Fluctuating Stress Laboratory Experiment: Hand Crank Generator

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Contact Information

University: University of Minnesota - Twin Cities

Team Members (with year of graduation): Aaron Schmitz (2013), Erik Sauber (2013), Michael Daniels (2013), Robert Jennings (2013), Stephanie King (2013), Zhengmu Wang (2013)

Faculty Advisers: Sam Strickling, Prof. Susan Mantell

Email Address: Michael Daniels (danie762@umn.edu), Erik Sauber (saub0015@umn.edu), Stephanie King (kingx809@umn.edu), Robert Jennings (jenni191@umn.edu), Aaron Schmitz (schm2225@umn.edu), Zhengmu Wang (wang1438@umn.edu)

Enlish

Project Information

Title: Fluctuating Stress Laboratory Experiment: Hand Crank Generator

Description:

We were tasked with creating a new laboratory experiment for the ME 3221 course at the University of Minnesota that demonstrated cyclic stresses and strains on a rotating shaft.

Products

WSN-9197, WSN-3214, LabVIEW

The Challenge

We were tasked with creating a new laboratory experiment for the ME 3221 course at the University of Minnesota that demonstrated cyclic stresses and strains on a rotating shaft.

The Solution

To measure the strains on the material surface we used National Instruments wireless data acquisition hardware and LabVIEW software.  The WSN-3214 was mounted on the rotating shaft’s crank and conditioned the signal from the strain gauges on the shaft.  The signal was then transmitted to the WSN-9197 for analysis on a computer.

The mechanical engineering curriculum at the University of Minnesota relies on ME 3221 to demonstrate how statics and dynamics dictate the design of machine components.  A key part of that subject is stresses and strains on a material under a cyclic load.  The existing apparatus for the cyclic stress experiment utilizes strain gauges placed on the crank of a stationary bicycle.  The strain gauges transmit the strain data through a set of slip rings to an analog to digital converter.  This lab apparatus is over ten years old and needs repairs to the bike hardware and data acquisition hardware and software.  This lab is a very important part of ME 3221 since the material is carried through the entire class and beyond.  As a result, it is critical that there is a working, intuitive apparatus.

Often when a lab needs a new experiment it will be presented as an engineering challenge for the Senior Design course, ME 4054W, at the University of Minnesota.  Our senior design group was tasked with creating a new lab that covered the same material as the old one with a different apparatus and updated data acquisition equipment from National Instruments.  We were allowed to choose what kind of apparatus to build as long as it demonstrated cyclic stresses.  Our group chose a hand crank generator for the apparatus where the stresses at the crankshaft would be measured. 

The Generator

Our new lab apparatus consists of a prebuilt hand crank generator with a custom made crank installed.  On the crank shaft we have installed two strain gauge rosettes that are located 90 degrees apart.  The strain gauges are wired to two National Instruments WSN 3214 wireless transmitters for signal conditioning.  The signal is sent to a National Instruments WSN 9791 receiver that is connected to a computer running LabVIEW.  A virtual interface in LabVIEW displays the strains and stresses from the strain gauges. The WSN 3214

Choosing Data Acquisition Hardware

One of the unique challenges we encountered when designing this lab apparatus was how to connect the strain gauges on the rotating shaft to the computer.  Two options were available to us.  We could have either used slip rings or a wireless transmitter.  Slip rings generate enough noise in a signal that it could cause a problem, especially since the signal would be conditioned after the slip rings, magnifying any noise created.  To overcome this we decided to use National Instruments wireless data acquisition hardware that was made specifically for strain measurements. This allowed us to avoid using slip rings as well as condition our signal without need to use additional amplifiers.  There are signal conditioning modules from other companies, such as Micro Measurements, that take data directly from strain gauges and feed it to the computer, but the National Instruments solution allowed us to do it wirelessly.

LabVIEW Virtual Interface

The existing bicycle lab is difficult to calibrate. Our solution includes a calibration function built right into the virtual interface.  The strain gauges on the crank shaft use the strain gages oriented axially on the shaft to obtain bending strain.  There are two gages located 90 degrees apart on the shaft for bending.  The calibration assumes that when a strain gage is located at the top of the shaft it is measuring the maximum bending strain and when it is located at the side it is zero.  This procedure takes less than 30 seconds and provides an accurate zero point for the lab procedure.  The virtual interface itself is composed of three graphs that display bending strain at 0 and 90 degrees and torsional strain.  Data is sent from the WSN 3214 in packets over a few seconds so a real time update is displayed on the graph when it is running.  The data is recorded on a .csv file for later analysis.

Lab Procedure

There are two components to the laboratory procedure after the system has been calibrated.  The first is to get an accurate set of data from a known load on the shaft.  A set of weights is attached to the fabricated aluminum crank and the handle is placed at the opposite end.  The student cranks at a constant speed to produce a smooth sinusoid in the strain graphs in the virtual interface graph.  The weighted crank provides a shifting load and makes for an easy calculation for the student.  The second part of the lab is to put the handle back on the aluminum crank and remove the weights.  The student then cranks the handle at an approximate constant speed.  Ideally, the strain graphs will be constant offset values due to the constant tangential input force.

Generator vs Bicycle

By using National Instruments wireless data acquisition hardware we were able to easily condition our strain gauge signal and make storing and transporting the apparatus much simpler.  The existing apparatus has a bundle of wires attached to the slip rings on the bike crank to the analog to digital converter on the computer permanently, which makes setup and transport cumbersome.  Additionally, the use of National Instruments products makes duplicating this new generator apparatus a simple task.   

Conclusion

Using National Instruments data acquisition software and LabVIEW software allowed us to create a new lab experiment for the ME 3221 course.  The new experiment effectively demonstrates fluctuating stresses on a rotating shaft. National Instruments was able to provide us with hardware that was made for conditioning and transmitting strain gauge information directly to LabVIEW.