18.1 - Project Proposal, Group 18
Introduction:
Atomic Force Microscopy (AFM) probes are devices that can be used to map out the surface topography of a sample and are typically used for measuring roughness at the nanometer level. In simple terms, they consist of a cantilever beam equipped with a strain gauge that moves across a surface in a tapping motion and measures the strain recorded at each point, translating that strain into a height. We want to build a large-scale model AFM probe using linkages to get a better idea and appreciation of the process on the macro scale, since with a real AFM probe, the movements are so small that it is not observable to humans.
Description of the problem:
A typical strategy employed by many AFM probes is to use intermittent motion and increment the probe downwards towards the surface in fixed steps. Then, once a significant strain is registered by the strain gauge, the number of downward steps is recorded, and the height (vertical offset) of the sample can easily be calculated based on the number of steps since each step is a fixed distance. In other words, our implementation would need to utilize intermittent motion to achieve this stepping motion. Additionally, because the cantilever will be contacting a surface, we will need to consider the force profiles that will be induced by this contact to ensure structural integrity. Furthermore, the primary movement that we would be concerned with would be a vertical translational (prismatic) motion, so avoiding unnecessary rotation and play is important for getting an accurate reading and correctly converting the number of steps to direct z-axis displacement.
Description of a proposed mechanism that could solve this problem:
The stepping/intermittent motion profile can be achieved using a Geneva drive, and the motion of the cantilever probe along the z axis (prismatic) can be achieved using a slider-crank mechanism - where the vertical displacement is represented by L1 in our lecture equations and Theta 4 would be fixed such that Link 1 is perpendicular to the surface being measured. Pictured below is an example of how we could implement the two systems together. The output of the Geneva drive would be the input to the 4 bar slider crank mechanism. To move the sample(s) along the horizontal direction to continue taking readings along the entire length of the sample, we plan to employ a gear train or belt to function as a conveyor belt.
Proposed scope of work for final project:
Our proposed deliverable would be a probe that can map out the topography of a simple sample involving primarily right angles (unit steps). For example, we can print out a handful of small, 1-inch edge length cubes and arrange them in any pattern we want (i.e., stacked towers, pyramids, etc.).
The development process will follow these steps:
Establish a prismatic motion profile using a slider crank mechanism such that the slider is displaced along the Z axis
Attach a cantilever with a strain gauge to the slider oriented such that the contact force between the sample and the cantilever would be in the Z direction
Create an intermittent motion profile using a Geneva drive or similar mechanism
Integrate the two systems such that the output of the Geneva drive is the input into the slider crank mechanism
Develop a conveyor belt mechanism or similar for moving items horizontally into the path of the cantilever using a gear train or belt mechanism
Implement software to read from the strain gauge and issue the following commands:
Stop stepping down once a strain is registered
Step back up
Move the conveyor belt
Map heights and generate discretized topographical plot
Additional analysis would be needed before final fabrication:
Verify Gruebler and Grashof Condition
This will ensure that the slider crank operates correctly
Complete Position and Velocity Analysis
This will allow us to determine the exact vertical displacement produced by each turn of our Geneva drive, as this is essential for accuracy and calibration
Test software
Use a simple testing configuration to ensure we can accurately read strain and issue timely commands to actuate/stop our motors
Preliminary Design
Example Kinematic Diagram
Example Animation of Motion Profile
Anthropic, "Geneva drive slider-crank animation," generated by Claude Sonnet 4.6 [Online]. Available: https://claude.ai . [Accessed: Mar. 11, 2026].
Gruebler Calculations:
Links, L = 4
Full Joints, J1 =4
Half Joints, J2 = 0
M = 3(L - 1) - 2*J1 - J2
M = 3(4 - 1) - 2*4 - 0
M = 1
Grashof Condition:
Link 1 Length, L1 = 140 mm (P)
Link 2 Length, L2 = 35 mm (S)
Link 3 Length, L3 =150 mm (L)
Link 4 Length, L4 =50 mm (Q)
S + L <= P + Q
35 + 150 <= 140 + 50
185 <= 190