Design Competition Pneumatic Crusher
The Impromptu portion of the CSB/SJU Engineering Design Competition requires students to build a structure out of simple materials and test it for strength by applying a downward force. To expedite the process of testing a large number of structures in a short period of time, a pneumatic crusher was built to record the applied pressure at the point where the structure gives out. A pneumatic system was chosen because of its relative low cost and the ease of slowing its response time so observers could witness a gradually collapse of the structure.
The apparatus consists of an air compressor of the type commonly used for air tools and inflating tires, a pneumatic ram, a gas pressure sensor, a data collection interface (Pasco 750), two solenoid gas valves, a computer and a specially designed electronic control unit that turns the valves on and off while the pressure is being measured. The Pasco interface accepts a large number of sensors. For this application we use a 700 kilopascal pressure sensor and a photogate. We also make use of a 5V DC output from the interface.
After the structure is placed on a platform directly below the ram, the operator starts the crushing process and the pressure measurements by clicking the start icon from Pasco software installed on the computer. The measurements begin and are graphed in real time while a 5V signal is sent to the control unit. The control unit simultaneously opens the valve from the compressor and closes the release valve on the ram. When the plunger on the ram has slowly crushed the structure and nears the platform, it blocks a photogate that sends a "stop" pulse to the interface while the control unit reverses the two valves, thus relieving the pressure in the ram and stopping the pressure measurements. The "strength" of the structure is indicated from the graph by the highest pressure recorded.
Student Placing Structure Being Tested
To make the competition a little more challenging, each structure is weighed before testing and, after the crushing pressure is determined from the graph, a pressure to mass ratio is calculated. The winning structure is determined by the largest ratio value. Because of this approach, contestants may choose to opt out of using all of the provided materials, thus lowering the mass in order to increase the magnitude of the ratio.
The apparatus has limitations. The maximum compressor pressure is 135 psi. The maximum ram pressure is 145 psi. The maximum sensor pressure is 700 kilopascals (about 100 psi). The sensor is the limiting factor and when the cross sectional area of the ram is taken into account, the downward force exerted by the ram is limited to about 80 pounds. Forces greater than 80 pounds will be recorded only as 80 pounds (pressure of 700 kilopascals). This means that the provided materials and design parameters must be carefully chosen to avoid a structure strength that is too great to be measured accurately.
Electronic Control Unit Schematic
College of Saint Benedict
Saint John’s University
Chair, Physics Department
SJU PEngl 107