Small Animal Loading Device ----Final Report

By: Akash Chauhan and Ruihe Zhang

Introduction❖ Project Descriptions and Objectives❖ Background Research❖ Alternative Design + Final Conceptual Design❖ Design Components

➢ Mechanical Components➢ Electrical Components

❖ Programming Logic❖ Final Testing + Result Analysis + Challenges Encountered ❖ Future Goals and Take Away

Project Descriptions and Objectives

❖ Create a prototype for a mouse loading device to conduct in vivo studies probing the behavior of bone cells under the influence of mechanical loading

❖ Objective:➢ Device should be able to vertically supply 3N force at frequency of 1 Hz➢ Device should apply cyclic loading for at least 15 minutes➢ Device must be portable➢ Device’s overall cost should be within $1000

Background Research

❖ Mouse placed on acetal plastic platform with leg sticking out from the edge of the platform placed between the compressional cups

❖ Sphere curvature on the compression cups➢ Better fit for the mouse’s leg

❖ Linear motion mechanism is needed to provide compression.

❖ Flexible top platform allows users to adjust height easily.

B.A.Christiansen, P.V.Bayly, and M.J.Silva, Journal of Biomedical Engineering, Constrained Tibial Vibration in Mice: A Method for Studying the Effects of Vibrational Loading of Bone

Background ResearchTop Platform

Force Sensor

Upper Compressional Cup

Mouse Leg

Lower Compressional Cup

Motor (Linear Actuator)

Bottom Platform

Basic Orientation:

V.A.Bhatia and K.L.Troy, SEM, A Portable Small-Scale Mechanical Loading and Testing Device: Validation and Application to a Mouse Tibia Loading Model

Alternative Design

❖ Electromechanical Actuator

❖ Scotch Yoke Mechanism

Final Conceptual Design❖ Justification for selecting actuator

design:➢ Less machining and mechanical

considerations

➢ Better compatibility with electrical systems and Arduino

➢ Easier to program with built in potentiometer

➢ Less wearing components (The pin inside

of the scotch yoke acts as a stress concentrator)

Mechanical Components

❖ Top/Bottom Platform and Threaded Rods➢ Basic Operating Mechanism

■ Use wrench to rotate nuts counterclockwise to loosen (the top ones) them

■ Use wrench to rotate nuts clockwise to

tighten them and lock the top platform in place

■ Note: Rotating the lower nuts counterclockwise would tighten its grip

Top Plate and Threaded Rods

Lower Nuts

Mechanical Components❖ Bracket Assembly

➢ Basic Operating Mechanism

■ Squeeze tweezer between the

space of the mounting bracket to loosen the bracket

■ Insert the bolt through the holes

and fasten the nuts onto the bolt

to mesh the mounting and top bracket together

➢ The top bracket assures stability of the

actuator as it moves and prevents vibration

Bracket Assembly

Mounting Bracket

Tweezer

Top Bracket

Mechanical Components❖ Upper Compressional Cups 3D

Printed➢ Basic Operating Mechanism

■ Squeeze force sensor through the

small rectangular gap (right above the sponge)

■ Push a small wire through the

holes on the upper cups to suspend the mouse’s leg Upper Compression Cup

Mechanical Components

❖ Lower Compressional Cups 3D Printed➢ Basic Operating Mechanism:

■ The sponge on the lower cup will

increase collision time between force

sensor and the leg to reduce the effect of sudden impact and overshoot

Lower Compression Cup

Sponge

Electrical Components: Overall Circuit Description

Loading Control

Position(Frequency) Control

Experiment Data Recording & Printing

Electrical Components: Linear Actuator❖ Function: Provide compressional force at a

controllable frequency. ❖ Progressive Automations, PA-14P, customized

BLDC linear actuator.➢ Connected with BLDC motor controller.➢ 3 inch stroke length➢ Max loading 150lbs➢ Max speed 2 inch/second

➢ Installed with an internal potentiometer to detect rod’s position.

➢ Charger: Operating with 12V and 5A

Electrical Components:Arduino

❖ Function: Giving command to linear actuator and using PID to control both compression loading and frequency.

❖ Arduino UNO:➢ Output voltage: 5V➢ Analog input (6 pins)➢ Digital & Digital PWM output (14 pins)

Electrical Components: Op-Amp Circuit❖ Function: Amplifying the signal sent from

sensor and sending it to Arduino. ❖ Components in this circuit:

➢ Force Sensor (FlexiForce A201)➢ Op-Amp Chip (MCP6004)➢ Three 1kΩ resistors.➢ One 100kΩ potentiometer.➢ 3V battery package.

Electrical Components: EEPROM Chip

❖ Function: This EEPROM chip circuit is used to record the experiment data instantaneously and store them for later use.

❖ Components in this circuit:➢ 24LC256 EEPROM chip➢ Arduino

Programming LogicWithout PID: With PID:

Constant motor speed

Changing motor speed

Final Testing and Results Analysis (Without PID)❖ Loading: Average Absolute Error

without PID for 30s:➢ 0.62±0.092 V➢ 3 ± 1.09N

Force (N) vs. TIme (s) without PID Control

Final Testing and Results Analysis (Without PID)

❖ Frequency: Number of cycles within 30 seconds without PID:➢ 31 cycles

➢ Inconsistent frequency each time interval

Oscilloscope Depicting Frequency Change

Final Testing and Results Analysis (With PID)

❖ Loading: Average Absolute Error with PID for 30s:➢ 0.62 ±0.1V➢ 3±1.12N

Force (N) vs. TIme (s) using PID Control

Final Testing and Results Analysis (With PID)

❖ Frequency: Number of cycles within 30 seconds with PID➢ 29 cycles➢ Consistent frequency in each time interval

Oscilloscope Depicting Frequency Change

Challenges Encountered and Measures Taken

❖ Excessive force applied by actuator >3N➢ Added sponges on the top and bottom compressional cups to reduce overshoot

❖ Large gain on Op-amp circuit (High Sensitivity)➢ Reduced resistance of potentiometer to optimize sensitivity of force sensor

❖ Misalignment between upper and lower compressional cups➢ Redesigned and 3D printed bottom compressional cups to achieve parallel alignment

Future Goals and Take AwayGoals:

❖ Measure the loading error: how much load exceeds 3N or how much load is required to meet 3N? How frequently does the error occur?

❖ Write a protocol of how to use the device.❖ Wrap up our design, especially the storage

of electrical components.

Take Away:

❖ Increase loading: Easy➢ Increase linear actuator’s speed

❖ Increase frequency:➢ Relatively hard➢ Increase linear actuator’s speed➢ Decrease the cyclic travelling distance

❖ Improve user interference:➢ Improve the linear actuator’s fixture.➢ Improve the upper compressional cups

and the mechanism for leg suspension.➢ Improve the top platform moving

mechanism. ➢ Improve coding: more comprehensive PID

control loop

Final Product