Saturday, August 14, 2010

The cyclic load fatigue tester

My project for most of the summer has been this cyclic load testing machine. It is an unpaid research project that will earn me some class credit. The idea is to create a machine that can apply a controlled force to bend the bicycle fork back and forth thousands of times, measuring the load and displacement and recording the two. The goal is to find out how well current specifications for bicycle forks, typically made of metal, predict failure in composite forks, such as the one mounted in the machine here.
Most of the electronic equipment is from Enfield, they supplied the PID (green) the servo valve (black just above the pid) and the pneumatic ram with a built in potentiometer to detect position. I fabricated the mounting structure for the ram, and the mount for the head tube was created by another student before I began this project. A PID is a device which outputs a control signal which is intended to control something so that the feedback signal matches the command signal. for example if you used it on an oven the control would control the heating elements, the command would be a signal equivalent to what the temperature sensor would read at the temperature you desired and the sensor would of course be the temperature sensor. In this application I wired the feedback to a load cell (a sensor which detects force) and the control to the servo valve that lets air in and out of the ram. Then I give the PID command signal corresponding to the load cell reading for the amount of force I want, and it adjusts the valve accordingly. To generate the command signal I have a LabJack U3, one of many data acquisition devices available. We chose this one because it is fast, has good sample code in many languages, and includes analog outputs, so we can use it for control as well as recording.
At this stage in the project I have the machine working, and I have figured out the commands to control it from VB or C++ (.NET 2008 versions), so it's mainly a matter of writing the software. There does seem to be an issue with the range on the load cell, but I have others I can try. I just got as far as making the force feedback work with the PID. In general my plan is first make it work, then make it better.