Shock Testing with Multi-tasking Motion Controller

Today's extreme mountain-bike riders subject suspension forks to high stress forces. After miles of ascents and descents on some of the world's toughest 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.

12/01/2005


AT A GLANCE
 
  • Weld quality test stand

  • Five axes of motion control

  • Closed-loop control

  • Position, force


Today's extreme mountain-bike riders subject suspension forks to high stress forces. After miles of ascents and descents on some of the world's toughest 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.

To test various welds for the best possible strength, Fox Racing Shox designed a new stress-testing machine using motion technologies from Emerson Control Techniques. Fox Racing Shox designs and manufactures high performance shock absorbers and racing suspension products for mountain bikes, motorcycles, ATVs, and snowmobiles.

Fox Racing Shox designed the mechanics of the new stress test machine with in-house expertise. Control Techniques helped design a control system to perform motion and data analysis for the machine. 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.

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. (Few standard motion controllers can do this without some customization, Control Techniques says.)

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 Fox Racing Shox stress test station for mountain bike forks, according to those involved in the application.






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