

On top of having a typical manual shaft collar brake, the double rotisserie has an additional failsafe brake for each axis of rotation. This was achieved by implementing Electroid electromagnetic brakes. These brakes are described as "always on" meaning that when power is supplied to them they provide a resistive torque to hold the rotary degrees of freedom stationary. When the user decides to rotate the subassembly, a button on the assembly is pressed which opens the circuit and cuts power to the desired brake. When the user achieves the desired angle, all that is required is for them to release the button to close the circuit. The brake subsequently reengages without the need for further intervention. Although it would have been more efficient to implement always off brakes, lead times on the order of months prevented this type of brake from being implemented. If the sponsor decides to do so, the design could be very easily retrofitted with Electroid always off brakes.

TPA Slides

Functional requirements:

• Provide smooth and easy translation under the load of the rotisserie.

• Easy to lock.

The linear rail and carriage assembly shown below offers approximately 20 cm of travel in the x direction which allows the user to position the microscope to the left and right. There was no option to purchase a handbrake for this assembly so we had to fabricate our own.

Thorlabs PWA090

Functional requirements:

• Provide the necessary level of vibration isolation due to typical operating room disturbances (foot traffic, HVAC equipment, elevators, vehicular traffic.)

• Securely mount to the bridge structure.

Photo of Thorlabs PWA090 isolator with inner diameter clamp for Thorlabs PWA090 transmissibility curve.

mounting

This component is absolutely crucial to the functionality of our design. Due to the very fine resolution that two photon microscopes are capable of, very small excitations could result in a blurry or unclear image. This is similar to taking photos on a digital camera. When the camera is zoomed all the way in, any sort of instability in the camera operators hands is very pronounced in the photo. Considering the single neuron resolution that two photon microscopes are capable of, it is reasonable that very small excitations would contribute to unclear images. This is why the need for ample vibration isolation is necessary for an effective design.

Each of the four isolators are mounted to a single leg of the bridge. They require a constant source of 276 kPa (40 psi) compressed air that is supplied by the operating room. The compressed air is used to ensure that the upper portion of the bridge structure is level via PID control. The location of the isolators in the design ensures that the upper portion of the bridge structure (any by extension, the microscope itself) are isolated from external excitations. Furthermore, the isolators also provide additional load bearing capabilities. Each isolator has a maximum allowable load of 52 kg.

Mount a laser pointer onto the isolated portion of the structure and shine the laser across the room on a sheet of grid paper. Record a video to compare the differences.

Design Testing

Two separate tests were conducted to assess the designs ability to meet the vibration criteria specified by the sponsor.

• 1. Mount an accelerometer onto the structure and another one on the floor. Induce vibrational excitation with stomping feet and tapping unisolated portion of the structure. Compute a fast Fourier transform (FFT) to quantify the differences in acceleration.

     1. Mount a laser pointer onto the isolated portion of the structure and shine the laser across the room on a sheet of grid paper. Induce vibrational excitation with vibrating cell phones and by stomping feet. Record a video to compare the differences.

The first test is more qualitative and serves as an easy method of roughly approximating the level of vibration and to assess if usable images could indeed be captured by a two photon microscope mounted to this device. The second test is more focused on actually quantifying the level of vibration that the microscope would exhibit.

Accelerometer test results:

The results of the accelerometer & FFT testing are show below. The isolators performed very well. There are clear reductions in the magnitude of accelerations. By realizing that acceleration and position are related by the second integral, it is possible to determine the magnitude of position by dividing the magnitude of acceleration by 2πf. Through simple arithmetic it is evident that we are on the micron scale, which is good enough to obtain a viable image with the TPM. Keep in mind that testing was carried out in the EBUII building of UC San Diego; right across the hall from a machine shop. It is highly likely that the vibrational disturbances in the testing location are significantly higher than what would typically be exhibited under normal O.R conditions.

Result of fast Fourier transform on accelerometer signal Result of fast Fourier transform on accelerometer signal

without isolation with isolation

Video test results:

We wanted to utilize a more straightforward method that would visually confirm what was seen by the FFT method.

The above video captures the laser showing movement due to vibrations of the structure.

The above video captures significantly less movement compared to the previous video.

The sway at the very end was from a team member accidentally bumping into the structure

when going to review the result of this test run.