Extreme mountain-bikers subject suspension forks to high stress forces. After miles on tough terrain, the forks have a tendency to fatigue. If failure occurs, it's typically on the critical weld between the top of the fork and the stem that fits into the frame headset.
Fox Racing Shox, which designs and manufactures high-performance shock absorbers and suspension products for mountain bikes, motorcycles, ATVs and snowmobiles, designed a new stress-testing machine to test various welds for the best possible strength.
Fox Racing Shox designed the mechanics of the stress test machine and Control Techniques helped design a control system to perform motion and data analysis. The application uses a Control Techniques MC-224 motion coordinator and five, "Motion Made Easy" EN-204 servo-drives with matched NT-330 servomotors coupled to five linear actuators.
A force transducer connects to each actuator and feeds back to the motion controller as a voltage proportional to force. The motion controller then communicates its data to a micro-PLC with touchscreen operator interface.
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 Linear motion actuators on the back of the test stand are matched with Control Technique's servo motors and ball-screw linear actuators. |
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Closed-loop control
After loading the machine with five sample forks, the operator enters a variety of test parameters through the touchscreen, including force, run time, and frequency (speed) of the motion. The PLC transfers this test data to the MC-224 motion controller.
The application required closed-loop control using the force feedback (voltage) and position feedback monitoring (encoder). The motion controller closes the loop from the force value at 2 ms for all axes. This gives the machine accurate control of the force applied to the forks, since the PID (proportional-integral-derivative) tuning terms are available.
The MC-224 tightly controls five axes of motion (with ability to handle 24 axes), applying a push-pull bending force to the bike forks under test. The force transducer coupled to the linear actuator provides accurate force feedback to the motion controller through an analog input. The MC-224 generates a true sine wave motion profile independent for each axis. Each sine wave is adjustable by the operator for amplitude (peak force) and frequency (move speed), which can be changed "on-the-fly."
End position captured
The motion controller's multi-tasking ability is used to separate the motion programs and the PLC communications program. As the metal in the fork fatigues over time, the ending stroke position of the push-pull will change. The final stroke position is captured for each axis. This critical data is sent to the PLC for later analysis, which ultimately results in better fork designs.
Closed-loop control for position and force was key to the success of the stress test station for mountain bike forks, according to those involved in the application.
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