6.2 Project Prototype

Project Prototype

Introduction

After contemplating the alternatives to the pill crusher design, the group decided to move forward with the Chinese tea grinder-inspired design. This is because it would achieve the same outcome of the pill being crushed with fewer moving parts and pure two-dimensional motion.

Description of Subsystems

The pill crusher design can be subcategorized into two codependent systems. The first of these involves the grinding wheel crank-slider. This is the system's main component and will be actuated with a motor. The same motor will power the other subsystem, the “end brushes.” As the grinding wheel moves back and forth across the design, pill dust will collect and slowly be pushed toward the edges of the crushing surface. The end brushes will move the fragments back towards the middle of the slot to allow the wheel to crush them further. These end brushes will move via two crank sliders, or one for each end of the crushing slot. 

Kinematic Analysis

Mobility Calculations

The mobility of the four-bar mechanisms in the design can be quantified using the Greubler-Kutzbach Equation. From this calculation, our design has 1 degree of freedom. Furthermore, it meets the Grashof condition to be considered a Class I kinematic system. In other words, the crank can make a full rotation. The calculations shown below only account for the grinding wheel subsystems, but similar calculations can be done for the other two crank sliders present in the design that yield the same results. 

Gruebler-Kutzbach Equation: 

𝑀 = 3 𝐿 − 1 − 2J1 − J2

= 3(4- 1) - 2(4) - 0 = 1 DOF

Grashof Condition:

𝑆 + 𝐿 ≤ 𝑃 + 𝑄

0 + 175 = 130 + 45

Class I KC - crank can make a full rotation

Position Analysis

The desired motion for this system is for the grinding wheel to oscillate in a straight line, covering a linear distance of 90mm. This can be exactly achieved using a crank slider mentioned above with no error. A graph of the x-position of the grinding wheel versus the input angle of the crank slider is shown below. 

Velocity Analysis

A graph of the velocity of the grinding wheel versus the input angle of the crank is shown below. 

Acceleration Analysis

A graph of the acceleration of the grinding wheel versus the input angle of the crank is shown below. 

Force/Mechanical Advantage Analysis

To determine the approximate mechanical advantage of the system, we use the formula 

MA=Fout/Fin=VFin/VFout=(in*rin) / Vslider

where in is the input angular velocity from the motor (2), rin is the length of the rod connected to the motor (44mm), and Vslider is the velocity of the crusher (l1_dot). 

It is important to note that the motor applies a pure torque on link 2 to rotate it, as opposed to a force at the length of the rod. However, since we are planning to set the motor to rotate at an angular velocity that we determine, it’s better to think of our input variables as an input angular velocity, as opposed to an input torque.

From velocity analysis, we see that the slider velocity is zero when the input angle is 0° or 180° degrees, in which the slider crank is completely extended or completely retracted, leading to a potentially infinite mechanical advantage.

Thus, we want most of the crushing to be at the opposite ends of our mortar, when the mechanism is completely extended and completely retracted, since that is where we will have the highest mechanical advantage of our mechanism. Naturally, the real mechanical advantage will be lower, since this analysis assumes the pill and all links are rigid.

We are limited by the deformation of the pill. Also, our mechanism will break if we exceed the yield strength of our links and bearing connections. Thus, we will leave enough room on the sides of the mortar to allow reasonable crushing, as well as ensure we can actually crush our object given our links are made of acrylic.

Animation

An animation of the approximate motion of the full system is shown below. All distances are in millimeters. Some design edits may need to be made based on the performance of future design iterations. 

Iteration Documentation

Here is a low-resolution prototype showing how the linkages are attached to the crank. 

Bill of Materials